Synthesis of AgInS2 quantum dots with tunable photoluminescence for sensitized solar cells

Synthesis of AgInS2 quantum dots with tunable photoluminescence for sensitized solar cells

Journal of Power Sources 341 (2017) 11e18 Contents lists available at ScienceDirect Journal of Power Sources journal homepage: www.elsevier.com/loca...

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Journal of Power Sources 341 (2017) 11e18

Contents lists available at ScienceDirect

Journal of Power Sources journal homepage: www.elsevier.com/locate/jpowsour

Synthesis of AgInS2 quantum dots with tunable photoluminescence for sensitized solar cells Chunqi Cai, Lanlan Zhai, Yahui Ma, Chao Zou*, Lijie Zhang, Yun Yang, Shaoming Huang** Zhejiang Key Laboratory of Carbon Materials, College of Chemistry and Material Engineering, Wenzhou University, Wenzhou, 325027, People's Republic of China

h i g h l i g h t s

g r a p h i c a l a b s t r a c t

 Tunable PL of AgInS2 QDs was achieved by the modulation on Ag/In ratios in QDs.  The highest PL QY 74% of AgInS2 QDs was attributed to the D-A pair recombination.  Exploiting AgInS2 QD with Long live PL and high QY, QDSCs achieved best PCE 2.91%.

a r t i c l e i n f o

a b s t r a c t

Article history: Received 15 September 2016 Received in revised form 2 November 2016 Accepted 27 November 2016

Synthesis of quantum dots (QDs) with high photoluminescence is critical for quantum dot sensitized solar cells (QDSCs). A series of high quality AgInS2 QDs were synthesized under air circumstance by the organometallic high temperature method. Feature of tunable photoluminescence of AgInS2 QDs with long lifetime and quantum yields beyond 40% has been achieved, which was mainly attributed to the donor-acceptor pair recombination, contributed above 91% to the whole emission profiles. After ligand exchange with bifunctional linker, water-soluble AgInS2 QDs were adopted as light harvesters to fabricate QDSCs, achieved best PCE of 2.91% (short-circuit current density of 13.78 mA cm2, open-circuit voltage of 0.47 V, and fill factor of 45%) under one full sun illumination. The improved photovoltaic performance of AgInS2 QDs-based QDSCs is mainly originated from broadened optoelectronic response range up to ~900 nm, and enhanced photoluminescence with long lifetime and high quantum yield beyond 40%, which provide strong photoresponse ~40% over the window below 750 nm. The synthetic approach combined with intrinsic defects created by intentionally composition modulation introduces a new approach towards the goal of high performance QDSCs. © 2016 Elsevier B.V. All rights reserved.

Keywords: AgInS2 quantum dots Quantum dot sensitized solar cell Photoluminescence Donor acceptor pair

1. Introduction Benefiting from solution processability, size and composition

* Corresponding author. ** Corresponding author. E-mail addresses: [email protected] (S. Huang).

(C.

http://dx.doi.org/10.1016/j.jpowsour.2016.11.101 0378-7753/© 2016 Elsevier B.V. All rights reserved.

Zou),

[email protected]

dependence bandgap, and enhanced multiple exciton generation, colloidal quantum dots (QDs) are among potential key players in the next generation of photovoltaic technologies [1e4]. Very recently, the recorded power conversion efficiency of 11.6% for ZnCu-In-Se QDSC has been achieved by Zhong group [5]. Exploiting QDs as solar harvesting materials constitutes a promising and fundamental building block toward QD-sensitized solar cells (QDSCs) [2,6], which involves light harvesting, disassociation of

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photoexcited electron-hole pair, and electron injection. A great deal of efforts has been made on various kinds of core and/or core/shell quantum dots and corresponding QDSCs, including CdS, CdSe, PbS, CdS/CdSe, and CdTe/CdSe. Among them, CdS/CdSe core/shell QDs were the glaring choices. Power conversion efficiencies (PCE) of 6.01% and 7.11% based on the QDs were approved successively by Kuang [7] and Wang [8] groups. However, highly toxic cadmium chalcogenide would put them into a dilemma in consideration of health and environmental issues. Candidates based on ternary I-IIIVI2 QDs without toxic cadmium, typical CuInS(Se)2 and AgInS(Se)2, become attractive and promising. CuInS(Se)2 has emerged as good replacements for light harvesting materials in QDSCs [1,9e13]. Zhong [12] and Hyeon [13] groups have demonstrated certified photovoltaic performance of 6.66% and 8.10%, respectively. Analogous to CuInS(Se)2, AgInS(Se)2 deserves the same advantages, high absorption coefficient, wider absorption range, tunable band gap, and big tolerance to composition. Many efforts have been made on the synthesis of AgInS2 QDs with high quality, but quite few on sensitizer in QDSCs [14e20]. Annealed AgInS2 QDs QDSCs exhibited better PCE of 0.8% [18], attributed to the fast charge transport from ligand free aqueous synthesis and the decrease of mid-gap defect states from post-synthesis annealing. AgInS2/In2S3 co-sensitizers based QDSCs by aqueous synthesis also achieved PCE of 0.7% [19]. Very recently, QDSCs based on the combination of aqueous CuInS2 QDs and thiol coadsorbents achieved efficiency 5.9%, and AgInS2 QDs based QDSCs showed efficiency 2.72% [20]. These PCE lag significantly behind those of CuInS2 QD based QDSCs. Therefore, developing AgInS2 based QDSC with good performance is critical and challenging target for their potential application. Previous research on binary and ternary QD based QDSCs has demonstrated their relatively poor performance stems partially from synthetic procedure and trap-state defects of QDs [5,12]. The former involves crystalline of QDs in aqueous synthesis and/or ligand exchange in oil synthesis, and the later concerns internal charge carrier recombination before carrier injection and photoexcited electron recombination from TiO2 to QDs after carrier injection [21,22], which both may worsen photocurrent and photovoltage, and thus lower PCE of QDSCs. Therefore, the choice of synthetic strategy on QDs is critical for better performance QDSCs. Various methods for the QDs synthesis and relevant attachment onto TiO2 have been focused and can be categorized into two major methods, in situ QDs synthesis and fabrication and the presynthesis QDs and assembly [1,22]. Since post-synthesized assembly approach by the self-assembly of mercaptopropionic acid (MPA)-capped water-soluble QDs onto an oxide film electrode was introduced by Zhong group [23], the pre-synthesis method of QDs take overwhelming advantages over in situ QDs fabrication, achieved most QDSCs with PCE more than 5% [5,6,8,12,24,25]. The organometallic high temperature synthetic method is the mature choice for pre-synthesis QDs and holds many merits [26], including controlled and tailored size, functional surface, and thus surface trap and optoelectronic properties of QDs. These features directly determine the electron quenching, trapping, and recombination process in QDSCs [27]. It should be highlighted that Schlenk technique or inert atmosphere protection is the standard procedure for the organometallic high temperature method because of high reactivity of precursors at evaluated temperature [10,28]. The complex reaction system exhibits poor air stability of QDs during synthesis, challenging their reproducibility and cost. Taking ternary I-III-VI2 QDs as sensitizer in QDSCs can make use of their narrower band gap than binary II-VI QDs, and also introduce complex surface trapping state for their ternary compositions [3]. By two methods, the overgrowth of wider band gap ZnS shell on QDs to form type-I core/shell structured QDs and/or alloyed QDs, trap-state defect can be minimized, luminescent emission

efficiency and stability of QDs also can be enhanced. As a matter of fact, the modulation on composition dependent structural defects, the surface defects and the internal defects, is another possible strategy [28e34], especially in ternary QDs, which tolerate a large range of nonstoichiometric compositions and complex crystal structure relevant with donor and acceptor trap states. The emission of highly luminescent CuInS2-based core/shell nanocrystals was attributed to radiative recombination involving transition from quantized electron level to localized hole state [28]. The determination of internal and surface defects on the performance of sizedependent CuInS2 QDs-based QDSCs was ascertained by timeresolved emission and transient absorption spectroscopy [9]. Hamanaka et al. [35] and Torimoto et al. [30] reported the PL mechanism of AgInS2 QDs, both attributed their origin to the donoracceptor (DA) pair recombination. One can improve PL and quantum yield (QY) of QDs by suppressing surface defects and enhancing internal defects, and achieve corresponding QDSCs with better performance. Herein, we report low-cost synthesis of AgInS2 QDs with tunable photoluminescence originated from gradient composition. A series of high quality AgInS2 QDs capped by oleylamine were synthesized by the thermal decomposition of organometallic precursors at high temperature [30,36]. Tunable PL of AgInS2 QDs with long lifetime was mainly attributed to the donor-acceptor pair recombination, which made contributions more than 91% to the whole emission profiles. All the AgInS2 QDs had the QY over 40%, and the highest QY of 74% was achieved for Ag/In ratio of 1.0. After ligand exchange mediated by bifunctional linker mercaptopropionic acid, watersoluble AgInS2 QDs were tethered on mesoporous TiO2 film electrode by pipetting QD aqueous solution onto the TiO2 film. The fabricated AgInS2 QDs-based QDSCs exhibit photoresponse extending to ~900 nm, achieving best PCE of 2.91% under 1 full sun illumination. 2. Experimental 2.1. Materials All chemicals were used as received without further purification. Sodium diethyldithiocarbamate trihydrate (Na(dedc), 99%), and n-hexane (95%) were obtained from J&K; Indium nitrate (In(NO3)3, 99.9%) from Alfa Aesar; Silver diethyldithiocarbamate (Ag(dedc), >98%) from TCI, Shanghai; Oleylamine (OLA, >80%) from Acros. 2.2. Synthesis of In(dedc)3 precursors The In(dedc)3 precursors were synthesized according to the previous reports [36]. In a typical synthesis of In(dedc)3, Na(dedc) (6 mmol) and In(NO3)3 (2 mmol) were respectively dissolved in 100 mL and 50 mL de-ionized water, then the In(NO3)3 solution was dropwise added to the Na(dedc) solution with magnetic stirring. The white product was washed 3 times with ethanol and dried under a vacuum at 60  C for 3 h. As-synthesized precursors were stored in desiccator at room temperature. 2.3. Synthesis of AgInS2 QDs with tunable composition AgInS2 QDs with tunable compositions can be achieved by varying the relative molar ratio of precursors in the source materials. In a typical synthesis of AgInS2 QDs (AIS-1.5), Ag(dedc) (0.15 mmol), In(dedc)3 (0.1 mmol) were loaded into a 50 mL rounded-bottom flask, which was then filled with 4 mL of oleylamine. After ultrasonic dispersing at room temperature for 2 min, 8 min of magnetic stirring at room temperature was followed. The

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rounded-bottom flask was immersed into oil bath and maintained at 180  C for 1 h and then allowed to cool to room temperature naturally. The red product was collected by centrifugation, washed several times with absolute ethanol and n-hexane. Quantitative elemental analyses of the QDs by EDS suggested the compositions of the QDs were roughly but systemically consistent with the fed Ag/In molar ratios. For simplicity, the sample for AgInS2 QDs with the ratio of Ag/In precursors 1.5 was referred AIS-1.5 and so on. 2.4. Ligand exchange of QD and fabrication of TiO2 photoanodes Ligand exchange of oil-soluble AgInS2 QD to water-soluble QDs was performed according to a literature method. Typically, 0.4 mmol MPA was first dissolved in 1.0 mL methanol, then the pH of the solution was adjusted to 11 with the use of 30% NaOH aqueous solution. The MPA solution was added into 15 mL AgInS2 QDs chloroform solution and stirred for 30 min to get the precipitation of the AgInS2 QDs. Then 10.0 mL de-ionized water was added into the mixture and kept the stirring for another 20 min. The solution was separated into two phases and the AgInS2 QDs were transferred into the superincumbent water from the underlying chloroform, the underlying phase was discarded and the aqueous phase containing the MPA-capped AgInS2 QDs was collected. The aqueous dispersion was further purified by centrifugation and decantation with the addition of acetone, and the precipitate was redissolved in 1.0 mL de-ionized water. The double layered mesoporous TiO2 photoanodes were fabricated on well-cleaned FTO glass. Before the 10.0 mm thick transparent TiO2 layer (particle size at 20 nm) was coated on the FTO substrate by successive screen-printing of home-made TiO2 paste, the cleaned FTO glass was treated with 40 mM TiCl4 aqueous solution for the formation of a compact TiO2 layer on substrate, followed by another screen-printing of 2.0 mm thick light scattering TiO2 layer (particle size at 200 nm). Finally, the film was heat treated in a hot plate at 500  C for 30 min. The AgInS2 QD sensitizers were immobilized on the TiO2 mesoporous films by pipetting 30 mL QD aqueous dispersion (absorbance of 3.0 and pH of 11.0) onto the film surface and maintaining 2e4 h before rinsed sequentially with de-ionized water and ethanol and then dried with nitrogen. After the immobilization, the sensitized TiO2 films were coated with ZnS by SILAR method, alternately dipping into 0.1 M Zn(NO3)3 and 0.1 M Na2S solution for 1 min, rinsing with de-ionized water between dips. 2.5. Assembling solar cells The sandwich-type cells were constructed by assembling the photoanode and the Cu2S/brass counter electrode using a 50 mm thick Scotch spacer. To prepare Cu2S/brass counter electrode, brass foil was immersed in HCl solution (1.0 M) at 70  C for 10 min and subsequently soaked into polysulfide electrode solution for 10 min. The polysulfide electrode solution was obtained by the dissolution of 2.0 M Na2S, 2.0 M S, and 0.2 M KCl in de-ionized water. A droplet (10 mL) of polysulfide electrolyte was injected into the cell device. The area of the cells was 0.25 cm2. For QDSCs fabricated under each condition, several cells were performed and tested in parallel. 2.6. Characterization The as-synthesized sample were drop cast on the Si low background sample holders and dried at room temperature in the air, then the powder X-ray diffraction (XRD) patterns of the prepared samples were recorded on Bruker D8 advance X-ray diffractometer with graphite monochromatized Cu Ka(l ¼ 1.5405 Å) radiation with a step of 0.02 at a scanning speed of 4 min1 in 2q ranging

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from 10 to 80 . Scanning electron microscopy (SEM) images were taken using a FEI Nova NanoSEM200 microscope. Composition analysis was performed by EDS (oxford INCA). The transmission electron microscopy (TEM) was carried out under JEOL JEM-2100F microscope operating at an accelerating voltage of 200 kV. UVevisible (UVevis) absorption spectra of the samples were recorded on a SHIMADZU UV-1800 spectrophotometer. Steadystate photoluminescence (PL) spectra and absolute quantum yield (QY) were measured using a Fluoromax-4 spectrofluorometer (HORIBA Jobin Yvon Inc.) equipped with a 150 W xenon lamp as the excitation source. The absolute QYs of the samples were determined by measuring emission and scattered light from the sample and reference in an integrating sphere [37e39]. All the samples were dispersed in hexane and placed in a cuvette inside the integrating sphere. The emitted and scatted radiation was collected at 90 angle from the excitation, and a baffle was placed beside the sample on the emission monochromator side to avoid the collection of directly scatted light. The PL decay dynamics were measured using time-correlated single photon counting (TCSPC) set-up from Jobin Yvon equipped with a 452 nm LED excitation source. Raman spectra were collected from inVia confocal Raman/PL system (Renishaw). The wavelength of the laser excitation was 532 nm (2.33 eV, 1 mW). Photocurrent density-photovoltage curves (J-V) of QDSCs were derived with a Keithley 2400 digital source meter (Keithley, USA) under AM 1.5G illumination (100 mW cm2) by Oriel Sol 3A Solar Simulator (94023A, Newport Stratford Inc., USA), calibrated with a standard crystalline silicon solar cell. The incident photon-to-current conversion efficiency (IPCE) was measured by using solar cell quantum efficiency measurement system (QEX10, PV Measurements, Inc.), and was calibrated with a NREL-certified Si diode before measurement. 3. Results and discussion Metal dithiocarbamate precursors were chose as the starting materials in this work for their stability and easy availability under air and sulfur self-containing [30,36]. Dodecanethiol and oleylamine are commonly used ligands in the synthesis of semiconductor nanocrystals and QDs [40], controlling the reactivity of metal ions and thus tailoring the phase and shape of targets. However, dodecanethiol was avoided in present work on purpose for predicted tightly adsorbance on QD surface, which would hinder electron transport in photovoltaic application [12]. Here, AgInS2 QDs were synthesized by thermolysis of metal dithiocarbamates in the presence of oleylamine at 180  C under atmosphere. The results of XRD analysis from Fig. 1a revealed broad diffraction peaks attributed to tetragonal AgInS2, regardless of varied compositions in AgInS2 QDs. Their diffraction peaks systematically shifted toward higher angles with the lower Ag/In ratios. The continuous minor diffraction peaks shifts of the AgInS2 QDs implied that no phase separation and separated nucleation happened in the present synthetic strategy. TEM observation (Figs. 1b, c, S1) showed that the as-synthesized AgInS2 QDs were homogeneous dots with an average diameter of 5 nm, which are very close to the domain size calculated from FWHM of the (112) peak in XRD pattern by Scherrer equation. High-resolution TEM images showed that an individual QD had a clear lattice fringe with interplanar spacing of 3.3 Å, which could be resolved as (112) lattice fringes and agrees well with those determined from diffraction peak at 26.9 in the XRD patterns (Fig. 1a), demonstrating crystalline nature of AgInS2 QDs. Further structural analysis of AgInS2 QDs was performed by confocal Raman system (Fig. S2). Fig. 2a shows absorption spectra of the resulting AgInS2 QDs prepared with different Ag/In ratios. No well-defined exciton absorption peaks were observed for nonstoichiometric AgInS2 QDs,

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Fig. 1. (a) XRD patterns and (b, c) typical TEM images of AgInS2 QDs with different Ag/In ratios.

although the QDs had homogenous size as shown in Fig. 1b, c and Fig. S1. The absorption band edges of the resulting AgInS2 QDs gradually shifted from 520 nm to 720 nm with increased Ag/In ratios, corresponding to the decrease of band gaps of the AgInS2 QDs [30]. In consideration with almost fixed size of all QDs shown in TEM, one could conclude that the shift of the absorption band edges should be attributed to variation in the composition of the AgInS2 QDs. Quantitative elemental analyses of the QDs by EDS shown in Fig. S3 and Table 1 further confirmed the above deduction. The compositions of the QDs were roughly but systemically consistent with the fed Ag/In molar ratios. Simultaneously, photoluminescence spectra of AgInS2 QDs (Fig. 2b) display broad red shift emission peaks, from 610 nm to 720 nm with higher Ag/In ratios, suggesting the decrease of band gap. In binary metal sulfide [41], the top of the valence band of non-

transition metal sulfides is primarily derived from S 3p orbitals, whereas the bottom of the conduction band is mainly derived from metal s orbitals. As for ternary sulfide [30,32,42], especially in I-IIIVI2 semiconductor, valence band is determined by hybridization of

Table 1 The elemental analyses of AgInS2 QDs with gradient composition (EDS). QDs

The ratios of Ag/In precursors

The ratios of Ag/In in QDs

AIS-1.5 AIS-1.2 AIS-1.0 AIS-0.7 AIS-0.5 AIS-0.25

1.5 1.2 1.0 0.7 0.5 0.25

1.13 1.01 0.85 0.55 0.40 0.33

Fig. 2. (a) UVevis absorption, (b) photoluminescence spectra and (c) QY of AgInS2 QDs with different Ag/In ratios.

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metal I cation d orbitals and anion p orbitals, while conduction band is composed by the association of s, p orbitals from metal III cation and p orbitals from anion. The higher valence band along with the increase of Ag/In ratio in QDs was reasonable, which resulted in narrower band gap and thus red shifted emission peaks. It should be noted that the difference of UVevis absorption and PL spectra became weaker until disappeared when Ag/In ratios reached 1.0 and above, suggesting the furthest saturated tolerance on crystal defects. The tendencies of UVevis absorption and PL spectra on Ag/In ratios also reminder us that the upper limited room for band gap adjustment by compositions. Depending on the ratio of Ag/In in AgInS2 QDs, the full-width at half-maximum intensity (fwhm) of the PL peaks from 105 nm to 150 nm and the Stokes shifts from 0.36 eV to 0.22 eV are considerably large. Usually, large fwhm and Stokes shifts are observed in the ternary I-III-VI semiconductor QDs due to the characteristic donor-acceptor pair transition or surface defect states [9,43]. The PL QY of the AgInS2 QDs reveals their sensitivity to the compositions of AgInS2 QDs, as shown in Fig. 2c. All the QDs had the QY above 40%, and the highest QY of 74% was achieved for Ag/In ratio of 1.0. The QYs of AgInS2 QDs were higher than the values reported AgInS2 QDs or NCs without alloying and/or shell coating [34e36,44,45], which implied the minor density of surface defect in QD. To understand the mechanism underlying the dependence of QY on compositions, PL decay dynamics of AgInS2 QDs were investigated by time correlated single photon counting (TCSPC) technique. As shown in Fig. 3, their PL decay curves can be fitted by biexponential equation I(t) ¼ A1 exp(-t/t1) þ A2 exp(-t/t2). Also, the effective value t* is used to estimate the PL decay time, which can be defined by t* ¼ (A1$t1þ A2$t2)/(A1þA2). As shown in the inset of Fig. 3, the PL relaxations can be decomposed into fast and slow decay components. The fast decay components were within tens nanoseconds with contributions of a fewer than 9%, while the slow decay parts were hundreds nanoseconds and made contributions more than 91% to the whole emission profiles, respectively. The fast decay components (38.7e54.3 ns) were attributed to the intrinsic recombination of core states and surface defect states. The slow decay component (276.3e573.9 ns) were ascribed to donoracceptor pair transition, especially in consideration that the different kinds of intrinsic defects were well-known deep trap states in ternary AgInS2 semiconductors for their enhanced configurable degree of freedom in atomic packing models [34,35]. Castro et al. attributed some donor and acceptor states for the

Fig. 3. PL decay curves of AgInS2 QDs with different Ag/In ratios. Inset shows the fit parameters of the decay curves.

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origin of fluorescence of CuInS2 QDs to sulfur vacancy and copper indium substitution as the donor, cupper vacancy as the acceptor [43]. Due to the identical tetragonal structure, AgInS2 stands the similar situation. Sulfur vacancy and interstitial silver create donor levels, and silver vacancy and interstitial sulfur act as acceptor levels in AgInS2 QDs [46,47]. One can comprehend that cation vacancy plays fundamental role in the determination of donoracceptor pair mechanism of PL [30]. The high QY and long PL lifetime for AgInS2 QDs could be achieved only on the assumption that internal defects, originated from donor-acceptor pair transition, played major roles in AgInS2 QDs instead of surface defects. After water-soluble ligand exchange in base circumstance, MPA-capped AgInS2 QDs were deposited and absorbed onto the surface of TiO2 nanoparticles, facilitated by their interaction with carboxyl groups [23]. EDS data (Fig. 4) collected from cross-section of AgInS2 QD sensitized TiO2 film exhibits the existence of Ag, In, and S elements. The 2D-projected elemental maps for three elements demonstrated the uniform distribution in depths of these elements among the entire TiO2 film. The adsorption and deposition of AgInS2 QDs on TiO2 film is verified by HRTEM shown in Fig. 5. Evenly distributed AgInS2 QDs attached to 20 nm TiO2 nanoparticles with clear boundary, marked with blue dotted circles. After the deposition of MPA-AgInS2 QDs onto mesoscopic TiO2 film electrodes used as photoanodes, three cycles of ZnS passivation layer using SILAR method were coated. Sandwich-type cells were constructed by assembling QD-sensitized TiO2 film electrode and Cu2S/brass counter electrode, separated with hollowed scotch spacer. Then polysulfide electrolyte was injected and inhaled for capillary effect. The J-V curves of the solar cells under the illumination of an AM 1.5 G solar simulator with an intensity of 100 mW cm2 (1 full sun) are shown in Fig. 6a, and the extracted photovoltaic parameters are collected in Table 2. It is noted that for the photovoltaic performance measurement at least four cells were constructed. The trend of the individual photovoltaic parameters as a function of Ag/In ratio in AgInS2 QDs is shown in Fig. S4. As the Ag/In ratio in QDs decreases, PCE reaches the maximum values at Ag/In ¼ 1.0, together with Jsc and Voc, while FF decreases slightly. The highest photocurrent and corresponding photovoltaic performance was obtained for AIS-1.0 QD with power conversion efficiency (PCE) of 2.91%. Among the QDSCs with different Ag/In ratios in AgInS2 QDs, AIS-1.0 QDs based cells exhibit the maximum short-circuit current density (Jsc) of 12.0 ± 3.61 mA cm2, open-circuit voltage (Voc) of 0.48 ± 0.04 V, and thus PCE of 2.77 ± 0.44%. The transparent enhanced Jsc of QDSCs for AIS-1.0 QD was thought introduced by increased efficient electron injection from both extended photoresponse range, which confirmed by red shifted absorption band edges in UVevis absorption spectra (Fig. 2a), and long lifetime PL with high QY (Figs. 2c and 3), while other QDSCs possessed either or neither of the both sources. Although these efficiencies are much lower than the recorded 7.06% for glaring CuInS2/ZnS based QDSCs and 11.6% for Zn-Cu-In-Se based QDSCs, the improvement of photovoltaic performance in AgInS2 QDs based QDSCs is still exciting, in consideration with poor performance of AgInS2 based QDSCs encumbered with the low quality of QDs. IPCE represents the percentage of incident photons that are converted to charge carriers and collected at the electrode surface. The IPCE spectra shown in Fig. 6b exhibit photoresponses roughly match with the absorption spectra of QDs. The photoresponse ranges in the IPCE spectra are wider than the corresponding absorption ranges of AgInS2 QDs, which can be ascribed to the light scattering effect by 200 nm TiO2 particles in mesoporous TiO2 layer. The IPCE spectrum of AIS-1.0 based QDSCs exhibits a strong photoresponse ~40% over the window below 750 nm, while the other

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Fig. 4. (a) SEM image, (b) EDS spectrum and (c-h) elemental distributions of typical cross-section of AgInS2 QDs sensitized TiO2 film with Ag/In ¼ 1. Na, Ca, Si and Sn elements are originated from sodium-calcium-silicate glass and FTO film, respectively.

Fig. 5. HRTEM images of AgInS2 QD sensitized TiO2 film. Blue dotted circles represent AgInS2 QDs randomly attached on TiO2 nanoparticles. (For interpretation of the references to colour in this figure legend, the reader is referred to the web version of this article.)

QDSCs show the weak photoresponse either in shorter wavelength window or low convert efficiency, consistent with the variation trend of Jsc values observed in the J-V measurements (Fig. S4). By integrating the product of the incident photon flux density and the cell's IPCE spectra, the calculated values for the above QDSCs are:

4.89, 7.62, 11.13, 6.75, 6.62, and 3.96 mA cm2, respectively, which are smaller than the Jsc values in J-V characteristics. The systematic discrepancy can be attributed to inefficient charge separation and collection for the lower monochromatic light intensities in IPCE than that of AM 1.5G illumination in J-V characteristics [48,49].

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harvesters to fabricate QDSCs, achieved best PCE of 2.91% under one full sun illumination. The QDSC showed a broadened IPCE action spectrum up to ~900 nm and strong photoresponse ~40% over the window below 750 nm. Together with enhanced PL with long lifetime and high QY, they contributed to the improved photovoltaic performance of QDSCs. The synthesis approach combined with intrinsic defects created by intentionally composition modulation would pave ways for low cost and facile synthesis of QDs towards high performance QDSCs. Acknowledgements This work was supported by the funds from the NSFC (51572199, 51420105002,51102186, 51302194, 61471270), the NSFZJ (LQ12E02006), and the Research Fund of College Student Innovation of Zhejiang Province (2014R424027). Appendix A. Supplementary data Supplementary data related to this article can be found at http:// dx.doi.org/10.1016/j.jpowsour.2016.11.101. References

Fig. 6. (a) J-V characteristics for AgInS2 QDSCs with different Ag/In ratios under 1 sun (AM 1.5) irradiation. (b) IPCE spectra of AgInS2 QDSCs.

Table 2 Photovoltaic parameters of QDSCs based on AgInS2 QDs with different Ag/In ratios. QDs a

AIS-1.5 AIS-1.2a AIS-1.0a AIS-0.7a AIS-0.5a AIS-0.25a AIS-1.0b a b

Jsc (mA cm2)

Voc (V)

5.66 ± 7.58 ± 12.0 ± 8.63 ± 5.99 ± 4.32 ± 13.78

0.36 0.43 0.48 0.42 0.42 0.35 0.47

0.43 1.29 3.61 0.88 0.75 0.46

± ± ± ± ± ±

FF (%) 0.04 0.04 0.04 0.05 0.05 0.02

0.58 0.56 0.51 0.50 0.52 0.47 0.45

± ± ± ± ± ±

PCE (%) 0.04 0.03 0.12 0.02 0.07 0.04

1.18 1.82 2.77 1.79 1.29 0.72 2.91

± ± ± ± ± ±

0.11 0.23 0.44 0.27 0.15 0.10

Average values of at least four different cells. Best performance of AgInS2 QD-based QDSCs.

4. Conclusions A series of high quality AgInS2 QDs with gradient composition were synthesized in the presence of oleylamine under air circumstance. Tunable photoluminescence of AgInS2 QDs with long lifetime were achieved by the modulation on the Ag/In ratio in QDs, whose origin attributed to the composition dependent band gap. The PL QYs beyond 40% revealed their sensitivity on the compositions of AgInS2 QDs and mainly attributed to the donor-acceptor pair recombination, which made contributions of above 91% to the whole emission profiles. After ligand exchange with bifunctional linker, MPA-capped AgInS2 QDs were adopted as light

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