Significantly Enhanced Open Circuit Voltage and Fill Factor of

Dec 7, 2012 - Nano Science and Technology Program, The Hong Kong University of Science ... that of DSSCs, open circuit voltage (Voc) and fill factor (...
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Significantly Enhanced Open Circuit Voltage and Fill Factor of Quantum Dot Sensitized Solar Cells by Linker Seeding Chemical Bath Deposition Keyou Yan,†,‡ Wei Chen,‡,§ and Shihe Yang*,†,‡ †

Nano Science and Technology Program, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong ‡ Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong § Michael Grätzel Centre for Mesoscopic Solar Cells, Wuhan National Laboratory for Optoelectronics and College of Optoelectronic Science and Engineering, Huazhong University of Science and Technology, Wuhan 430074, P. R. China S Supporting Information *

ABSTRACT: We have significantly improved open circuit voltage and fill factor with a Pt counter electrode of quasi-solid state quantum dot sensitized solar cells (QDSSCs) by achieving compact coverage of QDs on a TiO2 matrix through a linker seeding chemical bath deposition process, leading to 4.23% power conversion efficiency, nearly two times that with conventionally deposited control photoanode. The distinguishing characteristic of our linker seeding synthesis is that it does not rely on surface adsorption of precursor ions directly on TiO2 (TiO2∼Cdx) but rather nucleates special ionic seeds on a compact linker layer (TiO2-COORS-Cdx), thereby resulting in a full and even coverage of QDs on the TiO2 surface in large area. We have shown that the compact coverage not only helps to suppress recombination from electrolyte but also gives rise to better charge transport through the QD layer. This linker seeding chemical bath deposition method is general and expected to reinforce the hope of quasi-solid state QDSSCs as a strong competitor of dye-sensitized solar cells after further optimization and development.

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linker-assisted adsorption (LA),5,11,22−24 and (3) successive ionic layer adsorption and reaction (SILAR).15,25−27 Methods 1 and 3 depend on direct surface adsorption of ions to in situ deposit CdSe QDs on TiO2, which often leads to less than full coverage of TiO2 and gives rise to recombination, resulting in Voc between 0.45 and 0.55 V and FF below 40% with a Pt counter electrode. Although method 2 introduces bifunctional linkers to tether QDs to TiO2, the relatively rigid and large QDs compared to organic molecules (dyes) make them difficult to translocate along the nanoscale channels to the TiO2 surfaces, leading to channel obstruction and thus imperfect QD anchorage on TiO2.28 Efforts have been expended to combine linker attachment with in situ synthesis of sizecontrollable QDs by admixing the linker molecules, Cd2+ and S2− precursors28 but thwarted by the incompatibility of the acidic environment involved in the linker attachment to TiO2 and the basic condition needed for the size-controlled synthesis and dispersion of QDs. Therefore, finding an effective and reliable method to achieve a full and even coverage of QDs

INTRODUCTION Quantum dots (QDs) can be engineered to have tunable band gaps across a wide energy range by facilely changing their size, shape, and composition, making them attractive for developing future generation solar cells.1−9 Quantum dot sensitized solar cells (QDSSCs) represent a type of easy-to-fabricate and costeffective solar cells, and with the possibility of hot electron extraction and multiexciton generation, they may deliver higher performance than dye sensitized solar cells (DSSCs).5,6,10−12 To date, however, the performance of QDSSCs (typically 1− 5%) still lags behind that of DSSCs (10−12%).4,13−17 Although the photocurrent of QDSSCs has already been comparable to that of DSSCs, open circuit voltage (Voc) and fill factor (FF) are still too low, especially with a Pt counter electrode (around 0.5 V Voc and below 40% FF, respectively) .18,19 This could arise from charge recombination owing to the nonperfect anchorage of the QDs on the TiO2 surfaces since it is wellknown that recombination events can generate dark current and lead directly to low FF and Voc.18 Therefore, it is vital to improve the coverage of QDs on TiO2 to suppress and block the recombination, thereby boosting Voc and FF. There are three main methods for anchoring QDs on TiO2: (1) In situ growth of QDs by chemical bath deposition (CBD),20,21 (2) deposition of presynthesized colloidal QDs by © XXXX American Chemical Society

Received: September 22, 2012 Revised: December 6, 2012

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Na2SeSO3 and 15 mL of 25 mM CdCl2 at about 30 °C under argon atmosphere on a shaker. To build the S-doped CdSe (CdSeS) QD photoanode, to the above mixture solution was added about 0.5 mL of 25 mM Na2S. Before LS-CBD, we also used SILAR to deposit CdS as seeds for the subsequent CBD (CdS seeded CBD) growth. Although it seemed to work well just from the color change, the resulting open-circuit voltage (Voc) and fill factor (FF) were inferior to those of the LS-CBD deposited due presumably to poor adhesion arising from the sheer surface contact, as alluded to in previous reports (see detailed discussion in Supporting Information).27,30 Solar Cell Fabrication and Characterization. The photoanodes were assembled into solar cells with Pt-sputtered FTO counter electrodes and polysulfide electrolyte containing 1 M Na2S and 0.25 M S gelled by 0.75 g polyethylene glycol (PEG) 2000000 by using a 60 μm thick adhesive tape (Scotch brand). The gel electrolyte was employed to improve the performance stability of the cells. The N719 sensitized solar cell was fabricated as previously reported.31,32 In brief, the mesoscopic films were immersed into a 0.5 mM N719 solution (1:1 acetonitrile:tert-butyl alcohol) for 24 h and then retrieved and rinsed with acetonitrile. These N719-loaded photoelectrodes were assembled into solar cells with acetonitrile/ valeronitrile (85:15, volume ratio) electrolyte containing 0.6 M 1,2-dimethyl-3-propylimidazolium iodide, 0.03 M I2, 0.1 M guanidinium thiocyanate, and 0.5 M 4-tert-butylpyridine. The light source (Oriel solar simulator, 450-W Xe lamp, AM 1.5 global filter) was calibrated to 1 sun (100 mW cm−2) using an optical power meter (Newport, model 1916-C) equipped with a Newport 818P thermopile detector. J−V characteristic curves and intensity modulated photocurrent/photovoltage spectroscopy (IMPS/IMVS) were measured by the Zahner controlled intensity modulated photoresponse spectroscopy (C-IMPS) system. Incident photon to current conversion efficiencies (IPCEs) and electrical impedance spectroscopy (EIS) were measured on the Zahner system. FTIR was measured on TENSOR 27 of BRUKER OPTICS, and UV diffuse reflectance spectra were carried out on the same film samples using a Perkin-Elmer UV−vis spectrophotometer (model Lambda 20). The photoanode film area for the QDSSCs and DSSCs performance test was typically 0.25 cm2 using a black paint coated Al foil as a mask. Morphologies of the nanomaterials and subsequent nanostructures were directly examined on a JEOL6700F scanning electron microscope at an accelerating voltage of 5 kV. TEM observations were carried out on a JEOL 2010F microscope operating at 200 kV.

onto a mesoscopic TiO2 network in large scale should make a crucial stride in developing high performance QDSSCs. Herein, we demonstrate a linker seeding chemical bath deposition (LS-CBD) method for in situ deposition of QDs onto TiO2 based linker tethered ionic seeds, which essentially makes the best of the advantages of the above three methods while avoiding their adversities after optimization of the procedures. On the basis of the LS-CBD-deposited photoanodes coupled with a Pt counter electrode and a quasi-solid state electrolyte, we have achieved 0.593 V Voc, 49.5% FF, yielding a PCE up to 4.23% of S-doped CdSe (CdSeS)sensitized solar cell in a series of systematic experiments. Remarkably, compared to the performance of a control cell deposited by the combination of SILAR and CBD, these represent ∼20% increase in Voc, ∼40% increase in FF, and ∼80% increase in PCE. The key innovation of this work is, instead of surface adsorption of precursor ions directly on TiO2 (TiO2∼Cdx), the deliberate nucleation of special ionic seeds (not colloidal seeds) tethered by a compact linker layer (TiO2COORS-Cdx), resulting in a full and even coverage of QDs on the TiO2 surface in large area. Our equivalent circuit analysis and dynamic studies show that such conformal coverage of QDs on TiO2 not only enhances charge transport through the QD layer but also helps to suppress interfacial recombination, accounting for the dramatically boosted cell performance.

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EXPERIMENTAL SECTION Photoanode Preparation. Mesoporous microspheres of TiO2 were synthesized by a combination of sol−gel processes and hydrothermal treatment. Briefly, 60 mL of DI water was dropwise added into 180 mL of ethanol solution containing 4 mL of tetrabutyl titanate and 4 mL of oleic acid under vigorous stirring. After 60 min of the sol−gel process, the precipitate was washed with ethanol, collected, and transferred to a 200-mL autoclave with 30 mL of DI water and 90 mL of ethanol. Then 2 mL of 25% ammonium solution was dropwise added into autoclave and maintained in an oven at 160 °C for 16 h. The precipitate was washed with ethanol, and then 0.3−0.4 g of ethyl cellulose and 5 mL of terpineol were added. The mixture was transferred into a 10-mL beaker and heated to 80 °C to remove the ethanol under stirring. Then the beaker was heated to about 130 °C for encouraging agglutination of the microspheres and some terpineol was added to compensate for the evaporation of terpineol. Finally, the doctor blade technique was employed to obtain 14−15 μm films. In situ QD Anchoring by LS-CBD. The LS-CBD method was employed for the in situ deposition of QDs onto TiO2. Although linker assisted procedures for anchoring presynthesized QDs and for in situ deposition of QDs with a solution mixture of Cd2+, S2−, and the linkers28,29 have been reported previously, we found that it did not work as well as expected (see Supporting Information). Hence, for full and even coverage of QDs onto the TiO2 surface, we modified the procedures by introducing a linker-seeding step. In brief, the films were immersed in 10 mM 3-mercaptopropionic acid for 3 h to anchor a monolayer of 3-mercaptopropionic acid molecules by −COO-TiO2 binding at 40 °C under argon atmosphere (HS-(CH2)2-COO-TiO2). The films were transferred into a 25 mM CdCl2 solution (water:ethanol = 1:10) of about 50 mL for several hours at 40 °C under argon atmosphere (Cdx-S-(CH2)2-COO-TiO2). The anchored Cd2+ would serve as seeds for the subsequent CdSe deposition. The CBD process was performed in a mixture of 15 mL of 25 mM

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RESULTS AND DISCUSSION We chose in this work agglutinate mesoporous TiO2 microspheres with diameters ranging from 200 to 500 nm, which were made up of nanocrystals ∼18 nm in diameter, to build photoanode for the QDSSCs.31,33 The agglutination of all the microspheres together to form a robust network was shown to be beneficial to significantly improving electron transport.26 Compared with traditional nanoparticulate photoanodes characterized by only the pores of ∼40 nm, the agglutinate mesoporous microspheres with hierarchical channels (containing both ∼100 nm primary pores and ∼27 nm secondary pores) should be more permeable to the ionic precursors so that the latter could support the formation of a full QDs layer on the mesoscopic TiO2 surfaces. The LS-CBD process is illustrated in Scheme S1 of the Supporting Information, and the deposition process was B

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evidenced by the observation that throughout the deposition process the color change in the precursor solution always lagged much behind that of the film. This is crucial to the full and even coverage of QDs on TiO2. For comparison, a control experiment was conducted using SILAR for CdS seeds and CBD for CdSeS, which is referred to as CdS-seeded CBD. Shown in parts a−c of Figure 2 are cross sectional views of a bare TiO2 photoanode, LS-CBD-deposited photoanode, and CdS-seeded CBD-deposited photoanode. With reference to the bare TiO2 photoanode in Figure 2a, it is seen that LS-CBD could indeed ensure full and even coverage of QDs from top to bottom on the microsphere films (see Figure 2b). However, CdS-seeded CBD could only give a full coverage of the CdSeS QDs in the top part of the film, whereas some uncovered microspheres morphology can still be recognized in the bottom part (see Figure 2c). Photographs of the LS-CBD-deposited CdSe on the agglutinate microsphere film are shown in Figure S1a of the Supporting Information. From both the front view and back view of the film, a very even and uniform QD coverage is evident in such a large area, and more importantly, both sides of the film are nearly the same in terms of color uniformity and intensity, reflecting the effectiveness of the LS-CBD process. The corresponding TEM image in Figure S1b of the Supporting Information and EDX profile in Figure S1c of the Supporting Information gives more details about the TiO2 mesoporous microsphere film after the CdSe deposition. In order to microscopically trace the spatial distribution of CdSe in the microspheres film, we resorted to the elementary mapping technique. One can see from Figure S1b of the Supporting Information that CdSe is uniformly distributed in a way that matches well the spatial contrast of the TEM image, in keeping with the full and even coverage of CdSe on the TiO2 surface accomplished by the LS-CBD technique. In contrast, the QD layer on the same TiO2 mesoporous microspheres film deposited by the direct-CBD method under identical conditions except for skipping the linker-seeding step appears unsightly in terms of the uneven color intensity on both sides and the lighter color on the back side than on the front side (see Figure S2c of the Supporting Information). As for the CdS-seeded CBD method (see experimental details in the Supporting Information), although it was easy to deposit dense QDs on TiO2 judging from the color intensity, the film uniformity was not as good as that of the LS-CBD-deposited

monitored by FTIR spectra shown in Figure 1. To start with, a monolayer of 3-mercaptopropionic acids was compactly and

Figure 1. FTIR spectra taken to monitor the linker-seeding process consisting of (1) linker anchoring on TiO2 (curve c) and (2) ionic seed tethering by the linker (curve d).

evenly adsorbed onto the TiO2 surface through chemical bonding to form TiO2-OOCR-SH. This can be seen from the FTIR spectrum of curve c, in which the peak of −COOH at 1718 cm−1 has disappeared and is replaced by the two peaks at 1556 and 1401 cm−1, indicating the formation of −RCOOTiO2. Pivotally, in step 2, ionic seeds of Cd2+ were compactly tethered onto the TiO2 surface forming TiO2-OOCR-S-Cdx, which is consistent with the complete disappearance of the −SH peak at 2569 cm−1 (curve d). Scheme 1 highlights the Scheme 1. Schematic diagrams highlighting the differences between linker-tethered seeds (left) and directly adsorbed seeds (right)

differences between linker-tethered and directly adsorbed seeds. Then the subsequent CBD could ensure preferential growth of QDs on the TiO2 surface rather than in solution. This was

Figure 2. (a) Cross-sectional scanning electron microscopy images of TiO2 microsphere photoanode, (b) TiO2 microsphere photoanode with LSCBD-deposited CdSeS QDs, and (c) TiO2 microsphere photoanode with CdS-seeded CBD-deposited CdSeS QDs. Scale bars: 5 μm. C

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Figure 3. (a) Diffuse reflectance spectra of the films of CdSe, CdSe-ZnS, CdSeS, CdSeS-ZnS, and N719 sensitized solar cells and (b) diffuse reflectance spectra of the photoanodes prepared by LS-CBD and CdS-seeded CBD.

Figure 4. (a) J−V curves of the CdSe, CdSe-ZnS, CdSeS, CdSeS-ZnS, and N719 sensitized solar cells. (b) IPCE spectra of the CdSe, CdSe-ZnS, CdSeS, CdSeS-ZnS, and N719 sensitized solar cells. (AM 1.5G simulated sunlight with a power density of 100 mW cm−2). (c) Performance comparison between LS-CBD and CdS-seeded CBD-deposited CdSeS-sensitized solar cells. (d) Comparison of IPCE spectra of LS-CBD and CdSseeded CBD-deposited CdSeS sensitized solar cells.

film simply from scanning electron microscopy (Figure 2c), which was employed as a control sample in this work. Before presenting the photovoltaic performance testing results of the QDSSCs, we first show the light harvesting capability of the LS-CBD-deposited photoanodes, namely, the photoanodes with the TiO2 microspheres being coated with (1) CdSe, (2) CdSeS (see Figure S3 of the Supporting Information), (3) CdSe-ZnS, and (4) CdSeS-ZnS, plus control photoanodes deposited by the CdS-seeded CBD and sensitized by N719 (DSSC). Figure 3 shows the absorption spectra of photoanodes of these solar cells (the dashed line is the baseline based on bare TiO2). First, different from that of the DSSC, the photoanodes of QDSSCs exhibit gradually decreasing absorption from the UV region to the onsets at around 740 nm but without sharp excitonic features commonly observed in colloidal QDs due probably to a comparatively broad size distribution of the QDs deposited by LS-CBD. Second, compared with the nondoped photoanodes, the absorption

spectra of the S-doped photoanodes of QDSSCs show a ∼15 nm blue shift because the incorporated CdS component has a wider band gap. Besides, the ZnS coating yielded an additional ∼10 nm red shift. A similar red-shift was observed in CdSe/ ZnS core/shell nanoparticles and ascribed to an increased leakage of the exciton into the shell.34 At last, from the absorption spectrum shown in Figure 3b, we can see that the photoanode sensitized by CdS-seeded CBD-deposited CdSeS has similar onsets and absorption intensity, indicating a similar loading level of the QDs and thus similar light harvest capability to that of the LS-CBD-deposited photoanode. The similar light harvest capabilities of the photoanodes make our comparative performance and dynamic studies to be detailed below a meaningful endeavor. Figure 4a displays the photocurrent (J) vs voltage (V) characteristics of the LS-CBD-prepared four QDSSCs with the PEG gel polysulfide electrolyte. The extracted parameters are tabularized in Table 1. The CdSe-sensitized solar cell registered D

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Table 1. Performance Parameters of the CdSe, CdSe-ZnS, CdSeS, CdSeS-ZnS, and N719 Sensitized Solar Cellsa solar cells CdSe CdSe-ZnS CdSeS CdSeS-ZnS N719 Control experiment a

Jsc (mA cm−2)

Voc (V) 0.585 0.596 0.577 0.593 0.764 0.492

± ± ± ± ± ±

0.015 0.021 0.018 0.011 0.035 0.029

8.22 13.03 11.11 14.40 14.39 13.59

± ± ± ± ± ±

FF (%)

0.25 0.33 0.31 0.23 0.41 0.45

50.9 49.6 47.8 49.5 68.7 35.1

± ± ± ± ± ±

2.5 1.9 2.3 2.1 3.2 3.1

PCE (%)

JIPCE (mA cm−2)

± ± ± ± ± ±

8.01 12.56 10.45 13.33 13.35 13.31

2.45 3.85 3.06 4.23 7.56 2.35

0.35 0.20 0.29 0.18 0.61 0.46

The error estimate was based on 5 freshly prepared solar cells.

Table 2. Performance Cross Comparison of CdS/CdSe-ZnS Sensitized Solar Cells with Control Cells and Cells from Other Work solar cells CdSeS-ZnS CdS/CdSeS-ZnS CdS/CdSe-ZnS CdS/CdSe-ZnS CdS/CdSe-ZnS CdS/CdSe-ZnS Mn-CdS/CdSe-ZnS CdS/CdSe-ZnS CdS/CdSe-ZnS

method/counter electrode/electrolyte 2−

2−

LS-CBD/Pt/gel S /S x SILAR-CBD/Pt/gel S2−/S2−x SILAR/RGO-Cu2S/liquid S2−/S2−x SILAR/RGO-Cu2S/liquid S2−/S2−x SILAR/Pt/liquid S2−/S2−x SILAR/Pt/liquid S2−/S2−x SILAR/RGO-Cu2S/liquid S2−/S2−x CBD/Cu2S/liquid S2−/S2−x SILAR/Cu2S/liquid S2−/S2−x

Voc (V)

Jsc (mA cm−2)

FF (%)

PCE (%)

reference

0.593 0.492 0.516 0.520 0.460 0.504 0.558 0.575 0.579

14.40 13.59 17.2 18.4 11.3 15.6 20.2 13.68 15.77

49.5 35.1 47 46 31 47 47 63 57

4.23 2.35 4.19 4.40 1.60 3.70 5.42 4.92 5.21

this work control experiment 37 36 36 27 37 38 47

a short-circuit current density (Jsc) of 8.21 mA cm−2, Voc of 0.585 V, and FF of 50.9%, leading to a PCE of 2.45%. Despite the blue shift for the CdSeS photoanodes, the corresponding cells acquired Jsc of 11.11 mA cm−2, Voc of 0.577 V, and FF of 47.8%, yielding a 3.06% PCE, thanks to the uplifting of the conduction band edge arising from the S-doping.35 Further improvement made by the ZnS coating, due to the suppression of charge recombination arising from the climb of the conduction band edge, led to a Jsc of 13.03 mA cm−2, Voc of 0.59 V, and FF of 49.6%, amounting to a 3.85% PCE with the CdSe-ZnS QD shell, and a further increase to Jsc = 14.40 mA cm−2, Voc = 0.593 V, FF = 49.5%, and PCE = 4.23% with the CdSeS-ZnS QD shell. The IPCEs as a function of wavelength for the QDSSCs are shown in Figure 4b. To cross check and validate our Jsc data, we have calculated the total photocurrent from the IPCE curves by JIPCE = ∫ λe × IPCE(Pλλ/h), and the resulting values are listed in Table 1. In general, the JIPCE values are remarkably consistent with Jsc values from the J−V measurement. For the sake of comparison, critical performance indexes of the LS-CBD-prepared QDSSCs are tabulated side by side in Table 2 with those of the cells obtained by other deposition methods. More specifically, parts c and d of Figure 4 compare directly the performance characteristics of CdSeS-ZnS sensitized solar cells deposited by LS-CBD and by CdS-seeded CBD. Note that these comparisons are based on the use of a common Pt counter electrode. At least two performance features of our QDSSCs fabricated by LS-CBD are noteworthy. First, the LS-CBD fabricated QDSSCs consistently display higher and more reproducible Voc values (close to 0.6 V) than those prepared by conventional methods including CdS-seeded CBD, which are typically scattered in the range of 0.46−0.56 V.19,27,36−38 Second, the FF at around 50% with a Pt counter electrode is much higher than those of similar cells prepared by other methods (FF below 40%29). As a clear case of control experiment, although the photocurrent of the CdS-seeded CBD-deposited cell is only slightly lower than that of the LSCBD-deposited cell, the corresponding Voc and FF are

dramatically lower (0.492 V and FF 35.1% vs 0.593 V and FF 49.5%), resulting in an overall PCE only about half that of the latter. In general, Voc and FF are related to the shunt resistance (Rsh) and series resistance (Rs) in the typical equivalent diode circuit model of solar cells, which can be estimated from the derivative of the J−V curves through the equation: R = −(dJ/ dV)−1. Such resistance-voltage (R−V) curves are shown in Figure 5 together with the equivalent diode circuit scheme.

Figure 5. R−V characteristics of equivalent typical diode circuit used for the LS-CBD (red) and CdS-seeded CBD (black) deposited CdSeS-ZnS sensitized solar cell. The inset schematizes the equivalent circuit.

Bear in mind that the resistances close to short circuit and open circuit of R−V curves reflect, respectively, the shunt resistance (Rsh) and series resistance (Rs) in the equivalent diode circuit diagram, which in turn are related to the charge recombination and transport characteristics. The most striking feature of the R−V curves is that for the LS-CBD prepared cell the Rsh is as high as ∼4 times that of the cell prepared by CdS-seeded CBD, whereas the Rs is only 1/3 of it, suggestive of much lower charge recombination and much faster charge transport. While the suppressed charge recombination can be easily accredited to the LS-CBD method owing to the more compact coverage of QDs on the TiO2 surface, the enhanced charge transport is E

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vs 0.45 s of LS-CBD). It is known that the chemical capacitance Cμ is directly related to the difference of ECB−EFn via the relation Cμ∝exp[−(ECB − EFn)/kBT].19 The larger Cμ value of LS-CBD (12 mF cm−2 vs 8 mF cm−2) means a larger downward displacement of the TiO2 conduction band edge which would lower Voc. Consequently, the improved Voc and FF can be definitely attributed to the suppressed recombination due to the better quality of the LS-CBD film. Next, the QDSSCs were subjected to the IMPS/IMVS44−46 spectroscopic interrogation to examine more directly the charge transport and recombination from the characteristic response frequencies (τt = 1/2πf t and τr = 1/2πf r) (see Supporting Information). We have converted the electron transport time (τt) to electron diffusion coefficient (Dn ≈ d2/2.35τt), which is more meaningful for comparison between different devices. The resulting Dn and lifetime (τr) of the QDSSCs as a function of Jsc (proportional to photogenerated electron density) are shown in parts a and b of Figure 7, respectively. As probably a more useful parameter to gauge charge collection capability that combines the charge transport and recombination properties, electron diffusion length is employed here, which can be determined from the formula Ln = (Dnτr)1/2 (see Figure 7c). In general, the data are in line with the commonly observed

more puzzling since the same TiO2 network was employed in both cases. Nevertheless, this can be reconciled with the fact that the semiconducting QD layer on TiO2 is actually involved in the charge transport especially under illumination, a scenario unfound in typical DSSCs. In other words, the compact QD layer deposited by LS-CBD contributes to the much smaller Rs values associated with the much faster electron transport along the QD layer network.39 To further confirm the improved quality of the QDsensitized photoanode films prepared by LS-CBD, we studied the dynamic properties of the photogenerated carriers in the films especially at interfaces through EIS in the dark. As an illustration, EIS features of the LS-CBD-deposited CdSeS-ZnS sensitized solar cell and the CdS-seeded CBD-deposited CdSeS-ZnS sensitized solar cell are compared in Figure 6.

Figure 6. EIS Nyquist plots for the CdSeS sensitized solar cells prepared by LS-CBD and CdS-seeded CBD at forward bias 0.6 V in the dark. Fitted parameters are: for CdS-seeded CBD, Rs 2.86 Ohm cm2, Ws 0.925 Ohm cm2, RCE 38.5 Ohm cm2, Rr 15.5 Ohm cm2, and Cμ 8 mF cm−2; for LS-CBD, Rs 3.69 Ohm cm2, Ws 0.425 Ohm cm2, RCE 53.5 Ohm cm2, Rr 37.5 Ohm cm2, and Cμ 12 mF cm−2. The contribution of electrolyte was not considered.

According to Bisquert theory,18,40 the EIS curves of the two solar cells were fitted in terms of the equivalent circuit depicted in the inset of Figure 6. In the equivalent circuit, the series resistance Rs accounts for sheet resistance of the electrodes, contact resistance, and the resistance between the electrodes; Warburg diffusion element (Ws) at the highest frequencies relates to electron transport in porous film;19,41,42 RCE reflects charge transfer at the counter electrode at frequencies higher than the recombination arc, and Rr is associated with charge recombination at medium frequencies.19,42,43 The LS-CBDprepared film gives a well-defined semicircle arc associated with RCE and a somewhat deformed semicircle arc related to Rr. However, the CdS seeded CBD-prepared film appears to have the Rr arc merged into the larger RCE arc. Nevertheless, both EIS curves can be well fitted by the equivalent circuit referred to above, which are quite compelling. First, we can see that electron transport in the LS-CBD film is twice as good as the CdS-seeded CBD sample (Ws: 0.425 Ohm cm2 of LS-CBD vs 0.925 Ohm cm2 of CdS seeded CBD), which gives evidence that the QD layer has contribution to electron transport, perhaps by forming an electron transport channel along QDs. Second, charge recombination is better blocked in LS-CBD samples from the larger Rr value (37.5 Ohm cm2 of LS-CBD vs 15.5 Ohm cm 2 of CdS seeded CBD). The obtained recombination time (τn = RrCμ) also supports the suppressed recombination in the LS-CBD film arising from the more compact assembly of QDs on TiO2 (0.12 s of CdS seeded CBD

Figure 7. (a) Electron diffusion coefficients, (b) electron lifetimes, and (c) electron diffusion length comparison between LS-CBD and CdSseeded CBD-deposited CdSeS sensitized solar cells. F

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nonlinear dependence of Dn, τr, and Ln on Jsc arising from the multiple trapping−detrapping events within an exponential distribution of conduction band tail states. The measured parameters are collected in Table 3. For our interest, we want

efficient charge collection of the former than that of the latter and thus a more efficient photon to electron quantum yield. One last point of practical interest is the direct stability comparison of the quasi-solid state QDSSCs with their liquid QDSSCs counterparts. For this purpose, a simple stability test of the unsealed cells was carried out under normal conditions (ambient temperature and humidity). The efficiency of the gelelectrolyte-based cell can make a stable Voc and FF superior to liquid electrolyte. This can be clearly seen in Figure 8. After 8 h of illumination, Voc and FF have both retained over 90% of the initial values with the gel electrolyte, whereas only 70% of the initial Voc and 40% of the initial FF have been left with the liquid electrolyte. Although the photocurrents decreased with both of the electrolytes, the gel electrolyte could keep about 55% of the initial PCE after 8 h of illumination, whereas the liquid electrolyte could only retain less than 10% of the initial PCE. This result highlighted the benefit of the quasi-solid state electrolyte, in which the solvent was made difficult to volatilize and leak due to the PEG gelation effect (see Figure S4 of the Supporting Information, the upside down placement can still suspend the stirring bar), leading to the significantly enhanced stability of the QDSSCs.

Table 3. Dynamic Parameters of the CdSe, CdSe-ZnS, CdSeS, CdSeS-ZnS, and N719 Sensitized Solar Cells and Control Experiment at a Given Jsc (1 mA cm−2) solar cells

Dn (10−5 cm2 s−1)

τr (s)

Ln (μm)

estimated error

CdSe CdSe-ZnS CdSeS CdSeS-ZnS control experiment

1.51 1.71 1.91 1.94 1.42

0.32 0.90 0.64 1.12 0.21

25.6 41.1 30.1 46.2 18.4