Toward the Facile and Ecofriendly Fabrication of Quantum Dot

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Toward the Facile and Ecofriendly Fabrication of Quantum DotSensitized Solar Cells via Thiol Coadsorbent Assistance Jia-Yaw Chang,* Chen-Hei Li, Ya-Han Chiang, Chia-Hung Chen, and Pei-Ni Li Department of Chemical Engineering, National Taiwan University of Science and Technology, 43, Section 4, Keelung Road, Taipei 10607, Taiwan, Republic of China S Supporting Information *

ABSTRACT: This paper reports a facile and environmentally friendly approach to the preparation of highly efficient quantum dot-sensitized solar cells (QDSSCs) based on a combination of aqueous CuInS2 quantum dots (QDs) and thiol coadsorbents. The photovoltaic properties of the QDSSCs were found to be dependent on the type and concentration of the thiol coadsorbent. The incorporation of thiol coadsorbents results in improved JSC and VOC because (1) they provide disulfide reductants during the QD sensitization process and (2) the coadsorbent molecules are anchored on the TiO2 surface, thus affecting the movement of the conduction band of TiO2. To the best of the our knowledge, this is the first demonstrated use of various thiol coadsorbents as reducing agents in the fabrication of high-efficiency QDSSCs. CuInS2 QDSSCs fabricated with the assistance of thioglycolic acid coadsorbents exhibited efficiencies as high as 5.90%, which is 20 times higher than that of the control device without thiol coadsorbents (0.29%). In addition, the photovoltaic properties of a device fabricated using the colloidal CuInS2 QDs coated with different bifunctional linkers were investigated for comparison. The versatility of this facile fabrication process was demonstrated in the preparation of solar cells sensitized with aqueous AgInS2 or CdSeTe QDs. The AgInS2 QDSSC showed a conversion efficiency of 2.72%, which is the highest reported for Ag-based metal sulfides QDSSCs thus far. KEYWORDS: quantum dot-sensitized solar cells, copper indium sulfide, disulfide reductants, thiol coadsorbents, quantum dots as an example, Luo et al. reported that Mn2+-doped CdS/CISbased QDSSCs exhibited photovoltaic conversion efficiencies (PCEs) of 5.38%.10 Jara et al. reported that photovoltaic performance was enhanced with an increase in the size of CIS QDs, with a maximum efficiency of 2.52% for 4.3 nm.11 Guijarro et al. demonstrated the fabrication of Zn-CIS QDSSCs with a PCE of 2.01% under simulated AM 1.5 G illumination.12 Barpuzary et al. reported the efficiency of CIS/CdSe cosensitized solar cells to be 4.6% when using Cu2ZnSnS4 and MoS2 as the counter electrode.13 A recent report showed that the conversion efficiency of solar cells can be improved using presynthesized colloidal QDs.6,14−18 For example, Zhong et al. reported that cadmium-free CIS and CIS/ZnS-based QDSSCs deliver PCEs of up to 5.05% and 7.04%, respectively.16 In a subsequent work, the same group found that colloidal Mn:CdSeTe/Mn:ZnS QDSSCs exhibited impressive conversion efficiencies of up to 9.4%.7 In the ex situ growth approach, however, presynthesized colloidal QDs are often prepared in nonpolar organic solvents with the long-chain coordinating ligands, which might obstruct interfacial charge transfer at the QD/TiO2 interfaces and result

1. INTRODUCTION Dye-sensitized solar cells (DSSCs) are promising alternative photovoltaic devices for harnessing renewable energy because of their efficiency, ease of fabrication, solution processability, semitransparency, and low cost. Among various light absorbing materials, quantum dots (QDs) are attractive candidates for use in sensitized solar cells because of their unique features, including tunable optical properties, efficient multiple exciton generation, high absorption coefficients in the visible spectrum range, and large built-in dipole moment-facilitating charge carrier separation.1,2 Therefore, considerable efforts have been made to utilize QDs in QD-sensitized solar cells (QDSSCs).3−9 Two important methods have been developed for sensitizing mesoporous TiO2 (meso-TiO2) films with QDs in QDSSCs: (1) ex situ growth, in which the presynthesized colloidal QDs are attached to TiO2 by a linker-assisted assembly (LA) and (2) in situ deposition approach, in which TiO2 films are either immersed into a mixed solution of ionic precursors of QDs or dipped sequentially into cationic and anionic solutions, called chemical bath deposition (CBD) and a successive ionic layer adhesion and reaction (SILAR), respectively. The approach based on ex situ growth is more advantageous in that it enables accurate control of the size distribution, which in turn facilitates rational solar cell design and results in high crystallinity of the QDs deposited on TiO2 electrodes. Taking CuInS2 (CIS) QDs © XXXX American Chemical Society

Received: May 6, 2016 Accepted: July 4, 2016

A

DOI: 10.1021/acsami.6b05411 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces in poor affinity in QD penetration processes.19 An additional step involving ligand exchange via a short-chain mercapto-alkylcarboxyl ligand, called a bifunctional-linker,16,17,20,21 is required to displace the original long-chained capping ligands of QDs, decrease the distance between TiO2 and the QDs, and facilitate charge transfer from the QDs to the metal oxide matrix. However, this process can generate anion vacancies in QDs, further trapping electrons.22 A considerable amount of chemical waste (e.g., unreacted reagents, organic solvents used for washing and dispersing the QDs) is also generated after the preparation of QDs. On the other hand, the direct aqueous synthesis of the presynthesized colloidal QDs, already capped with a bifunctional-linker, provide straightforward access to self-assemble onto the TiO2 surface in photoanodes. Moreover, the use of water as a solvent is environmentally favorable because it is naturally abundant, nontoxic, noncorrosive, and nonflammable. Table S1 presents a summary of the use of direct aqueous synthesis QDs in QDSSCs and their corresponding photovoltaic parameters. The reported power conversion efficiencies (PCE) in the range of 0.01%−2.72% are relatively low. In addition, to improve the cell efficiency, an additional sensitization step, mainly including SILAR and CBD, is always required to deposit a second cadmium-based sensitizer (e.g., CdS or Mn:CdS) onto the mesoporous TiO2 electrodes after the first sensitization of aqueous colloidal QDs. A possible reason for the poor cell performance is the weak thiol linkage between the bifunctional linker and the QD surface. More specifically, a previous study showed that the thiol group of a bifunctional linker on the surface of QDs is susceptible to oxidation to form a disulfide (−S−S−) linkage, which is derived by the coupling of two thiol groups and subsequent desorption from the surface of the QDs.23 In other words, during sensitization of the QDs, disulfide formation at the interface between the QDs and TiO2 leads to easy detachment of the QDs from the TiO2 matrix, thereby reducing the lightharvesting capacity. Moreover, disulfide formation occurs, which limits the flux of the injected electrons from the QDs to TiO2, thus adversely affecting the photocurrent in the solar cells. Therefore, it is important to prevent, or at least suppress, disulfide formation during the sensitization of QDs. The precipitation of QDs, which results from the desorption of the bifunctional linker, can be prevented by excess amounts of free 3-mercaptopropionic acid (MPA) available in solution, which will replace the photochemically generated disulfides and maintain the solubility of the QDs in aqueous solution.23 Mann et al. reported that mixed monolayers of hexadecanoic acid and 16-mercaptohexadecanoic acid could enhance the adsorption of CdSe QDs to the TiO2 surface by suppressing disulfide formation caused by the dilution of thiol groups within the monolayer.24 This paper reports a straightforward and facile strategy to prepare high-performance QDSSCs based on a combination of aqueous CIS QDs and thiol coadsorbents without posttreatments such as thermal annealing or ligand exchange. The thiol coadsorbents used in this study were preferred because of their reducing ability; they have been widely employed in biological applications25−28 and can facilitate reductive cleavage of the disulfide linkages in the QD sensitization process. To the best of our knowledge, there is no report on the incorporation of coadsorbents in aqueous colloidal QDSSCs. The effects of various thiol coadsorbents on the photocurrent−voltage characteristics of the QDSSCs were investigated systemically.

Upon the incorporation of the thioglycolic acid (TGA) coadsorbent, the CIS-based cell showed efficiencies of up to 5.90%, which is the highest reported for aqueous CIS QDSSCs thus far.

2. EXPERIMENTAL SECTION 2.1. Materials. All chemicals were used as received without further purification. Thioglycolic acid (TGA, 97%), 3-mercaptopropionic acid (MPA, 99%), silver nitrate (AgNO3, 99.9%), selenium dioxide (SeO2, 99.4%), and copper(II) nitrate hemipentahydrate (Cu(NO3)2·2.5H2O, 98%) were obtained from Alfa-Aesar (Ward Hill, MA, U.S.A.). Copper(II) chloride dihydrate (CuCl2·2H2O, 99.99%), glutathione (GSH, 98%), sodium citrate (98%), dithiothreitol (DTT, 99%), indium(III) nitrate hydrate (In(NO3)3, 99.9%), sodium borohydride (NaBH4, ≥ 98.0%), and 2-mercaptoethanol (2ME, 99%) were purchased from Sigma-Aldrich (Milwaukee, WI, U.S.A.). Indium(III) chloride (InCl3, 99.99%), sodium sulfide nonahydrate (Na2S, 98%), cysteine (Cys, 99%), tellurium powder (99+%), and potassium chloride (KCl, 99%) were purchased from Acros Organics (Morris Plains, NJ, U.S.A.). Zinc acetate (Zn(OAc)2, 99%) was obtained from J. T. Baker (Phillipsburg, NJ, U.S.A.). 2.2. Synthesis of Aqueous CIS QDs. In a typical synthesis process, CuCl2 (1.25 × 10−2 mmol), InCl3 (5.00 × 10−2 mmol), and Na2S (8.13 × 10−2 mmol) were mixed in 20.0 mL of deionized water containing Cys (9.00 × 10−2 mmol) and sodium citrate (2.00 × 10−1 mmol). The mixed solution was transferred to a 30 mL vessel and reacted in a commercial 2.45 GHz single-mode microwave reactor (Monowave 300, Anton Paar GmbH) with 850 W power at 180 °C for 15 min. Finally, the Cys-capped CIS (abbreviated as Cys−CIS hereafter) QDs were filtered through Millipore membranes (0.2 μm pore size), precipitated with 2-propanol, separated via centrifugation at 6000 rpm for 15 min, washed with water, and dried at 40 °C. TGA (or GSH)-capped CIS (denoted as TGA−CIS or GSH−CIS, respectively) QDs were prepared using the same procedure described above, except for the use of TGA (9.00 × 10−2 mmol) or GSH (9.00 × 10−2 mmol) in place of Cys. 2.3. Synthesis of Aqueous AgInS2 QDs. In a typical synthesis process, AgNO3 (1.00 × 10−2 mmol), In(NO3)3 (6.00 × 10−2 mmol), and Na2S (1.60 × 10−1 mmol) were mixed in 15.0 mL of deionized water containing Cys (4.00 × 10−2 mmol) and NaOH (2.25 × 10−1 mmol). The mixed solution was transferred to a 30 mL vessel and reacted in a commercial 2.45 GHz single-mode microwave reactor (Monowave 300, Anton Paar GmbH) with 850 W power at 160 °C for 10 min. The purification of AgInS2 QDs was similar to that of CIS QDs. 2.4. Synthesis of Aqueous CdSeTe QDs. Cd2+ solution was prepared by mixing 10 mL aqueous CdCl2 solution (5.00 × 10−2 mmol) and 10 mL aqueous MPA solution (1.20 × 10−1 mmol) in a 30 mL reaction vessel, followed by titration with 1 M NaOH to pH 9.5. Se2− solution was prepared by mixing SeO2 (1.56 × 10−3 mmol) and NaBH4 (3.12 × 10−3 mmol) in 5 mL deionized water under argon atmosphere. Te2− solution was prepared by mixing Te powder (1.56 × 10−3 mmol) and NaBH4 (3.12 × 10−3 mmol) in 5 mL deionized water under argon atmosphere. For the preparation of CdSeTe QDs, freshly prepared Se2− solution (20 μL) and Te2− solution (20 μL) were quickly injected into the Cd2+ solution under argon atmosphere. The mixed solution was transferred to a 30 mL vessel and reacted in a commercial 2.45 GHz single-mode microwave reactor (Monowave 300, Anton Paar GmbH) with 850 W power at 150 °C for 10 min. The purification of CdSeTe QDs was similar to that of CIS QDs. 2.5. Fabrication of QDSSCs. The fluorine-doped tin oxide (FTO) glass substrates (15 Ω/sq resistance) were cleaned ultrasonically for 60 min using soapy water and then rinsed with water and ethanol. TiO2 films consisting of a transparent layer formed using TiO2 paste (18NRT, Dyesol) and a scattering layer formed using a mixture paste containing 30 wt % TiO2 nanoparticles (WER2-O paste, Dyesol) and 70 wt % TiO2 paste (18NR-T, Dyesol) were fabricated on FTO glass substrates by a screen-printing technique. The resulting film was B


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ACS Applied Materials & Interfaces annealed at 500 °C for 30 min to produce meso-TiO2 films with ∼8.8 μm thickness, as measured by scanning electron microscopy (not shown). To prepare the QD-sensitized films, the bare meso-TiO2 films were immersed into an aqueous suspension of CIS QDs (2 mg/mL) containing different kinds of thiol coadsorbents and maintained at 40 °C for 24 h in the dark. Subsequently, the CIS QD-sensitized films were rinsed thoroughly with methanol and dried under nitrogen gas. The CIS t-QD-sensitized films were then dipped into a 0.03 M Zn(CH3COO)2 methanol solution for 1 min and washed with methanol. The electrodes were then immersed into a 0.03 M Na2S methanol/water solution (7:3, volume ratio) for another 1 min and washed again, leading to the formation of a ZnS passivation layer on the QD-sensitized films. The above procedure constitutes a single SILAR cycle. Three SILAR cycles were carried out to deposit ZnS on top of the CIS QD-sensitized films, leading to the formation of QDsensitized photoanodes. The solar cells were assembled by sandwiching a polysulfide electrolyte between the Cu2S counter electrodes and QD-sensitized photoanodes using a 60-μm-thick Surlyn (DuPont 1702) as spacer. The Cu2S counter electrodes were prepared according to a previous report.29 The electrolyte employed in this study consisted of 1.8 M Na2S, 2.0 M S, and 0.2 M KCl in a water/methanol (7:3 by volume) solution. 2.6. Characterization and Measurements. Absorption spectra were recorded using a JASCO V-670 spectrophotometer equipped with an integrating sphere. Fluorescence emission spectra were measured at room temperature using a Fluorolog-3 spectrofluorometer (HORIBA JobinYvon, Japan). The emission decay profile was obtained by using a time-correlated single-photon counting (TCSPC) system (PicoHarp 300, Pico-Quant) and a detector router (PHR-800, Pico-Quant). A picosecond pulse diode laser (Pico-Quant) operating at 405 nm was used as the excitation light source. The X-ray diffraction (XRD) patterns were analyzed in the 2θ range of 20°−80° at room temperature, using a Bruker D8 Discover diffractometer operating at 45 kV and 40 mA with Cu Kα radiation (λ = 1.54 Å). Transmission electron microscopy (TEM) and scanning transmission electron microscopy-energy dispersive spectroscopy (STEM-EDS) were performed on a FEI Tecnai G2 F20 microscope (Philips, Holland), which was equipped with EDS and high-angle annular darkfield (HAADF) detectors at an accelerating voltage of 200 kV. Scanning electron microscopy (SEM) was carried out using a JSM 6500F (JEOL USA Inc., Peabody, MA, U.S.A.) field emission scanning electron microscope operating at an accelerating voltage of 15 kV. The current−voltage curves of the solar cells were measured using a Keithley Model 2400 source meter under one sun (AM 1.5 G, 100 mW/cm2) irradiation with a 150 W xenon solar simulator (Oriel, Model: 92250A). The active area of the solar cell was defined by a metal mask with a square aperture, 0.16 cm2 in area. The incident photon-to-current conversion efficiency (IPCE) was obtained using a Keithley 2400 multimeter under illumination with a 150 W xenon lamp and a monochromator (Oriel CornerstoneTM 130). Impedance measurements were performed using a Zennium electrochemical workstation (Zahner-Ennium) over the frequency range of 0.1 Hz to 100 kHz and at an AC amplitude of 10 mV at room temperature. Intensity-modulated photocurrent/photovoltage spectroscopy (IMPS/ IMVS) measurements were also carried out using a Zennium electrochemical workstation with a frequency response analyzer under a modulated blue light-emitting diode (λ = 457 nm) driven by a source supply (Zahner, PP211), which can provide both AC and DC components of the illumination. The modulated light intensity was set to 10% base light intensity, and the frequency ranged from 0.1 Hz to 1 kHz.

conventional heating approaches.30−34 Microwave irradiation has been particularly used to accelerate the homogeneous nucleation of multiple elements and to reduce the crystallization time, which in turn improves particle crystallinity and reduces the size distribution. Herein, the direct aqueous preparation of Cys−CIS QDs was accomplished by a microwave-assisted synthesis method that involved simply mixing the Cu and In reactants as well as Na2S as a sulfur source in the presence of sodium citrate and Cys. According to Pearson’s hard/soft acid/base (HSAB) principle,35 sodium citrate with multiple carboxylate groups is an effective Lewis base (electron donor) that preferentially chelates with hard Lewis acids such as In3+ to form a stable complex in the reaction. Cys stabilizer is a soft base necessary to modulate the reactivity of the soft acid Cu+. During heat treatment, all the reactants decompose gradually to yield reactive molecular species, called ‘‘monomers’’. Subsequently, a nucleation event occurs when the monomer concentration exceeds the critical supersaturation at elevated temperatures, followed by the growth stage, to afford the CIS QDs. Figure 1a shows a high-resolution TEM image of typical Cys−CIS QDs with high crystallinity. The corresponding QDs in the inset of Figure 1a reveal the interplanar distance of 0.322 nm, which can be indexed to the (112) plane of chalcopyrite CIS. EDS of Cys−CIS QDs confirmed the presence of Cu, In, and S, as shown in Figure 1b. Figure 1c shows XRD patterns of the Cys−CIS QDs; three diffraction peaks corresponding to the (111), (200), and (311) lattice planes of chalcopyrite CIS (JCPDS card no. 21-1272) were detected, which is in agreement with the TEM results. Figure S1 shows the absorbance and photoluminescence (PL) emission spectra of the Cys−CIS QDs prepared in the aqueous phase by microwave irradiation. The Cys−CIS QDs showed a broad absorption ranging from 300 to 700 nm and an emission maximum at 750 nm for λex = 430 nm. For sensitization of the QDs, the photoanodes were fabricated by immersing the meso-TiO2 films into the aqueous Cys−CIS QD solution containing different thiol coadsorbents, with the formula HS−R−COOH (carboxy-terminated alkylthiols, e.g., TGA, MPA, GSH, and Cys) or HS−R−OH (hydroxyterminated alkylthiols, e.g., DTT and 2ME), for 24 h. These thiol coadsorbents were chosen because low-molecular-weight thiol agents are used widely as disulfide reductants in biochemical systems.25−28 For example, tripeptide GSH, which is the most abundant intracellular thiol molecules inside cells, can trigger disulfide bond cleavage, so that GSH is oxidized to glutathione disulfide. TGA has been shown to efficiently break the disulfide bond of serum albumin and hair keratin.25,26 DTT can cleave disulfide bonds to yield free −SH groups by forming a very stable six-membered ring in the oxidized state. 2ME is also a common reducing agent for cleaving protein disulfide bonds between and within biological molecules. The present paper proposes a possible mechanism of disulfide breaking at the surface of Cys−CIS QDs via a “thiol−disulfide exchange” reaction with thiol coadsorbents. As shown in Scheme 1, nucleophilic attack by the sulfur atom of the thiol coadsorbents occurs at one of the thiol moieties in the Cys linker, which involves capping on the surface of the CIS QDs, followed by cleavage of the disulfide bridge in Cys−CIS QDs. Subsequently, a second thiol coadsorbent attacks the first to break the newly formed disulfide bridge, resulting in a new disulfide linkage between the two thiol coadsorbents.

3. RESULTS AND DISCUSSION Microwave irradiation has attracted increasing attention for the preparation of nanocrystals owing to its rapid volumetric heating with shorter time periods and higher reaction rates, high selectivity, and superior yields as compared to the C


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interfacial charge recombination in solar cells. A ZnS passivation layer was therefore deposited on the QD surface to improve the photovoltaic performance. Cys−CIS QDSSCs with the assistance of a TGA coadsorbent were chosen as an example for a detailed microscopic characterization of the TiO2 film sensitized with CIS QDs. The high-resolution TEM images in Figure 2 indicate the

Figure 2. HR-TEM image of CIS-sensitized TiO2 films with the TGA coadsorbent.

presence of the CIS QDs anchored on the TiO2 nanoparticles with well-defined lattice fringes of 0.316 and 0.351 nm, which were assigned to the (112) planes of CIS and the (101) planes of anatase phase TiO2, respectively. To better understand the elemental distribution of the CIS QDs throughout these TiO2 nanoparticles, high-angle annular dark-field (HAADF) scanning transmission electron microscopy (STEM) imaging, in conjunction with EDS elemental mapping, was performed, and the compositions of the as-prepared samples and elemental distributions were clarified. Figure 3a and b display the TEM and STEM images, respectively. As shown in Figure 3c−e, the Cu, In, S, Ti, and O signals emerging from the same region of the as-prepared samples indicate that the CIS QDs are distributed relatively homogeneously throughout the TiO2

Figure 1. (a) Representative high-resolution TEM image, (b) EDS, and (c) XRD pattern of Cys−CIS QDs. The XRD line patterns (bottom) correspond to bulk chalcopyrite CIS (JCPDS 85-1575).

According to the literature,36,37 ZnS semiconductors with a wide band gap can act as a passivation layer of QDs to prevent current leakage from the QDs to the electrolyte and to decrease

Scheme 1. Proposed Mechanism of Disulfide Cleavage on a QD Surface Using the Thiol Coadsorbent during the QD Sensitization Process

D


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Figure 3. (a) TEM and (b) HAADF-STEM images of the TiO2 film sensitized with CIS QDs, (c−g) STEM-EDS elemental mapping image of photoanodes with five panels: mapping of Cu, In, S, Ti, and O, respectively. Collected from the red square area in (b).

nanoparticles. These results confirm that the CIS QDs have been uniformly anchored onto the surface of the TiO2 nanoparticles with the assistance of the TGA coadsorbent. Photocurrent−voltage (J−V) curves of the Cys−CIS QDSSCs with or without the incorporation of thiol coadsorbents were measured under one-sun illumination (AM 1.5, 100 mW cm−2), as shown in Figure 4. Table 1 lists the

Table 1. Current−voltage Parameters of Cys−CIS QDSSCs Associated without (Control Group) or with Different Thiol Coadsorbents (0.5 M) coadsorbent

JSC (mA cm−2)

VOC (mV)

FF (%)

PCEa (%)

PCEb (%)

control 2ME DTT Cys GSH TGA MPA

1.25 1.40 2.05 4.46 6.93 12.82 2.62

442 384 478 544 628 640 574

53.2 49.3 54.5 56.8 56.4 54.1 56.0

0.29 0.04 0.53 1.38 2.46 4.44 0.84

0.28 0.04 0.53 1.32 2.37 4.20 0.80

± ± ± ± ± ± ±

0.02 0.00 0.03 0.07 0.07 0.25 0.06

a Performance of the champion cell. bAverage efficiency and standard deviations for the four devices.

cells treated with DTT and 2ME coadsorbents exhibited a conversion efficiency of 0.53% and 0.04%, respectively. When HS−R−COOH thiol coadsorbents (TGA, MPA, GSH, and Cys) were added to the sensitization solution, the cell efficiencies were enhanced significantly because of the improved photovoltaic parameters, such as JSC and VOC. In particular, Cys−CIS QDSSCs with the TGA coadsorbent yielded a maximum efficiency of 4.44% (JSC = 12.82 mA cm−2, VOC = 640 mV, and FF = 54.1%), compared to the other thiol coadsorbents. For comparison, control experiments were conducted to determine if the combination of QD and thiol coadsorbents coexisting in the QD sensitization process is essential for increasing the photovoltaic performance. In the first case, control photoanodes were prepared by dipping meso-TiO2 films in a 0.5 M TGA coadsorbent for 24 h, followed by dipping in

Figure 4. Current−voltage characteristic of Cys−CIS QDSSCs associated without (control group) or with different thiol coadsorbents (0.5 M) under AM1.5G light illumination of 100 mW cm−2.

performance parameters of the solar cells corresponding to Figure 4, including the open-circuit voltage (VOC), short-circuit photocurrent density (JSC), fill factor (FF), and PCE. The pristine Cys−CIS cells without the thiol coadsorbents gave efficiencies of only 0.29%, with a JSC of 1.25 mA cm−2 and VOC of 442 mV. The poor performance of the pristine cell is because of the lower JSC originating from the weak QD adsorption. The E


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in an efficiency of 2.93%, which is higher than that of the device fabricated by the direct aqueous synthesis of colloidal CIS QDs.38−40 As the sensitization time was increased from 0.5 to 3.0 h, the cell efficiency increased from 2.93% to 4.36%, and JSC was enhanced from 7.63 to 12.13 mA cm−2. When the photoanode sensitization time was extended to 24 h, the QDSSCs delivered PCEs of 5.90% with JSC values of 19.19 mA cm−2, while VOC remained at ∼650 mV. In Figure 5 and Table 2, JSC increased with increasing TGA concentration, suggesting

QD solution for another 24 h (cell A). In the second case, the photoanodes were fabricated by reversing the above-mentioned immersion sequence (cell B). Figure S2 and Table S2 show that the photovoltaic performance of cells A and B are very poor, suggesting that the presence of both QDs and the thiol coadsorbent is essential for the QD sensitization process. Another control device sensitized using the TGA coadsorbent without the incorporation of QDs was also fabricated (cell C). As expected, cell C exhibited a poor PCE of MPA > Cys > DTT > 2ME at a concentration of 4.0 M. In addition, in the case of 2ME, JSC increased slightly when the concentration of 2ME was less than 2.0 M and then showed significant improvement as the concentration reached 4.0 M, possibly because of the low reducing ability or high concentration threshold for the rupture of the disulfide linkage in the QD solution. In addition, the maximum VOC value of the QDSSC was in the order 1.0 M GSH (676 mV) > 4.0 M TGA (654 mV) > 4.0 M MPA (636 mV) > 4.0 M Cys (626 mV) > 4.0 M DTT (592 mV) > 4.0 M 2ME (590 mV). This suggests that GSH is the best thiol coadsorbent for improving the VOC value of the CIS QDDSCs. The photovoltaic results show that the coadsorption of the CIS QDs with thiol coadsorbents has a beneficial effect on the cell performance because of the systematic improvement of JSC and VOC. To further demonstrate the versatility of this simple fabrication approach, aqueous AgInS2 and CdSeTe QDSSCs were fabricated with the incorporation of TGA coadsorbents. Figure S6 shows the absorbance and PL emission spectra of AgInS2 and CdSeTe QDs prepared in the aqueous phase by microwave irradiation. Figure 7 and Table 3 present the J−V

aqueous AgInS2 QDs and TGA gave a promising conversion efficiency of 2.72%, which is more than 14 times that of the AgInS2 QDSSC without TGA (0.19%). To the best of our knowledge, this efficiency is the highest among those reported for QDSSCs with Ag-based metal sulfides such as AgInS241−44 and Ag2S.45−49 The aqueous CdSeTe QDSSC exhibited good photovoltaic performance with a JSC of 14.4 mA cm−2, VOC of 630 mV, and FF of 50.5%, delivering a PCE of 4.58%, which is significantly higher than that (0.11%) of the cell without TGA. A feasible interpretation of the JSC and VOC enhancement of cell with thiol coadsorbents is as follows. As shown in Figure 5, VOC increased linearly with increasing amounts of thiol coadsorbents, suggesting that the coassembly of the thiol coadsorbents and QDs on the surface of TiO2 leads to dependence of VOC enhancement on the thiol coadsorbent concentration. In principle, the theoretical maximum of the VOC of a photovoltaic device is related to the difference between the electron quasi-Fermi level in TiO2 under illumination conditions and the redox potential of the electrolyte, as given by50 VOC =

Table 3. Current−Voltage Parameters of AgInS2 and CdSeTe QDSSCs Associated without (Control Group) or with TGA Thiol Coadsorbents

CdSeTe (control) CdSeTe AgInS2 (control) AgInS2

JSC (mA cm−2)

VOC (mV)

FF (%)

PCEa (%)

PCEb (%)

0.64

384

46.0

0.11

0.11 ± 0.01

14.4 1.57

630 364

50.5 33.4

4.58 0.19

4.57 ± 0.12 0.18 ± 0.02

9.75

432

64.6

2.72

2.62 ± 0.10

(1)

where εCB is the conduction band edge energy level, kBT is the thermal energy, q is the elementary charge of the electrons, nc is the concentration of electrons in the conduction band, NCB is the effective density of states in the conduction band, and εredox is the redox potential of the electrolyte redox couple. The bandedge shift is observed if a sufficient amount of negative or positive ions accumulate on the TiO2 surface to induce a change in the potential across the junction. A similar enhancement of VOC was also observed in the DSSCs prepared by the combination of sensitizers and coadsorbents.51,52 The coassembly of sensitizers and coadsorbents bearing carboxylic acids, such as chenodeoxycholic acid, is proposed to afford a more favorable compact monolayer than that with the adsorption of sensitizers alone.51 Coadsorbents bearing carboxylic acids were reported to alter the conduction band and shield the trap states of TiO2, leading to enhanced VOC and JSC. Consequently, in the present case, the observed VOC enhancement upon the addition of thiol coadsorbents was attributed to the negatively shifted conduction band of TiO2, as depicted in Scheme 2a. Moreover, when HS−R−COOH, including TGA, MPA, GSH, and Cys, were used as the thiol coadsorbents, the VOC values of the cell improved significantly as compared to those of the cells employing HS−R−OH (e.g., DTT, 2ME) as thiol coadsorbents. This difference is likely due to the larger negatively charged adsorption of the carboxyl group of HS−R−COOH on the TiO2 surface causing a more negatively shifted conduction band of TiO2. In particular, the CIS QDSSC with the incorporation of 1.0 M GSH showed the highest VOC of 676 mV. This is because GSH, which consists of two carboxylic groups, offers a more negative shift of the conduction band edge of TiO2 toward greater negative potentials than that in the case of the other thiol coadsorbents. Typically, JSC is determined by the light-harvesting efficiency as well as the charge injection and charge collection efficiency, according to the following equation:

Figure 7. Current−voltage characteristic of AgInS2 and CdSeTe QDSSCs associated without (control group) or with different thiol coadsorbents under AM1.5G light under illumination of 100 mW cm−2.

coadsorbent

εCB kT ⎛ n ⎞ ε + B ln⎜ c ⎟ − redox q q ⎝ NCB ⎠ q

a

Performance of the champion cell. bAverage efficiency and standard deviations for the four devices.

curves and performance parameters of the AgInS2 and CdSeTe QDSSCs with or without the incorporation of thiol coadsorbents, respectively. The QDSSC sensitized with the

JSC = qΦ0ηLHηINJηCC G

(2) DOI: 10.1021/acsami.6b05411 ACS Appl. Mater. Interfaces XXXX, XXX, XXX−XXX

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ACS Applied Materials & Interfaces Scheme 2. (a) Energy Level Diagram and (b) Schematic Representation of QDSSCs with (Right Panel) or without (Left Panel) the Incorporation of Thiol Coadsorbents

where q is the elementary charge, Φ0 is the incident photon flux, ηLH is the light-harvesting efficiency, ηINJ is the charge injection efficiency, and ηCC is the charge collection efficiency. As a result, the improvement in the JSC of a cell can be attributed to several mechanisms: (i) A higher QD loading contributes to better light-harvesting efficiency and thus to enhanced photocurrent. Figure 5 shows that the JSC of the cell improves with increasing concentration of the thiol coadsorbents. In addition, a clear increase in the absorbance of the QD-sensitized TiO2 films as a function of the thiol coadsorbent concentration was detected (Figure 8a), suggesting that a larger amount of QDs are anchored to the TiO2 surface in the presence of a large amount of the TGA coadsorbent. The corresponding digital photographs of the bare TiO2 films and QD-sensitized TiO2 films are shown in Figure 8b. Therefore, the use of excess thiol coadsorbents would be expected to suppress disulfide formation, thereby preventing QD desorption from TiO2 and assisting in higher QD loading inside the TiO2 films, which is of great significance to promote the lightharvesting ability and increase JSC. (ii) A higher JSC could be attributed to a broader light-harvesting range. As shown in Figure 8a, apart from the increase in absorbance, it is also important to note that the spectra exhibit a progressive red-shift of the absorbance onset, so that a wider wavelength range can be used to harvest larger amounts of photons, with increasing TGA concentration. This is consistent with the IPCE results shown in Figure 8c. IPCE is determined by the light-harvesting efficiency, electron injection efficiency, and charge collection efficiency, and is directly related to JSC. In Figure 8c, the IPCE curves of the QDSSC with 0.1 M TGA cover the spectral range of 400 to 750 nm and exhibit maximum values of about 62% at around 520 nm. The maximum IPCE for the QDSSC with the 4.0 M TGA was ca. 70% in the wavelength range of 470−580 nm. The higher IPCE of the QDSSC contributed to the increase in the light-harvesting efficiency, which could result in the enhancement of JSC. Additionally, the IPCE spectrum showed that the QDSSC with the 4.0 M TGA exhibits an

Figure 8. (a) Absorption spectra of the TiO2 film sensitized without or with the QD solution along with different concentrations of TGA coadsorbents (0.1−6.0 M). (b) Digital photograph of the corresponding naked TiO2 films and QD-sensitized TiO2 films. (c) IPCE spectra of Cys−CIS QDSSCs with the incorporation of 0.1 and 4.0 M TGA coadsorbents.

obvious red-shift and tail broadening to a longer wavelength (750−880 nm) as compared to the QDSSC with 0.1 M TGA. The red-shifted absorbance is possibly a consequence of the formation of larger CIS QDs, leading to a reduction of quantum confinement. This might be due to partial ligand exchange between TGA and the Cys-linker capped on the surface of the QDs, leading to naked surfaces of tiny QDs and Ostwald ripening, which involves dissolution of the energetically less stable tiny QDs and growth of larger stable QDs. Another possible explanation for the change in absorbance is the impact of capping ligands on the photophysical properties of the QDs.53,54 A similar feature was recently observed: CdSe QDs exhibited a change in absorbance to longer wavelengths with increasing concentrations of MPA in solution.18 (iii) The increase in JSC could be attributed to the efficient injection of the resulting exited electrons from the QD to TiO2. As mentioned above, HS−R−COOH and HS−R−OH thiol coadsorbents could act as disulfide reductants in the QDs solution. As a result, a higher concentration of thiol coadsorbents enhances the cleavage of a greater number of disulfide bonds on the surface of Cys−CIS QDs to form free −SH groups, which increases the high flux of electrons into TiO2, leading to JSC improvement. This is because the thiol functional group of the bifunctional linker on the QDs surface is not only essential for anchoring the QDs on the TiO2 film but is also important as an electron-transporting bridge, which H


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GSH−CIS QDs (5.02 ± 0.18%). As discussed below, the higher JSC values can be explained by the rapid electron transfer and weaker charge recombination at the interface between the TiO2 matrix and the CIS QDs via the Cys linker, as shown in the emission-decay profile and electrochemical spectra, respectively. The beneficial effect would result from the bidentate functional moieties (thiol and amino groups) of the Cys linker, which could coordinate with the CIS QDs and facilitate rapid charge injection between the QDs and TiO2, while the TGA linker only possess a thiolated group tethered to the QDs surface. Although the polydentate linkage of the GSH linker shows high anchoring affinity to the QD surface, the complicated GSH molecular skeleton could adversely affect the efficiency of electron injection into the TiO2 matrix as compared to that in the case of the short-chain Cys linker. The rate constants of electron transfer (kET) could be estimated by comparing the decay lifetime of CIS/TiO2 and CIS/SiO2 using the following expression:6,7

delivers electrons from the excited QDs to the conduction band of the TiO2. (iv) The improved JSC can be correlated to the addition of thiol coadsorbents that suppress the aggregation of QDs effectively, thereby facilitating rapid electron injection and enhancing the photocurrent. This explanation might be analogous to the function of coadsorbents in DSSCs. Previous studies on DSSCs reported that dye aggregation was prevented effectively by the incorporation of coadsorbents such as 3phenylpropionic acid,55 4-guanidinobutyric acid,51 or chenodeoxycholic acid.56 This is because coadsorbent addition is beneficial for organizing and orienting the adsorption of sensitizers on the TiO2 surfaces, as the coadsorbent molecules occupy the empty spaces between the dye molecules and obstruct the formation of sensitizer aggregates. In the present case, the presence of thiol coadsorbents on TiO2 might lead to better orientation and self-assembly of the Cys−CIS QDs, leading to an increase in the electron injection yields (Scheme 2b). This process is expected to have a significant role in improving the generation of photoinduced electrons. To assess the influence of different bifunctional linkers on the photovoltaic performance of QDSSCs, another control experiment was conducted using TGA and GSH as bifunctional linkers in order to obtain TGA- and GSH-capped CIS QDs (labeled as TGA−CIS and GSH−CIS, respectively) under identical synthetic conditions. Figure 9 presents the J−V curves

kET =

1 1 − τ(CIS/TiO2 ) τ(CIS/SiO2 )

where τ(CIS/TiO2) and τ(CIS/SiO2) refer to the average lifetimes of the CIS QDs anchored on the TiO2 and SiO2 matrix, respectively. This is because when the CIS QDs are irradiated with light, the excited CIS QDs are capable of injecting electrons into the TiO2 matrix, leading to the quenching of emission, but not the transfer of photogenerated electrons into the insulating SiO2 matrix. The time-dependent PL decay profiles were recorded for TiO2 (or SiO2) films sensitized with CIS QDs with 450 nm diode laser excitation (Figure 10). The decay profiles were fitted to a multiexponential model, as expressed by eq 4: n

I (t ) =

n

⟨τ ⟩ = of the QDSSCs using CIS QDs with various bifunctional linkers as sensitizers. Table 4 lists the extracted device characteristics. The JSC values increased in the order Cys−CIS (19.19 mA cm−2) > TGA−CIS (18.02 mA cm−2) > GSH−CIS (17.43 mA cm−2), which is also reflected in the conversion efficiency of the cell. The cells sensitized with the Cys−CIS QDs yielded the highest PCE value on average (5.85 ± 0.05%) than as compared to those for TGA−CIS (5.25 ± 0.16%) and

FF (%)

PCEa (%)

PCEb (%)

TGA−CIS Cys−CIS GSH−CIS

18.02 19.19 17.43

634 650 636

47.4 47.3 46.9

5.41 5.90 5.20

5.25 ± 0.16 5.85 ± 0.05 5.02 ± 0.18

∑i = 1 αiτi2 n

∑i = 1 αiτi

(5)

Table 5 lists the corresponding parameters for the emission decays with reduced χ2 (≤1.1). The average lifetime of the Cys−CIS/TiO2 was estimated to be 0.66 ns, which is shorter than that obtained with the TGA−CIS/TiO2 (0.78 ns) and GSH−CIS/TiO2 (0.71 ns). As shown in Table 5, the rate constants changed by 0.78 × 109, 0.95 × 109, and 0.76 × 109 s−1 for TGA−CIS, Cys−CIS, and GSH−CIS QDs, respectively. These results suggest that more efficient charge injection from the excited Cys−CIS QDs to TiO2 leads to a shorter emission lifetime compared to that with GSH−CIS and TGA−CIS QDs. In addition, the higher rate constants of Cys−CIS compared to that of TGA− and GSH−CIS contributed to the higher photocurrent of the cell, as shown in the J−V curves of Figure 9. The interfacial resistance and electron recombination behaviors of the QDSSCs based on the CIS QDs capped with different bifunctional linkers were investigated further by EIS. Figure 11a presents the Nyquist plots of QDSSCs at frequencies ranging from 10−1 to 105 Hz recorded under dark

Table 4. Photovoltaic Parameters of QDSSCs Based on TGA−CIS, GSH−CIS, or Cys−CIS QDs Associated with the Same Concentration of TGA Coadsorbent VOC (mV)

(4)

where I refers to the normalized emission intensity, t is the time after pulsed-laser excitation, αi is the weighted coefficients of the components, and τi is the decay times. Subsequently, the average lifetime was calculated using eq 5:

Figure 9. Current−voltage characteristics of the QDSSCs based on TGA−CIS, GSH−CIS, or Cys−CIS QDs associated with the same concentration of TGA coadsorbent.

JSC (mA cm−2)

⎛ t⎞ ⎟ ⎝ τi ⎠

∑ αiexp⎜− i=1

photoanode

(3)

a Performance of the champion cell. bAverage efficiency and standard deviations for the four devices.

I


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Figure 10. Time-resolved emission kinetics of (a) TGA−CIS, (b) Cys−CIS, and (c) GSH−CIS QDs anchored on the TiO2 or SiO2 films measured under 400 nm excitation.

Table 5. Fluorescence Lifetime Parameters for Different Samples condition

α1 (%)

τ1 (ns)

α2 (%)

τ2 (ns)

α3 (ns)

τ3 (ns)

⟨τ⟩ (ns)

kET (109 s−1)

TGA−CIS/SiO2 TGA−CIS/TiO2 Cys−CIS/SiO2 Cys−CIS/TiO2 GSH−CIS/SiO2 GSH−CIS/TiO2

19.28 62.99 19.16 54.02 19.27 49.79

6.54 1.19 6.19 1.15 5.57 1.41

54.03 37.01 44.64 45.98 49.93 50.21

1.33 0.09 1.21 0.09 1.04 0.11

26.69 − 36.20 − 30.80 −

0.18 − 0.14 − 0.12 −

2.02 0.78 1.78 0.66 1.63 0.76

− 0.78 − 0.95 − 0.71

conditions with a forward bias of −0.55 V. In the equivalent circuit simulation, the sheet resistance (RS) accounts for the ohmic resistance of the substrate; RC1 and RC2 are related to the interfacial resistance at the counter electrode/the electrolyte and the charge recombination resistance at the TiO2/QDs/ electrolyte interface, respectively. The RC2 values for the GSH− CIS-, TGA−CIS-, and Cys−CIS-based cells are 99.5, 137.2, and 516.8 Ω, respectively (Table 6). This suggests that the degree of interfacial charge recombination of the Cys−CIS-based cell is weaker than that of the others. Figure S7 shows the Bode plots of GSH−CIS-, TGA−CIS-, and Cys−CIS-based QDSSCs. According to the EIS model, the electron life times (τn) can be determined from the Bode plot using the expression, τn = 1/(2π f max), where f max is the maximum frequency in the Bode plot. As shown in Table 6, the τn value increased in the order of Cys−CIS QDSSC (145.0 ms) > TGA−CIS QDSSC (96.2 ms) > GSH−CIS QDSSC (77.5 ms). The results suggest that QDSSC sensitized with Cys−CIS QDs has a longer electron lifetime, enabling the solar cell to achieve higher photovoltaic performance. Intensity-modulated photocurrent spectroscopy (IMPS) and intensity-modulated photovoltage spectroscopy (IMVS) were used to gain insights into the electron transport lifetime and

recombination lifetime that occurs at an illuminated electrode. In principle, IMPS records the periodic photocurrent response to the intensity-modulated light, which illustrates the kinetics of the charge transfer mechanisms under short-circuit conditions. Instead, the IMVS experiment was performed with the same perturbed light but under open-circuit conditions, which provides the electron lifetimes and recombination kinetics. Figure 11b and c show the electron transport time and recombination lifetime of the CIS QDs capped with different bifunctional linkers as a function of the light intensity from 30 to 150 W m−2 under illumination from a LED light source (λ = 457 nm). The electron transport time (τt) was estimated from the IMPS spectra in terms of the expression, τt = 1/(2π f IMPS), where f IMPS is the characteristic frequency at the lowest IMPS imaginary component. In addition, the recombination lifetime (τr) can be estimated from the equation, τr = 1/(2π f IMVS), where f IMVS is the characteristic frequency at the lowest IMVS imaginary component. The logarithm of the electron lifetime and recombination lifetime both decreased linearly with increasing photon flux, as shown in Figure 11b and c. At the same charge density, the τt value for the Cys−CIS-based cells was longer than those of the TGA−CIS- and GSH−CIS-based cells, while the τr value for the Cys−CIS QDSSC was shorter J


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Figure 11. (a) Nyquist plots of QDDSCs based on the as-prepared GSH−CIS, TGA−CIS, and Cys−CIS QDs in the dark; the lines show the fitting results by applying the appropriate equivalent circuit, which involves the constant phase element (CPE), sheet resistance (RS), and charge recombination resistance (RC), shown in the inset in the figure. Incident light intensity dependent on (b) electron transport time, (c) recombination lifetime, and (d) charge collection efficiency for GSH−CIS-, TGA−CIS-, and Cys−CIS-based cells. All measurements were carried out under the irradiation by a 457 nm LED.

collection during the illumination, which gives rise to a high charge transfer and the photocurrent density.

Table 6. Electrochemical Parameters of QDDSCs Sensitized with GSH−CIS, TGA−CIS, and Cys−CIS QDs photoanode

RS (Ω)

RC1 (Ω)

RC2 (Ω)

τn (ms)

GSH−CIS TGA−CIS Cys−CIS

22.00 25.67 22.99

47.78 46.37 47.24

99.5 137.2 516.8

145.0 96.2 77.5

4. CONCLUSIONS An environmentally friendly approach to the preparation of highly efficient QDSSCs based on the combination of aqueous CIS QDs and thiol coadsorbents is reported. The use of thiol coadsorbents during the QD sensitization process plays a crucial role in improving the photovoltaic performance of QDSSCs. The QDSSCs with the thioalkyl acid coadsorbent exhibited a much higher VOC and JSC than the cells coadsorbed with thiol coadsorbents bearing a hydroxyl group. A short QD sensitization time (30 min) leads to an efficiency of up to 2.93%. Moreover, the CIS QDSSC with the TGA thiol coadsorbent exhibited improved JSC and VOC, leading to efficiency levels as high as 5.90%, which is almost 20 times higher than that of the devices without thiol coadsorbents (PCE = 0.28%). The enhanced JSC was attributed to either the increased amount of adsorbed QDs or the efficient electron injection from the QDs into TiO2, or both, due to the reducing functionality of the thiol coadsorbents, which helps decrease disulfide formation on the QD surface. In addition, the increase in VOC could be attributed to the coassembly of thiol coadsorbents and QDs on the TiO2 surface, causing a shift in the TiO2 band-edge. The effects of various bifunctional linkers on the performance of QDSSC showed that the Cys−CIS QDSSC had a higher average efficiency of 5.85 ± 0.05% compared to the cells fabricated with TGA−CIS (5.25 ± 0.16%) and GSH−CIS QDs (5.02 ± 0.18%). This is because Cys−CIS QDs allowed for more efficient electron transport and less recombination than did the TGA−CIS and GSH−CIS

than that of the other two samples. The results suggest that Cys−CIS QDSSC has more efficient electron transport and less recombination than those of TGA−CIS and GSH−CIS QDSSC, consistent with the results of the time-dependent PL decay profiles shown in Figure 10. Figure 11d presents a plot of the charge-collection efficiency (ηcc) of CIS capped with different bifunctional linkers versus the incident light intensity. The ηcc value can be derived by IMPS and IMVS analysis through the following equation: τ ηcc = 1 − t τr (6) Here, ηcc follows an approximately linear relationship with the illumination intensity. The charge collection efficiency was higher in Cys−CIS QDSSC than that in TGA−CIS- and GSH−CIS-based cell. In particular, the ηcc values of Cys−CIS, TGA−CIS, and GSH−CIS were approximately 92.1%, 84.9%, and 79.7%, respectively, at a light intensity of 150 W m−2. When the light intensity was decreased to 30 W m−2, the Cys− CIS cell reached as much as 97.0%. A higher ηcc value was obtained via the Cys−CIS QDs acting as a sensitizer in photoanodes, suggesting the high charge K


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(9) Tian, J.; Lv, L.; Fei, C.; Wang, Y.; Liu, X.; Cao, G. A Highly Efficient (>6%) Cd1‑xMnxSe Quantum Dot Sensitized Solar Cell. J. Mater. Chem. A 2014, 2, 19653−19659. (10) Luo, J.; Wei, H.; Huang, Q.; Hu, X.; Zhao, H.; Yu, R.; Li, D.; Luo, Y.; Meng, Q. Highly Efficient Core−Shell CuInS2−Mn Doped CdS Quantum Dot Sensitized Solar Cells. Chem. Commun. 2013, 49, 3881−3883. (11) Jara, D. H.; Yoon, S. J.; Stamplecoskie, K. G.; Kamat, P. V. SizeDependent Photovoltaic Performance of CuInS2 Quantum DotSensitized Solar Cells. Chem. Mater. 2014, 26, 7221−7228. (12) Guijarro, N.; Guillén, E.; Lana-Villarreal, T.; Gómez, R. Quantum Dot-Sensitized Solar Cells Based on Directly Adsorbed Zinc Copper Indium Sulfide Colloids. Phys. Chem. Chem. Phys. 2014, 16, 9115−9122. (13) Barpuzary, D.; Banik, A.; Gogoi, G.; Qureshi, M. Noble Metalfree Counter Electrodes Utilizing Cu2ZnSnS4 Loaded with MoS2 for Efficient Solar Cells Based on ZnO Nanowires Co-Sensitized with CuInS2−CdSe Quantum Dots. J. Mater. Chem. A 2015, 3, 14378− 14388. (14) Pan, Z.; Zhao, K.; Wang, J.; Zhang, H.; Feng, Y.; Zhong, X. Near Infrared Absorption of CdSexTe1‑x Alloyed Quantum Dot Sensitized Solar Cells with More than 6% Efficiency and High Stability. ACS Nano 2013, 7, 5215−5222. (15) Jiao, S.; Shen, Q.; Mora-Seró, I.; Wang, J.; Pan, Z.; Zhao, K.; Kuga, Y.; Zhong, X.; Bisquert, J. Band Engineering in Core/Shell ZnTe/CdSe for Photovoltage and Efficiency Enhancement in Exciplex Quantum Dot Sensitized Solar Cells. ACS Nano 2015, 9, 908−915. (16) Pan, Z.; Mora-Seró, I.; Shen, Q.; Zhang, H.; Li, Y.; Zhao, K.; Wang, J.; Zhong, X.; Bisquert, J. High-Efficiency “Green” Quantum Dot Solar Cells. J. Am. Chem. Soc. 2014, 136, 9203−9210. (17) Li, W.; Pan, Z.; Zhong, X. CuInSe2 and CuInSe2−ZnS Based High Efficiency “Green” Quantum Dot Sensitized Solar Cells. J. Mater. Chem. A 2015, 3, 1649−1655. (18) Sahasrabudhe, A.; Bhattacharyya, S. Dual Sensitization Strategy for High-Performance Core/Shell/Quasi-shell Quantum Dot Solar Cells. Chem. Mater. 2015, 27, 4848−4859. (19) Li, W.; Zhong, X. Capping Ligand-Induced Self-Assembly for Quantum Dot Sensitized Solar Cells. J. Phys. Chem. Lett. 2015, 6, 796− 806. (20) Yang, J.; Oshima, T.; Yindeesuk, W.; Pan, Z.; Zhong, X.; Shen, Q. Influence of Linker Molecules on Interfacial Electron Transfer and Photovoltaic Performance of Quantum Dot Sensitized Solar Cells. J. Mater. Chem. A 2014, 2, 20882−20888. (21) Jumabekov, A. N.; Deschler, F.; Böhm, D.; Peter, L. M.; Feldmann, J.; Bein, T. Quantum-Dot-Sensitized Solar Cells with Water-Soluble and Air-Stable PbS Quantum Dots. J. Phys. Chem. C 2014, 118, 5142−5149. (22) Baker, D. R.; Kamat, P. V. Tuning the Emission of CdSe Quantum Dots by Controlled Trap Enhancement. Langmuir 2010, 26, 11272−11276. (23) Aldana, J.; Wang, Y. A.; Peng, X. Photochemical Instability of CdSe Nanocrystals Coated by Hydrophilic Thiols. J. Am. Chem. Soc. 2001, 123, 8844−8850. (24) Mann, J. R.; Watson, D. F. Adsorption of CdSe Nanoparticles to Thiolated TiO2 Surfaces: Influence of Intralayer Disulfide Formation on CdSe Surface Coverage. Langmuir 2007, 23, 10924−10928. (25) Katchalski, E.; Benjamin, G. S.; Gross, V. The Availability of the Disulfide Bonds of Human and Bovine Serum Albumin and of Bovine γ-Globulin to Reduction by Thioglycolic Acid. J. Am. Chem. Soc. 1957, 79, 4096−4099. (26) Hullmann, A.; Wood, J.; Kufner, F.; Schafer, S.; Nocker, B. Process for Permanent Shaping of Human Hair. U.S. Patent 8,753,616 B2, Jun 17, 2014. (27) Ballatori, N.; Krance, S. M.; Notenboom, S.; Shi, S.; Tieu, K.; Hammond, C. L. Glutathione Dysregulation and The Etiology and Progression of Human diseases. Biol. Chem. 2009, 390, 191−214. (28) Lukesh, J. C.; Palte, M. J.; Raines, R. T. A Potent, Versatile Disulfide-Reducing Agent from Aspartic Acid. J. Am. Chem. Soc. 2012, 134, 4057−4059.

QDs, as revealed by the emission decay profile, electrochemical spectra, and IMPS/IMVS measurements. In addition, the preliminary results showed that the addition of TGA coadsorbent improves the cell efficiency of aqueous AgInS2 and CdSeTe QDSSCs significantly as compared to that of the device without the thiol coadsorbents. Future refinement and optimization of AgInS2 and CdSeTe QDSSCs should result in significant improvement in the device performance. The proposed easy protocol may assist in the fabrication of highefficiency QDSSCs from different aqueous QDs.

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acsami.6b05411. Current−voltage characteristic of the cell treated with TGA coadsorbents before and after dipping the mesoTiO2 films. Time dependence of current−voltage characteristic of the Cys−CIS QDSSCs. Current−voltage characteristics of the Cys−CIS QDSSCs associated with different thiol coadsorbents. UV−visible and photoluminescence spectra of CdSeTe and AgInS2 QDs. (PDF)

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Tel.: +886-2-27303636. Fax: +886-2-27376644. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the Ministry of Science and Technology of the Republic of China for financially supporting this research under Contract Nos. 102-2628-M-011-001-MY3, 104-2119-M011-001, and 105-2119-M-011-002.

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REFERENCES

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