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J. Phys. Chem. C 2008, 112, 11600–11608
CdSe Quantum Dot-Sensitized Solar Cells Exceeding Efficiency 1% at Full-Sun Intensity Hyo Joong Lee,† Jun-Ho Yum,† Henry C. Leventis,‡ Shaik M. Zakeeruddin,† Saif A. Haque,‡ Peter Chen,† Sang Il Seok,§ 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, Department of Chemistry, Imperial College of Science Technology and Medicine, London SW72AZ, U.K., and KRICT-EPFL Global Research Laboratory, AdVanced Materials DiVision, Korea Research Institute of Chemical Technology, 100 Jang-Dong, Yuseong-Gu, Daejeon 305-600, Republic of Korea ReceiVed: March 25, 2008; ReVised Manuscript ReceiVed: May 7, 2008
Colloidal cadmium selenide (CdSe) quantum dots (QDs) have been prepared and exploited as inorganic dyes to sensitize a large-band-gap TiO2 layer for QD-sensitized solar cells. The optimized QD-sensitized solar cells exhibited an unprecedented incident photon-to-charge carrier generation efficiency of 36% and an overall conversion efficiency of over 1.7% at 0.1 sun and 1% at full sun intensity with a cobalt(II/III)-based redox system. The photovoltaic characteristics of CdSe QD-sensitized cells are compared with standard dye-sensitized solar cells, in which the former exhibited about half of the efficiency of the latter. From the kinetics of charge transfer monitored using transient spectroscopic and voltage decay measurements in the CdSe QD-sensitized cell, the regeneration yield of oxidized QDs was found to be close to almost unity, and the electron lifetime was longer in the CdSe QD-sensitized cell than in the dye-sensitized solar cell. Introduction Since a new architecture for the dye-sensitized solar cell (DSSC) was first reported by O’Regan and Gra¨tzel in 1991,1 it has attracted much attention throughout the world from both academic and industrial fields as a promising alternative to silicon-based solar cells.2 Currently, the highest efficiencies recorded by DSSCs have reached 11% and the long-term stability has improved by using hydrophobic dyes and robust electrolytes.3,4 For further improvement of the overall efficiency in DSSCs, various dye molecules have been designed and prepared as a key material for absorbing incident solar radiation, transferring charges over mesoporous metal oxide layers with high surface areas.5 Although the results obtained so far are very impressive, it is still feasible to improve yet further the efficiency and stability by optimizing the interfaces and engineering new materials. Inorganic semiconducting materials can be well suited for solar cell sensitization6,7 because they are robust, have tunable effective band gaps, and are easy to process; hence, they are ideal candidates for the optimization of a solar cell to achieve a maximum efficiency. Recently, sensitization of mesoporous metal oxide layers with various thin absorbers (CdS,8 CdSe,9 CdTe,10 CuInS2,11 etc.12) and quantum dots (QDs; CdS,13 CdSe,14 InP,15 InAs,16 Bi2S3,17 etc.18) has been proposed, and their photovoltaic properties have been tested. However, both thin absorber and QD-sensitized solar cells (QDSSCs) have shown comparatively lower efficiencies than expected and, accordingly, have not been fully explored and evaluated. Especially, QD-sensitized cells have been attracting much attention because of the recent popularity of the preparation of well-defined colloidal QDs by wet chemical synthesis19 and the * E-mail:
[email protected] (M.G.), mdkhaja.nazeeruddin@ epfl.ch (M.K.N.). Phone: +41-21-693-6124. Fax: +41-21-693-4311. † Swiss Federal Institute of Technology. ‡ Imperial College of Science Technology and Medicine. § Korea Research Institute of Chemical Technology.
demonstration of multiple excitons generated from single-photon absorption in colloidal QDs through impact ionization (an inverse Auger process).20 The latter phenomenon can lead to QDSSCs of over 100% incident photon-to-current conversion efficiency (IPCE) provided that we find a judicious way to collect the generated excitons before they recombine. Despite the great potential of QDs in solar cells, they still have not been demonstrated as an efficient inorganic dye for DSSCs13–18 and as a good sensitizer/electron acceptor and transporter in QD-polymer composite cells. In the latter structure, however, most of the photocurrent was generated by the polymer component.21 Therefore, progress in this field requires a breakthrough in understanding the working mechanism of QDSSCs and the realization of multiple exciton-extracted devices. In this article, we report a low-band-gap semiconducting material, CdSe QD, as a sensitizer in quantum-dot sensitized (qds)7 cells and examine both the overall device performance and the charge-transfer kinetics of this system. Experimental Method Chemicals for QD Preparation. Cadmium acetate dihydrate (99.99%), selenium powder (-100 mesh, 99.99%), technicalgrade trioctylphosphine oxide (TOPO; 90%), technical-grade trioctylphosphine (TOP; 90%), and chloroform were purchased from Aldrich. 3-Mercaptopropionic acid (MPA; 99+%), anhydrous toluene (99.8%), pyridine, and hexane were obtained from Fluka. Ethanol and methanol were of HPLC-grade. Synthesis of CdSe QDs. CdSe nanocrystal QDs were prepared according to the procedure developed by Peng et al.22 with few modifications. A selenium (Se) solution was prepared by mixing 0.40 g of Se powder, 10 mL of TOP, and 0.20 mL of toluene. A total of 20 g of technical-grade TOPO and 0.25 g of cadmium acetate dihydrate were placed in a three-neck, round-bottomed flask and heated to about 150 °C. After the solution was degassed and purged with nitrogen a few times at this temperature, it was heated to 300 °C. At this temperature,
10.1021/jp802572b CCC: $40.75 2008 American Chemical Society Published on Web 07/04/2008
CdSe Quantum Dot-Sensitized Solar Cells the Se solution was quickly injected into the reaction vessel through the rubber septum. The instant color change from yellow to red indicated the formation of CdSe nanocrystals. The heat was immediately removed from the reaction vessel, and small aliquots of the reaction solution were taken to monitor the reaction progress with a UV-vis spectrometer. Three different colloidal CdSe QDs with first exciton peaks of 520, 552, and 574 nm were prepared. When larger QDs were desired, the heat was restored until the first exciton peak at a desired wavelength was observed in the absorption spectrum. Then, the resulting solution was cooled to ∼50 °C and the CdSe QDs were precipitated with a copious amount of ethanol and collected by centrifugation and decantation. The precipitated CdSe QDs were recovered by adding a small amount of toluene and reprecipitated with ethanol. This purification process was repeated two more times. Ligand Exchange of QDs. About 0.20 g of TOPO-coated CdSe was dissolved in 40 mL of pyridine, then sonicated for clear dissolution, and refluxed at 90 °C overnight under dark conditions. Pyridine-coated CdSe was precipitated with hexane and collected by centrifugation and decantation (two times). The precipitate was dissolved in a mixture of pyridine and methanol (1:10, v/v), which was used for sensitization of TiO2 with CdSe QDs. Dye and Electrolyte Preparation. The ruthenium sensitizer (Z907Na), cis-[NaRuII(4-carboxylic acid-4′-carboxylate-2,2′-bipyridine)(4,4′-dinonyl-2,2′-bipyridine)](NCS)2, was prepared as reported earlier,23 and 0.3 mM Z907Na in acetonitrile/tert-butanol (1:1) was used for dye sensitization. The cobalt(II) complexes [Co(dbbip)2](ClO4)2 and [Co(O-phen)3](TFSI)2 [dbbip ) 2,6-bis(1′butylbenzimidazol-2′-yl)pyridine, O-phen ) 1,10-phenanthroline, and TFSI ) bis(trifluoromethanesulfonyl)imide] were synthesized according to the reported procedures.24,25 The cobalt electrolytes were prepared by dissolution of a proper amount of the Co2+ complex, 10% Co3+ complex, and 0.2 M LiClO4 in acetonitrile/ ethylene carbonate (4:6, v/v). The Co3+ complex was prepared by the addition of 10% NOBF4 into the electrolyte solution containing the Co2+ complex, or the Co3+ complex itself was synthesized and isolated as reported earlier.24d For comparison, a typical iodidebased electrolyte was prepared, which consisted of 0.6 M 1-butyl3-methylimidazolium dicyanoamide (BMII), 0.03 M I2, 0.5 M tertbutylpyridine, and 0.1 M guanidinium thiocyanate dissolved in a mixture of acetonitrile/valeronitrile (85:15, v/v). Fabrication of Dye- and QD-Sensitized Cells. Photoelectrodes consisted of a TiO2 film with a triple-layer structure. A compact blocking underlayer of spray-pyrolyzed titanium dioxide (ca. 150 nm thick) was deposited onto a cleaned conducting glass substrate (NSG; F-doped SnO2, resistance 15 Ω sq-1). A solution of titanium diisopropoxide bis(acetylacetonate) in ethanol was sprayed 16 times over the conducting glass surface, which was maintained at 450 °C. The treated glass plates were fired at 450 °C for 30 min more to remove remaining organic traces. Successive depositions of a 2-µm-thick (or thicker) transparent layer and a 4-µm-thick 60-nm lightscattering layer by screen printing and a final post-treatment with an aqueous solution of TiCl4 were then carried out according to typical procedures done in our laboratory for dye cells.26 Dye (Z907Na) sensitization followed the typical procedures,25 and QD derivatization of nanocrystalline oxide films was obtained by immersion of the TiO2 electrode into a solution of the linker molecule (0.1 M MPA in ethanol) for 12-16 h, then washing with ethanol, and dipping into a QD solution (pyridine-capped CdSe QDs dissolved in a mixture of pyridine and methanol) for 12-16 h. After rinsing the QD-sensitized
J. Phys. Chem. C, Vol. 112, No. 30, 2008 11601 electrode with a mixture of pyridine and methanol, that was assembled and sealed with a thin transparent hot-melt 25-µmthick Surlyn ring (DuPont) to the counter electrodes (Pt on FTO glass, chemical deposition of 0.05 M hexachloroplatinic acid in 2-propanol at 400 °C for 20 min). The electrolyte was injected into the interelectrode space from the counter electrode side through a predrilled hole, and then the hole was sealed with a Bynel sheet and a thin glass slide cover by heating. All of the processes and procedures in preparing electrodes and assembling those were the same as those in our typical dye-sensitized cells,26 except one step of QD attachment over TiO2 layers. Photocurrent-Voltage Measurements. The irradiation source for the photocurrent-voltage (I-V) measurement is a 450 W xenon light source (Osram XBO 450), which simulates the solar light. The incident light intensity was calibrated with a standard Si solar cell. The spectral output of the lamp matched precisely the standard global AM 1.5 solar spectrum in the region of 350-750 nm (mismatch
