Ternary organic solar cells

Ternary organic solar cells

CHAPTER THREE Ternary organic solar cells Xiaoling Ma, Fujun Zhang* Key Laboratory of Luminescence and Optical Information, Ministry of Education, Be...

5MB Sizes 1 Downloads 80 Views

CHAPTER THREE

Ternary organic solar cells Xiaoling Ma, Fujun Zhang* Key Laboratory of Luminescence and Optical Information, Ministry of Education, Beijing Jiaotong University, Beijing, China *Corresponding author.

3.1 Introduction The continuous increase in fossil fuelebased energy consumption has caused a global environmental problem, and the development of clean and renewable energy resources is an effective approach to solve the energy supply issue in the future [1e3]. Among the numerous renewable energy resources, solar energy is widespread and inexhaustible [4,5]. As a technology of converting sunlight to electricity, organic solar cells (OSCs) have attracted tremendous attention owing to their great potential for large-area production with low-cost materials and simple solution processing method. Currently, most highly efficient OSCs are fabricated with a bulk heterojunction structure in which electron donating and electron accepting materials are blended together within an active layer providing a large interfacial area for exciton dissociation [6,7]. The thickness of active layers is usually very thin (w100 nm) to ensure efficient charge transport and collection. However, the photon harvesting of thin active layers is insufficient due to the intrinsically narrow absorption range of organic materials. Increasing photon harvesting of active layers is the prerequisite issue for obtaining highly efficient OSCs. A detailed investigation on the donor absorption window indicates that the power conversion efficiency (PCE) of OSCs may exhibit approximately 35% improvement if the width of the absorption window can be increased from 200 to 400 nm [8,9]. Compared with the single-junction OSCs, tandem configuration solar cells can not only improve light absorption but also reduce thermalization loss of photonic energy by stacking two or more subcells with complementary absorption range [10e13]. Unfortunately, the tandem OSCs inescapably confront some serious technical challenges due to their complicated multilayers stacking in series; for

Solar Cells and Light Management ISBN: 978-0-08-102762-2 https://doi.org/10.1016/B978-0-08-102762-2.00003-3

© 2020 Elsevier Ltd. All rights reserved.

59

j

60

Xiaoling Ma and Fujun Zhang

instance, processing the intermediate layers as well as balancing the light absorption among each subcells makes it difficult in practical application. Alternatively, ternary OSCs featuring multiple light harvesting materials in one active layer have emerged as one of the most promising strategies to improve photovoltaic performance since ternary blends possess the advantage of improving photon harvesting ability as in tandem cells, while maintaining the straightforward fabrication that is utilized in single-junction devices [14]. In addition, the incorporation of the third component also can facilitate charge transport, exciton dissociation, and film morphology (molecular crystallinity, crystal orientation, domain size, and purity) [9,15e18]. The key photovoltaic parameters, short circuit current (JSC), open circuit voltage (VOC), and fill factor (FF) can be simultaneously or individually optimized to their maximum values by carefully selecting the third component and adjusting its content. Meanwhile, all the strategies for optimizing the performance of single bulk heterojunction OSCs, such as thermal annealing, solvent vapor annealing, hot solution, mixing solvent, solvent additive, and methanol treatment, can also be effectively applied in ternary OSCs [19e24]. Today, due to rapid development of donor and acceptor materials and device optimization technique, the PCE of ternary OSCs has exceeded 14% [25e27]. In this chapter, the development of ternary OSCs is summarized. In the early studies, fullerene derivatives such as PC61BM ([6,6]-phenyl-C61butyric acid methyl ester) or PC71BM were usually utilized as the dominate electron acceptors in ternary OSCs due to their high electron mobility and isotropy of charge transport. However, fullerene derivatives have several shortcomings, such as weak absorption in the visible region, limited tunability of energy levels, and poor stability [28]. Recently, nonfullerene acceptor materials with the advantages of strong absorptivity in specific region and tunable energy level have attracted much attention of researchers in photovoltaic region. Ternary OSCs can be mainly classified into three categories based on the combination of different acceptor materials: fullerene-based OSCs, fullerene- and nonfullerene-based OSCs, and nonfullerene-based OSCs. A detailed introduction about the fundamental working mechanisms (charge transfer, energy transfer, parallel-linkage, alloy model) of ternary OSCs is presented, followed by corresponding characterization techniques. The study direction of ternary OSCs also involves thick-film device, semitransparent device, and stability for future applications, which is discussed in this chapter. The current challenges and

61

Ternary organic solar cells

further prospects on ternary OSCs are also briefly analyzed in the last section.

3.2 Working mechanism of ternary OSCs It is known that the ternary OSCs can be classified into two categories according to the function of the third component: two donors/one acceptor (D1:D2:A1) and one donor/two acceptors (D1:A1:A2). Nowadays, there are four working mechanisms in both categories of ternary OSCs: charge transfer, energy transfer, parallel-linkage, and alloy model, which is closely related to the location of the third component in ternary active layer. The third component could (1) be fully embedded within one particular host domain; (2) form its own channels; (3) locate at the D/A interface; (4) alloy with the donor or acceptor materials [17], as depicted in Fig. 3.1.

3.2.1 Four working mechanisms In the case of charge transfer mechanism, the incorporation of the third component can provide more additional percolating pathways for efficient exciton dissociation and charge transport. The lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO) energy levels of the third component are properly located between the energy levels of the host donor and acceptor to form the cascade energy level alignment. In the D1:D2:A1 ternary system, for example, the third component (D2) acts as a bridge to transfer electrons to acceptor and deliver holes to D1, as shown in Fig. 3.2A. When the third component is located at the

Ternary blend OSCs

Embedded in one phase

Parallel-like-structure

Located at interfaces

Alloy structure

Cathode Electron transport layer Donor + Acceptor + Third component Hole transport layer Anode

Figure 3.1 Structures of ternary blend bulk heterojunction organic solar cells (OSCs) with four possible active layer morphologies. Reproduced with permission L. Lu, M.A. Kelly, W. You, L. Yu, Status and prospects for ternary organic photovoltaics. Nat. Photon. 9 (2015) 491e500. Copyright 2015, Nature.

62

Xiaoling Ma and Fujun Zhang

Figure 3.2 Schematic of the working mechanisms in ternary organic solar cells based on D1:D2:A1 ternary system. (A) The charge transfer mechanism. (B) The energy transfer mechanism. (C) The parallel-like mechanism. (D) The alloy model. The arrows indicate the possible charge carrier transfer and transport pathway.

interface between donor and acceptor, the holes or electrons generated in the third component can be efficiently collected by the corresponding electrode through the dominating charge carrier transport channels [29,30]. Thus, the observed VOC of ternary OSCs in such systems is basically pinned to the smaller VOC of the original binary OSCs. The energy transfer in ternary blend films should be a competing process compared with the charge transfer among the materials, and only one working mechanism is in a dominant position between energy transfer and charge transfer. In the D1:D2:A1 ternary system (Fig. 3.2B), where the third component only acts as the“energy donor,” all holes are only generated in dominating donor and the third component works just as a sunlight absorber (sensitizer) to extend the absorption window [29,31]. The photoexcited sensitizer can transfer

Ternary organic solar cells

63

energy to host donor or acceptor material via the F€ orster resonance energy transfer (FRET) process [31]. Substantial overlapping of the one material’s emission spectrum and the other material’s absorption spectrum is desirable for an efficient energy transfer. The parallel-linkage mechanism of ternary OSCs is significantly different from the charge transfer and energy transfer mechanisms. In parallel-linkage ternary OSCs, it is believed that the excitons generated in two donors/acceptors can migrate to their respective donor/ acceptor interface and then dissociate into free charge carriers in ternary active layers. In the D1:D2:A1 ternary system, this working mechanism is depicted in Fig. 3.2C. In the parallel-linkage ternary OSCs, the energy transfer and charge transfer are both absent between two donors or two acceptors, equivalent to a parallel connection of two individual binary OSCs. In such ternary systems, the VOC values of ternary OSCs lie between the measured values of the two individual binary OSCs and vary along with the composition of the ternary component, rather than being pinned by the smallest energy level difference between the donors and acceptors. Another working mechanism alloy model with the well-mixed D1/D2 or A1/A2 phases was proposed to explain the tunable VOC in ternary OSCs. It is supposed that the two donors or acceptors electronically couple into a new charge transfer state (CT state), which requires a good miscibility of the two donor or two acceptor materials. The CT state energy in alloy model varies along with the content of ternary blend composition, resulting shifted VOC of ternary OSCs. This working mechanism in a D1:D2:A1 ternary system is depicted in Fig. 3.2D. As discussed above, the four fundamental principles in ternary OSCs (charge transfer, energy transfer, parallel-like, alloy model) are clearly different from each other. The mechanism governs the photovoltaic process in ternary OSCs, and the relevant characterization methods are discussed in the following sections.

3.2.2 Characterization methods In a ternary system, both charge transfer and energy transfer can contribute to the improvement of JSC due to better photon harvesting and more efficient charge generation. Photoluminescence (PL) measurement is a convenient tool to investigate charge transfer or energy transfer between two different materials. Let us refer to the D1:D2:A1 ternary system: if energy transfer exists between two different band gap donors, one would expect increased emission intensity for the relatively low band gap donor with decreased emission intensity for the other donor, considering that the

64

Xiaoling Ma and Fujun Zhang

quantum yields of the two donors are similar. On the other hand, if charge transfer occurs between two donors, the emission intensity of one donor would be quenched without the increased emission intensity for the other donor. For example, the PL spectra of P3HT:SMPV1 films and solutions were investigated, as shown in Fig. 3.3A and B. It is apparent that the emission intensity of P3HT and SMPV1 are substantially decreased in blend films and solutions along with the incorporation of SMPV1 [32]. This result suggests that charge transfer occur between the two donors. As displayed in

Figure 3.3 Photoluminescence (PL) spectra of P3HT:SMPV1 films (A) and solutions (B) with different SMPV1 content under 490 nm light excitation. (C) PL spectra of P3HT:DIB-SQ films with different DIB-SQ content under 490 nm light excitation. (D) JeV curves of devices with P3HT, SMPV1, or P3HT:SMPV1 (1:1) as active layers. (E) JeV curves of devices with P3HT, DIB-SQ, or P3HT:DIB-SQ as active layer. (A) and (B) Reproduced with permission of American Chemical Society from Q. An, F. Zhang, L. Li, J. Wang, Q. Sun, J. Zhang, W. Tang; Z. Deng, Simultaneous improvement in short circuit current, open circuit voltage, and fill factor of polymer solar cells through ternary strategy. ACS Appl. Mater. Interfaces 7 (2015) 3691e3698. (C) Reproduced with permission of American Chemical Society from Q. An, F. Zhang, L. Li, J. Wang, J. Zhang, L. Zhou, W. Tang, Improved efficiency of bulk heterojunction polymer solar cells by doping low-bandgap small molecules. ACS Appl. Mater. Interfaces 6 (2014) 6537e6544. (D) Reproduced with permission of American Chemical Society from Q. An, F. Zhang, L. Li, J. Wang, Q. Sun, J. Zhang, W. Tang; Z. Deng, Simultaneous improvement in short circuit current, open circuit voltage, and fill factor of polymer solar cells through ternary strategy. ACS Appl. Mater. Interfaces 7 (2015) 3691e3698. (E) Reproduced with permission of American Chemical Society from Q. An, F. Zhang, L. Li, J. Wang, J. Zhang, L. Zhou, W. Tang, Improved efficiency of bulk heterojunction polymer solar cells by doping low-bandgap small molecules. ACS Appl. Mater. Interfaces 6 (2014) 6537e6544.

Ternary organic solar cells

65

Fig. 3.3C, the emission intensity of P3HT is gradually reduced while DIBSQ exhibits continuously improved emission intensity along with the increase of DIB-SQ content in P3HT:DIB-SQ blend films under 490 nm light excitation, suggesting efficient energy transfer from P3HT to SQ molecules [33]. It should be noticed that there are some possible errors with the PL measurement of film, induced by the sample thickness, measure angle, or molecular alignment, and so on. Zhang et al. first proposed a simple method, named as pure donor/acceptor device, to verify the charge transfer or energy transfer between two donors or acceptors [32,33]. These devices should be fabricated with only donors or acceptors as active layers. The JSC of P3HT: SMPV1-based devices is higher than that of P3HT- or SMPV1-based devices (Fig. 3.3D), which should be ascribed to efficient charge transfer between two donors. The JSC of P3HT:DIB-SQ-based devices is in between those of P3HT-based and DIB-SQ-based devices (Fig. 3.3E), suggesting that charge transfer between P3HT and DIB-SQ should be negligible. Yu et al. also fabricated a solar cell using only two donors to prove the charge transfer at the donors interfaces, exhibiting a relatively low PCE of 0.051% with the corresponding low JSC ¼ 0.192 mA cm2, VOC ¼ 0.65 V, and FF ¼ 40.8% [18]. Combining PL measurement and pure donor/acceptor device is an efficient and exact strategy to distinguish charge transfer and energy transfer between two different materials, which has been demonstrated in many systems by different groups [34,35]. F€ orster theory predicts a decrease in the excited-state lifetime of the FRET-donor with increasing FRET-acceptor concentrations, as FRET introduces an additional nonradiative decay path for the donor into the system. Taylor and coworkers studied the energy transfer between P3HT and SQ by femtosecond fluorescence upconversion technique [36]. The lifetime of neat P3HT film is 223 ps, which is decreased to 52.4 ps by incorporating 1% SQ and further to 9.9 ps with 5% SQ, suggesting the existence of energy transfer from P3HT to SQ. In PTB7-Th:BTR:PC71BM ternary system, the time-resolved transient photoluminescence spectra of the neat PTB7-Th, BTR, and their blend films were investigated by monitoring 760 nm light emission corresponding to the PTB7-Th emission peak [37]. The emission lifetime is increased from 0.08 to 0.18 ns along with the increased content of BTR, which further demonstrates the occurrence of FRET from BTR to PTB7-Th. The efficiency of FRET is given by: sDA s¼1  (3.1) sD

66

Xiaoling Ma and Fujun Zhang

where sD and sDA are the fluorescence lifetime of the FRET-donor in the absence and presence of the FRET-acceptor, respectively [38]. In P3HT: SQ:PC71BM system, the energy transfer efficiency is 77% or 96% for the blend films with 1% or 5% SQ [36]. The similar study of energy transfer efficiency was also carried out by Zhang’s group in SMPV1:DIB-SQ: PC71BM system. The energy transfer efficiency was about 56% or 67% for blend films SMPV1:SQ with weight ratio of 9:1 or 1:1, respectively [39]. Time-resolved transient absorption (TA) spectrum is another method to monitor the photophysical processes. Taylor and coworkers further investigated the photophysical processes of P3HT and SQ by TA spectra, as shown in Fig. 3.4A. TA spectra usually compose two main features: ground state bleach (GSB) with negative signature and excited state absorption (ESA) with positive signature. Neat P3HT film and blend P3HT:SQ films were excited at 500 nm to selectively excite P3HT. At early time, the

Figure 3.4 (A) Transient absorption (TA) spectra of neat P3HT film and P3HT:SQ blend films. The pumping wavelength was 500 nm with a fluence of 8 mJ cm2. The picosecond TA spectra of (B) neat PBDB-T, (C) neat PTB7-Th, and (D) blend PBDB-T:PTB7Th (80:20, wt/wt) films. (E) Transient absorption dynamics of neat PBDB-T, PTB7-Th, and their blend films by probing 585 and 720 nm, respectively. (A) Reproduced with permission of Springer Nature from J.S. Huang, T. Goh, X. Li, M.Y. Sfeir, E.A. Bielinski, S. Tomasulo, M.L. Lee, N. Hazari, A.D. Taylor, Polymer bulk heterojunction solar cells €rster resonance energy transfer. Nat. Photon. 7 (2013) 479e485. (E) Reproemploying fo duced with permission of Wiley-VCH from X. Ma, Y. Mi, F. Zhang, Q. An, M. Zhang, Z. Hu, X. Liu, J. Zhang, W. Tang, Efficient ternary polymer solar cells with two well-compatible donors and one ultranarrow bandgap nonfullerene acceptor. Adv. Energy Mater. 8 (2018) 1702854.

Ternary organic solar cells

67

negative signal contains three peaks at 520, 560, and 610 nm which well accord with the P3HT absorption with 0e2, 0e1, and 0e0 vibrational transitions, respectively. The positive signal at 650 nm represents the photoinduced ESA of P3HT. The TA signal is changed by introducing 1% or 5% SQ. The negative signal represents the GSB of SQ, suggesting that SQ is excited. The excitation of SQ is caused by the excited P3HT rather than the pumping wavelength, due to the SQ negligible absorption at 500 nm. Meanwhile, signals of the photoinduced absorption and stimulated emission in P3HT both substantially decay to zero. This suggests that the excitation energy is transfer from P3HT to SQ. The similar phenomenon can also be observed from PBDB-T:PTB7-Th:IEICO-4F-based OSCs [40], as shown in Fig. 3.4BeE. The TA spectra of neat PBDB-T and PTB7-Th films exhibit featured GSB and ESA signal. For the PBDB-T:PTB7-Th blend films, the GSB and ESA features are the overlapped features of two donors. The ESA peak of PTB7-Th in blend films shows an enhanced intensity and slow decay z1 ps compared to that in the neat PTB7-Th film. The energy transfer process from PBDB-T to PTB7-Th should occur due to the decreased PBDB-T signature and the simultaneously enhanced PTB7-Th signature. The lifetime of GSB signal by probing 585 nm is decreased from 0.49 to 0.47 ps by incorporating 20 wt% PTB7-Th. The lifetime of GSB signal by probing 720 nm is 4.5 ps for the neat PBT7-Th and 6.1 ps for PBDB-T:PTB7-Th blend films, respectively. The energy transfer from PBDB-T to PTB7-Th can be further confirmed from the enhanced lifetime of PTB7-Th GSB signal (720 nm) in the blend films. The efficient energy transfer can enhance the exciton utilization and contribute to PCE improvement. The concept of parallel-linkage was first proposed by You and coworkers, in which ternary blends can be regarded as two independent binary subcells [41]. Two groups of materials, TAZ:DTBT:PCBM (0.5:0.5:1 weight ratio) and DTffBT:DTPyT:PCBM (0.5:0.5:1 weight ratio), were employed for constructing parallel-linkage ternary OSCs. To keep the same quantity of individual components in corresponding ternary and binary OSCs, the thickness of binary and ternary active layers are fixed at w50 and w100 nm, respectively. The w50-nm-thick binary OSCs are defined as “subcell” for better understanding. The absorption spectra of parallel-linkage ternary OSCs are essentially linear combinations of the spectra of their two“subcells,” as shown in Fig. 3.5A and B. The EQE spectra of binary OSCs and ternary OSCs were also measured and are shown in Fig. 3.5C and D. The EQE spectra of parallel-linkage ternary

68

Xiaoling Ma and Fujun Zhang

(A) 0.6

TAZ “subcell” DTBT “subcell” PBHJ cell

0.4 0.3 0.2 0.1

400 70

500 600 700 Wavelength (nm)

0.2 0.1

800

400

(D) 70

60

60

50

50

40

40

EQE (%)

EQE (%)

0.3

0.0

0.0

(C)

30 20

0

800

20 TAZ “subcell” DTBT “subcell” PBHJ cell 400 500 600 700 Wavelength (nm) TAZ “subcell” Jsc: 6.53; Voc: 0.73

Current Density (mA/cm2)

(E) 10 8 6 4 2 0 -2 -4 -6 -8 -10 -12

500 600 700 Wavelength (nm)

30

DTBT “subcell” Jsc: 5.68; Voc: 0.81 PBHJ cell Jsc: 11.2; Voc: 0.77

0.0

0.2

0.4 0.6 Voltage (V)

DTffBT “subcell” DTPyT “subcell” PBHJ cell

10 0

800

400

(F) 10 8 6 4 2 0 -2 -4 -6 -8 -10 -12 -14 1.0

0.8

500 600 700 Wavelength (nm)

800

DTffBT “subcell” Jsc: 7.86; Voc: 0.89

Current Density (mA/cm2)

10

DTffBT “subcell” DTPyT “subcell” PBHJ cell

0.4 Absorption (a.u.)

Absorption (a.u.)

0.5

(B) 0.5

DTPyT “subcell” Jsc: 6.99; Voc: 0.83 PBHJ cell Jsc: 13.5; Voc: 0.87

0.0

0.2

0.4 0.6 Voltage (V)

0.8

1.0

Figure 3.5 Absorption spectra of the parallel-linkage ternary organic solar cells (OSCs) and their “subcells” based on (A) TAZ/DTBT and (B) DTffBT/DTPyT. EQE spectra of the parallel-linkage ternary OSCs and their “subcells” based on (C) TAZ/DTBT and (D) DTffBT/DTPyT. JeV curves of the parallel-linkage ternary OSCs and their “subcells” based on (E) TAZ/DTBT and (F) DTffBT/DTPyT. Reproduced with permission of American Chemical Society from L. Yang, H. Zhou, S.C. Price, Y. Wei, Parallel-like bulk heterojunction polymer solar cells. J. Am. Chem. Soc. 134 (2012) 5432e5435.

OSCs are approximately the sum of those of the individual “subcells” in the short wavelength range, where both donor polymers contribute to the formation of electronehole pairs. This suggests that most of the free charge

Ternary organic solar cells

69

carriers generated in each “subcell” of the PBHJ device are successfully collected by the respective electrodes. In the long-wavelength range, the EQE spectra of the parallel-linkage ternary OSCs are higher than that of the smaller band gap “subcell.” This is probably due to that the large band gap polymer with high mobility can serve as an additional charge transport channel in the parallel-linkage ternary OSCs, which facilitate the charge transport. In general, the JSC of the parallel-linkage ternary OSCs are significantly increased and almost identical to the sum of those for the two single “subcells”; the VOC values of the parallel-linkage ternary OSCs are between those of individual “subcells,” as shown in Fig. 3.5E and F. This should be attributed to that holes generated from individual donor polymers mainly travel through their corresponding polymer-connected channel to the anode, which is similar to the parallel connection of two single-junction binary OSCs. The parallel-linkage mechanism in ternary OSCs can also be verified from the PL spectra of the corresponding neat films or blend films. In PTB7:PBDT-TS1:PC71BM ternary system [42], both PL emission peaks of PTB7 and PBDT-TS1 are at about 780 nm due to their similar band gap of 1.8 eV. The PL emission intensity of PBDT-TS1 films is stronger than that of PTB7 films under the same excitation conditions. The emission intensity of PTB7:PBDT-TS1 blend films is gradually enhanced in the range from 750 to 850 nm along with the increased content of PBDT-TS1; and the emission peak of the blend films is located at between that of neat PTB7 and PBDT-TS1 film. This indicates neither charge transfer nor energy transfer exist between PTB7 and PBDT-TS molecules. In addition to D1:D2:A1 ternary system, the parallel-linkage mechanism has also been demonstrated between two acceptors. Sun et al. reported a ternary system based on PDBT-T1:PCBM:ITIC-Th as active layer [35]. The parallellinkage can be characterized by grazing incidence X-ray diffraction (GIXD) and resonant soft X-ray scattering (RSoXS) technology. When the ITIC-Th content increased to more than 30 wt%, the diffraction peak of PDBT-T1, PCBM, and ITIC-Th domain can be observed, suggesting that the used three materials can form their individual phase. For RSoXS measurements, 0% ITIC-Th blend shows a shoulder in the scattering at w0.014Å1, corresponding to a length scale of 45 nm for the phase separated domain. When the ITIC-Th content increased up to 50 wt%, the RSoXS profile shows two shoulders in the scattering at 0.013 Å-1 (48 nm) and 0.003 Å-1 (210 nm), indicating the formation of multilength scale morphology. The two length scales should be derived from the ITIC-Th

70

Xiaoling Ma and Fujun Zhang

and PCBM-rich domains. As a result, the PDBT-T1:PCBM:ITIC-Thbased OSCs work in parallel-linkage structure model. The alloy model (with the well-mixed D1/D2 or A1/A2 phases) was proposed by Thompson and Street et al. [43] to explain the tunable VOC of ternary OSCs. In this mechanism, the two donors or two acceptors should be well mixed to form the same frontier orbital energy levels, which are determined by blend composition. The VOC of OSCs is determined by the energy of interface band gap (EGI) minus the quasi-Fermi energies, where the EGI is the energy difference value between the HOMO energy level of donor and the LUMO energy level of acceptor. The reason of continuous change in VOC is the corresponding change of HOMO or LUMO energy levels. The photocurrent spectral response (PSR) was carried out by Thompson et al. to explore the electronic states in ternary blends and confirm the continuous change in the energy of the HOMO or LUMO of the complementary two-component materials. The PSR of the mixed acceptor system P3HT:PCBM:ICBA was measured and are shown in Fig. 3.6A. It has been reported that the energy below the optical band gap of each material is derived from the direct excitation at interface from the HOMO of donor to the LUMO of acceptor (Fig. 3.6A inset). Previous studies have shown that the actual interface band gap corresponds to a (B) 1.5

100 Fraction ICBA

donor 10

acceptor

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8 0.9 1

1 0.1 0.01 0.001

0.0001 0.8

1

1.2

1.4

1.6

Energy (eV)

1.8

2

1.4

0.85

1.35 PC=0.1

1.3 1.25

0.75

VOC

1.2

0.65

1.15 1.1

PC=0.01

0.55

1.05 2.2

1

Open-circuit voltage Voc(V)

0.95

1.45

Energy (eV)

Photocurrent (arb. units)

(A) 1000

0.45 0

0.2

0.4

0.6

0.8

1

Fraction ICBA

Figure 3.6 (A) Photocurrent spectral response (PSR) data for the P3HT:PCBM:ICBA ternary system plotted as a function of ICBA fraction in PCBM:ICBA pair. The inset represents the EGI or charge transfer transition. (B) Plot of the estimated interface band gap energy defined by PSR at photocurrent values of 0.1 and 0.01 compared to the values of VOC for P3HT:PCBM:ICBA ternary system. Reproduced with permission of American Chemical Society from R.A. Street, D. Davies, P.P. Khlyabich, B. Burkhart, B.C. Thompson, Origin of the tunable open-circuit voltage in ternary blend bulk heterojunction organic solar cells. J. Am. Chem. Soc. 135 (2013) 986e989.

Ternary organic solar cells

71

photocurrent of w104 of the peak value, where the shape of the PSR spectrum changes from an exponential at lower energy to a broader band at higher energy absorption of the heterojunction interface [44]. The interface absorption measured at photocurrent values of 0.1 and 0.01 (dashed lines in Fig. 3.6A) and the VOC of ternary OSCs were summarized in Fig. 3.6B. It can be observed that VOC is w0.55 V smaller than the interface band gap, which is consistent with the expected quasi-Fermi energies. The results indicate that the change in the EGI was accurately measured by VOC and that EGI changes continuously with the composition of two-component acceptors. The composition dependence of the band gap for an alloy model is often described by the extension of Vegard’s law [45,46]. EG1 ¼ ð1  XÞEG1 þ XEG1  bxð1  xÞ

(3.2)

in which b is known as the bowing factor and EG1 and EG2 are the band gaps of the used two materials. This model fits the data well, which can be observed in Fig. 3.6B. Similar measurements are also carried out in other ternary system based on P3HT75-co-EHT25/P3HTT-DPP-10%/PCBM as active layers. In this system, the VOC also can be continuously changed along with the different proportions of two donors. Either two donors or two acceptors can form an electronic alloy with changed HOMO and LUMO energy levels based on the average composition of two donors or acceptors, resulting in variation of VOC with composition in these two components. The alloy model can also be verified by varied HOMO and LUMO energy level of donors or acceptors, which can be calculated by cyclic voltammetry (CV) curves. In PBDB-T:ITCPTC:IDT6CN-M-based system [47], the LUMO energy levels of the acceptor blend films are gradually decreased along with the increase of ITCPTC content, indicating that the alloy state of ITCPTC and IDT6CN-M is formed in the acceptor blend films, as shown in Fig. 3.7A and B. In another ternary system based on PSTZ:IDIC:ITIC as active layer [48], the ITIC:IDIC blends exhibit the varied frontier orbital energies dependence on the composition of these two components, which indicate the forming of an electronic alloy between ITIC and IDIC, as shown in Fig. 3.7C. Recently, another strategy was proposed to distinguish if there exists an alloy model between two acceptors. The linearly varied VOC can be observed in ternary system when the two acceptors have similar chemical structure. A quasilinear correlation of VOC could be observed in the ternary system where the chemical nature of the two acceptors is drastically different. If the alloy states can be formed between two donors or

72

Xiaoling Ma and Fujun Zhang

(B) Ferrocene

(C) LUMO (eV)

HOMO energy level

Current (mA)

E (eV)

ITCPTC content

E (eV)

LUMO energy level

-2

-1

0

1

2

-2

-1

(D)

0

1

2

HOMO (eV)

(A)

ITIC contents

ITCPTC ratios (wt%)

(E)

0.3

INPIC-4F MeIC1

LUMO

MeIC1 content (wt%)

LUMO

INPIC-4F

MeIC1

LUMO+1

LUMO+1

Δ Energy (eV)

Voc(V)

LUMO+1 0.2 LUMO+1 0.1 0.0

-0.1

LUMO

LUMO

quasi-degenerate LUMOs

Figure 3.7 (A) Cyclic voltammetry plots of ITCPTC:IDT6CN-M blend films with different ITCPTC content. (B) The HOMO and LUMO energy levels of the acceptor blend films against the ITCPTC content in the acceptors. (C) Electronic energy levels of the ITIC: IDIC blend films with different ITIC contents. (D) Dependence of the VOC of the organic solar cells on the MeIC1 content, energy levels of the used materials, and schematic dynamic processes in the ternary active layers. (E) Pictorial representations of the frontier molecular orbitals of INPIC-4F and MeIC1 from the DFT calculations. Diagram of the frontier orbital energies of INPIC-4F and MeIC1. LUMO, lowest unoccupied molecular orbital; HOMO, highest occupied molecular orbital. (A) and (B) Reproduced with permission of Royal Society of Chemistry from M. Zhang, W. Gao, F. Zhang, Y. Mi, W. Wang, Q. An, J. Wang, X. Ma, J. Miao, Z. Hu, Efficient ternary non-fullerene polymer solar cells with PCE of 11.92% and ff of 76.5%. Energy Environ. Sci. 11 (2018) 841e849. (C) Reproduced with permission of Elsevier from W. Su, Q. Fan, X. Guo, X. Meng, Z. Bi, W. Ma, M. Zhang, Y. Li, Two compatible nonfullerene acceptors with similar structures as alloy for efficient ternary polymer solar cells. Nano Energy 38 (2017) 510e517. (E) Reproduced with permission of Royal Society of Chemistry from X.L. Ma, W. Gao, J.S. Yu, Q.S. An, M. Zhang, Z.H. Hu, J.X. Wang, W.H. Tang, C.L. Yang, F.J. Zhang, Ternary nonfullerene polymer solar cells with efficiency > 13.7% by integrating the advantages of the materials and two binary cells. Energy Environ. Sci. 11 (2018) 2134e2141.

two acceptors, the mixed new quasi frontier orbitals density (Ne) of alloy states strongly depends on the content of the third component. The VOCs of ternary OSCs can be calculated by the equation: VOCTernary ¼

f 1  N e1  V OCBinary1 þ f 2  N e2  V OCBinary2 f 1  N e1 þ f 2  N e2

(3.3)

in which V OCBinary1 , V OCBinary2 , and VOCTernary are the VOCs of corresponding binary OSCs and ternary OSCs; Ne1 and Ne2 are the Ne of acceptor 1 (donor

Ternary organic solar cells

73

1) and acceptor 2 (donor 2); f1 and f2 are the weight ratios of acceptor 1 (donor 1) and acceptor 2 (donor 2) in acceptors (donors). Using DFT quantum calculations, Ne can be obtained by Ne ¼ nl, where n is the molecular number of unit mass, and l is the number of quasi-degenerate LUMOs (<0.1 eV) of per molecular. This formula was proposed by Huang et al. and was verified in PTB7-Th:PC71BM:TPE-4PDI and PBDBT:ITIC-Th:TPE-4PDI systems [49,50]. Zhang et al. also employed the formula to confirm the formation of alloy model in PBDB-T:INPIC-4F: MeIC1-based ternary system [51]. The VOCs of the OSCs are gradually increased from 0.85 to 0.90 V along with the incorporation of MeIC1, as exhibited in Fig. 3.7D. The frontier molecular orbitals and frontier orbital energy diagrams of the used acceptors are shown in Fig. 3.7E. The Ne is 3.3  1020 mol g1 for INPIC-4F or 4.1  1020 mol g1 for MeIC1. According to Eq. (3.3), the calculated VOCs according to the equation are appropriate 0.86 V, 0.87 V, 0.88 V, and 0.89 V for ternary OSCs with 10 wt%, 30 wt%, 50 wt%, and 70 wt% MeIC1 in acceptors, which are well consistent with the measured VOCs.

3.3 The development of ternary OSCs In this section, we summarize the development of ternary OSCs based on different systems. According to the composition of acceptor material, the ternary system can be divided into three categories: fullerene-based OSCs, fullerene- and nonfullerene-based OSCs, and nonfullerene-based OSCs, which are discussed in the following.

3.3.1 Fullerene-based ternary OSCs In the early development process of OSCs, fullerene derivatives are usually used as acceptors due to their high electron mobility, large electron affinity, and isotropic charge transport. Ternary OSCs based on fullerene systems can be classified into two categories: donor 1:donor 2:fullerene and donor: fullerene 1:fullerene 2. 3.3.1.1 Donor 1:Donor 2:fullerene The early study of ternary OSCs is focused on P3HT:PCBM-based system. In 2005, Kim et al. have attempted to improve the compatibility of P3HT and PCBM by incorporating the third component F8BT into the blend films [52]. F8BT plays a role of compatibilizer to improve morphology of blend films, and both the JSC and the VOC of OSCs are improved. In

74

Xiaoling Ma and Fujun Zhang

2007, Heeger et al. investigated the performance of P3HT:PCBM-based OSCs with different P3HT molecular weight [53]. The best performance was obtained by using P3HT with an appropriate ratio between highmolecular-weight and low-molecular-weight components. It is known that P3HT possesses a wide band gap of w1.9 eV, and the photon harvesting was limited in short wavelength region. A polymer donor PCPDTBT possesses near-infrared (NIR) absorption and suitable energy levels and was incorporated into P3HT:PC61BM host system as the third component [54]. The absorption intensity of blend films is gradually enhanced in the long wavelength region along with the increase of PCPDTBT content. The EQE spectra are significantly enhanced due to the PCPDTBT sensitization of the NIR photoresponse, which have similar varying tendency with the absorption spectra. The extended absorption in long wavelength region and cascaded energy level between used materials contributes to the improved PCE from 2.5% to 2.8%. Similarly, the narrow band gap material Si-PCPDTBT was also used as the third component incorporating into P3HT:PCBM and P3HT:ICBA systems, and the PCE was improved to 4.0% and 5.1%, respectively [55,56]. Before 2014, most of study was focused on the P3HT system. Although the key parameters can be improved in ternary OSCs based on P3HT with various component, the PCE of P3HT-based ternary OSCs is still limited by relatively poor performance of binary reference device. Therefore, new approaches have been developed to improve device performance by incorporating either fullerene derivative acceptors or polymers and small-molecule donors as the third component into high-performance donoreacceptor copolymers. For example, Lu et al. incorporated a wide band gap polymer PID2 into PTB7:PC71BM system to improve the photon utilization in short wavelength region [18]. Along with the increase of PID2 content in donors, the absorption intensity of blend films in the wavelength range from 450 to 650 nm is gradually enhanced, while the absorption intensity from 650 to 750 nm is simultaneously decreased, as shown in Fig. 3.8A. This is consistent with the fact that the absorption peaks of PID2 and PTB7 are located at 610 and 683 nm, respectively. The photon harvesting of blend films can be adjusted by varying the PID2 content in donors. The incorporation of 10 wt% PID2 in donors lead to higher EQE values over the whole wavelength region (Fig. 3.8B), which can explain the improved JSC. In addition to enhanced photon harvesting, the improved JSC should be also attributed to improved charge separation and transport and decreased charge recombination, resulting from the cascade energy levels and optimized morphology of the ternary

75

Ternary organic solar cells

(A)

(B)

EQE (%)

Absorption (a.u.)

80 60 40 20 0 400

500

600

700

200

800

300

Wavelength (nm)

Intensity (a.u.)

10

(D)

0% 5% 10% 15% 100%

12

500

1011 1.4

1.6

1.8

2

(010)

1010

600

700

800

Wavelength (nm)

Intensity (a.u.)

(C)

400

0% 5% 10% 15% 100% 1010

109

109 0.5

1.0

qz

(-1)

1.5

2.0

0.5

1.0

1.5

2.0

qxy (-1)

Figure 3.8 (A) UV-vis absorption spectra of PTB7:PID2:PC71BM films with different PTB7-Th:PID2 weight ratios. (B) EQE curves of organic solar cells with different PID2 content. The out-of-plane (C) and in-plane (D) cuts of the PTB7-Th:p-DTS-(FBTTH2)2:PC71BM blend films with different p-DTS-(FBTTH2)2 content. Reproduced with permission of American Chemical Society from Ref. [59]. (A) and (B) Reproduced with permission of Springer Nature from L. Lu, T. Xu, W. Chen, E.S. Landry, L. Yu, Ternary blend polymer solar cells with enhanced power conversion efficiency. Nat. Photon. 8 (2014) 716e722.

active layers. The PCE is improved to 8.22%, with a JSC of 16.8 mA cm2, a VOC of 0.72 V, and an FF of 68.7%. The wide band gap material PID2 was also used as the third component incorporating into the PTB7-Th: PC71BM-based binary system [57]. The PCE was improved to 9.2% by incorporating 20 wt% PID2 in donors without other optimization treatment. The OSCs based on PTB7-Th:PC71BM have been well studied and afforded decent photovoltaic performance recently. Wang et al. designed ternary OSCs based on PTB7-Th:PffBT4T-2OD PC71BM as active layers [58]. The PffBT4T-2OD owns large absorption cross section, proper energy levels, and good crystallinity, which enhance exciton

76

Xiaoling Ma and Fujun Zhang

generation, charge dissociation, and transport and suppresses charge recombination of active layers, thus remarkably increasing the JSC and FF. Finally, a notable PCE of 10.72% is obtained for the OSCs with 15% weight ratio of PffBT4T-2OD. In addition to polymer molecular, a highly crystalline small molecule p-DTS-(FBTTH2)2 was also selected as the third component incorporating into PTB7-Th:PC71BM binary system, and the PCE of ternary OSCs was improved to 10.5% in optimized conditions [59]. Both the crystallinity and the face-on preferential orientation with respect to the substrate are enhanced when the small molecule is added, as shown in Fig. 3.8C and D. Ternary strategy has also been applied in all small molecular systems due to the well-defined molecular structure, less batch-to-batch variation and easier purification/synthesis of small molecular. A nematic liquid crystal material BTR as a morphology regulator was incorporated to DRCN5T:PC71BM host system [21]. The PCE of ternary OSCs was improved to 10.05% by mixing 1.5 wt% BTR, which should be attributed to optimized phase separation and complementary photon harvesting between DRCN5T and BTR. 3.3.1.2 Donor:fullerene 1:fullerene 2 Fullerene derivatives can also be used as the third component in ternary system. As a representative of fullerene derivatives, ICBA [60] is commonly used as the third component in D1:A1:A2 system. In 2011, ternary OSCs based on P3HT:PC61BM:ICBA as active layers were fabricated by Khlyabich et al. [61]. The VOC of ternary OSCs can be tuned between the VOCs values of the corresponding two binary OSCs. In 2014, ICBA as electron-cascade acceptor was incorporated into PTB7:PC71BM system to fabricate ternary OSCs by Zhan et al. [62], as shown in Fig. 3.9. The VOC of ternary OSCs is increased along with the incorporation of ICBA, which should be ascribed to higher LUMO energy level of ICBA compared with PC71BM. ICBA can play a bridging role between PTB7 and PC71BM, thus providing more routes for charge transfer at the donoreacceptor interface. From the atomic force microscopy (AFM) images, when the ICBA content is much lower than PC71BM content (e.g., 15% in fullerene), the morphology of the ternary active layer is similar to that of PTB7: PC71BM, which guaranteed suitable phase separation and efficient charge transport. The PCE of ternary OSCs was increased to 8.24% by incorporating 15wt% ICBA in acceptors, resulting from simultaneously improved JSC, VOC, and FF. Similarly, ICBA was also incorporated into a small molecular system BDT6T:PC71BM, and the PCE was improved from 5.74%

Ternary organic solar cells

77

Figure 3.9 (A) JeV curves of organic solar cells with the structure of ITO/PEDOT:PSS/ PTB7:ICBA:PC71BM/Ca/Al with different weight ratios of ICBA in fullerene under the illumination of an AM 1.5G solar simulator. (B) Energy levels of PTB7, ICBA, and PC71BM. AFM height images (CeE) and phase images (FeH) of PTB7:ICBA:PC71BM films spincoated on ITO/PEDOT:PSS substrates: (C and F) 0% ICBA, (D and G) 15% ICBA, and (E and H) 30% ICBA. Reproduced with permission of Royal Society of Chemistry from P. Cheng, Y. Li, X. Zhan, Efficient ternary blend polymer solar cells with indene-c60 bisadduct as an electron-cascade acceptor. Energy Environ. Sci. 7 (2014) 2005e2011.

to 6.43% by incorporating 15 wt% ICBA in acceptors [63]. In addition to ICBA, some other fullerene derivatives were also employed as the third component to improve device performance. For example, the PCE of P3HT- or PTBT-based OSCs was improved to 3.59% or 7.00% by employing mixed PC71BM and PC61BM as acceptors [64,65]. PC71BM is almost identical to PC61BM in terms of its molecular structure, frontier orbital energy levels, electron mobility, and electronic properties. The distorted geometry of PC71BM relative to PC61BM endows it with a much greater absorption coefficient in the visible region, allowing it to produce larger photocurrents compared to PC61BM. The PCE of P3HT:PC61BM-based OSCs was improved to 4.8% by incorporating PCBPy as the third component, resulting from the fact that PCBPy can self-organize into nanostructure to adjust morphology [66]. It is known that fullerene and its derivatives tend to diffuse out of the blend and aggregate into larger clusters

78

Xiaoling Ma and Fujun Zhang

or single crystals due to their spherical geometry. To restrain the molecular aggregation of fullerene and its derivatives, some fullerene derivatives with chemical modification are used as the third component in OSCs. In 2011, polymerizable fullerene derivatives PCBSD or PCBS were incorporated into P3HT:PC61BM blend by Cheng et al. [67] By chemical cross-linking of PCBSD, the morphology of ternary blend films can be fixed and stably preserved. An average PCE of 3.7% can still be obtained through longterm thermal treatment. In sharp contrast, the PCE of binary OSCs dropped dramatically to 0.69% under the same condition. Similar phenomenon can also be observed with PCBS as the third component. The thermal stability of OSCs can also be improved by incorporating some other cross-linkable fullerene derivatives as the third component [68e70].

3.3.2 Fullerene- and nonfullerene-based ternary OSCs Ternary OSCs based on the fullerene and nonfullerene as acceptors have exhibited rapid progress due to synthesis of various nonfullerene materials. The fullerene derivatives preserve the good electron-transport ability in ternary OSCs. However, there also exist some shortcomings, such as the weak absorption ability of fullerene and mismatching energy level between acceptor and a donor, leading to insufficient photon harvesting and large voltage loss. Compared with fullerene acceptors, nonfullerene acceptors exhibit strong photon harvesting ability in specific region, tunable energy levels, and low VOC loss. However, nonfullerene-based blend films usually suffer from severe phase separation due to the high crystallinity of nonfullerene acceptors. The combination of fullerene and nonfullerene materials can effectively solve the unfavorable morphological issue. The merits of fullerene and nonfullerene materials can be combined into 1 cell by ternary strategy, resulting better performance than the OSCs based on single acceptor component. Bo et al. firstly fabricated ternary OSCs with fullerene and nonfullerene materials (ITIC and PC71BM) as acceptors, and a polymer material (PPBDTBT) as donor [71]. The PCE of the optimized ternary OSCs is increased to 10.4% by incorporating 40 wt% PC71BM in acceptors, with simultaneously improved JSC and VOC, as shown in Fig. 3.10A. The ITIC-based binary blend films exhibit an inhomogeneous distribution with some large black and light region, resulting in low FF of OSCs, as shown in Fig. 3.10B. The incorporation of 40 wt% PC71BM can effectively suppress the large phase separation and a fairly uniform film morphology

79

(A)

(B)

(C) 0%

0

20%

40%

mc:PC BM (wt%) •

-3 -6 -9

60%

80%

100%

-12 -15

e

-3 Energy levels (eV)

Current density (mA cm-2)

Ternary organic solar cells

3.98

-4

0.2

0.4

0.6

Voltage (V)

(D)

0.8

Energy (eV)

h

5.96 h

Vacuum level (E) 5.6

5.43

5.51

-6

1.0

h

EICT+ of acceptors

5.4

LUMO

EICT-

3.41 3.78

-5

-18 0.0

e

e

5.2 LUMO

5.0

EICT+

EICT-

4.8

HOMO

EICT+ HOMO

Donor Acceptor A1 Acceptor A2

4.6

EICT- of acceptors

4.4 4.2

EICT+ of donors

4.0 3.0

3.5

4.0

4.5

5.0

5.5

6.0

Figure 3.10 (A) JeV characteristics of PPBDTBT:ITIC:PC71BM-based devices with different ITIC:PC71BM ratios. (B) TEM images of PPBDTBT:ITIC:PC71BM blend films with different PC71BM contents (wt%) in acceptors. (C) Energy-level orbitals of PPBDTBT, ITIC, and PC71BM. (D) Self-organized cascade junction of PTB7-Th:PC71BM: m-ITIC. (E) Pinning energy levels. (C) Reproduced with permission of Wiley-VCH from H. Lu, J. Zhang, J. Chen, Q. Liu, X. Gong, S. Feng, X. Xu, W. Ma, Z. Bo, Ternary-blend polymer solar cells combining fullerene and nonfullerene acceptors to synergistically boost the photovoltaic performance. Adv. Mater. 28 (2016) 9559e9566. (E) Reproduced with permission of Elsevier from C. Wang, X. Xu, W. Zhang, S.B. Dkhil, X. Meng, X. Liu, O. Margeat, A. Yartsev, W. Ma, J. Ackermann, Ternary organic solar cells with enhanced open circuit voltage. Nano Energy 37 (2017) 24e31.

with abundant donoreacceptor interfaces can be observed, resulting in higher JSC and FF of ternary OSCs. PC71BM can also form cascade-like energy level with PPBDTBT and ITIC, which facilitate charge transfer at the donoreacceptor interfaces, as shown in Fig. 3.10C. The same acceptors (ITIC and PC71BM) were also employed by Ding et al. and Liu et al. with different donors of PDTP4TFBT and PBDB-T, resulting in improved PCEs of 9.20% and 10.65%, respectively [72,73]. Fahlman et al. incorporated a nonfullerene acceptor m-ITIC as the third component into PTB7-Th:PC71BM-based system, resulting in simultaneously improved JSC and VOC compared with two PC71BM or m-ITIC-based binary OSCs [74], as shown in Fig. 3.10D and E. It should be noticed that the VOC of ternary OSCs are higher than two binary OSCs. Most of nonfullerene molecules were self-organized at the polymerefullerene interface due to the similar surface energy of nonfullerene acceptor m-ITIC and polymer

80

Xiaoling Ma and Fujun Zhang

(A)

(B) PBDB-T IT-M Bis[70]PCBM 1:1 1:1:0.2

1.0

EQE (%)

0.8 0.6 0.4

80 60 1:1:0 1:1:0.2 1:1:1

40 20

0.2 0.0

(C) 60 50

500 600 700 Wavelength (nm)

Binary Ternary

0 300 400 500 600 700 Wavelength (nm)

800

800

(D)

e 30

e

e

IEICO

Bis-PC BM 70

40 15 30 0 20 10 -15 0 300 400 500 600 700 800 9001000 Wavelength (nm)

PBDTTT-E-T

EQE (%)

70

400

ΔEQE(%)

Normalized Absorption (a.u.)

donor. The cascade energy alignment reduces bimolecular recombination and trap-assisted recombination via integer charge transfer states. A new ITIC-based nonfullerene acceptor IT-M was synthesized by Hou et al. [34]. The performance of PBDB-T:IT-M-based OSCs is improved by introducing Bis-PC70BM as the third component. A record PCE of 12.2% is achieved in ternary OSCs with a PBDB-T:IT-M:Bis-PC70BM ratio of 1: 1:0.2 [34]. The used materials exhibit apparently complementary absorption spectra, as shown in Fig. 3.11A. The EQE spectrum was enhanced in short wavelength range from 380 to 480 nm, as shown in Fig. 3.11B. It should be noticed that an increased response beyond 550 nm can also be observed,

Figure 3.11 (A) Optical absorption spectra of the solid thin films of PBDB-T, IT-M, BisPC70BM, PBDB-T:IT-M (1:1), and PBDB-T:IT-M:Bis-PC70BM (1:1:0.2). (B) EQE spectra of PBDB-T:IT-M:Bis-PC70BM-based OSCs with different Bis-PC70BM content. (C) EQE spectra of PBDTTT-E-T:IEICO binary organic solar cells (OSCs) and optimized ternary OSCs-based PBDTTT-E-T:IEICO:Bis-PC70BM as active layer. (D) Schematic of charge transfer in ternary active layers. (B) Reproduced with permission of Wiley-VCH from W. Zhao, S. Li, S. Zhang, X. Liu, J. Hou, Ternary polymer solar cells based on two acceptors and one donor for achieving 12.2% efficiency. Adv. Mater. 29 (2017) 1604059. (D) Reproduced with permission of Wiley-VCH from Y. Chen, Y. Qin, W. Yang, C. Li, H. Yao, N. Liang, X. Wang, W. Li, W. Ma, J. Hou, From binary to ternary: improving the external quantum efficiency of small-molecule acceptor-based polymer solar cells with a minute amount of fullerene sensitization. Adv. Energy Mater. 7 (2017) 1700328.

Ternary organic solar cells

81

which indicated that the charge generation between PBDB-T and IT-M interface is improved as well after incorporating Bis-PC70BM. BisPC70BM was also incorporated into PBDTTT-E-T:IEICO-based binary system by Hou et al. [75]. After blending with only 5% Bis-PC70BM as the third component, the PCE is increased to 10.21%, resulting from some obvious improvement in JSC (17.31e18.92 mA cm2) and FF (58% e65%). Rather than complement the absorption spectrum with sufficient amount of the third absorber, only 5% amount (w/w) of Bis-PC70BM in acceptor prefer acts as a sensitizer in enhancing the EQE for the host binary device (Fig. 3.11C), especially for the PBDTTT-E-T. Depending on strong electron affinity of fullerene sphere and middle-lying LUMO level, Bis-PC70BM could also effectively help extract electrons from PBDTTTE-T and successively deliver charges to IEICO within a cascade transfer process (Fig. 3.11D). With the motivation to improve the absorption in short wavelength range, Ding et al. fabricated ternary OSCs by incorporating appropriate amount of PC71BM into PTB7-Th:COi8DFIC system [76]. The PTB7-Th:COi8DFIC-based binary OSCs exhibit a PCE of 10.48%. The PCE of ternary OSCs is improved to 14.08% by incorporating 30 wt % PC71BM in acceptors, with simultaneously increased JSC of 28.20 mA cm2, FF of 71.0%, and VOC of 0.70 V. PC71BM showed a complementary optical absorption with host materials in the short wavelength range from 380 to 550 nm. The optimized ternary OSCs show stronger EQE response at 310e550 and 864e1050 nm wavelength range compared with PTB7-Th:COi8DFIC- and PTB7-Th:PC71BM-based binary OSCs, respectively, which can be explained from the absorption spectra of two acceptors. Most ternary OSCs are fabricated with polymer materials as donor, and the all-small-molecule OSCs have rarely been investigated. It is known that small molecules possess some remarkable merits, such as a well-defined chemical structure, facile synthesis, uniform batch-to-batch, easy purification, and allowing easier energy level control [77e80]. Nevertheless, unlike the conjugated polymer, small molecules have no long backbones to weave a continuous network skeleton. Meanwhile, small molecules with improved crystallinity tend to cause strong aggregation and large crystalline domains, which may deteriorate the photovoltaic process. Therefore, morphology modulation is a key issue in constructing highly efficient all-small-molecule ternary OSCs. Encouragingly, some successful all-small-molecule ternary systems have been reported. Hou et al. incorporated PC71BM into a DRTB-T:IDIC-based nonfullerene binary system, and a PCE of 10.48%

82

Xiaoling Ma and Fujun Zhang

is achieved [81]. The morphological characterization (Fig. 3.12AeG) demonstrates that the introduction of PC71BM limits the overgrowth of the crystalline, alleviate the phase separation, and form purer domains and more compact molecular stacking. The synergistic effect on the morphology and absorption of the PC71BM facilitates charge generation and suppresses charge recombination, as manifested by the significantly enhanced JSC; FF. Zhu et al. reported an all-small-molecule ternary OSC based on BTR:NITI:PC71BM as active layer [82]. The BTR:NITI:PC71BM (1:0.4: 1) ternary blend combines the advantages of both fullerene and nonfullerene acceptors. The PCE improvement should be attributed to the hierarchical morphology that was obtained from a balance between detailed phase separation and material crystallization, which showed that a subtle balance between the materials and interfaces is reached. The good transport property of PC71BM and the low energy loss of the nonfullerene are maintained, which increases the FF and VOC. PC71BM works not only as a transporting material but also as a phase mediator to control the film morphology.

3.3.3 Nonfullerene-based ternary OSCs Compared with fullerene- and nonfullerene-based system, nonfullerene system shows much better flexibility for composition modification, which

0.5 0.0

0.5 0.0

-1.0

qxy [Å-1]

-2.0

-1.0

-2.0

qz [Å-1]

-2.0

qz [Å-1]

qz [Å-1]

-1.0

0.0 0.5 1.0 1.5 2.0 2.4

0.5 0.0

(F)

0.5 0.0

-1.0

qxy [Å-1]

-2.0

0.5 0.0

-1.0

-2.0 0.0 0.5 1.0 1.5 2.0 2.4

(E) 0.0 0.5 1.0 1.5 2.0 2.4

(D)

(G)

(C)

0.5 0.0

-1.0

qxy [Å-1]

-2.0

5.0 4.5 4.0 3.5 3.0

Intensity.q2 (a.u.)

(B)

0.0 0.5 1.0 1.5 2.0 2.4

(A)

6 5 4 3

ternary film of SVA D:PC71BM D:NFA

2

10

-7 6 5 4 3 2

10

-8 6 5 4 3

0.01

2

3

4 5 678

0.1

2

3

4 5 6 7

q (nm-1)

Figure 3.12 TEM images (AeC) and two-dimensional GIWAXS pattern (DeF) of the DRTB-T:PC71BM-based binary blend film (A, D), the DRTB-T:IDIC-based binary blend film (B, E), and the DRTB-T:PC71BM:IDIC-based ternary blend film with SVA treatment (C, F). (G) The resonant soft X-ray scattering profiles of the DRTB-T:PC71BM, DRTB-T: IDIC binary films and DRTB-T:PC71BM:IDIC ternary film with SVA treatment. Reproduced with permission of Wiley-VCH from H. Zhang, X. Wang, L. Yang, S. Zhang, Y. Zhang, C. He, W. Ma, J. Hou, Improved domain size and purity enables efficient all-small-molecule ternary solar cells. Adv. Mater. 29 (2017) 1703777.

Ternary organic solar cells

83

provides numerous opportunities to improve the performance of OSCs. It has been well demonstrated in the recent year, and lots of encouraging results have been reported. 3.3.3.1 Donor: nonfullerene 1: nonfullerene 2 From the view of improving light harvesting ability of the active layer, a very popular material combination strategy in recent reports is to construct ternary blend films employing nonfullerene acceptors with different band gaps. Sun and coworkers reported a nonfullerene-based OSCs by employing two different nonfullerene acceptors, ITIC-Th and SdiPBI-Se, and the polymer donor PDBT-T1 [83]. These three materials show complementary absorption, covering a wide absorption range across the entire visible spectrum. A PCE of 10.3% is achieved for ternary OSCs by incorporating 50 wt % ITIC-Th content in acceptors, with an obviously enhanced JSC of 15.4 mA cm2. The main increase in JSC should be mainly ascribed to the enhanced absorption by ITIC-Th in the region between 700 and 800 nm. The two nonfullerene acceptors SdiPBI-Se and ITIC-Th exhibit high crystallinity in their binary system. However, in the ternary system with a 1:1 SdiPBI-Se:ITIC-Th ratio, the smearing of crystalline signals from two nonfullerene acceptors suggests that the multiple acceptors disrupt the original molecular packing and form a highly miscible phase. The physical nature, the absorption and frontier energy levels of this new mixture can be tuned by blending composition due to the high miscibility between the two acceptors. Hou et al. fabricated ternary OSCs with a wide band gap polymer donor J52, a medium-band gap acceptor IT-M and a low-band gap acceptor IEICO [84], which have complementary light absorption spectra in the entire visible light range reaching to the NIR region. The two acceptors exhibit excellent compatibility in ternary blend films due to the similar chemical structure. The overlap between emission spectra of IEICO and absorption spectra of IT-M also enable efficient energy transfer, providing another possible pathway for efficient charge generation. The PCE of ternary OSCs is improved to 11.1%, resulting from enhanced JSC of 19.7 mA cm2 and an FF of 67%. In addition to complementary absorption spectra of used ternary materials, a new selection criterion for obtaining highly efficient ternary OSCs is proposed from device perspective by Zhang et al. [85e87] based on the complementary photovoltaic parameters of the two binary OSCs. In PBDB-T: INPIC-4F:MeIC1 ternary system [51], the PCE was improved to 13.73% by incorporating 50 wt% MeIC1 in acceptors. The INPIC-4F- and

84

Xiaoling Ma and Fujun Zhang

MeIC1-based binary OSCs exhibit a rather high PCE of 12.55% or 11.53% with different advantages in terms of the photovoltaic parameters. The JSC of INPIC-4F-based binary OSCs is larger than that of MeIC1-based binary OSCs, and the FF and VOC are exactly the opposite. The advantages in terms of photovoltaic parameters of the two binary OSCs can be inherited into ternary OSCs by adjusting the MeIC1 content, resulting in a synchronously optimized JSC of 21.86 mA cm2, a VOC of 0.88 V, and an FF of 71.39%. This selection criterion has been verified in J71:ITIC:IT-M, PBDB-T: ITCPTC:IDT6CN-M, J71:ITIC:MeIC2 ternary system. A good compatibility of used materials is a precondition to integrate the advantages of two binary OSCs into one ternary OSC. Nonfullerene acceptors with similar chemical structures may exhibit good compatibility, and different photophysical and photochemical properties due to their slightly tailored functional groups. Combining two structurally similar nonfullerene acceptors into one ternary system should be a promising strategy to improve performance of OSCs, which have been demonstrated by several groups successfully. Zhang et al. reported an efficient ternary OSC with PBT1-C as donor, two structurally similar nonfullerene materials MeIC and MeIC2 as acceptors [88]. The optimized ternary OSCs with 30 wt% MeIC2 in acceptors achieve a PCE of 12.55%, which is much higher than that of 11.47% for MeIC-based binary OSCs and 11.41% for MeIC2-based binary OSCs. The good compatibility of used materials can also be evaluated by water contact angles (WCAs) of PBT1-C, MeIC, MeIC2, and MeIC:MeIC2 (1:1) films, as shown in Fig. 3.13A. The surface energy values of PBT1-C, MeIC, MeIC2, and MeIC:MeIC2 (1:1) films are gPBT1-C z 20.32 mN m1, gMeIC z 24.57 mN m1, gMeIC2 z 24.08 mN m1, and gMeIC: 1 MeIC2 z 24.14 mN m , respectively. The good compatibility of MeIC and MeIC2 can be further confirmed due to the similar surface energy of neat and blend acceptor films. The good compatibility of MeIC and MeIC2 are beneficial to form one alloyed acceptor for efficient electron transport in the ternary active layers. Recently, Yan et al. also reported a ternary OSC system with two structurally similar acceptors (ITCPTC and MeIC) and a polymer donor PM6 [89]. The optimal blend ratio of PM6: ITCPTC:MeIC (1:0.4:0.6) led to an impressive FF of 78.2% and PCE of 14.13%. Although these two small molecular acceptors exhibit similar absorption spectra, the synergistic effect of crystallinity and domain size/purity introduced by the two acceptors can be observed. From the RSoXS characterization (Fig. 3.13B), the ternary blend with 40% ITCPTC content in the acceptors provides a slightly larger domain size (23.5 nm) than 21.8 nm of the

Ternary organic solar cells

85

Figure 3.13 (A) The images of water contact angles of PBT1-C, MeIC, MeIC2, and MeIC: MeIC2 (1:1) films. (B) Resonant soft X-ray scattering of ternary blend films with different ITCPTC contents. (C) GIWAXS patterns of the neat PM6, ITCPTC, and MeIC thin films and the ternary blend films with different ITCPTC contents. (A) Reproduced with permission of Wiley-VCH from Q. An, J. Zhang, W. Gao, F. Qi, M. Zhang, X. Ma, C. Yang, L. Huo, F. Zhang, Efficient ternary organic solar cells with two Compatible non-fullerene materials as one alloyed acceptor. Small 14 (2018) 1802983. (C) Reproduced with permission of Royal Society of Chemistry from T. Liu, Z. Luo, Q. Fan, G. Zhang, L. Zhang, W. Gao, X. Guo, W. Ma, M. Zhang, C. Yang, Y. Li, H. Yan, Use of two structurally similar small molecular acceptors enabling ternary organic solar cells with high efficiencies and fill factors. Energy Environ. Sci. 11 (2018) 3275-3282

PM6:ITCPTC blend. The relative domain purities of PM6:ITCPTC, PM6: MeIC and the ternary blend with 40% ITCPTC content in the acceptors were calculated to be 0.85, 1, and 0.88, respectively. The 40% ITCPTC blend exhibits a reduced diffraction signal at the pep stacking position relative to the PM6:MeIC blend film, and a crystal coherence length (CCL) of 18.5 Å in the (010) direction falls in between the CCL of the two binary blend films, suggesting that the crystalline content of the blend film is reduced after introducing ITCPTC into the acceptors, as shown in Fig. 3.13C. The balance between domain size and relative domain purity in the ternary blend films contributes to the FF of the corresponding OSCs, and thus a higher value can be obtained.

86

Xiaoling Ma and Fujun Zhang

3.3.3.2 Donor 1: Donor 2: nonfullerene At present, the band gap of most of high-performance nonfullerene acceptors is relatively low, and their light absorption in short wavelength region is poor [14]. Even for the most efficient nonfullerene acceptorebased binary OSCs, the utilization of high-energy solar photons (400e550 nm) is insufficient [90e92]. Except for exploring high-performance wide band gap nonfullerene materials, the investigations on ternary OSCs with two donors and one acceptor can be a simple method to further improve light harvesting ability of active layer. The highly efficient ternary nonfullerene OSCs can be obtained if one nonfullerene acceptor can well work with two donors with different band gaps to cover more broad spectral range. Introducing a FRET process by incorporating a donor material with appropriate band gap is an effective way to improve photon utilization. An interesting report reveals that efficient energy transfer is possible to happen between all the three components. The wide band gap polymer donor PCDTBT was incorporated into PTB7-Th:ITIC blends to improve the light harvesting of the active layer [93]. It is found that the PCDTBT can be embedded compatibly in both PTB7-Th and ITIC. The emission spectrum of PCDTBT strongly overlaps the absorption spectra of both PTB7Th and ITIC (Fig. 3.14A) with a yellow shaded area. Introducing 20 wt% of energy donor PCDTBT in energy acceptors PTB7-Th and ITIC, the emission peak of PCDTBT disappears, while the PL peaks of PTB7-Th and ITIC appear, suggesting that the energy of PCDTBT is transferred to the PTB7-Th and ITIC (Fig. 3.14B). By doping appropriate amount of PCDTBT in the ternary system, photocurrent can be obviously enhanced. As a result, the ternary OSCs with a much higher PCE of 7.51% than the binary counterparts are obtained successfully. Moreover, a more efficient ternary system with two donors was constructed by Zhang et al. with the blends of PBDB-T:PTB7-Th:IEICO-4F [40]. In this system, the wide band gap polymer donor PBDB-T and the ultranarrow band gap nonfullerene acceptor IEICO-4F were used as host materials to fabricate binary OSCs. The medium band gap donor PTB7-Th was chosen as the third component to fabricate ternary OSCs. The combination of three materials can cover a large photon harvesting range from 300 to 1000 nm (Fig. 3.14C and D), resulting in a high JSC of 24.14 mA cm2. The potential dynamic processes between PBDB-T and PTB7-Th were investigated by measuring PL spectra and TA spectra, as shown in Fig. 3.14E and F. The emission intensity of PBDB-T is markedly quenched in the blend PBDBT:PTB7-Th films, even for the blend film with only 10 wt% PTB7-Th

Ternary organic solar cells

87

Figure 3.14 (A) Normalized UV-vis absorption spectra of PTB7-Th, ITIC, and PCDTBT films and the normalized photoluminescence (PL) spectrum of PCDTBT film. (B) PL spectra of neat PCDTBT film, 20 wt% PCDTBT:PTB7-Th film and 20 wt% PCDTBT:ITIC with an excitation wavelength of 500 nm. (Inset: the dual FRET effects diagram between PCDTBT with PTB7-Th and ITIC.) (C) Absorption coefficients of neat PBDB-T, PTB7-Th, and IEICO-4F. (D) Absorption spectra of blend films with different PTB7-Th content in donors. (E) PL spectra of neat PBDB-T, PTB7-Th, and blend PBDB-T:PTB7-Th films under 620 nm light excitation. (F) Transient absorption dynamics of neat PBDB-T, PTB7-Th and their blend films by probing 585 and 720 nm, respectively. (B) Reproduced with permission of Royal Society of Chemistry from P.Q. Bi, F. Zheng, X.Y. Yang, M.S. Niu, L. Feng, W. Qin, X.T. Hao, Dual forster resonance energy transfer effects in non-fullerene ternary organic solar cells with the third component embedded in the donor and acceptor. J.

88

Xiaoling Ma and Fujun Zhang

content. Meanwhile, emission intensity of PTB7-Th in the blend films is monotonously increased along with the increase of PTB7-Th content, which is even stronger than that of neat PTB7-Th film. This phenomenon indicates that energy transfer from PBDB-T to PTB7-Th should occur. To further confirm energy transfer dynamic process, TA dynamics of neat PBDB-T and the blend PBDB-T:PTB7-Th films were investigated by probing the GSB signal of PBDB-T and PTB7-Th. The energy transfer from PBDB-T to PTB7-Th can be further confirmed from the enhanced lifetime of PTB7-Th GSB signal (720 nm) in the blend films. The effective energy transfer between two donors facilitates exciton utilization, which should contribute to increased JSC of 24.14 mA cm2 and PCE of 11.62%. It was also be proved by Peng et al. that the energy transfer process exists between PBDB-T and PTB7-Th [94]. The optimized weight ratio was found to be 0.3:0.7:0.8 for PBDB-T:PTB7-Th:SFBRCN-based ternary blend, delivering a champion PCE of 12.27% with a high VOC of 0.93 V, JSC of 17.86 mA cm2, and FF of 73.9%. Recently, all-polymer solar cells have attracted much attention due to their outstanding thermal and mechanical stability and the facile modification of their chemical and optoelectronic properties to enable long-term operation [95e97]. N2200 and its derivatives are the most widely utilized polymer acceptors due to their broad light absorption, good electron affinity, and high mobility. However, their morphological problematic features of large aggregates formation, strong phase separation, and inhomogeneous internal phase composition may result in strong charge recombination. The insufficient coverage of the solar spectrum is also a problem in binary blend allpolymer solar cells. Ternary strategy appears to be a promising way to improve the efficiency of all-polymer solar cells. The light harvesting can be effectively enhanced by incorporating another light absorber. A wide band gap visible polymer PCDTBT was introduced as the third polymer into a low band gap donor/acceptor binary polymer blend PTB7-Th: P(NDI2ODT2) to increase the utilization of solar photons [98]. Owing to the complementary absorption bands of the ternary blends and the FRET

=

Mater. Chem. 5 (2017) 12120e12130. (F) Reproduced with permission of Wiley-VCH from X. Ma, Y. Mi, F. Zhang, Q. An, M. Zhang, Z. Hu, X. Liu, J. Zhang, W. Tang, Efficient ternary polymer solar cells with two well-compatible donors and one ultranarrow bandgap nonfullerene acceptor. Adv. Energy Mater. 8 (2018) 1702854.

Ternary organic solar cells

89

process between two donors, the PCE is improved to 6.7%, with an increased JSC of 14.4 mA cm2. Another particular challenging issue of state-of-the-art all-polymer solar cells is the lack of an efficient method to optimize the morphology of polymerepolymer blend films. Li et al. incorporated a high crystalline polymer donor PBDD-ff4T into PTB7-Th:N2200 blends [99]. PTB7-Th:PBDD-ff4T:N2200 blend film with 10 wt% PBDD-ff4T exhibits sharper and larger diffraction peak compared with that of PTB7-Th:N2200 blend film, indicating smaller lamellar stacking and pep stacking spacing of ternary blend films. PBDD-ff4T is fully embedded into PTB7-Th phase to form a donor alloy, which induced the crystallization of PTB7-Th. As a result, the ternary all-polymer OSCs with 10 wt% PBDD-ff4T without any extra treatments exhibits an optimized PCE of 7.2% with a VOC of 0.82 V, a JSC of 15.7 mA cm2, and an FF of 56%. The incorporation of a second polymer donor with simultaneously enhanced charge transport ability, suppressed aggregation tendency, and good miscibility between used materials is a promising strategy to increase the FF in all polymer solar cells. The BTAbased conjugated copolymers present high charge mobility and moderate planarity, which can facilitate charge transport [100,101]. The sensitizer polymer PBTA-BO was incorporated into PTzBI:N2200 system to fabricate ternary all-polymer OSCs by Huang et al. [102]. The resulting ternary allpolymer OSCs present significantly improved FF values of 78% and PCE of 10.12%. The charge mobility is improved and charge recombination is reduced in ternary active layers by incorporating appropriate amount of PBTA-BO, which should be attributed to the optimized microstructure morphology of the ternary blends. The intimate mixing properties of the two donors enable the formation of an alloylike blend in the present ternary systems, as verified by the linearly mutable VOC values. The ternary OSCs with PCE > 11% are summarized in Table 3.1. It can be observed that most of the highly efficient ternary OSCs are based on nonfullerene acceptor or the combination of fullerene and nonfullerene acceptors. The combination of ternary strategy with-high performance material will continually show great potential in future development.

3.4 The potential research directions of ternary OSCs In addition to PCE improvement, there are also some potential research directions of ternary OSCs for future application, such as thickfilm, semitransparent device, and stability, which will be discussed in this section.

90

Xiaoling Ma and Fujun Zhang

Table 3.1 Recent progress of ternary OSCs with PCE>11%. The third Binary blend [D:A] PCE [%] component

PCE [%]

References

PCE10:IDTBR p-DTS(FBTTh2)2:PC71BM PSTZ:ITIC J52:IT-M PffBT4T-2OD:PC71BM PTB7-Th:meta-TrBRCN PTB7-Th:PC71BM PBDB-T:ITIC FTAZ:IDTC J71:ITIC J71:IT-M PBDB-T:IEICO-4F PBDB-T:IDT6CN-M PTB7-Th:COi8DFIC PTB7-Th:BDTThIT-4F PTB7-Th:PC71BM PBDB-T:IT-M PBDTTT-EFT:IEICO-4F PBDB-T:IT-M PBDB-T:IT-M PBDB-T:SFBRCN PTB7-Th:F8IC PBT1-C:MeIC PBDB-T:NNBDT PTB7-Th:ITIC-2F PTB7-Th:COi8DFIC PBT1-C:ITIC-2Cl PBDB-T:IT-M PTB7-Th:3TT-FIC BTR:NITI PBDB-T:INPIC-4F PM6:ITCPTC PBDB-T-2F:IT-4Cl PTB7-Th:COi8DFIC

11.0 11.0 11.1 11.1 11.17 11.4 11.4 11.41 11.6 11.6 11.6 11.62 11.92 11.94 12.03 12.10 12.10 12.15 12.16 12.20 12.27 12.3 12.55 12.8 12.9 13.08 13.4 13.52 13.54 13.63 13.73 14.13 14.18 14.62

[103] [104] [48] [105] [106] [107] [108] [109] [110] [111] [85] [40] [47] [112] [113] [114] [109] [115] [116] [117] [118] [119] [88] [120] [121] [122] [123] [124] [125] [82] [51] [89] [25] [27]

d 8.77 8.06 9.4 10.57 10.15 9.21 10.03 10.4 10.7 10.68 10.25 10.51 10.72 9.61 10.10 11.10 10.63 10.89 10.80 9.39 10.9 11.47 11.7 12.1 11.47 11.1 11.71 12.21 9.03 12.55 13.04 13.45 10.48

IDFBR ZnP IDIC IEICO PCDTBT8 ITIC-Th BTR N2200 ITIC-Th-O MeIC2 ITIC PTB7-Th ITCPTC IEICO-4F IEICO-4F DR3TSBDT N2200 PCDTBT ITCN Bis-PC70BM PTB7-Th PC71BM MeIC2 FDNCTF IOTIC-2F BDTThIT-4F ICBA P1 PC71BM PC71BM MeIC1 MeIC IT-2Cl PC71BM

PCE, power conversion efficiency; OSCs, organic solar cells.

3.4.1 Thick active layerebased ternary OSCs It is known that ternary OSCs have attracted much attention due to that ternary organic blends can broaden the absorption range of OSCs without the use of complicated tandem cell structures. Despite their broadened

Ternary organic solar cells

91

absorption range, the light harvesting capability of ternary OSCs is still limited because ternary OSCs mostly use thin active layers of about 100 nm in thickness, which is not sufficient to absorb all photons in their spectral range and may also cause problems for future roll-to-roll mass production that requires thick active layers. Huang et al. reported a very efficient ternary OSC by incorporating a nematic liquid crystalline small molecule (BTR) into a PTB7-Th:PC71BM host system [108]. Compared with binary systems, a higher PCE of 10.37% is achieved for ternary OSCs with 25 wt% BTR content at the 250-nm-thick active layer. From GIWAXS characterization of blend films (Fig. 3.15A), the PTB7-Th (010) peak relating to pep stacking in the out-of-plane direction shifts to a higher q from 1.53 Å1 to 1.67 Å1 along with BTR content gradually increased to the optimal ratio (25%), and then shifts back with the further incorporation of BTR. This suggests that the blend film with 25% BTR has the most compact pep stacking with d-spacing of around 3.77Å. The more ordered molecular arrangement should facilitate hole transport, which can be confirmed from measured hole mobility according to the space charge limited current method. The ternary OSCs have a shorter charge extraction time than that of the binary devices with the same thick active layers (Fig. 3.15B), which is consistent with the change of charge mobility. Recently, Huang et al. reported an all-polymer OSC with PBTA-Si:PTzBI-Si:N2200 as ternary active layer [126]. The optimized all-polymer OSCs attain a PCE of 9.17% with an active layer thickness of 350 nm and maintain a PCE over 8% for thicknesses over 400 nm. Fig. 3.15C illustrates the dark JeV curves for device A (PBTA-Si:N2200based OSCs), device B (PTzBI-Si:N2200-based OSCs), and device C (PBTA-Si:PTzBI-Si:N2200-based OSCs); the thicknesses of active layer for those device is appropriately 300 nm. By fitting the dark JeV curves with a one diode replacement circuit [127], the diode ideality factor (n) was obtained, i.e., 1.86, 1.74, and 1.76 for devices A, B, and C, respectively. A lower ideality factor indicates weaker recombination induced by defect states. Although device C has an ideality factor similar to that of device B, the series resistance (Rs) of device C (1.46 U cm2) is lower than that of device B (Rs ¼ 1.74 U cm2) based on the JeV characteristics in the dark. Using the equivalent circuit model [128], the FF-containing series and parallel resistance losses were calculated, and the values of 69.37%, 73.85%, and 75.47% are obtained for devices A, B, and C, respectively. This result implies that device C, i.e., the optimized ternary OSC has a better potential to be fabricated and used in thick-film devices. The ternary strategy should be

92

Xiaoling Ma and Fujun Zhang

Figure 3.15 (A) In-plane and out-of-plane GIWAXS profiles for PTB7-Th:BTR:PC71BM blend films with various contents of BTR and 100 nm thickness. (B) Transient photocurrent of organic solar cells with PTB7-Th:BTR:PC71BM ratios of 1:0:1.1 and 0.75:0.25:1.1 with thin and thick active layers. (C) Dark JeV curves and the corresponding ideality factors for devices AeC with thicknesses of z300 nm (fitted by a one diode replacement circuit). (B) Reproduced with permission of American Chemical Society from G. Zhang, K. Zhang, Q. Yin, X. F. Jiang, Z. Wang, J. Xin, W. Ma, H. Yan, F. Huang, Y. Cao, Highperformance ternary organic solar cell enabled by a thick active layer containing a liquid crystalline small molecule donor. J. Am. Chem. Soc. 139 (2017) 2387e2395. (C) Reproduced with permission of Wiley-VCH from B. Fan, P. Zhu, J. Xin, N. Li, Y. Lei, W. Zhong, Z. Li, W. Ma, F. Huang, Y. Cao, High-performance thick-film all-polymer solar cells created via ternary blending of a novel wide-bandgap electron-donating copolymer. Adv. Energy Mater. 8 (2018) 1703085.

an efficient way to achieve high-performance thick-film devices that can meet the needs of future roll-to-roll production.

3.4.2 Semitransparent ternary OSCs Semitransparent OSCs have recently attracted extensive attention due to their potential applications in building integrated photovoltaics, wearable electronic power, and photovoltaic vehicles [129e131]. Ternary strategy can also be applied in semitransparent device to improve PCE and maintain

Ternary organic solar cells

93

average visible transmittance (AVT) of semitransparent OSCs. Sun et al. [132] fabricated a ternary semitransparent OSC based on PTB7-Th: PBT1-S:PC71BM as ternary active layer. The PCE of semitransparent OSCs is improved from 8.0% to 9.2%, which should be ascribed to enhanced light absorption and optimized film morphology. More importantly, the addition of a small amount of PBT1-S to the binary blend did not significantly change its AVT and neutral color perception, as shown in Fig. 3.16A and B. In addition to fullerene-based semitransparent system, ternary strategy was first applied in nonfullerene system by Zhang et al. [113]. The development of ultranarrow band gap nonfullerene materials provides more opportunity for the fabrication of semitransparent device. A semitransparent ternary OSC was fabricated with PTB7-Th:BDTThIT4F as host system and an ultranarrow band gap material IEICO-4F as the third component. The PCE of optimized semitransparent ternary OSCs is improved to 9.40% with AVT simultaneously improved to 24.6%, which should be among the highest values for semitransparent OSCs. The PCE improvement should be mainly attributed to enhanced photon harvesting in long wavelength region, which can be confirmed from the EQE spectra shown in Fig. 3.16C. The transmittance of Au/Ag electrode is relatively high in the spectral range from 300 to 600 nm, which is beneficial to prepare semitransparent OSCs. The AVT of semitransparent OSCs is increased from 24.2% to 24.6%, which can be calculated from the transmission spectra of corresponding OSCs shown in Fig. 3.16D. The ultranarrow band gap material IEICO-4F can also be used as third component in other ternary system with one narrow band gap material PTB7-Th as donor and two ultranarrow band gap materials (COi8DFIC and IEICO-4F) as acceptors [112]. The PCE of semitransparent OSCs is increased to 8.23% with simultaneously improved AVT of 20.78% by incorporating 15 wt% IEICO-4F in acceptors. Unlike the commonly reported role of the third component in improving the performance of ternary OSCs, IEICO-4F can induce a variation in the COi8DFIC molecular arrangement [122], resulting in extended photon harvesting in the NIR region, which can be confirmed from the EQE spectra of OSCs (Fig. 3.16E). To qualitatively show the transparency of semitransparent OSCs for human eyes, the photograph of a peony flower partly covered with semitransparent OSCs is exhibited in Fig. 3.16F. It is apparent that the peony flower covered with semitransparent OSCs can be clearly observed, suggesting the potential of semitransparent OSCs in the building-integrated photovoltaic field. In Zhang’s work [112,113], the PCE and AVT of semitransparent OSCs can be simultaneously improved

94

Xiaoling Ma and Fujun Zhang

Figure 3.16 (A) Optical transmittance of PTB7-Th:PC71BM, PBT1-S:PC71BM-, PTB7-Th: PBT1-S:PC71BM-based semitransparent organic solar cells (OSCs) covered by different Ag thickness (14 mm  15 nm). (B) The representation of color coordinates of PTB7Th:PC71BM, PBT1-S:PC71BM-, PTB7-Th:PBT1-S:PC71BM-based semitransparent OSCs under AM 1.5G illumination on the CIE chromaticity diagram. (C) EQE spectra of blended PTB7-Th:BDTThIT-4F:IEICO-4F films with different IEICO-4F contents. (D) Transmission spectra of corresponding films and semitransparent OSCs. (E) EQE spectra of PTB7Th:COi8DFIC:IEICO-4F-based semitransparent OSCs with different IEICO-4F contents. (F) Flower partly covered by semitransparent OSCs with (1) PTB7-Th:COi8DFIC; (2) the optimized ternary system; (3) PTB7-Th:IEICO-4F as the active layers. (B) Reproduced with permission of Wiley-VCH from Y. Xie, L. Huo, B. Fan, H. Fu, Y. Cai, L. Zhang, Z. Li, Y. Wang, W. Ma, Y. Chen, Y. Sun, High-performance semitransparent ternary organic solar cells. Adv. Funct. Mater. 28 (2018) 1800627. (D) Reproduced with permission of Elsevier from Z. Hu, J. Wang, Z. Wang, W. Gao, Q. An, M. Zhang, X. Ma, J. Wang, J. Miao, C. Yang, F. Zhang, Semitransparent ternary nonfullerene polymer solar cells exhibiting 9.40% efficiency and 24.6% average visible transmittance. Nano Energy 55 (2019) 424e432. (F) Reproduced with permission of Royal Society of Chemistry from X. Ma, Z. Xiao, Q. An, M. Zhang, Z. Hu, J. Wang, L. Ding, F. Zhang, Simultaneously improved efficiency and average visible transmittance of semitransparent polymer solar cells with two ultra-narrow bandgap nonfullerene acceptors. J. Mater. Chem. 6 (2018) 21485e21492.

by employing ternary strategy. Ternary strategy may provide more opportunity for fabricating efficient semitransparent OSCs.

3.4.3 Stability of ternary OSCs Many results confirm that appropriate incorporation of the third component is beneficial to improve device stability. Baran et al. fabricated a ternary OSC

Ternary organic solar cells

95

based on P3HT:IDTBR:IDFBR as active layer, and the PCE is improved from 6.3% to 7.7% with 30 wt% IDFBR in acceptors [103]. Combined with another low-band gap donor polymer PTB7-Th, the two mixed acceptors also lead to the ternary OSCs a higher PCE of 11.0%. The ambient stability of ternary and binary OSCs was tested. After 1200 h in air and under dark conditions (Fig. 3.17A), the ternary P3HT:IDTBR:IDFBR (1:0.7:0.3) device retains 80% of its PCE (6.1%), whereas P3HT:IDTBR performance retains 70% (4.3%). In addition, the P3HT:IDTBR, P3HT:IDFBR, and P3HT:IDTBR:IDFBR devices were exposed at operating conditions (unencapsulated, in air, AM1.5 radiation to illumination 100 mW cm2) for an initial 90 h test (Fig. 3.17B). The P3HT:IDTBR:IDFBR device exhibits the best air photostability, retaining 85% of its initial performance after 90 h. These results suggest that the addition of IDFBR to P3HT:IDTBR blend not only improves photovoltaic performance but also has a synergistic benefit on both storage lifetime and photostability. The stability of ternary OSCs was also investigated by Zhang et al. In nonfullerene-based ternary system PBDB-T:ITIC:N2200, [109] the optimized ternary OSCs exhibit excellent stability with about 80% initial PCE after 1000 h storage; however, only 74% or 42% initial PCEs can be retained for the corresponding binary OSCs after 1000 h storage (Fig. 3.17C). It means that the ternary active layers with 10 wt% N2200 exhibit a more stable morphology compared with other blend films, which can be confirmed from the GIXD (Fig. 3.17D) and TEM (Fig. 3.17E) experimental results. In fullerene-based ternary system PTB7-Th:BTR:PC71BM, the stability was investigated in inverted and conventional OSCs [37]. The decay trends of other photovoltaic parameters dependent on the storage time were fitted according to the power function (Y ¼ a(1 þ T)b), in which Y represents the key photovoltaic parameter, T represents the storage time, the coefficient a is the initial value of the corresponding photovoltaic parameter, and the absolute value of index (jbj) reflects the degradation rate. The smaller jbj values of the ternary OSCs indicate that the stability of the OSCs can be improved by employing the ternary strategy. The preserved percent of PCE of the inverted OSCs is 77.0% and 81.8% for the PTB7-Th:PC71BM-based OSCs and the optimized ternary OSCs, respectively. The preserved percent of PCE of the conventional OSCs is 65.5% and 70.2% for the corresponding cells. Obviously, the stability of the OSCs can be enhanced by employing BTR as a nucleating agent to form a more stable morphology.

96

Xiaoling Ma and Fujun Zhang

Figure 3.17 (A) Shelf storage lifetime (dark, in air) comparison of P3HT:IDTBR:IDFBR device efficiencies with other polymer:fullerene systems. Devices were exposed to ambient conditions over 1200 h duration or until high devices no longer showed any diode behavior. (B) Photostability of P3HT:IDTBR:IDFBR device and polymer:fullerene solar cells (in air, unencapsulated, under AM1.5 illumination at 1 sun) for 90 h. (C) Stability of PBDB-T:ITIC-, PBDB-T:ITIC:N2200-, PBDB-T:N2200-based OSCs without encapsulation under AM 1.5G illumination at intensity of 100 mW cm2. (D) Out-of-plane (solid line) and in-plane (dotted line) GIXD profiles of blend films with different N2200 content. (E) TEM images of blend films with different N2200 content. (B) Reproduced with permission of Springer Nature from D. Baran, R.S. Ashraf, D.A. Hanifi, M. Abdelsamie, N. €hr, S. Holliday, A. Wadsworth, S. Lockett, M. Neophytou, Reducing the Gasparini, J. A. Ro efficiency-stability-cost gap of organic photovoltaics with highly efficient and stable small molecule acceptor ternary solar cells. Nat. Mater. 16 (2017) 363e369. (E) Reproduced with permission of Elsevier from Q. An, F. Zhang, W. Gao, Q. Sun, M. Zhang, C. Yang, J. Zhang, High-efficiency and air stable fullerene-free ternary organic solar cells. Nano Energy 45 (2017) 177e183.

3.5 Challenges and outlooks As discussed in previous section, ternary strategy should be a simple, effective, and promising approach to realize performance improvement of OSCs. It should be noticed that the emerging of high-performance nonfullerene acceptors provides flexible options to rationally design and construct high-performance ternary OSCs. Results reported so far reveal that the

Ternary organic solar cells

97

fullerene:nonfullerene or nonfullerene materialsebased ternary OSCs should be the most promising ternary system, both of which have achieved a PCE more than 14%. In addition to increased photon harvesting, an appropriate incorporation of the third component can offer multiple benefits and synergistic effects, including optimizing film morphology, molecular arrangement, and enhancing charge carrier mobility as well as promoting exciton dissociation at the interface, thus resulting in performance improvement of OSCs. There are, however, still some challenges and limitations for the development of ternary OSCs: (1) compared with binary OSCs, the incorporation of the third component brings more complexities and variables into the binary system, and a deep understanding of the working mechanism in ternary OSCs is required for the further performance improvement. Advanced detection techniques such as TA spectroscopy should be employed to unravel the underling photophysical mechanism in ternary systems. (2) Deeper understanding of the VOC behavior for ternary systems is necessary. It was found that the VOC of ternary OSCs is sensitive to the film morphology. In alloy model, the VOC in ternary OSCs varies as a function of composition, while in some not particularly compatible systems, the VOC of ternary OSCs is mostly determined by the smallest energy level difference between donor and acceptor components. In some rare cases, the VOC in ternary OSCs is even higher than that of two binary OSCs. It is necessary to further investigate the origin of VOC in the ternary system and establish the relationship between morphology and device physics. (3) It is difficult to unify the guidelines of selecting the third component that matches well with the host materials. In addition to material properties (absorption spectrum, energy levels alignment, crystallinity, and surface energy) of the third component, the photovoltaic parameters of two binary OSCs should also be taken into consideration to construct effective ternary OSCs. In terms of those factors, different system may match different selection criteria, which makes the selection criteria ill-defined. (4) For donor materials, previous investigations mainly focused on developing low band gap polymer due to the poor light absorbing ability of fullerene acceptors. Nowadays, most of highperformance nonfullerene acceptors show strong absorption in long wavelength region (700e950 nm). Therefore, high-performance medium band gap or wide band gap donor materials should be further developed to match nonfullerene acceptors’ energy level and complement their absorption band. For future applications, more investigation on thick active layerebased ternary OSCs, the mechanisms of improved stability in ternary OSCs, and

98

Xiaoling Ma and Fujun Zhang

ternary semitransparent OSCs should be carried out. The PCE of 15% is commonly regarded as the target for enabling commercial viability. It is believed that the ternary OSCs based on fullerene:nonfullerene or nonfullerene materials as acceptors would be the most promising ternary system. The combination of high-performance materials and ternary strategy provide a bright prospect to push the performance of OSCs to the stage of commercial applications.

Acknowledgments This work was supported by National Natural Science Foundation of China (61675017, 61564003, 61377029).

References [1] H. Huang, L. Yang, B. Sharma, Recent advances in organic ternary solar cells, J. Mater. Chem. 5 (2017) 11501e11517. [2] A. Polman, M. Knight, E.C. Garnett, B. Ehrler, W.C. Sinke, Photovoltaic materials: present efficiencies and future challenges, Science 352 (2016) aad4424. [3] N.S. Lewis, Introduction: solar energy conversion, Chem. Rev. 115 (2015) 12631. [4] H. Li, K. Lu, Z. Wei, Polymer/small molecule/fullerene based ternary solar cells, Adv. Energy Mater. 7 (2017) 1602540. [5] W. Huang, P. Cheng, Y. Yang, G. Li, Y. Yang, High-performance organic bulk-heterojunction solar cells based on multiple-donor or multiple-acceptor components, Adv. Mater. 30 (2018) 1705706. [6] G. Yu, J. Gao, J.C. Hummelen, F. Wudl, A.J. Heeger, Polymer photovoltaic cells: enhanced efficiencies via a network of internal donor-acceptor heterojunctions, Science 270 (1995) 1789e1791. [7] J.J.M. Halls, C.A. Walsh, N.C. Greenham, E.A. Marseglia, R.H. Friend, S.C. Moratti, A.B. Holmes, Efficient photodiodes from interpenetrating polymer networks, Nature 376 (1995) 498e500. [8] T. Kirchartz, K. Taretto, U. Rau, Efficiency limits of organic bulk heterojunction solar cells, J. Phys. Chem. C 113 (2009) 17958e17966. [9] T. Ameri, P. Khoram, J. Min, C.J. Brabec, Organic ternary solar cells: a review, Adv. Mater. 25 (2013) 4245e4266. [10] Z.M. Beiley, M.G. Christoforo, P. Gratia, A.R. Bowring, P. Eberspacher, G.Y. Margulis, C. Cabanetos, P.M. Beaujuge, A. Salleo, M.D. Mcgehee, Semitransparent polymer solar cells with excellent sub-bandgap transmission for third generation photovoltaics, Adv. Mater. 25 (2013) 7020e7026. [11] J. You, L. Dou, Z. Hong, G. Li, Y. Yang, Recent trends in polymer tandem solar cells research, Prog. Polym. Sci. 38 (2013) 1909e1928. [12] O. Adebanjo, P.P. Maharjan, P. Adhikary, M. Wang, S. Yang, Q. Qiao, Triple junction polymer solar cells, Energy Environ. Sci. 6 (2013) 3150e3170. [13] T. Kim, H. Kim, J. Park, H. Kim, Y. Yoon, S. Kim, C. Shin, H. Jung, I. Kim, D.S. Jeong, Triple-junction hybrid tandem solar cells with amorphous silicon and polymer-fullerene blends, Sci. Rep. 4 (2014) 7154. [14] W. Xu, F. Gao, The progress and prospect of non-fullerene acceptors in ternary blend organic solar cells, Mater. Horiz. 5 (2018) 206e221. [15] P. Cheng, X.W. Zhan, Versatile third components for efficient and stable organic solar cells, Mater. Horiz. 2 (2015) 462e485.

Ternary organic solar cells

99

[16] J. Fang, Z. Wang, J. Zhang, Y. Zhang, D. Deng, Z. Wang, K. Lu, W. Ma, Z. Wei, Understanding the impact of hierarchical nanostructure in ternary organic solar cells, Adv. Sci. 2 (2015) 1500250. [17] L. Lu, M.A. Kelly, W. You, L. Yu, Status and prospects for ternary organic photovoltaics, Nat. Photon. 9 (2015) 491e500. [18] L. Lu, T. Xu, W. Chen, E.S. Landry, L. Yu, Ternary blend polymer solar cells with enhanced power conversion efficiency, Nat. Photon. 8 (2014) 716e722. [19] K. Sun, Z.Y. Xiao, S.R. Lu, W. Zajaczkowski, W. Pisula, E. Hanssen, J.M. White, R.M. Williamson, J. Subbiah, J.Y. Ouyang, A.B. Holmes, W.W.H. Wong, D.J. Jones, A molecular nematic liquid crystalline material for high-performance organic photovoltaics, Nat. Commun. 6 (2015) 6013. [20] Z.H. Chen, P. Cai, J.W. Chen, X.C. Liu, L.J. Zhang, L.F. Lan, J.B. Peng, Y.G. Ma, Y. Cao, Low band-gap conjugated polymers with strong interchain aggregation and very high hole mobility towards highly effi cient thick- film polymer solar cells, Adv. Mater. 26 (2014) 2586e2591. [21] M. Zhang, F.J. Zhang, Q.S. An, Q.Q. Sun, W.B. Wang, X.L. Ma, J. Zhang, W.H. Tang, Nematic liquid crystal materials as a morphology regulator for ternary small molecule solar cells with power conversion efficiency exceeding 10%, J. Mater. Chem. 5 (2017) 3589e3598. [22] X. Ma, F. Zhang, Q. An, Q. Sun, M. Zhang, J. Zhang, Dramatically boosted efficiency of small molecule solar cells by synergistically optimizing molecular aggregation and crystallinity, ACS Sustain. Chem. Eng. 5 (2017) 1982e1989. [23] Q.Q. Sun, F.J. Zhang, J. Wang, Q.S. An, C. Zhao, L.L. Li, F. Teng, B. Hu, A twostep strategy to clarify the roles of a solution processed pfn interfacial layer in highly efficient polymer solar cells, J. Mater. Chem. 3 (2015) 18432e18441. [24] Q.Q. Sun, F.J. Zhang, Q.S. An, M. Zhang, X.L. Ma, J. Zhang, Simultaneously enhanced efficiency and stability of polymer solar cells by employing solvent additive and upside-down drying method, ACS Appl. Mater. Interfaces 9 (2017) 8863e8871. [25] H. Zhang, H. Yao, J. Hou, J. Zhu, J. Zhang, W. Li, R. Yu, B. Gao, S. Zhang, J. Hou, Over 14% efficiency in organic solar cells enabled by chlorinated nonfullerene smallmolecule acceptors, Adv. Mater. 30 (2018) 1800613. [26] Z. Xiao, X. Jia, L. Ding, Ternary organic solar cells offer 14% power conversion efficiency, Sci. Bull. 62 (2017) 1562e1564. [27] H. Li, Z. Xiao, L. Ding, J. Wang, Thermostable single-junction organic solar cells with a power conversion efficiency of 14.62%, Sci. Bull. 63 (2018) 340e342. [28] C.B. Nielsen, S. Holliday, H.Y. Chen, S. Cryer, I. Mcculloch, Non-fullerene electron acceptors for use in organicsolar cells, Accounts Chem. Res. 48 (2015) 2803e2812. [29] H. Fu, Z. Wang, Y. Sun, Advances in non-fullerene acceptor based ternary organic solar cells, Solar RRL 2 (2017) 1700158. [30] Q. An, F. Zhang, J. Zhang, W. Tang, Z. Deng, B. Hu, Versatile ternary organic solar cells: a critical review, Energy Environ. Sci. 9 (2016) 281e322. [31] J.S. Huang, T. Goh, X.K. Li, M.Y. Sfeir, E.A. Bielinski, S. Tomasulo, M.L. Lee, N. Hazari, A.D. Taylor, Polymer bulk heterojunction solar cells employing forster resonance energy transfer, Nat. Photon. 7 (2013) 480e486. [32] Q. An, F. Zhang, L. Li, J. Wang, Q. Sun, J. Zhang, W. Tang, Z. Deng, Simultaneous improvement in short circuit current, open circuit voltage, and fill factor of polymer solar cells through ternary strategy, ACS Appl. Mater. Interfaces 7 (2015) 3691e3698. [33] Q. An, F. Zhang, L. Li, J. Wang, J. Zhang, L. Zhou, W. Tang, Improved efficiency of bulk heterojunction polymer solar cells by doping low-bandgap small molecules, ACS Appl. Mater. Interfaces 6 (2014) 6537e6544.

100

Xiaoling Ma and Fujun Zhang

[34] W. Zhao, S. Li, S. Zhang, X. Liu, J. Hou, Ternary polymer solar cells based on two acceptors and one donor for achieving 12.2% efficiency, Adv. Mater. 29 (2017) 1604059. [35] T. Liu, X. Xue, L. Huo, X. Sun, Q. An, F. Zhang, T.P. Russell, F. Liu, Y. Sun, Highly efficient parallel-like ternary organic solar cells, Chem. Mater. 29 (2017) 2914e2920. [36] J.S. Huang, T. Goh, X. Li, M.Y. Sfeir, E.A. Bielinski, S. Tomasulo, M.L. Lee, N. Hazari, A.D. Taylor, Polymer bulk heterojunction solar cells employing f€ orster resonance energy transfer, Nat. Photon. 7 (2013) 479e485. [37] X. Ma, F. Zhang, Q. An, Q. Sun, M. Zhang, J. Miao, Z. Hu, J. Zhang, A liquid crystal material as the third component for ternary polymer solar cells with an efficiency of 10.83% and enhanced stability, J. Mater. Chem. 5 (2017) 13145e13153. [38] T. Goh, J.S. Huang, E.A. Bielinski, B.A. Thompson, S. Tomasulo, M.L. Lee, M.Y. Sfeir, N. Hazari, A.D. Taylor, Coevaporated bisquaraine inverted solar cells: enhancement due to energy transfer and open circuit voltage control, ACS Photonics 2 (2015) 86e95. [39] Q.S. An, F.J. Zhang, Q.Q. Sun, J. Wang, L.L. Li, J. Zhang, W.H. Tang, Z.B. Deng, Efficient small molecular ternary solar cells by synergistically optimized photon harvesting and phase separation, J. Mater. Chem. 3 (2015) 16653e16662. [40] X. Ma, Y. Mi, F. Zhang, Q. An, M. Zhang, Z. Hu, X. Liu, J. Zhang, W. Tang, Efficient ternary polymer solar cells with two well-compatible donors and one ultranarrow bandgap nonfullerene acceptor, Adv. Energy Mater. 8 (2018) 1702854. [41] L. Yang, H. Zhou, S.C. Price, Y. Wei, Parallel-like bulk heterojunction polymer solar cells, J. Am. Chem. Soc. 134 (2012) 5432e5435. [42] M. Zhang, F. Zhang, J. Wang, Q. An, Q. Sun, Efficient ternary polymer solar cells with a parallel-linkage structure, J. Mater. Chem. C 3 (2015) 11930e11936. [43] R.A. Street, D. Davies, P.P. Khlyabich, B. Burkhart, B.C. Thompson, Origin of the tunable open-circuit voltage in ternary blend bulk heterojunction organic solar cells, J. Am. Chem. Soc. 135 (2013) 986e989. [44] R.A. Street, K.W. Song, J.E. Northrup, S. Cowan, Photoconductivity measurements of the electronic structure of organic solar cells, Phys. Rev. B Condens. Matter 83 (2011) 616e619. [45] A. Rockett, The Materials Science of Semiconductors, Springer, 2008, ISBN 978-0387-25653-5. [46] J.C. Wu, J. Zheng, C.L. Zacherl, P. Wu, Z.K. Liu, R. Xu, Hybrid functionals study of band bowing, band edges and electronic structures of cd1exznxs solid solution, J. Phys. Chem. C 115 (2011) 19741e19748. [47] M. Zhang, W. Gao, F. Zhang, Y. Mi, W. Wang, Q. An, J. Wang, X. Ma, J. Miao, Z. Hu, Efficient ternary non-fullerene polymer solar cells with PCE of 11.92% and ff of 76.5%, Energy Environ. Sci. 11 (2018) 841e849. [48] W. Su, Q. Fan, X. Guo, X. Meng, Z. Bi, W. Ma, M. Zhang, Y. Li, Two compatible nonfullerene acceptors with similar structures as alloy for efficient ternary polymer solar cells, Nano Energy 38 (2017) 510e517. [49] Y. Chen, P. Ye, Z.G. Zhu, X. Wang, L. Yang, X. Xu, X. Wu, T. Dong, H. Zhang, J. Hou, Achieving high-performance ternary organic solar cells through tuning acceptor alloy, Adv. Mater. 29 (2016) 1603154. [50] Y. Chen, P. Ye, X. Jia, W. Gu, X. Xu, X. Wu, J. Wu, F. Liu, Z.G. Zhu, H. Huang, Tuning Voc for high performance organic ternary solar cells with non-fullerene acceptor alloys, J. Mater. Chem. 5 (2017) 19697e19702. [51] X.L. Ma, W. Gao, J.S. Yu, Q.S. An, M. Zhang, Z.H. Hu, J.X. Wang, W.H. Tang, C.L. Yang, F.J. Zhang, Ternary nonfullerene polymer solar cells with efficiency > 13.7% by integrating the advantages of the materials and two binary cells, Energy Environ. Sci. 11 (2018) 2134e2141.

Ternary organic solar cells

101

[52] Y. Kim, S. Cook, S.A. Choulis, J. Nelson, J.R. Durrant, D.D.C. Bradley, Effect of electron-transport polymer addition to polymer/fullerene blend solar cells, Synthetic Met 152 (2005) 105e108. [53] W. Ma, J.Y. Kim, K. Lee, A.J. Heeger, Effect of the molecular weight of poly(3hexylthiophene) on the morphology and performance of polymer bulk heterojunction solar cells, Macromol. Rapid Commun. 28 (2007) 1776e1780. [54] M. Koppe, H.J. Egelhaaf, G. Dennler, M.C. Scharber, C.J. Brabec, P. Schilinsky, C.N. Hoth, Near ir sensitization of organic bulk heterojunction solar cells: towards optimization of the spectral response of organic solar cells, Adv. Funct. Mater. 20 (2010) 338e346. [55] T. Ameri, J. Min, N. Li, F. Machui, D. Baran, M. Forster, K.J. Schottler, D. Dolfen, U. Scherf, C.J. Brabec, Performance enhancement of the P3HT/PCBM solar cells through nir sensitization using a small-bandgap polymer, Adv. Energy Mater. 2 (2012) 1198e1202. [56] T. Ameri, T. Heumuller, J. Min, N. Li, G. Matt, U. Scherf, C.J. Brabec, Ir sensitization of an indene-c60 bisadduct (icba) in ternary organic solar cells, Energy Environ. Sci. 6 (2013) 1796e1801. [57] L. Lu, C. Wei, X. Tao, L. Yu, High-performance ternary blend polymer solar cells involving both energy transfer and hole relay processes, Nat. Commun. 6 (2015) 7327. [58] F. Zhao, Y. Li, Z. Wang, Y. Yang, Z. Wang, G. He, J. Zhang, L. Jiang, T. Wang, Z. Wei, Combining energy transfer and optimized morphology for highly efficient ternary polymer solar cells, Adv. Energy Mater. 7 (2017) 1602552. [59] J. Zhang, Y. Zhang, J. Fang, K. Lu, Z. Wang, W. Ma, Z. Wei, Conjugated polymersmall molecule alloy leads to high efficient ternary organic solar cells, J. Am. Chem. Soc. 137 (2015) 8176e8183. [60] Y. He, H.Y. Chen, J. Hou, Y. Li, Indene-c(60) bisadduct: a new acceptor for highperformance polymer solar cells, J. Am. Chem. Soc. 132 (2010) 1377e1382. [61] P.P. Khlyabich, B. Burkhart, B.C. Thompson, Efficient ternary blend bulk heterojunction solar cells with tunable open-circuit voltage, J. Am. Chem. Soc. 133 (2011) 14534e14537. [62] P. Cheng, Y. Li, X. Zhan, Efficient ternary blend polymer solar cells with indenec60 bisadduct as an electron-cascade acceptor, Energy Environ. Sci. 7 (2014) 2005e2011. [63] T.Y. Huang, D. Patra, Y.S. Hsiao, S.H. Chang, C.G. Wu, K.C. Ho, C.W. Chu, Efficient ternary bulk heterojunction solar cells based on small molecules only, J. Mater. Chem. 3 (2015) 10512e10518. [64] Y. Santo, I. Jeon, K.S. Yeo, T. Nakagawa, Y. Matsuo, Mixture of [60] and [70] PCBM giving morphological stability in organic solar cells, Appl. Phys. Lett. 103 (2013), 073306. [65] S.J. Ko, W. Lee, H. Choi, B. Walker, S. Yum, S. Kim, T.J. Shin, H.Y. Woo, J.Y. Kim, Improved performance in polymer solar cells using mixed PC61BM/ PC71BM acceptors, Adv. Energy Mater. 5 (2015) 1401687. [66] L. Chen, K. Yao, Y. Chen, Can morphology tailoring based on functionalized fullerene nanostructures improve the performance of organic solar cells? J. Mater. Chem. 22 (2012) 18768e18771. [67] Y.J. Cheng, C.H. Hsieh, P.J. Li, C.S. Hsu, Morphological stabilization by in situ polymerization of fullerene derivatives leading to efficient, thermally stable organic photovoltaics, Adv. Funct. Mater. 21 (2011) 1723e1732. [68] C.P. Chen, C.Y. Huang, S.C. Chuang, Highly thermal stable and efficient organic photovoltaic cells with crosslinked networks appending open-cage fullerenes as additives, Adv. Funct. Mater. 25 (2015) 207e213.

102

Xiaoling Ma and Fujun Zhang

[69] A. Diacon, L. Derue, C. Lecourtier, O. Dautel, G. Wantz, P. Hudhomme, Crosslinkable azido c60-fullerene derivatives for efficient thermal stabilization of polymer bulk-heterojunction solar cells, J. Mater. Chem. C 2 (2014) 7163e7167. [70] D. He, X. Du, W. Zhang, Z. Xiao, L. Ding, Improving the stability of P3HT/ PC61BM solar cells by a thermal crosslinker, J. Mater. Chem. 1 (2013) 4589e4594. [71] H. Lu, J. Zhang, J. Chen, Q. Liu, X. Gong, S. Feng, X. Xu, W. Ma, Z. Bo, Ternary-blend polymer solar cells combining fullerene and nonfullerene acceptors to synergistically boost the photovoltaic performance, Adv. Mater. 28 (2016) 9559e9566. [72] M. An, F. Xie, X. Geng, J. Zhang, J. Jiang, Z. Lei, D. He, Z. Xiao, L. Ding, A highperformance D-A copolymer based on dithieno[3,2-b:20 ,30 -d]pyridin-5(4h)-one unit compatible with fullerene and nonfullerene acceptors in solar cells, Adv. Energy Mater. 7 (2017) 1602509. [73] Q. Liu, J. Toudert, L. Ciammaruchi, G. Martínezdenegri, J. Martorell, High opencircuit voltage and short-circuit current flexible polymer solar cells using ternary blends and ultrathin ag-based transparent electrodes, J. Mater. Chem. 5 (2017) 25476e25484. [74] C. Wang, X. Xu, W. Zhang, S.B. Dkhil, X. Meng, X. Liu, O. Margeat, A. Yartsev, W. Ma, J. Ackermann, Ternary organic solar cells with enhanced open circuit voltage, Nano Energy 37 (2017) 24e31. [75] Y. Chen, Y. Qin, W. Yang, C. Li, H. Yao, N. Liang, X. Wang, W. Li, W. Ma, J. Hou, From binary to ternary: improving the external quantum efficiency of small-molecule acceptor-based polymer solar cells with a minute amount of fullerene sensitization, Adv. Energy Mater. 7 (2017) 1700328. [76] X. Ding, Ternary organic solar cells offer 14% power conversion efficiency, Sci. Bull. 62 (2017) 1562e1564. [77] J. Zhou, Z. Yi, X. Wan, G. Long, Z. Qian, N. Wang, Y. Liu, L. Zhi, G. He, C. Li, Solution-processed and high-performance organic solar cells using small molecules with a benzodithiophene unit, J. Am. Chem. Soc. 135 (2013) 8484. [78] L. Yang, S. Zhang, C. He, J. Zhang, H. Yao, Y. Yang, Y. Zhang, W. Zhao, J. Hou, New wide band gap donor for efficient fullerene-free all-small-molecule organic solar cells, J. Am. Chem. Soc. 139 (2017) 1958e1966. [79] B. Kan, M. Li, Q. Zhang, F. Liu, X. Wan, Y. Wang, W. Ni, G. Long, X. Yang, H. Feng, A series of simple oligomer-like small molecules based on oligothiophenes for solution-processed solar cells with high efficiency, J. Am. Chem. Soc. 137 (2015) 3886e3893. [80] D. Deng, Y. Zhang, J. Zhang, Z. Wang, L. Zhu, J. Fang, B. Xia, Z. Wang, K. Lu, W. Ma, Fluorination-enabled optimal morphology leads to over 11% efficiency for inverted small-molecule organic solar cells, Nat. Commun. 7 (2016) 13740. [81] H. Zhang, X. Wang, L. Yang, S. Zhang, Y. Zhang, C. He, W. Ma, J. Hou, Improved domain size and purity enables efficient all-small-molecule ternary solar cells, Adv. Mater. 29 (2017) 1703777. [82] Z. Zhou, S. Xu, J. Song, Y. Jin, Q. Yue, Y. Qian, F. Liu, F. Zhang, X. Zhu, Highefficiency small-molecule ternary solar cells with a hierarchical morphology enabled by synergizing fullerene and non-fullerene acceptors, Nat. Energy 3 (2018) 952e959. [83] T. Liu, Y. Guo, Y.P. Yi, L.J. Huo, X.N. Xue, X.B. Sun, H.T. Fu, W.T. Xiong, D. Meng, Z.H. Wang, F. Liu, T.P. Russell, Y.M. Sun, Ternary organic solar cells based on two compatible nonfullerene acceptors with power conversion efficiency > 10%, Adv. Mater. 28 (2016) 10008e10015. [84] R.N. Yu, S.Q. Zhang, H.F. Yao, B. Guo, S.S. Li, H. Zhang, M.J. Zhang, J.H. Hou, Two well-miscible acceptors work as one for efficient fullerene-free organic solar cells, Adv. Mater. 29 (2017) 1700437.

Ternary organic solar cells

103

[85] Z.H. Hu, F.J. Zhang, Q.S. An, M. Zhang, X.L. Ma, J.X. Wang, J. Zhang, J. Wang, Ternary nonfullerene polymer solar cells with a power conversion efficiency of 11.6% by inheriting the advantages of binary cells, ACS Energy Lett. 3 (2018) 555e561. [86] M. Zhang, W. Gao, F.J. Zhang, Y. Mi, W.B. Wang, Q.S. An, J. Wang, X.L. Ma, J.L. Miao, Z.H. Hu, X.F. Liu, J. Zhang, C.L. Yang, Efficient ternary non-fullerene polymer solar cells with pce of 11.92% and ff of 76.5%þ, Energy Environ. Sci. 11 (2018) 841e849. [87] J.X. Wang, W. Gao, Q.S. An, M. Zhang, X.L. Ma, Z.H. Hu, J. Zhang, C.L. Yang, F.J. Zhang, Ternary non-fullerene polymer solar cells with an efficiency of 11.6% by simultaneously optimizing photon harvesting and phase separation, J. Mater. Chem. 6 (2018) 11751e11758. [88] Q. An, J. Zhang, W. Gao, F. Qi, M. Zhang, X. Ma, C. Yang, L. Huo, F. Zhang, Efficient ternary organic solar cells with two Compatible non-fullerene materials as one alloyed acceptor, Small 14 (2018) 1802983. [89] T. Liu, Z. Luo, Q. Fan, G. Zhang, L. Zhang, W. Gao, X. Guo, W. Ma, M. Zhang, C. Yang, Y. Li, H. Yan, Use of two structurally similar small molecular acceptors enabling ternary organic solar cells with high efficiencies and fill factors, Energy Environ. Sci. 11 (2018) 3275e3282. [90] Z. Zheng, Q. Hu, S. Zhang, D. Zhang, J. Wang, S. Xie, R. Wang, Y. Qin, W. Li, L. Hong, A highly efficient non-fullerene organic solar cell with a fill factor over 0.80 enabled by a fine-tuned hole-transporting layer, Adv. Mater. (2018) 1801801. [91] S. Zhang, Y. Qin, J. Zhu, J. Hou, Over 14% efficiency in polymer solar cells enabled by a chlorinated polymer donor, Adv. Mater. 30 (2018) 1800868. [92] J. Sun, X. Ma, Z. Zhang, J. Yu, J. Zhou, X. Yin, L. Yang, R. Geng, R. Zhu, F. Zhang, Dithieno[3,2-b:2’,3’-d]pyrrol fused nonfullerene acceptors enabling over 13% efficiency for organic solar cells, Adv. Mater. 30 (2018) 1707150. [93] P.Q. Bi, F. Zheng, X.Y. Yang, M.S. Niu, L. Feng, W. Qin, X.T. Hao, Dual forster resonance energy transfer effects in non-fullerene ternary organic solar cells with the third component embedded in the donor and acceptor, J. Mater. Chem. 5 (2017) 12120e12130. [94] X. Xu, Z. Bi, W. Ma, Z. Wang, W.C.H. Choy, W. Wu, G. Zhang, Y. Li, Q. Peng, Highly efficient ternary-blend polymer solar cells enabled by a nonfullerene acceptor and two polymer donors with a broad composition tolerance, Adv. Mater. 29 (2017) 1704271. [95] L. Gao, Z.G. Zhang, L. Xue, J. Min, J. Zhang, Z. Wei, Y. Li, All-polymer solar cells based on absorption-complementary polymer donor and acceptor with high power conversion efficiency of 8.27%, Adv. Mater. 28 (2016) 1884e1890. [96] Y. Guo, Y. Li, O. Awartani, H. Han, J. Zhao, H. Ade, Y. He, D. Zhao, Improved performance of all-polymer solar cells enabled by naphthodiperylenetetraimide-based polymer acceptor, Adv. Mater. 29 (2017) 1700309. [97] T. Kim, J.H. Kim, T.E. Kang, C. Lee, H. Kang, M. Shin, C. Wang, B. Ma, U. Jeong, T.S. Kim, Flexible, highly efficient all-polymer solar cells, Nat. Commun. 6 (2015) 8547. [98] H. Benten, T. Nishida, D. Mori, H.J. Xu, H. Ohkita, S. Ito, High-performance ternary blend all-polymer solar cells with complementary absorption bands from visible to near-infrared wavelengths, Energy Environ. Sci. 9 (2016) 135e140. [99] W.Y. Su, Q.P. Fan, X. Guo, B. Guo, W.B. Li, Y.D. Zhang, M.J. Zhang, Y.F. Li, Efficient ternary blend all-polymer solar cells with a polythiophene derivative as a hole-cascade material, J. Mater. Chem. 4 (2016) 14752e14760. [100] S.C. Price, A.C. Stuart, L.Q. Yang, H.X. Zhou, W. You, Fluorine substituted conjugated polymer of medium band gap yields 7% efficiency in polymer-fullerene solar cells, J. Am. Chem. Soc. 133 (2011) 4625e4631.

104

Xiaoling Ma and Fujun Zhang

[101] K. Li, Z.J. Li, K. Feng, X.P. Xu, L.Y. Wang, Q. Peng, Development of large bandgap conjugated copolymers for efficient regular single and tandem organic solar cells, J. Am. Chem. Soc. 135 (2013) 13549e13557. [102] Z.Y. Li, L. Ying, R.H. Xie, P. Zhu, N. Li, W.K. Zhong, F. Huang, Y. Cao, Designing ternary blend all-polymer solar cells with an efficiency of over 10% and a fill factor of 78%, Nano Energy 51 (2018) 434e441. [103] D. Baran, R.S. Ashraf, D.A. Hanifi, M. Abdelsamie, N. Gasparini, J.A. R€ ohr, S. Holliday, A. Wadsworth, S. Lockett, M. Neophytou, Reducing the efficiencystability-cost gap of organic photovoltaics with highly efficient and stable small molecule acceptor ternary solar cells, Nat. Mater. 16 (2017) 363e369. [104] N. Li, G. Ke, Y. Jiang, Q. Rong, X. Hu, Y. Dong, L. Feng, X. Peng, T.P. Russell, G. Zhou, Small-molecule solar cells with simultaneously enhanced shortcircuit current and fill factor to achieve 11% efficiency, Adv. Mater. 29 (2017) 1700616. [105] R. Yu, S. Zhang, H. Yao, B. Guo, S. Li, H. Zhang, M. Zhang, J. Hou, Two wellmiscible acceptors work as one for efficient fullerene-free organic solar cells, Adv. Mater. 29 (2017) 1700437. [106] W. Li, J. Cai, F. Cai, Y. Yan, H. Yi, R.S. Gurney, D. Liu, A. Iraqi, T. Wang, Achieving over 11% power conversion efficiency in pffbt4t-2od-based ternary polymer solar cells with enhanced open-circuit-voltage and suppressed charge recombination, Nano Energy 44 (2018) 155e163. [107] W. Wu, G. Zhang, X. Xu, S. Wang, Y. Li, Q. Peng, Wide bandgap molecular acceptors with a truxene core for efficient nonfullerene polymer solar cells: linkage position on molecular configuration and photovoltaic properties, Adv. Funct. Mater. 28 (2018) 1707493. [108] G. Zhang, K. Zhang, Q. Yin, X.F. Jiang, Z. Wang, J. Xin, W. Ma, H. Yan, F. Huang, Y. Cao, High-performance ternary organic solar cell enabled by a thick active layer containing a liquid crystalline small molecule donor, J. Am. Chem. Soc. 139 (2017) 2387e2395. [109] Q. An, F. Zhang, W. Gao, Q. Sun, M. Zhang, C. Yang, J. Zhang, High-efficiency and air stable fullerene-free ternary organic solar cells, Nano Energy 45 (2017) 177e183. [110] P. Cheng, J. Wang, Q. Zhang, W. Huang, J. Zhu, R. Wang, S.Y. Chang, P. Sun, L. Meng, H. Zhao, Unique energy alignments of a ternary material system toward high-performance organic photovoltaics, Adv. Mater. 19 (2018) 1801501. [111] J. Wang, W. Gao, Q. An, M. Zhang, X. Ma, Z. Hu, J. Zhang, C. Yang, F. Zhang, Ternary non-fullerene polymer solar cells with efficiency of 11.6% by simultaneously optimizing photon harvesting and phase separation, J. Mater. Chem. 6 (2018) 11751e11758. [112] X. Ma, Z. Xiao, Q. An, M. Zhang, Z. Hu, J. Wang, L. Ding, F. Zhang, Simultaneously improved efficiency and average visible transmittance of semitransparent polymer solar cells with two ultra-narrow bandgap nonfullerene acceptors, J. Mater. Chem. 6 (2018) 21485e21492. [113] Z. Hu, J. Wang, Z. Wang, W. Gao, Q. An, M. Zhang, X. Ma, J. Wang, J. Miao, C. Yang, F. Zhang, Semitransparent ternary nonfullerene polymer solar cells exhibiting 9.40% efficiency and 24.6% average visible transmittance, Nano Energy 55 (2019) 424e432. [114] T. Kumari, S.M. Lee, S.H. Kang, S. Chen, C. Yang, Ternary solar cells with a mixed face-on and edge-on orientation enable an unprecedented efficiency of 12.1%, Energy Environ. Sci. 10 (2017) 258e265. [115] M.J. Xiao, K. Zhang, S. Dong, Q.W. Yin, Z.X. Liu, L.Q. Liu, F. Huang, Y. Cao, High-performance ternary nonfullerene polymer solar cells with both improved

Ternary organic solar cells

[116]

[117] [118]

[119] [120] [121] [122]

[123] [124]

[125]

[126]

[127] [128]

[129] [130]

105

photon harvesting and device stability, ACS Appl. Mater. Interfaces 10 (2018) 25594e25603. W. Jiang, R. Yu, Z. Liu, R. Peng, D. Mi, L. Hong, Q. Wei, J. Hou, Y. Kuang, Z. Ge, Ternary nonfullerene polymer solar cells with 12.16% efficiency by introducing one acceptor with cascading energy level and complementary absorption, Adv. Mater. 30 (2017) 1703005. W.C. Zhao, S.S. Li, S.Q. Zhang, X.Y. Liu, J.H. Hou, Ternary polymer solar cells based on two acceptors and one donor for achieving 12.2% efficiency, Adv. Mater. 29 (2017), 7 1604059. X.P. Xu, Z.Z. Bi, W. Ma, Z.S. Wang, W.C.H. Choy, W.L. Wu, G.J. Zhang, Y. Li, Q. Peng, Highly efficient ternary-blend polymer solar cells enabled by a nonfullerene acceptor and two polymer donors with a broad composition tolerance, Adv. Mater. 29 (2017) 1704271. S. Dai, T. Li, W. Wang, Y. Xiao, T.K. Lau, Z. Li, K. Liu, X. Lu, X. Zhan, Enhancing the performance of polymer solar cells via core engineering of nir-absorbing electron acceptors, Adv. Mater. 30 (2018) 1706571. B. Kan, Y.Q.Q. Yi, X.J. Wan, H.R. Feng, X. Ke, Y.B. Wang, C.X. Li, Y.S. Chen, Ternary organic solar cells with 12.8% efficiency using two nonfullerene acceptors with complementary absorptions, Adv. Energy Mater. 8 (2018) 1800424. J. Lee, S.J. Ko, M. Seifrid, H. Lee, C. McDowell, B.R. Luginbuhl, A. Karki, K. Cho, T.Q. Nguyen, G.C. Bazan, Design of nonfullerene acceptors with near-infrared light absorption capabilities, Adv. Energy Mater. 8 (2018) 1801209. M. Zhang, Z. Xiao, W. Gao, Q. Liu, K. Jin, W. Wang, Y. Mi, Q. An, X. Ma, X. Liu, C. Yang, L. Ding, F. Zhang, Over 13% efficiency ternary non-fullerene polymer solar cells with cocked-up absorption edge by incorporating a medium band gap acceptor, Adv. Energy Mater. 8 (2018) 1801968. Y.P. Xie, F. Yang, Y.X. Li, M.A. Uddin, P.Q. Bi, B.B. Fan, Y.H. Cai, X.T. Hao, H.Y. Woo, W.W. Li, F. Liu, Y.M. Sun, Morphology control enables efficient ternary organic solar cells, Adv. Mater. 30 (2018) 1803045. N. Li, Y. Kan, H. Wang, K. Gao, B. Xu, Q. Rong, R. Wang, J. Wang, F. Liu, J. Chen, Ternary non-fullerene polymer solar cells with 13.51% efficiency and a record-high fill factor of 78.13%, Energy Environ. Sci. (2018) https://doi.org/ 10.1039/C8EE01564C. H.H. Gao, Y.N. Sun, X.J. Wan, X. Ke, H.R. Feng, B. Kan, Y.B. Wang, Y.M. Zhang, C.X. Li, Y.S. Chen, A new nonfullerene acceptor with near infrared absorption for high performance ternary-blend organic solar cells with efficiency over 13%, Adv. Sci. 5 (2018) 1800307. B. Fan, P. Zhu, J. Xin, N. Li, Y. Lei, W. Zhong, Z. Li, W. Ma, F. Huang, Y. Cao, High-performance thick-film all-polymer solar cells created via ternary blending of a novel wide-bandgap electron-donating copolymer, Adv. Energy Mater. 8 (2018) 1703085. C. Waldauf, M.C. Scharber, P. Schilinsky, J.A. Hauch, Physics of organic bulk heterojunction devices for photovoltaic applications, J. Appl. Phys. 99 (2006) 104503. N. Li, D. Baran, G.D. Spyropoulos, H. Zhang, S. Berny, M. Turbiez, T. Ameri, F.C. Krebs, C.J. Brabec, Environmentally printing efficient organic tandem solar cells with high fill factors: a guideline towards 20% power conversion efficiency, Adv. Energy Mater. 4 (2014) 1400084. Y. Li, G. Xu, C. Cui, Y. Li, Flexible and semitransparent organic solar cells, Adv. Energy Mater. 8 (2017) 1701791. Q. Tai, F. Yan, Emerging semitransparent solar cells: materials and device design, Adv. Mater. 29 (2017) 1700192.

106

Xiaoling Ma and Fujun Zhang

[131] Y. Cui, C. Yang, H. Yao, J. Zhu, Y. Wang, G. Jia, F. Gao, J. Hou, Efficient semitransparent organic solar cells with tunable color enabled by an ultralow-bandgap nonfullerene acceptor, Adv. Mater. 29 (2017) 1703080. [132] Y. Xie, L. Huo, B. Fan, H. Fu, Y. Cai, L. Zhang, Z. Li, Y. Wang, W. Ma, Y. Chen, Y. Sun, High-performance semitransparent ternary organic solar cells, Adv. Funct. Mater. 28 (2018) 1800627.