CdSe Co-Sensitized Solar Cells Based on a New SnO2

Article pubs.acs.org/JPCC

CdS/CdSe Co-Sensitized Solar Cells Based on a New SnO2 Photoanode with a Three-Dimensionally Interconnected Ordered Porous Structure Junyan Xiao, Qingli Huang, Jing Xu, Chunhui Li, Guoping Chen, Yanhong Luo, Dongmei Li,* and Qingbo Meng* Key Laboratory for Renewable Energy (CAS), Beijing Key Laboratory for New Energy Materials and Devices, Institute of Physics, Chinese Academy of Sciences, Beijing 100190, China ABSTRACT: Highly ordered SnO2 inverse opal films with different thicknesses are prepared by our newly developed doctor-blading technique combined with liquid-phase deposition, which have been applied in CdS/CdSe co-sensitized solar cells for the first time. Up to 4.37% of light-to-electricity conversion efficiency with a high open-circuit photovoltage of 700 mV has been achieved under AM 1.5 (100 mW cm−2) illumination. A careful comparison of two SnO2 photoanodes (inverse opal structure and conventional disordered film composed of submicrometer particles) is performed by analysis of optical and photoelectrochemical properties and electrochemical impedance spectra. The comparison reveals that the highly ordered SnO2 inverse opal structure can effectively reduce the charge recombination and increase the open-circuit photovoltage, fill factor, and cell performance.

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INTRODUCTION In recent years, sensitized mesoscopic solar cells have received remarkable attention because of their cost-effectiveness and easy fabrication process.1 Inorganic semiconductor quantum dots (QDs), as a new kind of light harvesting materials, offer some advantages of high molar extinction coefficients, sizedependent bandgaps, photostability, and low cost. Thus, the sensitized solar cell based on QD sensitizers, so-called quantum dot-sensitized solar cells (QDSCs), has become one of the most popular research topics in the next generation of solar cells.2 The core part of QDSCs is a nanocrystalline wide bandgap semiconductor photoanode, which can provide a high surface area framework for QD deposition and acts as a photogenerated electron diffusion pathway to the conducting substrate as well. Nanocrystalline TiO2 photoanode has been widely used in QDSCs; the highest efficiencies reported in the past few years are in the range of 6∼7%.3−5 In fact, nanostructured SnO2 is an ideal substitutive material, its electron mobility and photostability are higher than those of TiO2, and it also has a more favorable conduction band edge, facilitating efficient charge injection and charge collection.6,7 However, the cell performance of SnO2-based QDSCs is still far from satisfactory; Jsc is higher, but Voc and FF are apparently lower (Voc < 500 mV, FF < 0.5) even after surface passivation treatment, mainly because of strong recombination reaction arising from low porosity and lack of material generality of SnO2 nanoparticle films.6−8 Therefore, developing new SnO2 photoanodic structures is crucial for enhancing the cell performance of SnO2-based QDSCs. Highly ordered, multiscale porous SnO2 materials are undoubtedly a good choice because of some fascinating textural © 2014 American Chemical Society

features, i.e., periodic pore distribution, tunable pore size and structure, large specific surface area, and so on. Typically, inverse replica materials derived from colloidal crystal templates, so-called “inverse opals”, possess some additional advantages, for example, an open and interconnected macropore structure, nanosized wall component, etc. An inverse opal structure as an optical element was first introduced into the photoanode of the sensitized solar cells by Mallouk and coworkers.9 The first QDSC based on TiO2 inverse opal structure was reported by Toyoda et al., which can present 3.5% efficiency with a high Voc of 710 mV.10,11 Currently, the inverse opal structure in the sensitized solar cells can be used as (1) the scattering layer on short wavelength photons because of defects in the photonic crystal layer, (2) back light reflection layer corresponding to the stop band of the photonic crystal because of the PC (or inverse opal) layer as a dielectric mirror, and (3) an absorption enhancement due to resonant modes.9 However, no SnO2 inverse opal structures have been employed in sensitized solar cells so far. A routine three-step fabrication method is usually adopted to obtain inverse opal structures: (1) prepare the template with close-packed, uniformly sized spheres; (2) fill the interstitial spaces with a fluid precursor; and (3) remove the template to afford a porous inverse replica. Each step is indispensable to producing high-quality inverse opal structures.12−17 Recently, a new blade-coating technique combined with new liquid-phase deposition (LPD) has been developed in our lab, which can provide good quality opal structure templates on a large scale, Received: December 5, 2013 Revised: January 24, 2014 Published: February 4, 2014 4007

dx.doi.org/10.1021/jp411922e | J. Phys. Chem. C 2014, 118, 4007−4015

The Journal of Physical Chemistry C

Article

and the fabrication procedure is time-saving.18 Herein, the SnO2 inverse opal (io-SnO2) photoanodes for CdS/CdSe cosensitized solar cells have been developed based on this method, as shown in Scheme 1. By optimization of the film

powder (50 mM) in an aqueous solution of Na2SO3 (125 mM) at 80 °C for 1 h; the reaction mixture was then filtered to remove the residual Se powder.18,19 The electrode substrate is fluorine-doped tin oxide conducting glass (FTO, Pilkington; thickness, 2.2 mm; sheet resistance, 14 Ω/square). Before use, FTO glass was first washed with a mild detergent, rinsed with distilled water several times, rinsed with ethanol in an ultrasonic bath, and finally dried under air stream. Preparation of SnO2 Inverse Opal Electrodes. SnO2 inverse opal electrodes were prepared according to our previous work with a slight modification.20 Briefly, monodisperse polystyrene (PS) microspheres (285 nm diameter) were synthesized by emulsifier-free emulsion polymerization then dispersed in ethanol/water mixture (5: 2 v/v) to afford the PS colloidal suspension with a concentration of 10 vol %. The PS opal templates on FTO glass were prepared by using doctorblading technique and the above PS colloidal suspension. Each coating layer was about 1 μm thick, and the template thickness was controlled by the coating times. Then the as-prepared PS opal templates were immersed in 0.1 M SnCl2·2H2O ethanol solution for 1 min and then dried at 80 °C for 15 min to improve the hydrophilicity of the films. SnO2 was filled into the templates by using our newly developed liquid-phase deposition (LPD) method in which 0.1 M SnF2 aqueous solution and lab-made CaO2·8H2O were adopted at 60 °C for 15 min.20 The PS templates were removed by calcination at 450 °C for 1 h, leaving behind io-SnO2 electrodes. Finally, the io-SnO2 electrodes were immersed in 40 mM TiCl4 aqueous solution at 70 °C for 40 min and then calcinated at 500 °C for 30 min.21,22 Preparation of SnO2 Microsphere Electrodes. For comparison, SnO2 microsphere electrodes were also prepared. The SnO2 microspheres were obtained according to the literature.23 Briefly, SnF2 aqueous solution was heated at 60 °C for 24 h while keeping the pH value to 3. The precipitated SnO2 microspheres were collected, washed with water, and dried in the air. The SnO2 microsphere slurry was obtained by mixing the SnO2 microspheres, ethyl cellulose, and α-terpineol by ball milling. SnO2 microsphere (m-SnO2) films were prepared by doctor-blading technique on FTO glass, dried at 80 °C, and then calcined at 450 °C for 30 min. As-prepared SnO2 microsphere electrodes were immersed in 40 mM TiCl4 aqueous solution at 70 °C for 40 min and were finally calcinated at 500 °C for 30 min. Preparation of CdS/CdSe Co-Sensitized Electrodes. CdS and CdSe QDs were assembled in sequence on the SnO2 photoanodes by chemical bath deposition (CBD) technique at 10 °C.24 CdS was deposited with an aqueous mixture solution of 20 mM CdCl2, 66 mM NH4Cl, 140 mM thiourea, and 230 mM ammonia for 1 h. After being washed with water completely, the CdS-sensitized SnO2 film was immersed in an aqueous solution of 26 mM CdSO4, 40 mM NTA, and 26 mM Na2SeSO3 for 4−12 h. Finally, the CdS/CdSe co-sensitized photoanodes were passivated with ZnS by twice dipping into 0.1 M Zn(CH3COO)2 and Na2S aqueous solution for 1 min alternately. Fabrication of CdS/CdSe Co-Sensitized Solar Cells. Insitu Cu2S counter electrode on brass foil was prepared according to the literature.25 The polysulfide electrolyte was composed of 1 M Na2S and 1 M S in water. The solar cell was fabricated by sandwiching a Cu2S counter electrode, a drop of polysulfide electrolyte, and CdS/CdSe co-sensitized photoanode (photoanode area, 0.28 cm2) using a Surlyn film as

Scheme 1. Preparation Steps of Inverse Opal SnO2-Based CdS/CdSe Co-sensitized Solar Cells: (a) Fabricating PS Opal Templates on FTO Glass by Doctor-Blading; (b) Filling PS Opal Templates by Liquid Phase Deposition; (c) Removing PS Templates by Calcination; (d) Sensitizing with CdS/CdSe QDs by Chemical Bath Deposition; (e) Assembling into Solar Cells

thickness and CdSe deposition time, up to 4.37% of light-toelectricity conversion efficiency has been achieved, higher than that of the known SnO2-based QDSCs as well as that of the TiO2 inverse opal-based QDSCs.7,11 Furthermore, a series of optical, photoelectrochemical, and electrochemical characterizations were investigated to reveal that this highly ordered SnO2 inverse opal structure can effectively reduce the interfacial charge recombination process, thus enabling cell performance that is better than conventional submicrometer SnO2 particle photoanodes.

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EXPERIMENTAL SECTION Materials. Sodium nitrilotriacetate (NTA) and selenium powder were from Alfa Aesar Chemicals; styrene, K2S2O8, SnCl2·2H2O, SnF2, CaCl2, H2O2, CdCl2·2.5H2O, NH4Cl, Na2S· 9H2O, CdSO4·8/3H2O, Zn(CH3COO)2·2H2O, Na2SO3, thiourea, ammonia, brass foil, and sulfur were from Sinopharm Chemical Reagent Co. Ltd. All the chemicals were used without further purification. All the solutions for the electrolyte and QD deposition were prepared by using Milli-Q high-purity water (Millipore Model RG). Na2SeSO3 was prepared by heating Se 4008



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the SnO2 microsphere film is also presented; the particle diameters are primarily in the range of 200−500 nm, and some aggregates are also observed (Figure 1c). When the two SnO2 films are deposited by CdS and CdSe QDs in sequence, the QDs are uniformly dispersed on the surface of SnO2 films without obvious aggregation, as shown in Figure 1b,d. To the CdS/CdSe QD-sensitized io-SnO2 film, the inverse opal framework can be well-maintained; however, the inverse opal wall becomes thicker. Furthermore, the morphology of the CdS/CdSe QD-sensitized film becomes rougher, but the pores in the film are not blocked in comparison with the bare SnO2 films, suggesting that the electrolyte can effectively penetrate into the photoanodes. The microstructure and crystal phase of CdSe-sensitized inverse opal SnO2 were further investigated by TEM.24 As shown in Figure 2a, the inverse opal SnO2 exhibits spheric shell

spacer. Under illumination, a mask with a window of 0.15 cm2 was clipped on the photoanode side to define the active area of the cell. Characterization of Electrodes and QDSCs. Film thicknesses were determined by a surface profiler (KLATencor). The morphologies of the films were obtained with scanning electron microscopy (SEM, FEI, XL30 S-FEG) and transmission electron microscopy (TEM, FEI Tecnai F20 Supertwin). Samples for TEM investigation were prepared by scrapping the CdSe-sensitized inverse opal SnO2 film from the FTO substrate and dispersing it in ethanol, followed by transferring several drops of the suspension solution onto a carbon-coated copper grid. The surface areas of io-SnO2 and mSnO2 films were obtained by using N2 adsorption−desorption method on an ASAP 2020 apparatus (Micromeritics). The transmittance and reflectance spectra of the photoanodes were obtained on a UV-2550 spectrophotometer with an integral sphere (Shimadzu). The cells were illuminated under 100 mW cm−2 (AM 1.5) (Oriel Solar Simulator 91192), and the incident light intensity was measured by a radiant power/energy meter (Oriel 70260). Photocurrent density−photovoltage (J−V) characteristics for the cells were recorded on a Princeton Applied Research Model 263A potentiostat−galvanostat. Incident-photon-to-current conversion efficiency (IPCE) was carried out by direct current (DC) method using a lab-made IPCE setup under 0.3−0.9 mW cm−2 monochromic light illumination without bias illumination.26 The electrochemical impedance spectroscopy (EIS) was characterized on a Zahner IM6e electrochemical workstation in the dark over different bias voltage (50−700 mV) in the frequency range of 105−0.1 Hz.21 The obtained impedance spectra were fitted with Zview software in terms of an appropriate equivalent circuit.

Figure 2. TEM (a) and HRTEM (b) images of sensitized inverse opal SnO2. The scale bars in panels (a) and (b) are 100 and 5 nm, respectively.

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structure, which are covered by CdSe QDs. In the highresolution TEM (HRTEM) image of Figure 2b, the wall thickness of the SnO2 spheric shell is estimated to be 20 nm, and distinct lattice fringes in the HRTEM image suggest good crystallinities of the SnO2 and CdSe. First, two sets of lattice spacings of 0.335 and 0.264 nm are found, corresponding to the lattice planes (110) and (101) of tetragonal cassiterite SnO2 (JCPDS No. 41-1445), respectively. Second, the CdSe clusters consist of crystal grains with the lattice spacing of 0.330 nm, corresponding to the (101) plane of hexagonal wurtzite CdSe (JPCDS No. 65-3415). For comparison, the SnO2 microsphere film was also prepared. To consider the possible effect of different starting materials on the cell performance, SnO2 microspheres were prepared by using SnF2 aqueous solution and reaction temperature (60 °C) that were the same as those used in the preparation of the inverse opal SnO2.11,27 The textural structures of io-SnO2 and m-SnO2 films were investigated by N2 adsorption and desorption. The BET surface areas of ioSnO2 and m-SnO2 films are 45.7 and 14.3 m2 g−1, respectively. In fact, under the same volume condition, the two electrodes are supposed to have similar surface areas because the mesoporous structure of io-SnO2 film was damaged when preparing the sample for the BET measurement. As we know, the perfect inverse opal occupies only 25.95% of its total volume; it is thus proposed that the density of the io-SnO2 film is about one-third the density of the m-SnO2 film. UV−Visible Spectra of SnO2 Electrodes with and without QD Sensitization. UV−visible reflectance and transmittance spectra were used to analyze the optical

RESULTS AND DISCUSSION Microstructure of SnO2 Inverse Opal Electrodes. Figure 1a presents the cross-sectional SEM image of the inverse opal structure SnO2 film with a thickness of about 2 μm. As can be seen, the io-SnO2 film is basically uniform and highly ordered, and the diameter of the SnO2 shells is estimated to be 260 nm.20 In addition, high-quality and hierarchical porous structures are also obtained. For comparison, the SEM image of

Figure 1. Cross-sectional SEM images of (a) bare io-SnO 2 photoelectrode and (b) CdS/CdSe co-sensitized io-SnO2 photoelectrode; top view SEM images of (c) bare m-SnO2 photoelectrode and (d) CdS/CdSe co-sensitized m-SnO2 photoelectrode. 4009



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bare film. Therefore, according to eq 1, the effective refractive index n increases while d and θ remain unchanged, leading to the longer reflection wavelength (the so-called red-shifted effect).28 Generally, the application of an inverse opal photoanode with reflection peak at 400 nm into conventional dye-sensitized solar cells is supposed not to help utilize more incident light.15,29 However, this red-shifted reflection peak position of the photoanode exactly matches the absorption range of CdSe QDs in CdS/CdSe co-sensitized solar cells.18,24 Besides, the two kinds of CdS/CdSe co-sensitized SnO2 films are found to exhibit very weak transmittance, whereas the ioSnO2 film can exhibit reflectance that is slightly stronger than that of m-SnO2 film in the short wavelength region (350−600 nm), as shown in Figure 3b. It is thus suggested that simply increasing the photoanodic thickness may not be beneficial for utilizing more incident light, which will be further confirmed by the following investigation. Cell Performance of QDSCs Based on SnO2 Inverse Opal Structure. When CdS/CdSe QD-decorated SnO2 photoanode is incorporated with polysulfide electrolyte and Cu2S counter electrode to give a sandwich-type cell, the influence of various CdSe deposition time on the photovoltaic performance is first determined. Here, the CdS deposition time is fixed at 1 h while the thickness of io-SnO2 electrode is kept at 3 μm. The J−V characteristic curves of QDSCs with different CdSe deposition time are shown in Figure 4a, and photovoltaic parameters (the short-circuit photocurrent density (Jsc), open-

properties of bare io-SnO2 and CdS/CdSe co-sensitized ioSnO2 photoanodes. The UV−visible reflectance and transmittance spectra of m-SnO2 and its CdS/CdSe co-sensitized mSnO2 films with thickness the same as that of the io-SnO2 photoelectrode are also presented in Figure 3. To reduce

Figure 3. UV−visible reflectance spectra (a) and transmittance spectra (b) of bare SnO2 films and CdS/CdSe co-sensitized SnO2 films with inverse opal structure and microsphere structure.

interference, an analogous sandwiched-type configuration is used for UV−vis measurement, consisting of a 4 μm SnO2 film deposited on the cover glass, pure water, and bare cover glass.18 The CBD time for CdS and CdSe QDs were adopted as 1 and 8 h, respectively. As shown in Figure 3a, before sensitization, both io-SnO2 and m-SnO2 films exhibit strong light scattering ability. For the io-SnO2 film, an apparent reflection peak is observed at the wavelength of 400 nm due to the characteristics of photonic crystal structure, which is different than the indiscriminate reflection of m-SnO2. Correspondingly, the CdS/CdSe cosensitized io-SnO2 film exhibits a red-shifted reflection peak at 630 nm. Here, the possible reason for this phenomenon is given according to the follwing Bragg equation: λ = 2nd cos θ

(1)

where n is the effective refractive index of io-SnO2 films (bare or CdS/CdSe co-sensitized film), d the {111} plane spacing of the fcc structure, and θ the angle between the vector of the incident light and the normal to the {111} plane. For io-SnO2 systems, λ represents the centered wavelength of the reflected light. For CdS/CdSe QD-covered io-SnO2 film, its refractive index is supposed to increase in comparison with that of the

Figure 4. Photocurrent−photovoltage (J−V) characteristic curves of (a) 3 μm io-SnO2 QDSCs with different CdSe growth time and (b) ioSnO2-based QDSCs with different thicknesses under 8h CdSe deposition. 4010



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circuit voltage (Voc), fill factor (FF), and efficiency (η)) are presented in Table 1. As we can see in Figure 4a and Table 1, Table 1. Parameters of Photovoltaic Performances of 3 μm io-SnO2 CdS/CdSe QDSCs with Different CdSe Deposition Time CdSe deposition time (h)

Jsc (mA cm−2)

Voc (mV)

FF

η (%)

4 6 8 10 12

6.45 7.65 8.16 8.17 7.27

571 601 688 614 606

0.420 0.506 0.635 0.599 0.556

1.55 2.32 3.57 3.01 2.45

with increasing the CdSe deposition time from 4 to 6 to 8 h, the Jsc, Voc, FF, and η values of the QDSCs gradually increase, and an η value of 3.57% can be achieved when the CdSe deposition time is 8 h. However, when the the deposition time increases beyond 8 h, Jsc does not increase any more, whereas the Voc and FF values begin to decline, resulting in poor cell performance. Therefore, the best CdSe deposition time is 8 h for the CdS/CdSe co-sensitized io-SnO2 photoanodes and will be adopted in the following discussion. Under the optimal CdSe deposition time, the influence of ioSnO2 photoanode thickness on the cell performance is also investigated, as shown in Figure 4b and Table 2. As can be seen, Table 2. Photovoltaic Parameters for CdS/CdSe CoSensitized QDSCs Based on io-SnO2 Photoanodes with Different Thicknesses thickness (μm)

Jsc (mA cm−2)

Voc (mV)

FF

η (%)

1 2 3 4 5

5.92 7.15 8.16 10.13 9.91

666 675 688 700 663

0.606 0.624 0.635 0.616 0.521

2.39 3.01 3.57 4.37 3.43

when the film thickness increases from 1 to 4 μm, the cell performance gets better mainly due to the enhancement of the Jsc; however, their Voc values are quite similar, which is in good accordance with the other QDSCs based on highly ordered photoanodes.30 When the film thickness is 5 μm, the Jsc does not increase any more whereas the Voc and FF become poor, suggesting that a strong recombination reaction exists in the device. Comparison of Cell Performances with Different SnO2 Structures. To understand the influence of different SnO2 structures on cell performance, the IPCE spectra of two devices were investigated. As shown in Figure 5a, in the wavelength range of 370−600 nm, the IPCE value of io-SnO2-based QDSC is slightly lower than the m-SnO2 counterpart, which is in agreement with the slightly higher reflectivity of the inverse opal structure. However, in the wavelength range greater than 600 nm, the IPCE value of the inverse opal structure SnO2based QDSC is higher. Obviously, light absorption can be enhanced on either the blue or red side, which corresponds to the reflection peak of inverse opal structure SnO2 film (630 nm in the current case) due to the resonant modes. This result is in accordance with previous studies.9,31,32 According to IPCE spectra, the QDSCs with two SnO2 structures can present almost the same integral Jsc values, 9.70 and 9.66 mA cm−2 for io-SnO2 and m-SnO2 photoanodes, respectively, which are still

Figure 5. (a) IPCE spectra of QDSCs with different SnO 2 photoanodes; J−V characteristic curves (b) and dark current−voltage characteristic curves (c) of QDSCs based on different SnO 2 photoanodes.

lower than that of CdS/CdSe-based QDSCs based on ∼20 nm SnO2 nanoparticles because of a relatively narrow light absorption range and strong front reflection.7 Figure 5b presents the J−V characteristics of QDSCs based on io-SnO2 and m-SnO2 photoanodes with the same thickness (4 μm) and the same CdSe deposition time (8 h). In fact, there is no obvious difference between the measured Jsc of these two photoanodic structures, in good agreement with the integral Jsc values from IPCE measurements. However, compared to the m-SnO2 structure (Jsc = 9.64 mA cm−2, Voc = 489 mV, FF = 0.511, η = 2.41%), remarkable improvements in Voc and FF can be obtained in the case of inverse opal structure SnO2 photoanode, which results in an 80% increase in the conversion 4011



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efficiency. Here, the m-SnO2 and the io-SnO2 film from PS templates are suggested to have similar conduction band position; thus, the improvement in the photovoltage of SnO2based QDSCs is mainly assigned to the different microstructure of the SnO2 photoanodes. Further comparison in dark current−voltage characteristics of the two devices reveals that the dark current of the QDSCs based on m-SnO2 photoanode is much higher than that of the QDSCs based on io-SnO2 photoanode, as shown in Figure 5c. It is thus suggested that strong charge recombination between photoinjected electrons and Sx2− ions will bring about unsatisfactory FF for the m-SnO2 photoanode in comparison with io-SnO2 photoanode. Besides, for the SnO2 microsphere photoanode, the dark current onset occurs at a potential lower than that of the inverse opal photoanode, indicating that a relatively higher recombination rate occurs in the m-SnO2 structure, leading to a lower Voc, as shown in Figure 5b. Investigation of Charge-Transport and Recombination Processes. Under the identical S22−/S2− electrolyte and counter electrode, the charge transport and recombination processes are primarily determined by the photoanode microstructures. Currently, electrochemical impedance spectroscopy (EIS) is a useful tool to study the electron transport and charge recombination in the photoanodic film.33 Here, Nyquist plots of io-SnO2- and m-SnO2-based CdS/CdSe QDSCs were obtained under dark conditions at different biases. Figure 6a,b presents two representative plots at the applied forward biases of 250 and 500 mV for the two SnO2 photoanodes. As we can see, the obtained EIS spectra consist of two semiarcs in which the semiarc in the high-frequency range represents the charge-transfer behavior at the counter electrode−electrolyte interface while the semiarc in the lowfrequency range represents the chemical capacitance (CSnO2) of nanostructured SnO2 photoanode and the recombination resistance (Rrec‑SnO2) between the photoelectrode and polysulfide electrolyte.21 A good fitting to the above EIS data is obtained by using the equivalent circuit of a simplified transmission line model, in which transport resistance of the SnO2 photoanode is negligible, as shown in Figure 6c.7,21 Figure 7a presents the relationship between Rrec‑SnO2 and different applied biases (Vappl). In the applied bias range from 0 to 350 mV, similar Rrec‑SnO2 values of two SnO2-based CdS/ CdSe QDSCs are observed; however, Rrec‑SnO2 of io-SnO2-based CdS/CdSe QDSCs are obviously higher when the Vappl is larger than 400 mV. Good diode characteristics is thus deduced for the io-SnO2 photoanode.34 Therefore, a slower charge recombination rate is suggested in io-SnO2-based QDSCs, in good agreement with the dark current−voltage characteristics. As we know, in sensitized solar cells, the current crossing the sample at a certain potential can be calculated from eqs 2 and 3:35,36 i = Jsc −

∫0

V

dV RTotal

RTotal = R S,tot + R rec

Figure 6. Nyquist plots of io-SnO2- and m-SnO2-based CdS/CdSe QDSCs under dark conditionsat lower applied forward bias (250 mV) (a) and higher applied forward bias (500 mV) (b) and the equivalent circuit model for fitting the EIS (c). Inset: the abscissa expanded in high-frequency ranges. Scattered points, experimental data; solid line, fitted curves. Rs, series resistance; RCE, charge-transfer resistance at the counter electrode−electrolyte interface; Rrec‑SnO2, recombination resistance at the photoanode−electrolyte interface; CCE, double-layer capacitance at the counter electrode; CSnO2, chemical capacitance of SnO2 photoanode.

Voc =

∫0

Jsc

(R S,tot + R rec)di

(4)

For the io-SnO2- and m-SnO2-based QDSCs, their Jsc and RS,tot values are almost the same, whereas the Rrec‑SnO2 of io-SnO2 structure is significantly larger, thus resulting in higher Voc. This difference is in good agreement with the experimental Voc (700 mV versus 489 mV). According to a previous study, the dominant capacitance is the chemical capacitance (CSnO2) of photoanodes at high voltages.34 Here, the dependence of chemical capacitance (CSnO2) on different applied biases (Vappl) is presented in Figure 7b. This capatitance is the total chemical capacitances of the systems, induced by conduction band state, monoenergetic surface states, and bulk trap states in the SnO2 photoanode, as

(2) (3)

where Jsc is the short-circuit current and RS,tot is the total series resistance, which is the sum of Rs and RCE. Alternatively, the open-circuit voltage (Voc) can be obtained as follows: 4012



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Cμ(cb) = e 2

nc kBT

Cμ(trap) = e 2

NL exp[(E − EC)/kBTC] kBTC

(6)

(7)

where Cμ is the chemical capacitance per unit volume, e the absolute elementary charge, nc the concentration of free electrons in the conduction band (Ec), kB Boltzmann constant, T the temperature, NL the total volume density of the bulk trap states, and Tc a parameter with temperature units that determines the depth of the distribution below the lower edge of the Ec. Considering slight differences in SnO2 materials and QD sensitizers in these two systems, it is suggested that the io-SnO2 and m-SnO2 photoanodes have nearly the same nc, Ec, and NL, thus leading to similar Cμ at the same applied bias. One issue needs to be addressed: all the chemical capacitances in the above equations are chemical capacitances per unit volume (Cμ) instead of CSnO2. In other words, under the identical volumes, the io-SnO2 and m-SnO2 photoanodes have the same chemical capacitance (CSnO2). However, because of different densities of the two SnO2 photoanodes, the SnO2 volume in the m-SnO2 film is supposed to be ∼2 times greater than that in the io-SnO2 film under the same geometrical area and thickness conditions. Accordingly, the CSnO2 value of the m-SnO2 film is ∼2 times higher than that of the io-SnO2 film, in good agreement with the experimental results, as shown in Figure 7b. Because of this difference in CSnO2, a significant upward shift of the quasi-Fermi level in the io-SnO2 film is suggested in comparison with that in the m-SnO2 film when the same amount of photogenerated electrons inject into the conduction bands of two electrodes, thus resulting in a higher Voc.

CONCLUSIONS

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AUTHOR INFORMATION

SnO2 inverse opal structure was prepared by using a doctorblading technique combined with liquid-phase deposition and has been introduced into CdS/CdSe co-sensitized solar cells for the first time. By optimization of the film thickness and CdSe deposition time, the unique morphology can provide the conversion efficiency of 4.37% under AM 1.5 (100 mW cm−2) illumination with a high Voc of 700 mV. In addition, a detailed comparison between the inverse opal structure and conventional disordered film composed of submicrometer particles was investigated by optical and photoelectrochemical properties and electrochemical impedance spectra, suggesting that highly ordered SnO2 inverse opal structure can effectively reduce the charge recombination and increase the open-circuit photovoltage and fill factor. This work opens a new method for enhancing the application of the highly ordered SnO2 porous structures in various research fields such as solar cells, gas sensors, fuel cells, and Li-batteries.

Figure 7. (a) Charge-transfer resistances and (b) chemical capacitances (CSnO2) obtained from fitting EIS data of the io-SnO2and m-SnO2-based CdS/CdSe QDSCs at different applied biases; (c) electron energy diagrams representing the conduction band state (Ec) and bulk trap states in the m-SnO2 and io-SnO2 photoanodes. EF is the quasi-Fermi energy of the electrons in SnO2 photoanodes. Here, the Ec is assumed to be unchanged with respect to the redox level, Eredox.

shown in Figure 7c. In general, monoenergetic surface states in the SnO2 film are considered to be eliminated after TiCl4 treatment.21 Therefore, the chemical capacitance in our study is basically related to the conduction band state and bulk trap states of the SnO2 photoanode, which can be expressed as follows:37 Cμ = Cμ(cb) + Cμ(trap)

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Corresponding Authors

*Tel.: (86) 10-82649242. E-mail: [email protected]. *Tel.: (86) 10-82649242. E-mail: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors thank the financial support from the Natural Science Foundation of China (21173260, 51072221, 51372270, and 91233202), the Ministry of Science and Technology of China (973 Project, 2012CB932903 and 2012CB932904), Beijing Science and Technology Committee (Z131100006013003), and the Knowledge Innovation Program of the Chinese Academy of Sciences.

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