Efficient CdSe Quantum Dot-Sensitized Solar Cells Prepared by an

NANO LETTERS

Efficient CdSe Quantum Dot-Sensitized Solar Cells Prepared by an Improved Successive Ionic Layer Adsorption and Reaction Process

2009 Vol. 9, No. 12 4221-4227

HyoJoong Lee,*,† Mingkui Wang, Peter Chen, Daniel R. Gamelin,§ Shaik M. Zakeeruddin, Michael Gra¨tzel,* and Md. K. Nazeeruddin* Laboratory for Photonics and Interfaces, Institute of Chemical Sciences and Engineering, School of Basic Sciences, Swiss Federal Institute of Technology, CH-1015 Lausanne, Switzerland Received July 28, 2009; Revised Manuscript Received October 17, 2009

ABSTRACT In pursuit of efficient quantum dot (QD)-sensitized solar cells based on mesoporous TiO2 photoanodes, a new procedure for preparing selenide (Se2-) was developed and used for depositing CdSe QDs in situ over TiO2 mesopores by the successive ionic layer adsorption and reaction (SILAR) process in ethanol. The sizes and density of CdSe QDs over TiO2 were controlled by the number of SILAR cycles applied. After some optimization of these QD-sensitized TiO2 films in regenerative photoelectrochemical cells using a cobalt redox couple [Co(o-phen)32+/3+], including addition of a final layer of CdTe, over 4% overall efficiencies were achieved at 100 W/m2 with about 50% IPCE at its maximum. Light-harvesting properties and transient voltage decay/impedance measurements confirmed that CdTe-terminated CdSe QD cells gave better charge-collection efficiencies and kinetic parameters than corresponding CdSe QD cells. In a preliminary study, a CdSe(Te) QD-sensitized TiO2 film was combined with an organic hole conductor, spiro-OMeTAD, and shown to exhibit a promising efficiency of 1.6% at 100 W/m2 in inorganic/organic hybrid all-solid-state cells.

Since 1991,1 dye-sensitized solar cells (DSSCs) have attracted tremendous attention in both academic and industrial research laboratories. The approach of DSSC technologies to commercial entrance has been possible mainly because of an improved understanding of their operating principles and a continuous introduction and testing of new materials.2 The development of new molecular dyes and their counterparts for charge separation, i.e., metal oxides (electron transporter) and redox couples (hole conductor), have played key roles in enhancing overall efficiencies and device stabilities.3 In the search for next-generation sensitizers, inorganic quantum dots (QDs) have been suggested and tested with a high expectation of having advantages over molecular dyes:4 (1) facile tuning of effective band gaps down to the IR range by changing their sizes and compositions, (2) higher stability and resistivity toward oxygen and water over molecular counterparts, (3) new possibilities for making multilayer or hybrid sensitizers, and (4) new * Corresponding authors: [email protected], [email protected], and [email protected]; phone, +41-21-693-6124; fax, +4121-693-4311. † Current address: Department of Chemistry, Pohang University of Science and Technology (POSTECH), Gyeongbuk, South Korea. § On sabbatical leave from the Department of Chemistry, University of Washington, Seattle, WA. 10.1021/nl902438d CCC: $40.75 Published on Web 11/05/2009

 2009 American Chemical Society

phenomena such as multiple excition generation and use of energy transfer-based charge collection as well as direct charge transfer schemes. The huge interest in colloidal QDs and their applications over the past decade has also imparted momentum to this research area.5 Despite intense interest since a first report of TiO2/InP photoelectrochemical cells,6a QDs have so far not excelled as sensitizers in metal oxide solar cells, in part due to a lack of efficient hole carriers in either liquid- or solid-type cells.6 QD preparation on mesoporous metal oxides has focused on two approaches: (1) colloidal QDs capped with surface ligands have been attached to metal oxide surfaces through linker molecules or other attractive forces;6a-e (2) QDs grown directly onto TiO2 electrodes in chemical bath deposition (CBD) processes under normal or hydrothermal conditions. In the CBD approaches, dissolved cationic and anionic precursors are reacted slowly in one bath7 or separated into two beakers and the bare electrode dipped alternatively into each, to grow the target QDs. This latter process has been dubbed “successive ionic layer adsorption and reaction” (SILAR).6f,g,8 The SILAR process has been used to prepare various inorganic-semiconductor-modified electrodes, particularly of metal sulfides, but notably not of metal selenides or tellurides because of difficulties preparing stable Se2- and

Te2- precursors.9 To date, there have been no successful applications of the SILAR process for preparation of metal selenide- and telluride-modified mesoporous oxides.10 This limitation has restricted QD-sensitized solar cells prepared by the SILAR process to a narrow range of materials such as CdS and PbS,6f,g,9 while metal selenide sensitizers have been prepared usually by electrochemical plating11 or CBD techniques based on slow release of selenide from Na2SeO3 in the presence of metal cations.7 Compared to the SILAR process, this CBD process for preparing CdSe-modified electrodes is inefficient (takes a few hours or overnight), poorly controlled (in size and density of the QDs), and unselective (not only deposited over the electrode surfaces but also nucleated in bulk solution as well as on the side walls of the deposition container). In both principle and practice, the SILAR process could now be considered as a best way to allow deposition of well-defined compositionmodulated (doped, alloyed, or multilayered) QD layers onto mesoporous metal oxides in the solution process, as demonstrated recently with colloidal QDs, where very precisely controlled multilayers were deposited over QD cores by alternating injection of cationic and anionic precursors.12 There is therefore an urgent need for effective and general preparative methodologies that will allow deposition of metal selenide and telluride QDs onto oxides by the SILAR process. In this Letter, we report the development of a new procedure for preparing selenide and telluride ions, by reducing the corresponding dioxide precursor in ethanol, that allows SILAR growth of CdSe and CdSe(Te) QDs over mesoporous TiO2 films. We demonstrate application of these CdSe QDs as sensitizers in photoelectrochemical cells using a cobalt complex, Co(o-phen)32+/3+, as a regenerative redox couple, and report promising overall efficiencies of 4.2% at 100 W/m2. To carry out the SILAR process for the deposition of metal selenides, it is necessary to prepare selenide ions in solution and keep them stable for an extended period. While suitable experimental conditions were sought for this process, it was observed that SeO2 could be reduced by NaBH4 in ethanol, as described in eq 1. SeO2 + 2NaBH4 + 6C2H5OH f Se2- + 2Na+ + 2B(OC2H5)3 + 5H2 + 2H2O

(1)

When 2 equiv of NaBH4 was added to an ethanol solution of SeO2 under inert atmosphere (N2 or Ar purging), a gradual color change from deep red to transparent was observed (Figure S1 in Supporting Information), indicating that SeO2(+4) was being reduced to Se2-(-2). This transparent solution was then transferred into a glovebag under inert gas,13 where the SILAR process was performed following the usual procedure; the optimized TiO2 film/FTO electrode was successively immersed into two different solutions for about 30 s each, one consisting of 0.03 M Cd(NO3)2 dissolved in ethanol and the other containing the in situ generated 0.03 M Se2- in ethanol. Following each immersion, the films were rinsed for 1 min or more using pure ethanol 4222

Figure 1. Absorption spectra of ∼2 µm thick film made of 20 nm TiO2 after SILAR deposition of CdSe QDs (one-six cycles) and photographs of the corresponding films (left inset). TEM images of CdSe QDs/TiO2 particles after six cycles of the SILAR process (right inset, scale bar indicated).

to remove excess precursor, and the electrode was dried before the next dipping. This immersion cycle was repeated several times, from one to six complete cycles. With each SILAR cycle, the CdSe deposits became larger in size and denser in distribution, as evidenced by the absorption spectra shown in Figure 1; the absorption onset moved gradually down to around 700 nm, and its magnitude increased with each cycle. This successive deposition of CdSe over mesoporous TiO2 was accompanied by a series of color changes visible to the naked eye, i.e., quantum size effects, as shown in the inset of Figure 1. These color changes closely resemble the color changes observed when colloidal QDs of CdSe are grown by the hot-injection method, showing a yellow color just following nucleation, and then proceeding through orange, red, and dark red during growth.5 On the basis of these observations, the SILAR process described here was concluded to successfully deposit CdSe QDs onto the surfaces of mesoporous TiO2 films. The sharp excitonic features observed in colloidal QDs were not detected here, reflecting a comparatively broad size distribution in the SILAR-deposited QDs, as always observed with SILAR deposition of other QDs.6f,g,9a,b Narrow spectral features are not critical to the performance of these QDs as solar cell sensitizers. The current procedure for QD deposition could be extended to the preparation of other metal selenide QDs over TiO2 layers simply by changing the cation precursor solution; ZnSe and PbSe QDs were deposited successfully after just a few SILAR cycles, and their absorption spectra obtained (Figure S2 in Supporting Information). The procedures described here are therefore generally applicable for deposition of metal selenide QDs over flat or mesoporous metal oxides. A similar approach was also explored for the preparation of tellurides. Using NaBH4 to reduce TeO2, a pale pink solution was obtained that was then used for the deposition of metal tellurides.14 With the aim of making a type II heterostructured QD sensitizer to assist charge Nano Lett., Vol. 9, No. 12, 2009

separation,15 CdTe was deposited onto CdSe QDs in the last SILAR cycle, and this modification was found to increase the overall efficiency of the QD-sensitized cells significantly (vide infra). The important criteria for making efficient QD sensitizers over mesoporous metal oxide films by in situ deposition are that (1) QDs should grow conformally and homogeneously along the surface, without clogging the oxide mesopores, and (2) it should be possible to control the final QD sizes to achieve efficient charge separation from QDs to electron and hole acceptors. To investigate CdSe QD growth by the SILAR process developed here, the morphological changes of a TiO2 film were monitored by collecting SEM and TEM images before and after deposition of a CdSe6 (where the number indicates the number of SILAR cycles applied, which was optimized for photovoltaic performance; see below) QD layer as described above. From comparison of parts a and b of Figure S3 in Supporting Information, there is no indication of blocking of the TiO2 mesopores after deposition of CdSe by the SILAR process 6 times. Instead, the bare TiO2 surface appears to be covered by a homogeneous thin layer of smaller particles, the CdSe QDs. Similar results were obtained after deposition of CdSe5Te1 (Figure S3c in Supporting Information). Closer inspection of the CdSe6/TiO2 sample by highresolution TEM (Figure 1 inset) reveals more clearly the growth of well-separated ∼2.5-5 nm diameter hemispherical dots over the TiO2 surfaces. The rather low precursor concentrations (30 mM) apparently generate well-separated nucleation sites such that the final deposits do not overlap each other, as reported recently for PbS QDs grown on TiO2.6g The flexibility of this SILAR deposition process, including the ability to control the size and distribution density of the deposited CdSe QDs by changing precursor concentrations and the number of SILAR cycles, makes it very promising and versatile for general deposition of metal chalcogenide QDs directly onto mesoporous substrates. In particular, the current SILAR process was carried out in ethanol, not under the usual aqueous conditions. The low surface tension of the alcohol solvent leads to better penetration into the mesopores with fast removal from mesopores during drying, which facilitates conformal QD growth along the oxide surfaces and thus results in improved overall efficiencies.6g,9d To check the photovoltaic performances of these QDsensitized electrodes in regenerative device structures, three samples of CdSe5 were prepared in parallel following the procedures described above. Two of these were then subjected to one additional SILAR cycle, one with CdSe and one with CdTe (named CdSe6 and CdSe5Te1, respectively). Those three electrodes were then each assembled with a platinized counter electrode, and a liquid electrolyte of the cobalt redox couple16 was injected between the two electrodes as for DSSCs prepared in our group. The active areas of the films (0.159 cm2) were accurately defined using metallic masks to avoid the aperture effect reported recently.17 Increasing the number of SILAR cycles from five to six increased the CdSe QD sizes, leading to a higher short-circuit current (Jsc) and a lower open-circuit voltage (Voc) because Nano Lett., Vol. 9, No. 12, 2009

Table 1. Photovoltaic Data Sets for CdSe5-, CdSe6-, and CdSe5Te1-Sensitized Mesoporous TiO2 Cells Prepared in Parallel Using Co(o-phen)32+/3+ as a Redox Couple, and the Best CdSe5Te1 QD- and Z907Na Dye-Sensitized Cells Constructed under the Same Conditions Are Included for Comparisona Jsc (mA/cm2)

Voc (V)

FF

efficiency (%)

CdSe5 3.27 (0.55) 0.65 (0.59) 0.50 (0.84) 1.08 (2.95) CdSe6 3.93 (0.67) 0.61 (0.55) 0.49 (0.82) 1.19 (3.26) CdSe5Te1 4.39 (0.79) 0.64 (0.58) 0.55 (0.82) 1.57 (4.06) CdSe5Te1 best 4.94 (0.83) 0.67 (0.60) 0.54 (0.78) 1.77 (4.18) Z907Na dye 6.38 (1.14) 0.57 (0.49) 0.59 (0.72) 2.20 (4.38) CdSe5Te1 solid 2.15 (0.37) 0.70 (0.65) 0.55 (0.59) 0.84 (1.56) a The data in the last row are based on a CdSe5Te1-sensitized solidstate cell with spiro-OMeTAD. All the data were collected under both simulated 1 sun and 0.1 sun intensities (the numbers in parentheses indicate the data obtained at 0.1 sun).

of the increased light absorption but decreased band gap of the larger particles. The short-circuit current increased by about 20% and open-circuit voltage decreased by about 10% in the CdSe6 cell vs CdSe5, thus yielding about 10% increase of overall efficiency (Table 1). The increase in Jsc from CdSe5 to CdSe6 arose mainly from the enhanced charge collection in the far-red region, seen clearly in the IPCE data of Figure 2a, although the incident light-harvesting power also increased over the entire visible region (Figure 1). Above six SILAR cycles, the overall efficiency saturated at the value of the CdSe6 cell and then decreased gradually, probably due to the limited pore space within the TiO2 film for adsorbing more CdSe and its effect on diffusion of the cobalt complex. In making an efficient QD-sensitized mesoporous solar cell, it thus appears important to balance the relationships between the average pore size of the oxide film, the amount of deposited sensitizer, and intrinsic diffusion properties of the redox couples. Important results were obtained when the sixth deposition over CdSe5 was done to form CdTe instead of CdSe. When CdTe was used as the last layer, the short-circuit current always increased significantly (about 40%) while retaining almost the same value in open-circuit voltage. With this approach, CdSe5Te1 QD-sensitized cells working in a regenerative mode were made that yielded over 4% overall efficiency at simulated 100 W/m2 (Table 1). The beneficial effect obtained from termination with a CdTe layer may come from improved hole transfer from the CdSe cores to the CdTe outer layers because of the cascade band structures (the inset of Figure 2a).15 To understand the origin of this effect, the light-harvesting properties of the same series of films were checked and compared with results from IPCE measurements. The CdSe6 electrode showed superior absorbing power in the visible range relative to either the CdSe5Te1 or the CdSe5 electrodes. The CdSe5 and CdSe5Te1 electrodes were similar, with the latter slightly better above 500 nm (Figure 2b). In IPCE measurements, however, the CdSe5Te1 electrode gave the best result among the three cells, followed by the CdSe6 and then the CdSe5 electrodes (Figure 2a). In particular, although the absorbance did not change at all below 500 nm and increased only slightly above 500 nm when CdSe5Te1 was prepared from 4223

Figure 2. (a) Comparison of IPCE data for CdSe5-, CdSe6-, and CdSe5Te1-sensitized cells (inset: energy diagram of the bulk band offsets for the active components involved). (b) Absorption spectra of CdSe5-, CdSe6-, and CdSe5Te1-sensitized TiO2 films.

Figure 3. Comparison of (a) recombination rate constants and (b) effective diffusion lengths for CdSe6- and CdSe5Te1-sensitized TiO2 cells with Co(o-phen)32+/3+ redox couple, obtained by transient photovoltage and photocurrent decay measurements.

the CdSe5 film, the IPCE value increased about 30% over the entire range (ca. 40% increment in integrated total shortcircuit current) with inclusion of the CdTe termination layer. This observation indicates that the rather thin CdTe layer is not contributing much to light harvesting but increases charge collection efficiency substantially. Further deposition of CdTe did not further enhance the overall cell efficiency, perhaps because of increased pore filling, which would affect diffusion of the cobalt redox couple within the mesoporous TiO2 films. Therefore, the number of SILAR cycles used for CdTe deposition was restricted to one here. This interpretation was verified by the observation that QD sensitizers prepared with more CdTe cycles (two to three) continued to be effective when TiO2 films with larger pore sizes (∼48 nm) were used (data not shown). To compare kinetic parameters of the CdSe6- and CdSe5Te1-sensitized cells, transient photovoltage and photocurrent decay measurements were performed using the two best QD-sensitized cells. Figure 3a shows the recombination rate constants (kre) as a function of open circuit voltage measured for these two cells. At identical open circuit voltages, kre for the CdSe5Te1 cell is lower than that of the CdSe6 cell, indicating that recombination is retarded by the use of CdTe as a last deposition layer. The two cells show 4224

slightly different TiO2 conduction band edge energies (Ec) with respect to the redox potential of the electrolyte, as indicated by impedance measurements (see Figure S5a in Supporting Information).18 The difference in recombination rate constants is therefore accounted for by formation of a barrier on the QD or bare TiO2 surface in the presence of the last CdTe layer, which retards recombination between the photogenerated conduction band electrons and the oxidized cobalt electrolyte. As a result, a longer electron diffusion length (Le) was measured in the CdSe5Te1 device than in the CdSe6 device (Figure 3b). The larger electron diffusion length is directly related to a higher charge collection yield, consistent with the higher Jsc of the CdSe5Te1 device. Electrochemical impedance spectroscopy measurements in the dark also confirmed that the presence of CdTe on CdSe retards interfacial electron recombination with the Co(o-phen)33+ complex (Figure S5b in Supporting information). A higher electron diffusion length (Figure S5c in Supporting Information) was thus achieved in the CdSe5Te1 device. Our findings support earlier statements concerning the importance of surface barriers in blocking unwanted electron transfer from QDs to electrolytes for enhancing the performance of QD-sensitized cells.4c Although popular with DSSCs,18 the above approach to obtaining kinetic parameters from functional cells has been applied very rarely to QD-sensitized mesoporous solar cells.6e The absence of such data in the literature likely reflects the lack of efficient and reproducible QD-sensitized cells with well-defined components. To perform such measurements, it was necessary to focus first on finding ways of increasing the overall efficiencies in such cells under regenerative conditions. The good power conversion efficiencies (>4%), stabilities, and reproducibilities achieved here have made it possible to extract meaningful kinetic parameters from stateof-the-art transient photovoltage/photocurrent decay and impedance measurements and to compare the results across different QD-sensitized cells. After optimization, the CdSe5Te1-cell structure described above reached over 4% overall efficiency at 100 W/m2 using Nano Lett., Vol. 9, No. 12, 2009

Figure 4. (a) J-V curve and (b) IPCE data for the best CdSe5Te1-sensitized cell obtained in this study. For comparison, IPCE data collected for sodium cis-dithiocyanato(4-carboxylate-4′-carboxylic acid-2,2′-bipyridine)(4,4′-dinonyl-2,2′-bipyridine)ruthenium(II) (coded as Z907Na) dye-sensitized cell under the same experimental conditions are included in (b) as well as for a CdSe5Te1-sensitized solid-state cell with spiro-OMeTAD as the hole-transport layer.

the cobalt redox couple as the hole conductor. This high efficiency was achieved in a very reproducible way from batch to batch. The best data obtained so far are presented in Figure 4. Although a somewhat decreased overall efficiency of 1.8% was observed under ∼1 sun conditions due to slow diffusion of the cobalt complex,9b a well-defined current-voltage curve was obtained under simulated 100 W/m2, where the effects of slow diffusion were less severe. The best data were obtained from cells that had been aged for 7 days or more, rather than from freshly prepared cells (Table S2 in Supporting Information). All of the cells tested in the present study showed a gradual 20-30% increase of overall efficiency during the 1-2 weeks following cell preparation, after which their performance maintained its best value for about 1 month before slowly decreasing due to leakage of the electrolyte solvent.19 The gradual increase in overall efficiency observed with aging came entirely from an incremental increase of the short-circuit current (Jsc) with time. We hypothesize that this gradual increase of Jsc arises from surface passivation of the SILAR QDs, which possess no surface passivating ligands following initial deposition. Importantly, growth of these metal selenide QDs directly onto the TiO2 helps to maintain a stable interface in the present SILAR-based QD cells, in contrast with the unstable interfaces often obtained with colloidal QDs attached indirectly to oxide surfaces.6e The excellent efficiencies of these cells are reflected in the high IPCE values they yield (Figure 4b). Although a rather thin photoanode was used for these data (2.3 µm thick transparent and 5.8 µm thick scattering film), about 50% light-to-current conversion was seen over a broad visible range. To estimate how well this best QD cell performs in the cell architecture used, these results were compared with those from a standard dye cell prepared with the same cell architecture; the Z907Na dye was tested under the same conditions as used for the CdSe5Te1 cell, and its photovoltaic parameters are summarized in Table 1. The best QDsensitized cell approaches about 75% of the standard dye cell’s performance in short-circuit current (Figure 4b). This direct comparison between dye- and QD-sensitized cells constructed in the same cell architecture makes it possible Nano Lett., Vol. 9, No. 12, 2009

to estimate the present status of QD sensitizers. The 75% relative generation of currents in the QD-sensitized cell is very promising. If a more efficient hole conductor than the cobalt complex used here can be identified, analogous to polyiodide in DSSCs, even greater efficiencies will undoubtedly be achieved in such QD-sensitized cells. Finally, it would be most attractive if the principles of QD sensitization learned here for liquid cells could be translated to all-solid-state QD-sensitized cells, because of the greater intrinsic stability of the latter. Limited results have been reported so far in QD- or extremely thin absorber (ETA)-sensitized solid state cells.6g,11a,20 With the best QD sensitizer found in this study, CdSe5Te1, solid-state cells were fabricated with spiro-OMeTAD as the hole conductor. Spiro-OMeTAD has been used successfully in solid-type DSSCs.21 Although not fully optimized, this solid-state CdSe5Te1 QD cell gave over 30% IPCE and 1.6% overall efficiency at 92 W/m2 through a relatively thin 1.8 µm TiO2 film (Table 1 and Figure 4b). The cell was stable for a few months under atmospheric room light. These preliminary results are promising and suggest that it may be possible to achieve even greater efficiencies if optimized conditions for better pore filling of the hydrophobic organic hole conductor (spiro-OMeTAD in chlorobenzene) into hydrophilic inorganic mesopores can be identified. This problem was recognized recently in cells prepared with PbS QDs and spiro-OMeTAD.6g As one possible solution, hydrophobic molecular dyes or surface-passivating molecules such as decylphosphonic acid (DPA) and hexadecylmalonic acid (HDMA) could be helpful in making a more intimate contact between the QD-sensitized TiO2 surface and spiro-OMeTAD, leading to improved charge collection at these interfaces. Such possibilities are now under active investigation in our laboratory. In summary, we have shown that well-defined and controllable SILAR deposition of CdSe and CdSeTe QDs in situ onto mesoporous TiO2 films can be used to fabricate efficient QD-sensitized solar cells in both liquid- and solidtype structures. The flexibility to change the final surface deposition from CdSe to CdTe has been recognized as an advantage of this in situ SILAR approach, yielding a large 4225

enhancement in photocurrent attributable to favorable hole transfer to the CdTe layer and diminished unproductive recombination with electrolytes by surface barrier. On the basis of the results reported recently by others4c,7b,10 and obtained here, it appears essential to control the surface states of QD sensitizers in order to optimize the charge collection efficiencies of such electrodes. Finally, the best QD sensitizers identified here yielded overall cell efficiencies comparable as those by a standard dye (Z907Na) cell in the same configuration. This promising result emphasizes the importance of developing more efficient hole carriers for application in QD-sensitized cells, analogous to the I-/I3- redox couple of DSSCs. Acknowledgment. This work was supported by the SOHYD and GRL project. H. J. Lee thanks Professor SuMoon Park and the National Center for Nanomaterials Technology at POSTECH, South Korea for the HR-TEM measurements, and Peter Chen thanks the Taiwan Merit Scholarships Program (TMS-094-2A-026). D.R.G. is grateful to the Sloan Foundation and the University of Washington for partial support during sabbatical leave. The 30 and 60 nm TiO2 particles were a gift from Showa Titanium Company to L.P.I.

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Supporting Information Available: Details on the experimental procedure, impedance data under dark condition for transport/recombination parameters, and several supporting images. This information is available free of charge via the Internet at http://pubs.acs.org. References (1) O’Regan, B.; Gra¨tzel, M. Nature 1991, 353, 737. (2) (a) Hagfelt, A.; Gra¨tzel, M. Acc. Chem. Res. 2000, 33, 269. (b) Kroon, J. M.; Bakker, N. J.; Smit, H. J. P.; Liska, P.; Thampi, K. R.; Wang, P.; Zakeeruddin, S. M.; Gra¨tzel, M.; Hinsch, A.; Hore, S.; Wurfel, U.; Sastrawan, R.; Durrant, J. R.; Palomares, E.; Pettersson, H.; Gruszecki, T.; Walter, J.; Skupien, K.; Tulloch, G. E. Prog. PhotoVoltaics 2007, 15, 1. (3) (a) Robertson, A. Angew. Chem., Int. Ed. 2006, 45, 2338. (b) Wang, P.; Zakeeruddin, S. M.; Moser, J.-E.; Nazeeruddin, M. K.; Sekiguchi, T.; Gra¨tzel, M. Nat. Mater. 2003, 2, 402. (c) Bai, Y.; Cao, Y.; Zhang, J.; Wang, M.; Li, R.; Wang, P.; Zakeeruddin, S. M.; Gra¨tzel, M. Nat. Mater. 2008, 7, 626. (d) Martinson, A. B. F.; Hamann, T. W.; Pellin, M. J.; Hupp, J. T. Chem.sEur. J. 2008, 14, 4458. (4) (a) Nozik, A. J. Physica E 2002, 14, 115. (b) Kamat, P. V. J. Phys. Chem. C 2008, 112, 18737. (c) Hodes, G. J. Phys. Chem. C 2008, 112, 17778. (d) Schaller, R. D.; Klimov, V. I. Phys. ReV. Lett. 2004, 92, 186601. (e) Luther, J. M.; Law, M.; Beard, M. C.; Song, Q.; Reese, M. O.; Ellingson, R. J.; Nozik, A. J. Nano Lett. 2008, 8, 3488. (f) Huynh, W. U.; Dittmer, J. J.; Alivisatos, A. P. Science 2002, 295, 2425. (g) Sadhu, S.; Patra, A. ChemPhysChem 2008, 9, 2052. (h) Luque, A.; Martı´, A.; Bett, A.; Andreev, V. M.; Jaussaud, C.; van Roosmalen, J. A. M.; Alonso, J.; Ra¨uber, A.; Strobl, G.; Stolz, W.; Algora, C.; Bitnar, B.; Gombert, A.; Stanley, C.; Wahnon, P.; Conesa, J. C.; van Sark, W. G. J. H. M.; Meijerink, A.; van Klink, G. P. M.; Barnham, K.; Danz, R.; Meyer, T.; Luque-Heredia, I.; Kenny, R.; Christofides, C.; Sala, G.; Benı´tez, P. Sol. Energy Mater. Sol. Cells 2005, 87, 467. (5) (a) Murray, C. B.; Norris, D. J.; Bawendi, M. G. J. Am. Chem. Soc. 1993, 115, 8706. (b) Alivisatos, A. P. Science 1996, 271, 933. (c) Burda, C.; Chen, X. B.; Narayanan, R.; El-Sayed, M. A. Chem. ReV. 2005, 105, 1025. (6) (a) Zaban, A.; Mic´ic´, O. I.; Gregg, B. A.; Nozik, A. J. Langmuir 1998, 14, 3153. (b) Yu, P.; Zhu, K.; Norman, A. G.; Ferrere, S.; Frank, A. J.; Nozik, A. J. J. Phys. Chem. B 2006, 110, 25451. (c) Robel, I.; Subramanian, V.; Kuno, M.; Kamat, P. V. J. Am. Chem. Soc. 2006, 128, 2385. (d) Leschkies, K. S.; Divakar, R.; Basu, J.; Enache-Pommer, 4226

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E.; Boercker, J. E.; Carter, C. B.; Kortshagen, U. R.; Norris, D. J.; Aydil, E. S. Nano Lett. 2007, 17, 1793. (e) Lee, H. J.; Yum, J.-H.; Leventis, H. C.; Zakeeruddin, S. M.; Haque, S. A.; Chen, P.; Seok, S. I.; Gra¨tzel, M.; Nazeeruddin, Md. K. J. Phys. Chem. C 2008, 112, 11600. (f) Plass, R.; Pelet, S.; Krueger, J.; Gra¨tzel, M.; Bach, U. J. Phys. Chem. B 2002, 106, 7578. (g) Lee, H. J.; Leventis, H. C.; Moon, S.-J.; Chen, P.; Ito, S.; Haque, S. A.; Torres, T.; Nu¨esch, F.; Geiger, T.; Zakeeruddin, S. M.; Gra¨tzel, M.; Nazeeruddin, Md. K. AdV. Func. Mater. 2009, 19, 2735. (a) Niitsoo, O.; Sarkar, S. K.; Pejoux, C.; Ruhle, S.; Cahen, D.; Hodes, G. J. Photochem. Photobiol., A 2006, 181, 306. (b) Diguna, L. J.; Shen, Q.; Kobayashi, J.; Toyoda, T. Appl. Phys. Lett. 2007, 91, 023116. (c) Okazaki, K.-i.; Kojima, N.; Tachibana, Y.; Kuwabata, S.; Torimoto, T. Chem. Lett. 2007, 36, 712. (a) Pathan, H. M.; Lokhande, C. D. Bull. Mater. Sci. 2004, 27, 85. (b) Park, S.; Clark, B. L.; Keszler, D. A.; Bender, J. P.; Wager, J. F.; Reynolds, T. A.; Herman, G. S. Science 2002, 297, 65. (a) Vogel, R.; Hoyer, P.; Weller, H. J. Phys. Chem. 1994, 98, 3183. (b) Lee, H. J.; Chen, P.; Moon, S.-J.; Fre´de´ric, S.; Sivula, K.; Bessho, T.; Gamelin, D. R.; Comte, P.; Zakeeruddin, S. M.; Seok, S. I.; Gra¨tzel, M.; Nazeeruddin, Md. K. Langmuir 2009, 25, 7602. (c) Tachibana, Y.; Akiyama, H. Y.; Ohtsuka, Y.; Torimoto, T.; Kuwabata, S. Chem. Lett. 2007, 36, 88. (d) Chang, C.-H.; Lee, Y.-L. Appl. Phys. Lett. 2007, 91, 053503. (e) Sun, W. T.; Yu, Y.; Pan, H. Y.; Gao, X. F.; Chen, Q.; Peng, L. M. J. Am. Chem. Soc. 2008, 130, 1124. While preparing this manuscript, we have found that a good trial for realizing a selenide-used SILAR process and its results were released recently: (a) Lee, Y.-L.; Huang, B.-M.; Chien, H.-T. Chem. Mater. 2008, 19, 1626. (b) Lee, Y.-L.; Lo, Y.-S. AdV. Func. Mater. 2009, 19, 1. where the reacting cationic (metal salt) and anionic (from Na2SeO3) precursors (which have been dissolved in one beaker, this is a typical way used so far) were kept separated in two beakers (this is a new trial) not to contact each other. Then the typical SILAR process was done to deposit CdSe. But, it took a longer time (ca. 1 h) in turn for selenide adsorption due to a slow release from Na2SeO3 and needed a higher temperature (50 °C) than in the case of metal sulfide (less than a few minutes at room temperature). Anyway, this is a rather advanced way for preparing and adsorping selenide thus resulting in over 4% overall efficiency with polysulfide electrolytes though many advantages of the typical SILAR process were not fully taken for deposition of CdSe. (a) Le´vy-Cle´ment, C.; Tena-Zaera, R.; Ryan, M. A.; Katty, A.; Hodes, G. AdV. Mater. 2005, 17, 1512. (b) Tena-Zaera, R.; Katty, A.; Bastide, S.; Le´vy-Cle´ment, C. Chem. Mater. 2007, 19, 1626. (a) Battaglia, D.; Li, J. J.; Wang, Y. J.; Peng, X. G. Angew. Chem., Int. Ed. 2003, 42, 5035. (b) Li, J. J.; Wang, Y. A.; Guo, W. Z.; Keay, J. C.; Mishima, T. D.; Johnson, M. B.; Peng, X. G. J. Am. Chem. Soc. 2003, 125, 12567. (c) Battaglia, D.; Blackman, B.; Wang, Y. J.; Peng, X. G. J. Am. Chem. Soc. 2005, 127, 10889. To keep selenide unchanged, it is necessary to maintain an inert atmosphere during the SILAR process. If the oxygen level increases, selenide starts to reoxidize gradually and finally precipitates as black selenium. (a) Law, W.-C.; Yong, K.-T.; Roy, I.; Ding, H.; Hu, R.; Zhao, W.; Prasad, P. N. Small 2009, 5, 1997. (b) Rogach, A. L.; Franzl, T.; Klar, T. A.; Feldmann, J.; Gaponik, N.; Lesnyak, V.; Shavel, A.; Eychmller, A.; Rakovich, Y. P.; Donegan, J. F. J. Phys. Chem. C 2007, 111, 14628. Kim, S.; Fisher, B.; Eisler, H.-J.; Bawendi, M. J. Am. Chem. Soc. 2003, 125, 11466. The cobalt redox couples have been proven recently to be the firstgeneration noncorrosive and well-defined hole conductor for testing QD-sensitized cells (refs 6b, 6e, and 9b). One weak point to be overcome is a sublinearity of photocurrent vs light intensity due to slow diffusion of cobalt complexes. Therefore, the comparison among the samples described in the text was done at 100 W/m2 to exclude the diffusion limitation problems in photocurrents. (a) Ito, S.; Nazeeruddin, Md. K.; Liska, P.; Comte, P.; Charvet, R.; Pechy, P.; Jirousek, M.; Kay, A.; Zakeeruddin, S. M.; Gra¨tzel, M. Prog. PhotoVoltaics 2006, 14, 589. (b) Park, J.; Koo, H.-J.; Yoo, B.; Yoo, K.; Kim, K.; Choi, W.; Park, N.-G. Sol. Energy Mater. Sol. Cells 2007, 91, 1749. (a) Wang, M.; Chen, P.; Humphry-Baker, R.; Zakeeruddin, S. M.; Gra¨tzel, M. ChemPhysChem 2009, 10, 290. (b) Wang, M.; Gra¨tzel, C.; Moon, S.-J.; Humphry-Baker, R.; Rossier-Iten, N.; Zakeeruddin, S. M.; Gra¨tzel, M. AdV. Funct. Mater. 2009, 19, 2163. Nano Lett., Vol. 9, No. 12, 2009

(19) After some leakage of electrolyte solvent in the assembled cell was observed, it was reinjected and the performance of the cell restored. Therefore, the stability of current QD cells is maintained as far as electrolyte solvent kept between two electrodes under room light and atmospheric conditions. (20) (a) Itzhaik, Y.; Niitsoo, O.; Page, M.; Hodes, G. J. Phys. Chem. C 2009, 113, 4254. (b) Belaidi, A.; Dittrich, T.; Kieven, D.; Tornow, J.;

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Schwarzburg, K.; Lux-Steiner, M. Phys. Status Solidi PRL 2008, 2, 172. (c) Larramona, G.; Chone´, C.; Jacob, A.; Sakakura, D.; Delatouche, B.; Pe´re´, D.; Cieren, X.; Nagino, M.; Bayon, R. Chem. Mater. 2006, 18, 1688. (21) Schmidt-Mende, L.; Gra¨tzel, M. Thin Solid Films 2006, 500, 296.

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