Enhancing the Charge Separation in Nanocrystalline Cu2

Letter pubs.acs.org/JPCL

Enhancing the Charge Separation in Nanocrystalline Cu2ZnSnS4 Photocathodes for Photoelectrochemical Application: The Role of Surface Modifications Néstor Guijarro, Mathieu S. Prévot, and Kevin Sivula* Laboratory for Molecular Engineering of Optoelectronic Nanomaterials, Institute of Chemical Sciences and Engineering, École Polytechnique Fédérale de Lausanne (EPFL), Station 6, 1015-Lausanne, Switzerland S Supporting Information *

ABSTRACT: Cu2ZnSnS4 (CZTS) colloidal inks were employed to prepare thin-film photocathodes that served as a model system to interrogate the effect of different surface treatments, viz. CdS, CdSe, and ZnSe buffer layers along with methylviologen (MV) adsorption, on the photoelectrochemical (PEC) performance using aqueous Eu3+ redox electrolyte. PEC experiments revealed that ZnSe and CdSe overlayers outperform traditional CdS, and the additional surface modification with MV was found to further boost the charge extraction. By analyzing the photocurrent onset behavior and measuring the open circuit photopotentials, insights are gained into the nature of the observed improvements. While a more favorable conduction band offset rationalizes the improvement offered by CdSe, charge transfer through midgap states is invoked for ZnSe. Improvement offered by MV treatment is clearly caused by both the shifting of the flat-band potential and a charge-transfer mediation effect. Overall, this work suggests promising alternative surface treatments for CZTS photocathodes for PEC energy conversion. SECTION: Energy Conversion and Storage; Energy and Charge Transport

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electron transfer from the CZTS, and the potential losses over the interfaces for the device operation in forward bias. In such a way, the optimized output parameters (i.e., short-circuit photocurrent, open-circuit voltage, and fill factor) can be achieved. In contrast, a PEC electrode is typically polarized in reverse bias, and a high surface area nanostructured semiconductor-liquid junction is often employed to afford a large surface area for catalysis. Moreover, decreasing the overpotential required for photocurrent onset is generally more important than increasing the photocurrent density when dealing with the implementation of the photocathode in a PEC tandem cell.5,15 In this scenario, light absorption by the overlayer may be tolerable. Moreover, absorbing different ionic or molecular species on semiconductor surfaces provides a further strategy to modify PEC electrodes that is less accessible to investigation for PV cells.16,17 Surprisingly, while great efforts have been devoted to the design of buffer layers in p-type quaternary chalcogenide PV devices, the particular optimization of the surface modification for PEC application has been mostly overlooked to date. In this work, CZTS nanocrystal (NC) thin films prepared from a colloidal route serve as model system to investigate the effect of different surface treatments on the PEC performance

he p-type quaternary chalcogenide Cu2ZnSnS4 (CZTS) has drawn much attention as a promising material for solar energy conversion by virtue of its earth-abundant composition, favorable optical properties, and suitability for low-cost solution-based processing.1 In addition to traditional photovoltaics (PV),2 CZTS has recently shown promise for the direct solar-to-chemical energy conversion in photoelectrochemical (PEC) devices.3,4 Indeed, a PEC tandem cell employing a water-oxidizing n-type photoanode with a waterreducing CZTS photocathode could potentially exhibit a solarto-hydrogen conversion over 15%.5 As with PV devices, it is well known that the PEC performance of p-type multinary copper-based chalcogenides relies on the presence of n-type buffer layers. Work from the groups of Domen and Matsumura demonstrated the critical role of the typically employed CdS and the less commonly used ZnS as overlayers on CZTS to promote charge separation through the resulting p-n heterojunction, which improves the PEC photocurrent in the presence of a water-reducing catalyst.3,4,6−12 Very recently, Gunawan et al. also reported the use of In2S3 as a buffer layer for a CuInS2 photoelectrode.13 To date, the choice of the buffer layers for PEC electrodes has been driven by previous results with PV devices.14 Nevertheless, the requirements for the buffer layer in a PEC device are distinct from a PV device due to differences in device operation. The ideal PV design demands highly crystalline and planar layers with a wide band gap n-type buffer layer that balances the competitive light absorption, the photogenerated © XXXX American Chemical Society

Received: September 19, 2014 Accepted: October 23, 2014

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dx.doi.org/10.1021/jz501996s | J. Phys. Chem. Lett. 2014, 5, 3902−3908

The Journal of Physical Chemistry Letters

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CZTS NC thin films were alternately immersed in a 50 mM Cd(CH3COO)2 (cadmium source) or 50 mM Zn(CH3COO)2 (zinc source) ethanol solution and a 50 mM Na2S (sulfur source) (or a 30 mM selenide) ethanol solution, depending on the desired overlayer. (See the SI for complete details.) Figure 1b shows the absorption spectra of CZTS thin films before and after five SILAR cycles of CdS, CdSe, or ZnSe. (Spectra as a function of SILAR cycles are shown in Figure S4 in the SI.) Upon modification, a new absorption feature appeared with an onset close to the band gap energy of the corresponding buffer layer, providing evidence of the successful growth of the binary chalcogenide. Likewise, Raman spectra further confirmed the deposition of the different overlayers on the CZTS films (Figure S6 in the SI). The precise control over the amount of deposited overlayer was achieved by adjusting the number of SILAR cycles. In particular, for CdS, a linear growth rate of ∼1.33 nm per cycle was found. (See Figure S5 in the SI.) Moreover the onset of the absorption was found to red-shift for each layer as the thickness increased, consistent with the weakening of quantum confinement (Figure S4 in the SI). To gain some insight into the distribution of the overlayer throughout the nanoporous film, a small portion of the films were mechanically detached from the conductive substrate and examined by scanning transmission electron microscopy (STEM) together with energy-dispersive X-ray (EDX) elemental mapping. As observed in the case of a CdSe overlayer (Figure 1c−e), the particles are evenly covered by both Cd and Se, verifying near-conformal coating. We note that for thicker overlayers some faceting of the binary chalcogenides was observed. (See Figures S7−S10 in the SI.) We next investigated the effect of surface treatments on the PEC performance of the CZTS electrodes in aqueous 50 mM Eu(NO3)3 electrolyte, which serves as sacrificial electron acceptor. Linear sweep voltammograms under chopped light illumination (AM1.5G 100 mW cm−2) for the bare and surfacemodified CZTS electrodes are displayed in Figure 2a for the bare CZTS (black curve), CZTS/buffer layer (blue curves), and CZTS/buffer layer/MV (red curves). Each case is discussed below. Cathodic photocurrents that gradually increased as the potential was swept more negative26 were observed with the bare CZTS electrode during illumination, confirming its p-type PEC behavior. The photocurrent density for the bare CZTS matches well with that reported by Kameyama et al.19 but is smaller than reported by Riha et al.,20 which could be attributable to the differing film thickness or illumination conditions employed in that work. As expected, higher photocurrents were observed after modifying the CZTS surface with CdS, CdSe, or ZnSe (five SILAR cycles shown in Figure 2a). Similar to the CZTS/CdS case,27−29 a type-II heterojunction alignment explains the photocurrent increase in the CZTS/CdSe electrode given the reported conduction band (CB) position of CdSe.30 Because it is expected that the kinetics of Eu3+ reduction are similarly facile for both the CZTS/CdSe and CZTS/CdS electrodes, we can assume that the rate of electron extraction at the CZTS/chalcogenide interface would limit the photocurrent density. Given that this rate is known to be proportional to the offset between the donor and acceptor energy levels31,32 and that the corresponding CB energy offsets at the CZTS/CdSe and CZTS/CdS interfaces are calculated to be 400 and 140 meV, respectively (see Figure 2b for diagram and SI for details), a more efficient electron extraction, and hence, higher photocurrent values,

in aqueous electrolyte. Colloidal routes allow fine-tuning the composition, circumventing the deleterious phase segregation found in traditional approaches.18 The PEC properties of CZTS NCs19,20 mimic those of compact films21,22 typically employed as photocathodes for water reduction but benefit from their straightforward preparation. Herein, CZTS surface modification with CdS, CdSe, and ZnSe layers and the subsequent surface adsorption of methylviologen (MV) are investigated to gain insight into factors controlling charge separation and recombination at the electrode/electrolyte interface. CZTS colloidal NCs with an average size of 15.8 ± 0.6 nm were synthesized following the wet chemical route reported by Guo et al.2 with minor changes. The NCs exhibited the expected optical, crystallographic properties, and composition. (See Figures S1 and S2 in the Supporting Information (SI).) CZTS NC thin films were prepared by doctor blading a concentrated “ink” solution in chloroform (∼50 mg/mL) onto a F:SnO2 (FTO)-coated glass and the subsequent annealing under vacuum at 275 °C to remove the organic capping ligands, as demonstrated by FT-IR. (See Figure S3 in the SI.) The thickness of thin films was ca. 70 nm. Figure 1a shows a representative scanning electron microscopy (SEM) image of the surface of a CZTS thin film. The NCs appear densely packed, giving a compact but nanoporous film.20

Figure 1. (a) Top-view SEM image of a CZTS NC thin film after annealing treatment. (b) Absorption spectra of CZTS films before and after modification with five SILAR cycles of CdS, CdSe, and ZnSe. STEM image (c) and the corresponding EDX elemental mapping (d,e) for the CZTS NCs.

CZTS NC thin films were next modified by coating with CdS, CdSe, or ZnSe. While the deposition of metal chalcogenide buffer layers is commonly carried out by chemical bath deposition routes,6,8 this method has proven difficult in coating meso- and nanoporous structures by clogging pores and effectively reducing the active area of the electrodes.23,24 In contrast, the successive ionic layer adsorption and reaction (SILAR) approach has demonstrated a more conformal coating throughout nanoporous structures.25 In the present work, the 3903



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CTZS, the apparently more effective electron extraction compensated for this photon loss. Overall, these results suggest that ultrathin CdSe overlayers are a promising alternative to CdS for CZTS PEC electrodes. The use of ZnSe as a buffer layer proved to outperform the conventional CdS overlayer as well. This is surprising considering that the CZTS/ZnSe interface is predicted to exhibit a type-I alignment that would prevent, rather than promote, charge separation.35 Thus, a different mechanism must be invoked to rationalize the improved performance. In the previously investigated Cu(In,Ga)Se2/ZnSe system, it was argued that the accumulation of charges at the buffer layer/ active layer interface can shift the bands of ZnSe downward, providing an energetically viable pathway for electron extraction.35 Alternatively, we hypothesize that recently reported high density of midgap states in ZnSe NCs36 might as well play a role in charge separation, providing a channel for electrons to flow from the CZTS to the interface with the electrolyte. It is worth noting the presence of progressively higher photocurrent transient spikes as the potential is swept cathodically, even in bare CZTS. (See Figure S11 in the SI.) Although in a classical photoelectrode the spikes disappear as the depletion layer broadens, the opposite behavior, like the one observed here, can result from two plausible origins: first, the recombination of photogenerated holes with reduced species of the redox couple (Eu2+), generated at the electrode/ electrolyte interface during illumination (as observed for higher photocurrent density);37 second, the recombination of photogenerated carriers at interfacial states (also possible at low photocurrent density).38 Interestingly, while CdS and CdSe modification scarcely exhibits transient spikes for low photocurrents, ZnSe indeed shows noticeable spikes for the 5-SILAR cycle modification, suggesting that a higher degree of trapping and recombination occurs at the junction in this case. Regardless of the origin of the transients, the improved performance of the ZnSe and CdSe buffer layers to that of the CdS suggests these are promising buffer layers for further development, especially ZnSe, considering its lack of cadmium. Next, to investigate the strategy of using positively charged molecular species to enhance PEC performance for CZTS, we employed MV, a well-known positively charged electron scavenger, as a surface modifier for the NC-based thin films. CZTS/buffer layer electrodes were modified by immersion in a 50 mM aqueous solution for 30 min, followed by thorough rinsing. The adsorption of MV on the chalcogenide leads to a striking enhancement of the photocurrents, especially in the potential range below 0.15 V versus Ag/AgCl (see Figure 2a, red curves), whereas saturation photocurrents remain similar after the treatment. (See Figure S11 in the SI.) We note that the enhanced photoresponse is repeatable over many samples, and the enhancement is stable, as demonstrated from chronoamperometry and photopotential measurements (vide infra). Interestingly, transient spikes appear to be more pronounced upon MV adsorption. Following our previous reasoning, this suggests that while the photocurrent increases, the deposition of the MV layer is accompanied by the introduction of interfacial states involved in charge trapping and recombination. The photocurrent increase in similar systems employing the adsorption of positively charged molecular species has been explained, in electrostatic terms, by an induced downward shift of the energy bands, which increases the driving force for

Figure 2. (a) Linear sweep voltammograms under chopped light illumination for CZTS before and after deposition of different buffer layers and adsorption of MV, scanned cathodically at a scan rate of 2 mV s−1 and using simulated 1 sun illumination (AM1.5G). (b) Estimated band-energy positions for different materials. (See the SI,) All photoelectrochemical measurements were performed in Ar-purged aqueous 50 mM Eu(NO3)3 electrolyte.

would be expected for CdSe. Indeed we observe higher photocurrents over the entire potential range tested when using CdSe compared with CdS. Note that the photoelectrochemical (PEC) performance strongly depends on the amount of CdS or CdSe deposited. In particular, deposition of five SILAR cycles corresponds to the optimum thickness for the modification in all cases. (See Figure S11 in the SI.) Interestingly, for CdSe and CdS, this is the same thickness where optical absorption onset of the overlayer reaches its final bulk value (Figure S4 in the SI), suggesting that thinner overlayers provide less driving force for electron extraction due to a higher CB position caused by quantum confinement effects. Apart from the CB position, the absorption of photons in the buffer layer could also affect the PEC response. In fact, incident photon-to-current efficiency (IPCE) measurements reported for both PV and PEC configurations have demonstrated that buffer layers filter the light reaching the active layer without contributing to the photocurrent, as accounted for by the systematic decrease in the IPCE for energies higher than the band gap of the buffer layer.11−13,33 Therefore, strong light absorption in the overlayer would limit the overall performance of the photocathode. This parasitic absorption explains, in part, why the photocurrent decreases after five SILAR cycles (both charge transport resistance through the buffer layer and blockage of the nanoporous structure34 can also be factors). Interestingly, while the CdSe overlayer absorbed more photons than CdS (Figure 1b), decreasing the amount of light available to the 3904



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electron extraction.17 Alternatively, the MV could also contribute to the improved PEC performance by acting as charge-transfer mediator (an electron scavenger).39,40 As depicted in Figure 2b, the predicted cascade energy alignment across the buffer layer/MV/electrolyte interface could facilitate rapid electron extraction from the chalcogenide overlayer, which would reduce electron−hole recombination losses at the CZTS/buffer layer interface and give rise to higher photocurrents. In fact, MV has already proven to act as an efficient mediator between solution-dispersed dyes41 or CdSe39 and Pt nanoparticles, enhancing photocatalytic activity. Similarly, thiolcontaining molecules have shown to drastically enhance charge separation by mediating the hole transfer between the photoactive material and a hole acceptor.42 Because distinguishing between the aspects of band energy shift and chargetransfer mediation is of fundamental importance to the understanding of the origin of the photocurrent increase in our system, we further investigated the PEC properties by open-circuit photopotential measurements and by analyzing the photocurrent onset behavior, which can be considered a crude estimate of the flat band potential. The open-circuit photopotential (EOC) is defined as the difference between the steady-state potential under illumination (Eph) and the rest potential in dark (Edark).43 Figure 3 displays

the equilibration of the electrode with the redox couple (as nc,dark depends on Edark, which is set by the electrolyte redox couple). The higher EOC of the CdSe modified film, compared with the CdS, supports the view that a buffer layer with a lower CB position will increase the driving force for electron extraction and increase the number of charges accumulating in the electrode. The intermediate value of the ZnSe EOC supports the explanation that midgap states are involved in the transfer, as opposed to an explanation involving the a shifting of the bands in response to an accumulation of charges. In all cases, EOC attained a maximum at five SILAR cycles according to the optimum for photocurrent production. Because no current is flowing during the open-circuit measurements, the decrease in photocurrent and photopotential as the buffer-layer thickness increases cannot be due to an increase transport resistance but can be attributed to the aforementioned low-light intensity on the CZTS due to the light-filtering effect and the reduction of active surface area. After the MV treatment, the electrodes delivered higher EOC values. This is consistent with the explanation that the PEC performance is enhanced due to shifting of the bands as previously described for the adsorption of positively charged species on GaAs electrodes.17 Estimates of the flat-band potential, EFB, confirm this view. While the Mott−Schottky method is typically used to measure EFB, our results using this approach were inconsistent, likely because of the nanocrystalline nature of the films, which deviates from the Schottky junction model required. However, EFB can be roughly estimated as the potential corresponding to the onset of photocurrent. This method has been commonly employed for nanocrystalline electrodes and gives results close to those obtained for the crystalline films using other methods.19,21,44 However, the presence of interfacial states and surface trapping and recombination can affect the determination of EFB, yielding, for example, more positive flat-band potential for n-type materials.45 Despite this, the analysis reported herein attempts to gain insight into the effects of the overlayer deposition on the estimated EFB of CZTS films by comparing the trends in the changing EFB as a function of the surface treatment. Linear extrapolation of jph versus E (Figure 4) for potentials near the photocurrent onset potential gives an estimate of EFB. The bare CZTS exhibits a EFB of +0.25 V versus Ag/AgCl, and after the CdS deposition EFB shifted negatively to +0.18 V, whereas the MV modification displaced it in the positive direction by 120 mV (to +0.30 V). This suggests that CdS modification shifts the CZTS energy bands to higher energy (unfavorable for low onset potentials), whereas the MV moves them to lower energy, therefore facilitating charge separation at more anodic potentials. It must be pointed out that while the EFB shift caused by charged molecules or dipoles adsorption can be rationalized in electrostatic terms,17,25 the change in EFB when depositing the buffer layers can also be rationalized by the semiconductor−semiconductor Fermi level equilibration through the heterojunction formation.46,47 Estimating EFB using the steady-state photocurrent values found in chronoamperometry measurements confirmed the trend in the shift of EFB, with the addition of a 550 nm cutoff filter to avoid an ntype response due to CdS excitation (Figure S13 in the SI). In addition, error bars shown in Figure S13 in the SI, which result from testing multiple electrodes, indicate the reproducibility of our measurements. Comparing the different overlayers in Figure 4, we note that the ZnSe coating shifts EFB in the positive direction in contrast

Figure 3. Photopotential values versus the number of SILAR cycles obtained for transient illumination of CZTS thin film modified with CdS, CdSe, and ZnSe, with (open symbol) and without (solid symbol) methylviologen. All measurements were carried out in Ar-purged aqueous 50 mM Eu(NO3)3 electrolyte under 1 sun irradiation.

the EOC values obtained for the buffer-layer modified CZTS films with and without the MV treatment. We note that the EOC values were taken after a stabilization time under illumination for 10 min. (See Figure S12 in the SI for details with CdS.) As expected, the addition of the chalcogenide buffer layer increases the EOC compared with the bare CZTS film (zero SILAR cycles). We note that the optimum (five SILAR cycles) EOC from the CdSe buffer layer is greater than the CdS. This suggests that the EOC is not set by the type II heterojunction, as would be expected in a PV cell. Indeed, for nanocrystalline photoelectrodes in electrolyte, the EOC can be simply considered as proportional to log(1 + nc,ph/nc,dark), where nc,ph and nc,dark are the concentrations of majority carriers (holes) that accumulate in the valence band of the CZTS electrode under illumination and in the dark, respectively.43 Indeed, we observe that our EOC values are less than would be expected in CZTS/buffer layer PV cells. This can be attributed, in part, to 3905



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reaching the MV, which allows the MV to funnel electrons from the buffer layer to the electrolyte. Despite the enhancement afforded by the surface modification, the values of the photocurrent are far below the theoretical maximum current density attainable with our electrodes (6.5 mA cm−2, as estimated from the absorption spectrum of the bare CZTS film and the total irradiance on the electrode). This discrepancy primarily arises from the poor connectivity between the NCs, as expected from the low-temperature annealing, which limits the hole transport and facilitates recombination at the grain boundaries. Furthermore, the large density of surface states expected for such extremely high surface area NC films may participate actively as recombination centers, dramatically limiting the yield of charge separation.20 Despite this, we note that the addition of Pt as a water reduction catalyst to our CdS- and MV-treated CZTS electrodes gave similar photocurrents in nonsacrificial aqueous electrolyte to those observed with sacrificial Eu3+/2+ (see Figure S14 in the SI), suggesting that the increased photocurrent observed in our work can also improve water-reduction photocurrents. In summary, we report promising alternative surface treatments to enhance photocurrent onset potential and overall charge separation in CZTS films. Photoelectrochemical measurements performed in aqueous redox Eu3+ electrolyte demonstrated that ZnSe and especially CdSe coatings outperform conventional CdS-treated CZTS thin films for photocurrent production, and ZnSe also shows a more favorable flatband potential. Surprisingly, we found that additional surface modification with MV further boosts the PEC response. Opencircuit photopotential measurements and an analysis of the photocurrent onset gave further insight into the roles of the different overlayers. While a larger CB band offset, which increases the driving force for charge separation, explains the increased performance of the CZTS/CdSe electrode compared with the CdS, charge transfer through midgap states or bands realignment during illumination likely facilitate superior charge separation when ZnSe buffer layers are used. Our results also clearly demonstrated that MV adsorption plays a double role; that is, it shifts the onset of photocurrents to more positive potential, while it operates as an efficient electron-transfer mediator funneling the electrons from the buffer layer to the electrolyte. In such a way, the faster electron withdrawal would reduce the recombination at the CZTS/buffer layer interface, leading to higher photocurrents. Altogether, these findings emphasize that carefully tuning the CZTS surface holds the key to further enhancement of the charge separation at the CZTS electrode/electrolyte interface. We expect that these results will open new avenues for boosting the solar-to-chemical conversion efficiency in CZTS-based photoelectrodes in combination with an inexpensive catalyst for water reduction.

Figure 4. Flat-band (EFB) measurement for bare (a) and sequentially modified CZTS films with CdS, CdSe, and ZnSe (b−d) with and without MV by linear extrapolation of the jph versus E (jph intercepts the x axis at the estimated EFB). Dashed lines correspond to the linear fitting.

with CdS and CdSe. The subsequent adsorption of MV on the ZnSe further shifts EFB to +0.37 V, which is more positive than either of the Cd-based overlayers and accordingly the most promising for low overpotential operation in a tandem cell. It is important to remark that the shift of EFB brought by the MV treatments on CdSe and ZnSe (40 and 80 mV, respectively) is much less pronounced compared with CdS/MV. This is in contrast with the nearly identical improvement in EOC offered by the MV treatment and the superior photocurrent enhancement observed with the MV treatment in these cases (Figure 2a). These findings highlight the aforementioned alternative role of MV as mediator in charge transfer. Further insights into this aspect can be gained by examining the slope of the photocurrent onset as seen in Figures 4 and Figure S13 in the SI. Indeed, in the absence of variation in photon absorbance or carrier transport in the electrode active layer, the slope of the photocurrent onset is inversely related to the charge-transfer resistance at the electrode−electrolyte interface.48 Thus, the larger observed value of the slope after MV treatment, especially for CdSe, suggests a more facile electron extraction consistent with the view that MV acts also as a charge-transfer mediator to facilitate charge extraction. While it is difficult to quantify the improvement offered by both the shift in the EFB and the charge-transfer mediation offered by the MV treatment, given the smaller observed EFB shift but the greater improvement of the photocurrent afforded by the MV treatment on CdSe and ZnSe as compared with CdS, we can conclude that both effects are measurably important to the overall observed performance enhancement. In fact, it is particularly illustrative that despite displaying the smallest EFB shift, the CdSe-modified samples give rise to the better performance at more positive potentials, which is clearly accounted for by the higher jph versus E slope (Figure 4). It is worth stressing that while the MV treatment significantly enhances the output parameters of the electrodes when combined with the buffer layer the effect is virtually negligible when applied to the bare CZTS. In the absence of a buffer layer, the MV, that is, he adsorbed redox species, can act as a recombination center. However, the introduction of the buffer layer in-between the CZTS and MV precludes the holes

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ASSOCIATED CONTENT

* Supporting Information S

Detailed experimental procedures. Additional morphological, compositional and photoelectrochemical characterization of CZTS thin films, before and after surface modification. Estimate of energy bands position. Figures S1−S14 as described in the main text. This material is available free of charge via the Internet at http://pubs.acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: kevin.sivula@epfl.ch. 3906



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Notes

and Cadmium Selenide: Efficient Photoelectrochemical Oxidation to the Disulfide and Flat-Band Potential of the Semiconductor As a Function of Adsorbate Concentration. J. Am. Chem. Soc. 1986, 108, 3570−3577. (17) Hilal, H. S.; Turner, J. A. Controlling Charge-Transfer Processes at Semiconductor/Liquid Junctions. Electrochim. Acta 2006, 51, 6487− 6497. (18) Zhou, H.; Duan, H.-S.; Yang, W.; Chen, Q.; Hsu, C.-J.; Hsu, W.C.; Chen, C.-C.; Yang, Y. Facile Single-Component Precursor for Cu2ZnSnS4 with Enhanced Phase and Composition Controllability. Energy Environ. Sci. 2014, 7, 998−1005. (19) Kameyama, T.; Osaki, T.; Okazaki, K.-i.; Shibayama, T.; Kudo, A.; Kuwabata, S.; Torimoto, T. Preparation and Photoelectrochemical Properties of Densely Immobilized Cu2ZnSnS4 Nanoparticle Films. J. Mater. Chem. 2010, 20, 5319−5324. (20) Riha, S. C.; Fredrick, S. J.; Sambur, J. B.; Liu, Y.; Prieto, A. L.; Parkinson, B. A. Photoelectrochemical Characterization of Nanocrystalline Thin-Film Cu2ZnSnS4 Photocathodes. ACS Appl. Mater. Interfaces 2011, 3, 58−66. (21) Scragg, J. J.; Dale, P. J.; Peter, L. M.; Zoppi, G.; Forbes, I. New Routes to Sustainable Photovoltaics: Evaluation of Cu2ZnSnS4 as an Alternative Absorber Material. Phys. Status Solidi B 2008, 245, 1772− 1778. (22) Pawar, S. M.; Pawar, B. S.; Moholkar, A. V.; Choi, D. S.; Yun, J. H.; Moon, J. H.; Kolekar, S. S.; Kim, J. H. Single Step Electrosynthesis of Cu2ZnSnS4(CZTS) Thin Films for Solar Cell Application. Electrochim. Acta 2010, 55, 4057−4061. (23) Shen, Q.; Kobayashi, J.; Diguna, L. J.; Toyoda, T. Effect of ZnS Coating on the Photovoltaic Properties of CdSe Quantum DotSensitized Solar Cells. J. Appl. Phys. 2008, 103, 084304. (24) Guijarro, N.; Lana-Villarreal, T.; Mora-Seró, I. n.; Bisquert, J.; Gómez, R. CdSe Quantum Dot-Sensitized TiO2 Electrodes: Effect of Quantum Dot Coverage and Mode of Attachment. J. Phys. Chem. C 2009, 113, 4208−4214. (25) Rühle, S.; Greenshtein, M.; Chen, S. G.; Merson, A.; Pizem, H.; Sukenik, C. S.; Cahen, D.; Zaban, A. Molecular Adjustment of the Electronic Properties of Nanoporous Electrodes in Dye-Sensitized Solar Cells. J. Phys. Chem. B 2005, 109, 18907−18913. (26) In bulk semiconductors, this behavior can be ascribed to the widening of the depletion layer under reverse bias conditions (i.e., increasing the electric field in the semiconductor near the liquid junction) as the potential is shifted, which enhances a photogenerated charge separation and collection. However, in the case of NC thin films, a depletion layer cannot be borne within the “nanosized” particles, which are smaller than the space-charge region and effectively screened by ions in solution. In this scenario, a more negative applied potential corresponds to a larger driving force for hole collection, which, in turn, reduces the electron−hole pair recombination and ameliorates the electron extraction. (27) Bao, W.; Ichimura, M. First-Principles Study on Influences of Crystal Structure and Orientation on Band Offsets at the CdS/ Cu2ZnSnS4 Interface. Int. J. Photoenergy 2012, 2012, 619812. (28) Bär , M.; Schubert, B.-A.; Marsen, B.; Wilks, R. G.; Pookpanratana, S.; Blum, M.; Krause, S.; Unold, T.; Yang, W.; Weinhardt, L.; Heske, C.; Schock, H.-W. Cliff-Like Conduction Band Offset and KCN-Induced Recombination Barrier Enhancement at the CdS/Cu2ZnSnS4 Thin-Film Solar Cell Heterojunction. Appl. Phys. Lett. 2011, 99, 222105. (29) Santoni, A.; Biccari, F.; Malerba, C.; Valentini, M.; Chierchia, R.; Mittiga, A. Valence Band Offset at the CdS/Cu2ZnSnS4 Interface Probed by X-ray Photoelectron Spectroscopy. J. Phys. D: Appl. Phys. 2013, 46, 175101. (30) Nozik, A. J.; Memming, R. Physical Chemistry of Semiconductor−Liquid Interfaces. J. Phys. Chem. 1996, 100, 13061−13078. (31) Marcus, R. A. on the Theory of Oxidation-Reduction Reactions Involving Electron Transfer I. J. Chem. Phys. 1956, 24, 966−978. (32) Canovas, E.; Moll, P.; Jensen, S. A.; Gao, Y.; Houtepen, A. J.; Siebbeles, L. D.; Kinge, S.; Bonn, M. Size-Dependent Electron

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS N.G. thanks the European Commission’s Framework Project 7 for the financial support through a Marie-Curie Intra-European Fellowship (COCHALPEC, Project 326919). We thank the CIME (Prof. C. Hebert) and the LPMC (Prof. R. Gaal) at EPFL for assistance in electron microscopy and Raman spectroscopy, respectfully. We also thank X. Yu for the help with the TEM measurements.

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Enhancing the Charge Separation in Nanocrystalline Cu2

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