CdS Nanowire Architecture for Improved

Nov 15, 2013 - Since 1-D architectures are expected to offer better charge transport, .... layer of CdSe and CdS offers the best combination to achiev...
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Sequentially Layered CdSe/CdS Nanowire Architecture for Improved Nanowire Solar Cell Performance Hyunbong Choi,† James G. Radich,‡ and Prashant V. Kamat*,†,‡ Notre Dame Radiation Laboratory, †Department of Chemistry and Biochemistry, and ‡Department of Chemical & Biomolecular Engineering, University of Notre Dame, Notre Dame, Indiana 46556, United States S Supporting Information *

ABSTRACT: The power conversion efficiency of semiconductor nanowire (NW) based solar cells as compared to quantum dot solar cell (QDSC) has remained lower, and efforts to improve the photovoltaic performance of semiconductor NWs continue. We have now succeeded in using a layered architecture of CdS and CdSe NWs for improving the photovoltaic performance of nanowire solar cell (NWSC). The photoanode designed with sequentially deposited films of CdSe and CdS NWs delivered a power conversion efficiency of 1%. This efficiency of CdSe/CdS composite is an order of magnitude improvement over single nanowire system (CdS or CdSe) based solar cell. The improvement seen in the CdSe/CdS composite film is attributed to charge rectification and improvement of electron and hole separation and transport in the opposite direction. Impedance spectroscopy demonstrates the beneficial effect of type II structure in CdSe/CdS sequential deposition through lower transport resistance, which remains a dominating effect in dictating the overall performance of the NWSC.

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for solar cell development.20 Many interesting photophysical and photochemical properties of metal chalcogenide NWs have been extensively explored in recent years.31−34 Yet examples of II−VI semiconductor nanowires as being a major player in solar cells are relatively few as compared to the quantum dot based solar cells. The photovoltaic performance of NWSCs lags behind that of QDSC with the maximum reported photoconversion efficiency in the range of 1−3%, and challenges remain to overcome the limitations imposed by charge recombination processes.19,20,35−37 In a recent study we demonstrated that by tethering a hole scavenging carbazole to CdSe NWs, a significant improvement in the photovoltaic performance could be realized.20 One approach to maximize the photovoltaic performance of NWSCs is to couple two different semiconductor NWs so that they absorb different regions of incident photons and also offer charge rectification through the deployment of type II band structure. Similar approaches of coupling CdS and CdSe semiconductors has proven to be useful toward improving charge separation in QDSC.38−43 In a recent study Lee et al. reported improved charge separation in CdSe/CdS based QDSC achieved through the attainment of stepwise band-edge levels.42,43 In addition, capping the CdS or CdSe quantum dots with a thin ZnS layer suppresses electron recombination with the redox couple at the electrolyte interface.44−46 Although such strategies have assisted in improving the photovoltaic

INTRODUCTION The ability to tune the photoresponse of semiconductor nanostructures by size and surface manipulation offers new ways to design next generation solar cells.1−12 Quantum dot sensitized solar cells (QDSC) have received a lot of attention in recent years, and power conversion efficiency greater than 6% has been attained.13 The recent surge in the photovoltaic efficiency of perovskite based solar cell further rejuvenated interest in designing extremely thin absorber (ETA) solar cells.14−18 One-dimensional semiconductor nanostructures, viz. CdSe nanowires, have also been considered as candidates for solar cells by our group and others.19−23 The direct band gap (∼1.74 eV at 300 K), higher absorption cross section, visiblelight absorption, and the ability to undergo charge separation make CdSe nanowires an excellent choice for the design of next-generation solar cells.24−29The one-dimensional architecture allows tuning of the effective band gap by varying the radius and the length of the wire through quantum confinement effects.24 Semiconductor quantum dots, because of their small dimensions, are unable to sustain long-lived charge separation. Type II semiconductor heterostructures (e.g., TiO2/CdSe) with matching band energies that enable better charge separation are often required to attain better charge separation in QDSC. Since 1-D architectures are expected to offer better charge transport, it should be possible to employ them directly in NWSC.30 Thus, the semiconductor NWs deposited on a transparent electrode can serve as both photoabsorber and charge transporter in an interconnected NW assembly. Using this approach, we recently reported CdSe based NW electrodes © 2013 American Chemical Society

Received: October 15, 2013 Revised: November 11, 2013 Published: November 15, 2013 206

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performance in liquid junction QDSC,11,12,47,48 little effort has been made toward combining two different NW architecures in solar cells. Scheme 1 shows sequentially deposited layers of CdSe and CdS NWs on conducting glass electrode and the schematic

electron microscopy images of the NWs employed in the present study are shown in Figures 1A and 1B. Lowmagnification images show thin, straight NWs with lengths exceeding 2 μm. The high-resolution image shows that the wires are crystalline with a mean diameter of 15 nm. When the NWs are deposited as films on OTE, they can directly absorb incident photons and induce charge separation. We deposited CdS and CdSe nanowires onto the OTE surface using electrophoretic deposition (EPD). Unlike the spin-cast or drop-cast methods, EPD offers the convenience of depositing semiconductor nanowire films with controlled thickness in either single or multiple layers. Two OTEs were inserted in a 1 cm cuvette containing NW suspension in acetonitrile/toluene (2:1 v/v). The electrodes were separated by 0.5 cm, and a dc electric field (60 V) was applied. The negatively charged NWs are driven toward the electrode connected to the positive terminal. The asymmetric charging of nanoparticles and NWs and the polar solvent in the immediate vicinity render them negatively charged.49−51 Using this approach, we were able to deposit CdS and CdSe NW films onto the OTE surface. For bilayer films we sequentially deposited the two NWs with CdS NWs followed by CdSe NWs to obtain OTE/CdS/CdSe and CdSe NWs followed by CdS to obtain OTE/CdSe/CdS electrodes. Images of OTE/CdS, OTE/CdSe, OTE/CdS/CdSe, and OTE/CdSe/CdS electrodes were acquired using scanning electron microscopy are shown in Figures 1C−F, respectively. The distinction between the morphology of CdS NWs and CdSe NWs is evident in these images. For example, the images in Figures 1C,F show similar surface morphology since the top layer is made up of CdS NWs. Similarly, images in Figures 1D,E closely resemble since the top layer is made up of CdSe NWs. CdSe NWs appear more aggregated after EPD onto OTE. In all these cases we observe a closely woven network of NWs that serves as the backbone of electrophoretically deposited films on OTE. Similar morphology was also noted in NW films with different thicknesses, where film thickness is dependent on deposition time.

Scheme 1. Electron Transfer Process in CdSe NWs/CdS NWs

representation of type II band energy structure. The band energy alignment in such a coupled NW architecture is expected to facilitate charge separation. In order to elucidate the charge transfer dynamics of such composite semiconductor NW films and their role in maximizing photovoltaic performance, we have now carried out sequential deposition of CdS and CdSe nanowires on optically transparent conducting glass electrodes (OTE). The results of photovoltaic and electrochemical impedance spectroscopy (EIS) experiments using both OTE/CdSe NWs/CdS NWs and OTE/CdS NWs/CdSe NWs assemblies demonstrate the beneficial effect of charge rectification in the NWSC through appropriate band alignment.

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RESULTS AND DISCUSSION Synthesis and Characterization of CdS NWs and CdSe NWs. CdS and CdSe nanowires were synthesized using a literature method.24 The high- and low-resolution transmission

Figure 1. TEM images of (A) CdS NWs and (B) CdSe NWs. Inset shows the HRTEM images. Inset scale bar is 5 nm. SEM images of (C) CdS NWs, (D) CdSe NWs, (E) CdS NWs/CdSe NWs, and (F) CdSe NWs/CdS NWs on OTE. 207

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Figure 2. (A) Absorption spectra of OTE/CdS NWs/CdSe NWs. Thickness of CdS was kept constant at 1.15 μm. Thickness of CdSe NW layer is (a) 0, (b) 140, (c) 275, (d) 410, and (e) 550 nm. (B) Absorption spectra of OTE/CdSe NWs/CdS NWs. Thickness of CdSe NW layer was set to ∼310 nm. Thickness of CdS NWs layer is (a) 0, (b) 325, (c) 650, (d) 975, and (e) 1350 nm. Inset shows the cross-sectional SEM image.

Figure 3. (A) J−V spectra and (B) IPCE spectra of (a) CdSe NWs, (b) CdS NWs, (c) CdS NWs/CdSe NWs, (d) CdSe NWs/CdS NWs, and (e) CdSe NWs + CdS NWs. The thickness of nanowires is the same as in Table 1.

Absorption Characteristics of CdS NWs with CdSe NWs. Figure 2 shows the absorpion spectra of two sets of electrodes. In the first set we deposited CdS NWs layer of thickness 1.15 μm followed by the deposition of CdSe NWs layer of varying thickness. This electrode is referred as OTE/ CdS/CdSe. In the second set of electrodes the ordering of layers was reversed (maintaining thickness of the first layer of CdSe NW constant at 300 nm and varying the thickness of CdS NWs layer) to obtain the OTE/CdSe/CdS electrode. The absorption spectra of OTE/CdS/CdSe electrodes in Figure 2A show dominance of CdS NWs absorption below 510 nm corresponding to the bandgap of ∼2.4 eV with a shoulder around 460 nm. The CdSe NWs exhibit absorption extending into the red region with an onset at 720 nm. Collectively, both CdS and CdSe NWs exhibit rather broad absorption in the entire visible region. The increasing absorption in the red region represents increasing thickness of CdSe NWs. The absorption spectra of OTE/CdSe/CdS electrode in Figure 2B are dominated by the absorption of the first CdSe NWs layer. The increasing thickness of CdS NWs increases electrode absorbance at wavelngths below 500 nm. The insets show the cross-sectional SEM image of the two electrodes with CdS NWs and CdSe NWs layers in two different configurations. The two layers are clearly distinguishable, and a mapping analysis of

the cross section allows us to estimate the thickness of each layers. Although the spectral features of OTE/CdS/CdSe and OTE/CdSe/CdS in Figures 2A,B appear to be similar, the first layer, viz. CdS NWs in OTE/CdS/CdSe or CdSe NWs in OTE/CdSe/CdS, absorbs most of the incident photons when excited through the transparent side of the electrode. Any fraction of photons that are not absorbed by the first layer are subsequently absorbed by the second layer. In order to evaluate the synergy between the two semiconductor nanowires following bandgap excitation, we constructed solar cells with photoanodes of the two different configurations. A set of electrodes were also prepared by electrodepositing a premixed suspension of CdS and CdSe NWs. This electrode is referred to as OTE/(CdSe + CdS). The SEM image confirmed the presence of integrated NWs of two semiconductors. Under these deposition conditions both CdS and CdSe NWs are dispersed throughout the film and allow close interaction between two nanowires. Both CdS and CdSe nanowires within the premixed NW film absorb incident photons uniformly throughout the visible region, in contrast to the sequential absorption by CdS and CdSe NWs electrode with layered structure. SEM and absorption characteristics of the OTE/ (CdSe + CdS) electrode are provided in Figure S1. 208

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increased power converson efficiency of η = 1.08% for OTE/ CdSe/CdS cell and η = 0.38% for OTE/CdS/CdSe photoanode. It is interesting to note that the superior performance was seen when the CdS NWs were deposited onto the CdSe NWs. We also compared the photovoltaic performance of a film deposited from premixed CdS + CdSe, which exhibited a photoconversion efficiency of 0.12%. Comparison of these photoconversion efficiencies shows that the extended light absorption by the two semiconductors in the composite is not the primary reason for the improvement in CdS/CdSe NW layered film. The ordered layer of CdS and CdSe seems to be an important factor in achieving higher photoconversion efficiency. The photoreponse of the NWSCs was recorded in terms of incident photon to carrier generation efficiency (IPCE) at different incident light wavelengths and are shown in Figure 3B. The maximum IPCE for the solar cell employing CdSe NWs alone corresponds to about 8% and matches the absorption band of the nanowires. The CdS NWs photoanode responds only below 550 nm with maximum IPCE of 20%. The electrodes modified with CdS NWs and CdSe NWs bilayers show two maxima around 480 and 640 nm. These photoresponse features confirm the participation of both the CdS and CdSe NWs to the overall photocurrent generation process in the layered films. However, the maximum efficiencies of these composites vary from 10% to 30%. The NWSC employing OTE/CdSe/CdS photoanode exhibits photoresponse with maximum IPCE of ∼30% around 500 nm. It should be noted that the absorbance of the three NSWCs containing both CdS and CdSe was matched at 480 nm (Abs ∼ 1.74). These observations indicate that the difference in IPCE cannot solely be attributed to varying degree of light absorption by the CdS and CdSe NWs. As discussed in the J−V characteristics section, the ordered layer of CdSe and CdS offers the best combination to achieve improvement in charge separation and hence the collection of photogenerated electrons and holes by the OTE and electrolyte, respectively. If indeed the layered structure is responsible for controlling the overall photovoltaic performance of bilayer NWSC, we should be able to observe the effect of NW layer thickness on the overall photovoltaic performance. A set of photoanodes consisting of OTE/CdS/CdSe and OTE/CdSe/CdS were prepared by varying the thickness of the second NW layer while maintaining the first layer (closest to OTE) constant. Figure 4 shows the dependence of solar cell parameters (short-circuit

Photovoltaic Performance of NWSC. The light energy harvesting aspects of NWSC were evaluated using a liquid junction photoelectrochemical cell with NW film as the photoanode under AM 1.5 simulated solar illumination. The counter electrode was Cu2S/RGO,52 and the electrolyte consisted of 0.1 M Na2S, 0.1 M S in deionized H2O. The J− V characteristics of the NWSC employing five different photoanodes (CdSe NWs, CdS NWs, CdS NWs/CdSe NWs, and CdSe NWs/CdS NWs and premixed CdSe NWs + CdS NWs films deposited via EPD on OTE) are presented in Figure 3A. The typical active area of the unmasked photoanode was 0.18 cm2. The short-circuit current density (Jsc), open-circuit voltage (Voc), fill factor (f f), and overall conversion efficiency (η) of CdSe NWs are summarized in Table 1. For a single layer Table 1. Comparison of NWSC Performance Parametersa support/nanowire

Jsc (mA cm−2)

Voc (V)

ff

η (%)

CdSe NWs CdS NWs CdS NWs/CdSe NWSb CdSe NWs/CdS NWsc CdSe NWs+CdS NWsd

1.00 1.04 2.00 4.34 1.16

0.393 0.510 0.477 0.507 0.298

0.34 0.37 0.38 0.49 0.34

0.13 0.20 0.36 1.08 0.12

a Performance of NWSCs was measured with 0.18 cm2 working area under AM 1.5 illumination. Electrolyte: 0.1 M Na2S and 0.1 M S in deionized H2O. f f and η correspond to fill factor and power conversion efficiency, respectively. The thickness of CdSe NWs and CdS NWs are ∼310 and ∼1100 nm, respectively. bThe thickness of CdS NWs/CdSe NWs is 1100 nm/274 nm. cThe thickness of CdSe NWs/CdS NWs is 310 nm/975 nm. dThe weight ratio of CdSe NWs+CdS NWs is 1:1.

NWSC we observe similar photocurrent of 1.0 mA/cm2. However, the photovoltages (393 mV for CdSe and 510 mV for CdS NWSC) and photoconversion efficiencies (0.13% for CdSe and 0.20% for CdS NWSC) highlight the difference between the two semiconductor NWs. Despite the broader absorption of CdSe NWs, its performance remains slightly lower than that of CdS NWs. The low efficiency of semiconductor NWSC has been discussed in our earlier study.20 The two composite electrodes OTE/CdSe/CdS and OTE/ CdS/CdSe exhibited short-circuit current density that is greater than single nanowire cells. The Jsc of NWSC employing OTE/ CdSe/CdS cell is 4 times greater than reverse ordered, viz. OTE/CdS/CdSe photoanode. Concurrently, we observe

Figure 4. Dependence of solar cell parameters on the thickness of CdS NWs (black line) and CdSe NWs (red line): (A) short-circuit current (Jsc), (B) open-circuit voltage (Voc), (C) fill factor ( f f), and (D) overall power conversion efficiency (η) under AM 1.5 illumination. The counter electrode was Cu2S/reduced graphene oxide with 0.1 M S/0.1 M Na2S in deionized water as electrolyte. For the OTE/CdSe/CdS cell, the thickness of CdSe NWs was set to 300 nm (red line). For the OTE/CdS/CdSe cell, the thickness of CdS NWs was set maintained at 1000 nm (black line). 209

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Figure 5. EIS spectra and corresponding resistance parameters for electron transport through the NWs and recombination at the electrode electrolyte interface. (A) and (B) depict spectra at varying bias potentials between Voc (measured under illumination) and Jsc for OTE/CdS/CdSe and OTE/CdSe/CdS electrodes. (C) and (D) demonstrate the increase in transport resistance, RTrans, and resistance to recombination at the electrode−electrolyte interface, RRec, when the OTE/CdS/CdSe sequence is compared to OTE/CdSe/CdS. The increase in RTrans is significantly higher in CdS/CdSe relative to RRec.

current density (Jsc), open-circuit voltage (Voc), fill factor (f f), and overall conversion efficiency (η)) on the thickeness of CdSe NWs and CdS NWs. In the case of OTE/CdS/CdSe photoanode, we kept the thickness of the CdS NWs layer to 1000 nm, and the thickness of CdSe NW layer was varied. Similarly for the OTE/CdSe/CdS electrode, the CdSe NW layer thickness was kept constant at 300 nm and the thickness of NW layer was varied. In both cases we see a comaparable trend of increasing photocurrent and photoconversion efficiency with increasing thickness of the second layer up to about 1 μm. At greater thicknesses we see a decreasing trend in these parameters. A slightly different scenario emerges for the Voc dependence on the thickness of the second NW layer. Increasing thickness of CdS NWs layer (in the case of OTE/CdSe/CdS) resulted in the increased Voc while increasing thickness of CdSe NWs layer (in the case of OTE/CdS/CdSe) caused a decrease in Voc. These results suggest that there is synergy between the two semiconductor layers, and the performance can be optimized by controling the thickness of two semiconductor NW layers. On the basis of the absorption features of the two semiconductor NWs, we would have expected OTE/CdS/ CdSe to perform better since it favors sequential excitation of both CdS NWs and CdSe NWs. (CdS NWs is excited first with blue/green light, and the filtered red light is absorbed by CdSe NWs layer.) In the case of OTE/CdSe/CdS the visible light is mostly absorbed by the CdSe NWs with only a fraction of incident light absorbed by the adjacent CdS NWs layer. Yet, the OTE/CdSe/CdS photoanode performance is superior compared to OTE/CdS/CdSe photoanode. These observations

further ascertain our assessment that the ordering of the two layers with proper band alignment is more important than the light absorption management. The CdSe NW layer close to the OTE is the preferred choice as it facilitates charge rectification in the CdSe/CdS bilayer electrode (Scheme 1). We further examined the electrochemical response of the electrodes using EIS to probe these aspects of charge rectification in type II band structure. Electron Transport and Recombination Properties in Layered Structure. EIS is a useful tool in probing the electron transport through solar cells.53 Early work on dye-sensitized mesoscopic TiO2 electrodes enabled the mechanistic understanding of electron transport through the porous TiO2 layer as well as interfacial recombination processes at the electrode− electrolyte interface. In this study EIS was employed to elucidate the charge transport characteristics of the layered NWSCs with different orientation of CdS and CdSe NW loading. The elimination of mesoscopic TiO2 transport layer in the NWSCs required a slightly alternate approach in modeling the electrode response. In the case of layered CdS and CdSe NWSCs, the lack of electron injection to metal oxide layer necessitates electron transport through the NW network to FTO contact where charges are collected. Figure 5 depicts the EIS spectra for the OTE/CdS/CdSe (A) and OTE/CdSe/CdS (B) measured at different forward bias potentials in the dark. At each bias potential the evolution of two convoluted semicircles appears in the spectrum, which we attribute to the transport resistance through the NWs (RTrans) and the resistance to recombination (RRec) at the electrode− electrolyte interface. The spectra demonstrate that CdSe NWs/ 210

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Fermi level (and thus recombination) at the photoanode, we conclude that the lower Voc in the OTE/CdS/CdSe orientation results from increased electron−hole recombination at the CdS/CdSe interface. EIS and J−V resistance analyses together demonstrate the lower Voc is a consequence of the charge injection barrier developed at the CdS/CdSe junction rather than increased electron recombination with oxidized electrolyte species. Thus, OTE/CdSe/CdS orientation enables charge rectification and improvement in collection of photogenerated charges.

CdS NWs ordering resulted in lower overall impedance at each bias potential. Fitting the spectra to an equivalent circuit model (Figure S3) allowed us to calculate resistance values for RTrans and Rrec. The resistance values are plotted as a function of bias potential in Figures 5C and 5D respectively for OTE/CdS/ CdSe and OTE/CdSe/CdS to clearly illustrate the effect of NW sequential ordering. The plots of Rrec and RTrans vs bias potential reveal competing processes of electron transport through NWs and electron recombination with oxidized species of the electrolyte in determining overall NWSC performance. Both resistance components are expected to decrease in response to more negative bias potential as the Fermi level of the electrode approaches the flat-band potential. The higher Fermi level facilitates both favorable electron injection into oxidized electrolyte species and higher conductivity of the semiconductor layers as the energy of the electrons approaches the conduction band. In the case of OTE/CdS/CdSe ordering we observed only a slightly higher resistance to electron recombination at the electrode−electrolyte interface at each potential evaluated. However, the values for RTrans were strikingly higher in the OTE/CdS/CdSe electrode as compared to OTE/CdSe/CdS, suggesting electron transport through OTE/CdS/CdSe dictates the relative photoelectrochemical performance of the NWSCs. The type II band alignment of OTE/CdSe/CdS depicted in Scheme 1 is much more conducive to transport of photogenerated electrons through the NW electrode than the reverse case whereby electron injection into CdS NWs from CdSe NWs is not energetically favored. This ordering of OTE/CdSe/CdS and development of type II band structure thus enables a low-resistance pathway for photogenerated electrons to traverse the absorber layer. Photoelectrochemical and impedance data can be qualitatively compared by analysis of J−V characteristics at the Jsc and Voc values. Shunt and series resistance in an ideal photodiode are proportional to the slope of the J−V curve at the Jsc and Voc, respectively. The shunt resistance is a measure of recombination through electron leakage from the transport pathway through the circuit. On the other hand, series resistance is a direct measure of the ohmic resistances exerted by the contacts and other interfacial junctions of the cell. The shunt (Rsh) and series (Rs) resistances can be approximated by calculating the derivative of the J−V curve at the Jsc and Voc points as shown in eqs 1 and 2.

⎡ dV ⎤ R sh = ⎢ ⎥ ⎣ dJ ⎦ J

(1)

⎡ dV ⎤ Rs = ⎢ ⎥ ⎣ dJ ⎦V

(2)

sc

oc

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CONCLUSION The solution processed CdS NWs and CdSe NWs offer the convenience of 1-D architectures. Better charge transport properties are expected to provide improvement in the photovoltaic performance. However, the solution processed semiconductor nanowires, unlike their counterpart, quantum dots, exhibit relatively poor efficiency. The sequential deposition strategy adopted for the design of NWSC shows some improvement in the power conversion efficiency, but not to the level of QDSC. The configuration of sequential ordering of the CdSe and CdS NWs is important as shown by the electron transport properties in the impedance spectral analysis. Whereas the CdS and CdSe nanowires have failed to deliver viable efficiencies for photovoltaic device configuration, they can still serve as building blocks in constructing photoactive anodes. The need for improved electron transport and absorption characteristics through the rational integration of multiple semiconductor NWs and or QDs is necessary to improve the photovoltaic performance. Efforts are currently underway to modify the NW surface with hole transport relays to minimize the charge recombination process. One such effort of utilizing surface bound squaraine dye as a hole acceptor/ transporter as well as sensitizer has recently been reported.54

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EXPERIMENTAL SECTION Materials. All reactions were carried out under a nitrogen atmosphere. Solvents were distilled from appropriate reagents. CdS NWs and CdSe NWs were synthesized using the procedure described in the literature. Electrophoretic Deposition. A known amount of nanowires suspension in toluene/acetonitrile (2/1, v/v) (0.1, 0.2, 0.3, and 0.4 mg/mL for 137, 274, 412, and 549 nm thickness of CdSe NWs, respectively, and 0.2, 0.3, 0.4, and 0.5 mg/mL for 325, 650, 975, and 1305 nm thickness of CdS NWs, respectively) was transferred to a 1 cm cuvette in which the FTO/ZnO and a counter electrode (another FTO) were kept at a distance of 4 mm. A direct current voltage (60 V/cm) was applied between the electrodes using a Fluke 415 power supply. The deposition of the film can be monitored visibly as the nanowires are driven to the electrode and the solution becomes colorless. Each layer was sequentially deposited in this way. The FTO/ZnO electrode coated with CdSe NWs and CdS NWs sequentially is referred as OTE/CdSe/CdS. Optical and Photoelectrochemical Measurements. All experiments were carried out at room temperature. Absorption spectra were measured with a Varian Cary 50-Bio UV−vis spectrophotometer. Transmission electron microscopy (TEM) of CdS NWs and CdSe NWs was performed on a Titan 80-300 (FEI Company, 300 kV). Scanning electron microscope (SEM) images were obtained using a Hitachi S-4500 FESEM. A Princeton Applied Research model PARSTAT 2263 was used

Analysis of the shunt resistances of the OTE/CdS/CdSe and OTE/CdSe/CdS NWSCs at Jsc shows nearly 1.6 times increase in Rsh in the OTE/CdS/CdSe cell. This increase in Rsh is in good agreement with the 1.5 times increase in Rrec in OTE/ CdS/CdSe cell calculated from the increase in Rrec measured at flat-band bias potentials. When the derivative of the J−V curve is evaluated at Voc, the value for Rs is ∼3 times higher in the OTE/CdS/CdSe cell orientation relative to OTE/CdSe/CdS compared to an average of 3.7 times higher from RTrans over all bias potentials. (Details for resistance calculations can be found in Table S1.) Furthermore, since Voc is proportional to the 211

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for recording J−V characteristics. A Newport Oriel QE Kit (QE-PV-SI) was used for measuring IPCE values. Solar Cell Fabrication. FTO glass plates (Pilkington TEC Glass-TEC 8, Solar 2.3 mm thickness) were cleaned in a detergent solution using an ultrasonic bath for 30 min and rinsed with water and ethanol. The ZnO compact layer was first deposited on the FTO glass substrate by the spin-coating method with 0.1 M zinc acetate and 0.1 M MEA in methanol. The nanowires were deposited on the FTO using the electrophoretic deposition technique. The counter electrode was prepared by doctor blading Cu2S and reduced graphene oxide (RGO) composite on FTO glass. 0.1 M sodium sulfide and 0.1 M sulfur dissolved in water was used as the liquid electrolyte. The nanowires deposited FTO electrode and Cu2S/ RGO counter electrode were assembled into a sealed sandwichtype cell by heating at 80 °C with a hot-melt ionomer film (Surlyn SX 1170-25, Solaronix) as a spacer between the electrodes. A drop of electrolyte solution (electrolyte of 0.1 M sodium sulfide and 0.1 M sulfur dissolved in water) was placed over a hole drilled in the counter electrode of the assembled cell and was driven into the cell via vacuum backfilling. Finally, the hole was sealed using additional Surlyn and a cover glass (0.1 mm thickness). Electrochemical Impedance Spectroscopy. Potentiostatic EIS spectra were obtained using Gamry PCI4750 potentiostat in the frequency range of 100 kHz−100 mHz with 10 mV rms perturbation in potential. A Gamry EChem Analyst was used to construct the equivalent circuit model and fit the corresponding data.

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ASSOCIATED CONTENT

S Supporting Information *

Absorption spectra of OTE/CdS+CdSe electrode obtained by premixing two nanowire solutions and its photoelectrochemical parameters; EIS equivalent circuit model and diode J−V analysis. 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: [email protected] (P.V.K.). Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The research described herein was supported by the University of Notre Dame SAPC program. P.V.K. acknowledges the support of by the Division of Chemical Sciences, Geosciences, and Biosciences, Office of Basic Energy Sciences, of the U.S. Department of Energy through Award DE-FC02-04ER15533. This is contribution NDRL 4986 from the Notre Dame Radiation Laboratory. J.R. acknowledges for the Eilers Sustainable Energy Fellowship provided by cSEND, University of Notre Dame.

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REFERENCES

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