CuInS2 solar cells with tunable synthesis of CuInS2 quantum dots

CuInS2 solar cells with tunable synthesis of CuInS2 quantum dots

Materials Science in Semiconductor Processing 24 (2014) 117–125 Contents lists available at ScienceDirect Materials Science in Semiconductor Process...

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Materials Science in Semiconductor Processing 24 (2014) 117–125

Contents lists available at ScienceDirect

Materials Science in Semiconductor Processing journal homepage: www.elsevier.com/locate/mssp

Size-dependent polymer/CuInS2 solar cells with tunable synthesis of CuInS2 quantum dots Wenjin Yue n, Mingyang Lan, Guoqiang Zhang, Wenshan Sun, Songming Wang, Guangjun Nie nn School of Biochemical Engineering, Anhui University of Polytechnic, Wuhu 241000, PR China

a r t i c l e i n f o

abstract

Available online 30 March 2014

This paper reports the size-dependent performance in polymer/CuInS2 solar cells with tunable synthesis of chalcopyrite CuInS2 quantum dots (QDs) by the solvothermal method. The CuInS2 QDs of 3.2–5.4 nm in size are fine tuned by the reaction time in the solvothermal process with the slow supply of In3 þ ions during the crystallization, and the band gaps increased with QDs sizes decreasing according to the results from the characterization of sizes, morphologies, component elements, valence states and band gaps of CuInS2 QDs. We fabricated MEH-PPV/CuInS2 solar cells, and the photoactive layer of device displayed sizedependent light-harvesting, charge separation and transport ability. Moreover, the solar cells exhibit size-dependent short circuit current (Jsc) and open circuit voltage (Voc), with higher performance in both Jsc and Voc for smaller CuInS2 QDs, resulting in the maximum power conversion efficiency of ca. 0.12% under the monochromic illumination at 470 nm; CuInS2 QDs actually serve as an effective electron acceptor material for the MEH-PPV/CuInS2 solar cells with the wide spectral response extending from 300 to 900 nm. & 2014 Elsevier Ltd. All rights reserved.

Keywords: CuInS2 Quantum dots Solvothermal process Solar cells Charge transfer

1. Introduction Hybrid polymer-based solar cells (PSCs) consisting of organic conjugated polymers as the electron donor (D) and inorganic semiconductor nanocrystals as the electron acceptor (A) in a bulk-heterojunction (BHJ) architecture are promising to meet the increasing global energy consumption [1–4] because they combine the properties of inorganic and organic materials [5,6]. Wide bandgap metal oxide (TiO2 or ZnO) nanostructures are used as the electron acceptor to prepare various hybrid PSCs [7–12]. However, one of the major efficiency-limiting factors in these devices is the narrow absorption band of conjugated polymers with mismatching n

Corresponding author. Tel.: þ86 553 2871254. Corresponding author. Tel.: þ86 553 2871255. E-mail addresses: [email protected] (W. Yue), [email protected] (G. Nie). nn

http://dx.doi.org/10.1016/j.mssp.2014.03.019 1369-8001/& 2014 Elsevier Ltd. All rights reserved.

solar spectrum [2] because most of conjugated polymers have larger band gap than the maximum photon flux of AM 1.5 solar spectrum [4]. Quantum dots (QDs) provide the possibility of accessing the novel materials and devices that benefit from the unique physical properties between those of molecular and bulk states due to quantum size effects (e.g., slower exciton cooling, multiple exciton generation, and the development of discrete, well-separated energy states in the reduced dimensions) [13,14], which is applied to tune and optimize the band gap (Eg) of low band-gap semiconductor to obtain broad spectral absorption spanning from the visible to near-infrared region (NIR). For example, solar cells based on inorganic semiconductor QDs with radii smaller than the Bohr exciton radii of the material are predicted to have the maximum thermodynamic power conversion efficiency up to 66% [15–19], as a result, semiconductor QDs have now been considered for polymer-based solar cells [20–26].

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Ternary CuInS2 has a band gap of 1.5 eV and an absorption coefficient of 105 cm  1, which is considered as an ideal material for efficient solar cells [27–30]; moreover, CuInS2 exhibits good radiation stability, and the p- or n-type conductivity depending on the stoichiometry can be easily controlled during its synthesis [31]. The Wannier–Mott bulk exciton Bohr radius of CuInS2 is 4 nm [32]. Several methods are available for the preparation of CuInS2 nanoparticles smaller than 5 nm, including thermal decomposition of single-source precursors [33,34], photochemical decomposition of single-source precursor [35], microwave assistant decomposition of single-source precursors [36], solvent thermolysis of organometallic precursors [37] and surfactant-assisted chemical reactions [32,38,39]. A solvothermal technique is the convenient method to prepare nanocrystals, and has been reported for synthesis of chalcopyrite CuInS2 QDs [40,41] and zincblende CuInS2 QDs [42]. Up to now, the CuInS2 QDs have received the control over the size in the range of 2–8 nm [39–41,43], but very few reports on the device performance of solar cells with tunable-sized CuInS2 QDs are available. Particularly, the band gaps of QDs strongly depend on their sizes [5,6,39,43,44]; the intrinsic correlation between the QDs size and the performance of hybrid PSCs is not clear yet, which is crucially important for the optimization of solar cells based on QDs. In this paper, the chalcopyrite CuInS2 QDs with tunable sizes are successfully synthesized by solvothermal method and the size-dependent performance in MEH-PPV/CuInS2 hybrid solar cells is revealed. We find the solvothermal process for controlled synthesis of highly pure chalcopyrite CuInS2 QDs within a wide time window of 6–18 h via the slow supply of In3 þ ions during crystallization. Results show that the smaller CuInS2 QDs exhibit the richer copper element and larger band gap, moreover, the photovoltaic layer containing small-sized CuInS2 QDs displays larger D/A specific interface area contributing to charge separation and more ideal interpenetrating networks channel beneficial to charge transport, finally resulting in the maximum power conversion efficiency (η) of ca. 0.12% under the monochromic illumination at 470 nm with the higher open voltage (Voc) and current density (Jsc). It should be identified that the present study is significantly different from that in our previous paper, although it displayed the similar synthesis method of CuInS2 QDs [41]. First, the QDs size in present study (3.2, 3.7 and 5.4 nm) is more than that (3.2 and 5.4 nm) in previous paper, which is in favor of demonstrating the influence of QDs size on device performance more systematically. Second, device architecture of solar cells is largely different. CuInS2 QDs are used as the additive in aligned MEH-PPV/TiO2 solar cells to form ternary MEH-PPV–CuInS2/TiO2 device in previous paper; however, in present paper, CuInS2 QDs are simply blended with polymer to form MEH-PPV/CuInS2 solar cells because the device is fabricated more conveniently and used more widely. Third, the highlighted contents are different. In previous paper, we mainly emphasized on the influence of the presence of CuInS2 on device performance. However, the size-dependent properties of QDs and photoactive layer contributing to device performance of solar cells are discussed in present paper. Finally, the size-dependent regularity on device performance

is extremely different. The additive of large-sized CuInS2 QDs contributed to the higher device performance in previous paper, whereas the device containing small-sized CuInS2 QDs displayed higher device performance in present paper, which may originate from different charge transport mechanisms. 2. Experimental 2.1. Chemicals Indium acetate (Aldrich), octadecylamine (Alfa Aesar), poly(2-methoxy-5-(2-ethyl hexyloxy)-1,4-phenylene vinylene) (MEH-PPV, average Mn ¼40,000–70,000, Aldrich) and poly(3,4-ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS, Clevios P HC V4, H.C. Starck) were commercially obtained. Other chemicals, including copper acetate monohydrate (Analytically Pure), thiourea (Analytically Pure), ethanol (Analytically Pure) and chlorobenzene (Chemically Pure), were purchased from the Sinopharm Chemical Reagent Co., Ltd. All the chemicals were used as received without further purification, except for the distillation of chlorobenzene under the reduced pressure before use. 2.2. Synthesis of CuInS2 QDs CuInS2 QDs were synthesized by the solvothermal method similar to previous reports [41,45]. In this experiment, the size of chalcopyrite CuInS2 QDs was controlled by the reaction time (t) in the solvothermal process at 160 1C. Typically, Cu(Ac)2  H2O (0.05 mmol) was first dissolved in absolute ethanol (50 mL) at room temperature, resulting in a blue solution; then, In(Ac)3 (0.05 mmol) was dispersed into the blue solution, leading to a blue suspension. Afterward, 0.60 mmol octadecylamine was dissolved into the suspension by ultrasonic treatment and a sapphire dispersion was formed. Finally, 0.2 mmol CS(NH2)2 was added into the dispersion and a brown-colored dispersion was rapidly produced. The brown dispersion was transferred into a Teflon-lined stainless steel autoclave of 60 mL capacity and maintained at 160 1C for different times (t¼6, 12, and 18 h) to carry out the solvothermal reactions. After the autoclave naturally cooled to room temperature, the product was collected by centrifugation (10,000 rpm, 8 min), washed several times with absolute ethanol and dried under vacuum at 60 1C for 6 h prior to other measurements. 2.3. Assembly of solar cells MEH-PPV/CuInS2 solar cells were fabricated as follows. The PEDOT:PSS suspension that was passed through a 0.45 μm filter was first spin-coated onto indium tin oxide (ITO) ( r15 Ω/□, Shenzhen Laibao Hi-Tech Co., Ltd., China) coated glass substrate in ambient conditions, followed by the annealing at 140 1C for 30 min under nitrogen atmosphere to eliminate the water, producing a PEDOT:PSS film (ca., 40 nm thick) on ITO. Then, a MEH-PPV–CuInS2 photoactive layer was deposited from chlorobenzene (with MEH-PPV content of 5 mg/mL and CuInS2 content of 5 mg/mL) over the PEDOT:PSS layer by spin-coating.

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Finally, 0.2 nm LiF and 100 nm Al films were subsequently deposited onto the photoactive layer by thermal evaporation. The devices were transferred into a glovebox (O2 o1 ppm, H2Oo1 ppm) for sealing. The active area of the device was defined by the size of Al electrode (1  4 mm2). 2.4. Characterization The power X-ray diffraction (XRD) patterns of the products were measured on a MXP18AHF X-ray diffractometer with monochromated Cu-Kα radiation (λ¼1.54056 Å). The high resolution transmission electron microscopy (HRTEM) observations were carried out on a JEOL-2010 transmission electron microscopy. X-ray photoelectron spectroscopy (XPS) analyses were performed on an ESCALAB 250 XPS, using Al Kα X-rays as the excitation source, and all the XPS peaks were calibrated by using C 1s (284.60 eV) as the reference. Cyclic voltammetric (CV) measurements were performed on an IM6e electrochemical workstation (Zahner Co., Germany) at scan rate of 50 mV/s, with platinum plate as a counterelectrode. In the CV measurements, tetrabutylammonium perchlorate dissolved in acetonitrile (0.1 M) was used as a supporting electrolyte, and the potential (E) was recorded versus an Ag/AgCl reference electrode. The working electrode was prepared by drying a drop of CuInS2 QDs dispersion (0.04 mg/mL) in ethanol on a glassy carbon disc that had been freshly polished, cleaned, and dried. The electrolyte solution was first thoroughly deoxygenated by bubbling high purity nitrogen for 15 min, and nitrogen atmosphere was maintained during the CV measurements. The film samples of MEH-PPV and MEH-PPV–CuInS2 composites for optical measurements were spin-coated (1500 rpm, 60 s) on freshly cleaned quartz substrates from chlorobenzene solutions, where a solution with MEH-PPV concentration of 5 mg/mL was applied to prepare the pristine polymer film and the MEH-PPV–CuInS2 composite films were deposited from chlorobenzene (with MEH-PPV content of 5 mg/mL and QDs content of 5 mg/mL). Prior to the optical measurements, the films were kept in vacuum overnight at room temperature. Absorption spectra were recorded under ambient conditions on a UV 2550 spectrophotometer (Shimadzu), while photoluminescence (PL) spectra were measured under ambient conditions on a F-7000 spectrofluorometer (Hitachi) with an excitation at 480 nm. 3. Results and discussion 3.1. Properties of different-sized CuInS2 QDs. 3.1.1. Sizes and morphologies Fig. 1 shows the XRD patterns of the CuInS2 QDs synthesized at different reaction times (t) for the solvothermal process. The intense diffraction peaks at 2θ¼27.91, 46.51 and 55.01 match well with the crystal faces (112), (204) or (220), and (116) or (312) of tetragonal chalcopyrite CuInS2 (JCPDS PCPDFWIN #85-1575, a¼5.523 Å and c¼11.133 Å). No characteristic peaks of impurity phases (e.g., CuS and In2S3) are observed. The XRD results clearly indicate the influence of the solvothermal reaction time t on the size of CuInS2 QDs. On the

Fig. 1. XRD patterns of the CuInS2 QDs synthesized in the different solvothermal reaction times.

Table 1 Sizes and elements of the chalcopyrite CuInS2 QDs synthesized at different reaction times (t). t (h)

DXRDa (nm)

DTEMb (nm)

Cu:In:Sc

6 12 18

3.2 3.7 5.4

3–4 4–5 6–8

1.40:1.00:1.73 1.24:1.00:1.54 1.09:1.00:1.51

a b c

Diameter determined by XRD. Diameter determined by TEM. XPS data.

basis of the full width half-maximum (FWHM) of (112) peak (FWHM¼2.51, 2.21 and 1.51 for t¼6, 12 and 18 h, respectively), the average sizes (S) of the CuInS2 QDs are calculated by the Scherrer formula S¼Kλ(Bcosθ)  1, where K is a constant (0.9), λ is the wavelength of the X-ray (1.54056 Å), B is the FWHM (in radians), and θ is the Bragg angle of (112) peak. As shown in Table 1, the size of CuInS2 QDs with the diameter in the range of 3.2–5.4 nm increases with increasing t, similar to the findings in the surfactant-assisted chemical reactions [39,43]. The as-prepared CuInS2 QDs were studied with a transmission electron microscope (TEM). As shown in Fig. 2, all the CuInS2 crystals are almost spherical, and the longer reaction time t results in the larger CuInS2 QDs (Table 1). Note, in our previous report, the chalcopyrite CuInS2 QDs with the sizes of 3.2 and 5.4 nm were obtained by the similar synthesis procedure for 6 h and 18 h, respectively [41]. In this experiment, we found that the time for the solvothermal reaction plays a key role in the growth of CuInS2 QDs. A shorter time for 12 h (within 6– 18 h) leads to the QDs size of 3.7 nm (within 3.2–5.4 nm); however, longer time for 24 h results in the particles larger than 8 nm and some are irregular in shape (Fig. 2d). It is a fact that the reactant In(Ac)3 has a poor solubility in ethanol that serves as the solvent for solvothermal reaction even at the boiling point of ethanol [45]. Therefore, the tuned sizes of the chalcopyrite CuInS2 QDs within a wide time window (6–18 h) are reasonably related to the slow supply of In3 þ ions during crystallization.

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Fig. 2. TEM images of the CuInS2 nanoparticles synthesized in 6 h (a), 12 h (b), 18 h (c) and 24 h (d).

3.1.2. Component elements and valence states XPS measurements of the as-synthesized CuInS2 QDs were carried out to get the more accurate information on composition (Fig. 3). The survey spectrum indicates the presence of Cu, In, S and C as well. Note that the high intensity of C is likely due to the high concentration of capping agents and the contamination as a result of the sample exposure to atmosphere. The XPS analysis shows that the binding energies of Cu 2p3/2 and In 3d5/2 are 932 eV and 445 eV, respectively, and the S 2p core level spectrum has two peaks at 162 eV for Cu–S and at 163 eV for In–S, which agrees well with the reported data for CuInS2 [46,47]. With increasing the size of CuInS2 QDs, the stoichiometric ratio of Cu/In decreased (Table 1), indicating higher In content existed in the larger QDs, which is similar to the findings in the synthesis of CuInS2 nanoparticles by surfactantassisted chemical reactions [43]. The time-dependent In content in the CuInS2 QDs further supports the correlation between the QDs size and the slow supply of In3 þ ions during crystallization. 3.1.3. Band gaps According to the theoretical model based on the finitesize effect of quantum dots [48], the edge shifts of valence [ΔEv] and conduction [ΔEc] bands in the CuInS2 QDs, relative to those of the bulk CuInS2, are calculated according to the relationships ΔEc ¼ħ2π2/(2meD2) and ΔEv ¼ħ2π2/

(2mhD2), where D is the particle diameter, me is the effective electron mass, and mh is the effective hole mass. For calculation, the values of 0.16 for me and 1.3 for mh are taken for CuInS2 [49]; moreover, the average diameters of the CuInS2 QDs from XRD data and the band gap of 1.5 eV with Ev ¼  5.6 eV and Ec ¼  4.1 eV for bulk CuInS2 [49] are used. The calculated Ev,c [¼Ev  ΔEv], Ec,c [ ¼Ec þΔEc] and Eg,c [¼ Ec  Ev] values of the CuInS2 QDs are shown in Table 2. Optical band gap (Eg,o) is determined by UV–vis absorption spectra (inset of Fig. 4), which are approximated using the direct band gap method [50,51], by plotting the squared absorbance versus energy and extrapolating to zero [52] (Fig. 4a). Clearly, Eg,o of CuInS2 QDs (Table 2) is significantly blue-shifted as compared to that of bulk CuInS2 (1.53 eV) due to the quantum size effect [53]. Eg,o increases with decreasing quantum dots size, which agrees with Eg,c. Electrochemical band gap (Eg,e) of different-sized CuInS2 QDs is determined by cyclic voltammetry (CV) [42]. With decreasing the QD sizes, Eox shifts slightly in the positive direction but Ered moves remarkably in the negative direction (Fig. 4b), similar to the observations in the cases of CuInS2 [43] and CdTe [54] nanoparticles. According to Ec,e ¼  (Eox þ4.71) eV and Ev,e ¼ (Ered þ4.71) eV, where Eox and Ered are relative to the Ag/AgCl reference electrode, the valence band (Ev,e), conduction band (Ec,e) levels and the electrochemical band

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Fig. 3. XPS spectra of the as-synthesized CuInS2 QDs with different sizes: (a) survey spectrum, (b) Cu 2p spectrum, (c) In 3d spectrum, and (d) S 2p spectrum.

Table 2 Band gaps of different-sized chalcopyrite CuInS2 QDs determined by different methods. D (nm)

Ev,ca (eV)

Ec,ca (eV)

Eg,ca (eV)

Eg,ob (eV)

Ev,ec (eV)

Ec,ec (eV)

Eg,ec (eV)

3.2 3.7 5.4

 5.63  5.62  5.61

 3.87  3.93  4.02

1.76 1.69 1.59

1.94 1.77 1.65

 5.84  5.76  5.76

 3.84  4.00  4.13

2.00 1.76 1.63

a Calculated data based on the finite-size effect of quantum dots, and Eg,c is calculated band gap. b Determined by absorption spectra, and Eg,o is optical band gap. c Determined by CV measurements, and Eg,e is electrochemical band gap.

gap Eg,e(¼Ec,e  Ev,e) for the CuInS2 QDs are evaluated (Table 2), which agrees well with the Eg,c. 3.2. MEH-PPV/CuInS2 solar cells 3.2.1. Properties of photoactive layers As we know, in polymer-based solar cells, the energy conversion process has four fundamental steps in the commonly accepted mechanism, that is, absorption of light, generation of excitons, diffusion of the excitons, dissociation of the excitons with the generation of charge, charge transport and collection [55]. Obviously, device performance depends largely on the process occurred in photoactive layers such as the generation of excitons, dissociation of the excitons and charge transport, which

is related to the absorption spectra, PL spectra and morphologies of the photoactive layer. The light-harvesting abilities of photoactive layers in MEH-PPV/CuInS2 solar cells consisted of polymer and different-sized CuInS2 QDs are characterized by absorption spectra (shown in Fig. 5a). Compared to the absorbance of pristine MEH-PPV film in 400–600 nm [56], the photoactive layer displays stronger light-harvesting ability with the absorbance increasing within 300–900 nm originated from the complementary absorption contribution of CuInS2 QDs to MEH-PPV (inset of Fig. 4a). Moreover, the increase in quantum dots size would lead in the increased absorbance, which may result from the smaller band gap of larger-sized CuInS2 QDs contributing to stronger lightharvesting ability. Polymer exciton separation ability is another important factor influencing device performance. Normally, we characterize it by polymer fluorescence (PL) quenching efficiency, which is calculated by comparing the maximum emission intensity of the photoactive layer to that of pristine polymer film. As shown in Fig. 5b, the PL spectra of MEH-PPV–CuInS2 composites display the emission profile similar to that of pristine MEH-PPV; however, the PL intensity of the composites is reduced. The quenching of the MEH-PPV emission by adding CuInS2 QDs indicates the occurrence of polymer excitons separation resulting in the charge transfer from polymer to QDs [42]. As the size of the CuInS2 QDs decreases from 5.4 nm, 3.7 nm to 3.2 nm, the PL quenching efficiency increases from 16.80%, 25.65% to 42.89%, respectively. The increased PL quenching efficiency

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Fig. 4. (a) The determination of the band gap by direct band gap method. (b) Cyclic voltammogram for the CuInS2 QDs with different sizes as shown. Inset of (a) shows UV–vis absorption spectra in ethanol.

is attributed to the larger MEH-PPV/CuInS2 specific interfacial area for the polymer exciton separation in the case of smaller CuInS2 QDs because the mass concentration of CuInS2 QDs in the bulk heterojunction is comparable for the particle in spite of the size, but for smaller particles, the same mass concentration corresponds to an increased quantity of particles and a higher surface-to-volume ratio. Besides the exciton separation, the effective charge transport is responsible for the generation of photocurrent, which depends mainly on the morphology of the photoactive layer. Fig. 6 shows that the TEM and HRTEM images of the photoactive layer contained different-sized CuInS2 QDs. TEM images (Fig. 6a–c) demonstrate that the CuInS2 QDs aggregate into QDs rich domains (dark contrast) dispersing randomly in MEH-PPV rich matrix (light areas). HRTEM images (Fig. 6d–f) show that QDs rich domains are formed by the individual CuInS2 QDs in good contact with each other, having a uniform width almost consisted of one or two quantum dots. With increasing the QDs size, the width increased, i.e., 4–7 nm for 3.2 nm QDs, 5–8 nm for 3.7 nm and 8–10 nm for 5.4 nm QDs. Smaller particles form larger specific interface area with polymer and more effective interpenetrating networks channels owe to the same mass concentration corresponding to an increased quantity of particles. The individual QDs size could be clearly observed from HRTEM with the spacing distance ca. 0.32 nm which matched the (112) planes of tetragonal

Fig. 5. Absorption spectra (a) and room temperature photoluminescence spectra (excitation at 480 nm) (b) of MEH-PPV and MEH-PPV-CuInS2 (R¼ 1/1) composite films spin-coated on quartz substrates. The CuInS2 QDs have different sizes as shown.

chalcopyrite CuInS2, suggesting no crystallographic structure and size change in CuInS2 QDs even after they were mixed with MEH-PPV to form MEH-PPV–CuInS2 composites. 3.2.2. Device performance Fig. 7a shows the typical J V curves of such devices with the MEH-PPV/CuInS2 weight ratio of R¼1/1 under monochromatic illumination of 15.85 mW/cm2 at 470 nm. It should be noted that the J–V curves in dark for all the devices pass through the origin where no potential results in no current (inset of Fig. 7a), similar in form to those of the heterojunction solar cells [57]. Table 3 presents the averaged overall photovoltaic performance of three individual devices for each sample. It could be seen that the performance of hybrid solar cells significantly correlates with the sizes of CuInS2 QDs. On one hand, reducing the QDs size enhances the device Voc. It is well known that the Voc value in hybrid polymer/ nanoparticle solar cells is directly related to the energy A difference of HOMO level of polymer and the Ec level of A nanoparticle components [4,21,57,58]. In this experiment, Ec becomes higher with decreasing the size of CuInS2 QDs (Table 2), contributing to the increase in the difference D A between EHOMO (5.3 eV) and Ec of CuInS2 QDs (inset of Fig. 8b), leading to larger Voc for smaller CuInS2 QDs. On the other hand, reducing the sizes of QDs significantly enhances Jsc of MEH-PPV/CuInS2 solar cells. To get deep insight into the size-dependent Jsc, we analyzed the photocurrent action responses of the MEH-PPV/CuInS2 solar cells

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Fig. 6. TEM (a-c) and HRTEM (d-f) images of the MEH-PPV-CuInS2 composites with 3.2 nm (a, d), 3.7 nm (b, e) and 5.4 nm (c, f) CuInS2 QDs. The HRTEM images were taken from the marked yellow regions the corresponding TEM images; the green circles in (d-f) indentify individual quantum dots, while the red dashed lines indentify the MEH-PPV/CuInS2 interface. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

in terms of IPCE spectra. As shown in Fig. 7b, a very broad spectral response extending from 300 to 900 nm is revealed despite of different-sized CuInS2 QDs, with the shape almost similar to the absorption spectrum of respective CuInS2 QDs (inset of Fig. 4a). The response in the region 600–900 nm, where there is no absorption in the MEH-PPV film (Fig. 5a), corresponds to the contribution from the CuInS2 QDs, while the response below 600 nm is the combined contribution from both MEH-PPV and CuInS2 QDs. Therefore, the presence of the CuInS2 QDs provides the faintly complementary absorption contributing to the device photocurrent. With decreasing the QDs sizes, the IPCE values increase. The measured IPCE values (Fig. 7b and Table 3) for the different-sized CuInS2 QDs at 470 nm close to the maximum absorption of MEH-PPV are in the order 3.243.745.4 nm, which agrees with the calculated results according to the relation IPCEcal ¼1240Jsc/λPin (Table 3), where λ (nm) is the incident photon wavelength, Jsc (μA/cm2) is the photocurrent of the device from the J–V measurements, and Pin (W/m2) is the incident power. The size-dependent photocurrent is the same to that of the PL quenching efficiency, indicating that Jsc is dominantly correlated with the exciton dissociation at the MEH-PPV/CuInS2 interface other than light-harvesting ability. On the whole, the decrease in quantum dots would lead to the increase in Voc and Jsc, contributing to the improved device performance. The maximum efficiency (η) nearly to 0.12% is obtained with 3.2 nm QDs, which is much higher

than the similarly structured devices consisting of MEHPPV/CuInS2 nanocrystals [59] or MEH-PPV/zincblende CuInS2 QDs [42], P3HT/CdSe QDs [60] and MEH-PPV/CdSe QDs [24]. 3.2.3. Charge transfer mechanism Solar cells were fabricated by spin-coating MEH-PPV– CuInS2 hybrids as photoactive layers directly onto PEDOT: PSS layers. Morphological characteristics of photoactive layers in MEH-PPV/CuInS2 solar cells are indicated in Fig. 8a. CuInS2 QDs aggregate to form continuous and highly condensed interpenetrating networks in the photoactive layers, contributing to the formation of charge transport channels in MEH-PPV/CuInS2 solar cells. On the basis of the energy alignments between MEH-PPV (HOMO¼  5.3 eV and LUMO¼ 3.0 eV) [61] and CuInS2 QDs (Table 2), the MEH-PPV/CuInS2 interface is energetically favorable for the dissociation of excitons photogenerated in MEH-PPV, resulting in the charge transfer from polymer to CuInS2 QDs (Fig. 5b) with the electrons being injected into the CuInS2 QDs as acceptor and the holes remaining in the polymer as donor; moreover, as indicated by the action spectra (Fig. 7b), the CuInS2 QDs are excited together with MEH-PPV to contribute to the photocurrent generation, by injecting holes onto the polymer with electrons remaining on the CuInS2 QDs [42]. It is concluded that the chalcopyrite CuInS2 QDs are effective electron acceptor, as well as the assistant light-absorbing

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material, for the MEH-PPV/CuInS2 devices with wide spectral response extending from 300 to 900 nm, in which the charge transfer processes occur by the way similar to the case of zincblende CuInS2 QDs [42]. It is noted that the size-dependent intensity of the IPCE is opposite to the light-harvesting abilities changed with QDs size in the photoactive layer (Fig. 5a), which is obviously different from previous reports [62,63]. In our device, polymer is still the main light-harvesting material and CuInS2 acts as the electron acceptor; as a result, the increased light-harvesting ability with increasing QDs size contributes to Jsc slightly. However, small-sized QDs resulting in larger specific interface area are beneficial to charge separation (Fig. 5b); moreover, small-sized QDs producing more effective interpenetrating networks are beneficial to charge transport (Fig. 6), which are apparently different from the unsatisfactory morphology of CdSe QDs dispersing in P3HT matrix in the CdSe/P3HT device [62] and CdSe naonoparticles capped by TOPO dispersing in PFT

matrix in the CdSe/PFT device [63]. Therefore, the higher photocurrent for the device consisting of the smaller CuInS2 QDs mainly correlates with the increased MEHPPV/CuInS2 interface area for charge separation, the formation of interpenetrating networks beneficial to charge transport, which offsets the deficiency in light-harvesting ability. It is noted that the performance of the MEH-PPV/ CuInS2 solar cells in this study are not high enough, although much higher than previous reports [42,49,59]. The low performance may be rationalized as follows. First, the charge transfer efficiency from the polymer to the CuInS2 QDs in the solid state is still rather low, as indicated by the PL quenching efficiencies (ca. 17–42%) (Fig. 5b), which may owe to the fact that capping agent molecules on CuInS2 QDs do not facilitate the charge transfer at MEHPPV/CuInS2 interface [21]. Moreover, the collection efficiency of the photogenerated electrons is not high. We envision that for higher power conversion efficiency of the

Fig. 7. J–V (a) curves under the monochromatic illumination of 15.85 mW/cm2 at 470 nm and IPCE spectra (b) of the MEH-PPV/CuInS2 (R¼ 1/1) solar cells with the sizes of CuInS2 QDs as shown. The inset of (a) shows the J–V properties of the devices in dark.

Fig. 8. Architecture (a) and the energy level diagram (b) in the MEH-PPV/ CuInS2 (R¼ 1/1) solar cells. In plot (a), black dots illustrate the aggregates of CuInS2 QDs, red regions represent MEH-PPV matrix, and arrows show the transport direction of electrons and holes. In plot (b), Voc is for the solar cells based on those composites. (For interpretation of the references to color in this figure legend, the reader is referred to the web version of this article.)

Table 3 Device performance of MEH-PPV/CuInS2 solar cells based on different-sized CuInS2 QDs under monochromatic illumination (15.85 mW/cm2) at 470 nm. QDs sizes (nm)

Voc (V)

Jsc (mA/cm2)

FF (%)

η (%)

IPCEmeas (%)

IPCEcal (%)

3.2 3.7 5.4

0.5007 0.016 0.425 7 0.017 0.3687 0.005

0.1587 0.008 0.0407 0.004 0.0187 0.005

20.957 1.00 20.127 1.00 20.177 2.00

0.1177 0.002 0.025 7 0.001 0.0117 0.001

2.147 0.13 0.59 7 0.07 0.26 7 0.08

2.64 70.14 0.6770.07 0.30 70.08

W. Yue et al. / Materials Science in Semiconductor Processing 24 (2014) 117–125

photovoltaic device based on the polymer/CuInS2 QDs, it is necessary to carry out the interfacial modification of QDs and improve the compatibility of organic/inorganic interface, as well as the application of an aligned photoelectrode for charge transport and collection. 4. Conclusions Chalcopyrite CuInS2 QDs with diameters of 3.2–5.4 nm are synthesized by the solvothermal approach, where the timedependent composition and tuneable size reasonably correlate with the slow supply of In3þ in the solvothermal process. The smaller CuInS2 QDs are richer in copper element and exhibit slightly lower valance band (Ev) but remarkably higher conduction band (Ec) energy level, resulting in a larger band gap. In MEH-PPV/CuInS2 solar cells, the CuInS2 QDs act as an effective electron acceptor material for the device with wide spectral response extending from 300 to 900 nm. Decreasing the size of CuInS2 QDs produces the MEH-PPV/CuInS2 solar cells with higher Voc and Jsc. The increased Voc for the smaller QDs is the result of the enlarged energy level difference between the polymer and CuInS2 QDs due to Ec of QDs shifts to the higher level, while the higher Jsc mainly correlates with the increased MEH-PPV/CuInS2 specific interfacial area for the more effective charge transfer from polymer to CuInS2 and the formation of more effective interpenetrating networks for charge transport, which offsets the deficiency in light-harvesting ability. Acknowledgments This work was supported by the National Natural Science Foundation of China (51202002) and 2012 National Undergraduate Innovation Entrepreneurship Project in Local University (3110404212). References [1] R.M. Swanson, Science 324 (2009) 891–892. [2] B.R. Saunders, M.L. Turner, Adv. Colloid Interface Sci. 138 (2008) 1–23. [3] H.W. Hillhouse, M.C. Beard, Curr. Opin. Colloid Interface 14 (2009) 245–259. [4] M. Skompska, Synth. Met. 160 (2010) 1–15. [5] C. Burda, X. Chen, R. Narayanan, M.A. El-Sayed, Chem. Rev. 105 (2005) 1025–1102. [6] Y.W. Jun, J.S. Choi, J. Cheon, Angew. Chem. Int. Ed. 45 (2006) 3414–3439. [7] W.J.E. Beek, M.M. Wienk, R.A.J. Janssen, Adv. Funct. Mater. 16 (2006) 1112–1116. [8] Y.Y. Lin, T.H. Chu, S.S. Li, C.H. Chuang, C.H. Chang, W.F. Su, C.P. Chang, M.W. Chu, C.W. Chen, J. Am. Chem. Soc. 131 (2009) 3644–3649. [9] Y.-C. Huang, W.-C. Yen, Y.-C. Liao, Y.-C. Yu, C.-C. Hsu, M.-L. Ho, P.-T. Chou, W.-F. Su, Appl. Phys. Lett. 96 (2010) 123501. [10] J.-S. Huang, C.-Y. Chou, C.-F. Lin, Sol. Energy Mater. Sol. Cells 94 (2010) 182–186. [11] D. Bi, F. Wu, Q. Qu, W. Yue, Q. Cui, W. Shen, R. Chen, C. Liu, Z. Qiu, M. Wang, J. Phys. Chem. C 115 (2011) 3745–3752. [12] F. Wu, W. Shen, Q. Cui, D. Bi, W. Yue, Q. Qu, M. Wang, J. Phys. Chem. C 114 (2010) 20225–20235. [13] M. Nirmal, L. Brus, Acc. Chem. Res. 32 (1999) 407–414. [14] C.B. Murray, C.R. Kagan, Annu. Rev. Mater. Sci. 30 (2000) 545–610. [15] V. Aroutiounian, S. Petrosyan, A. Khachatryan, K. Touryan, J. Appl. Phys. 89 (2001) 2268. [16] A.J. Nozik, Physica E 14 (2002) 115–120. [17] H.J. Queisser, Physica E 14 (2002) 1–10. [18] R.P. Raffaelle, S.L. Castro, A.F. Hepp, S.G. Bailey, Prog. Photovolt. Res. Appl. 10 (2002) 433–439. [19] V.I. Klimov, J. Phys. Chem. B 110 (2006) 16827–16845.

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