Diffusion-Limited Recombination in Dye-Sensitized TiO 2 Solar Cells
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N15.12.1
Diffusion-Limited Recombination in Dye-Sensitized TiO2 Solar Cells Nikos Kopidakis, Kurt D. Benkstein, Jao van de Lagemaat, and Arthur J. Frank National Renewable Energy Laboratory, Golden, CO 80401, U.S.A. ABSTRACT The effect of doping on the electron transport dynamics and recombination kinetics in dyesensitized solar cells was investigated. A simple electrochemical method was developed to dope TiO2 nanoparticle films with Li. Increasing the doping levels is found to slow electron diffusion. The electron diffusion time exhibits a light intensity dependence at all doping levels consistent with a multiple electron-trapping model involving native and doping-induced traps. Importantly, the diffusion time and recombination lifetime of photocarriers are observed to increase in unison with increased doping. This is the first observation that electron diffusion limits recombination with the redox electrolyte under normal working conditions of the dye cell. A model is presented that accounts for the observation. The implications of this mechanism on cell performance are also discussed. INTRODUCTION Since its discovery in 1991 [1], dye-sensitized nanocrystalline TiO2 solar cells have received much attention because of their low cost and high efficiency [2,3]. A dye-sensitized solar cell typically consists of a highly porous film composed of sintered TiO2 crystallites of 20 nm diameter sandwiched between a collecting F:SnO2 electrode and a Pt counter electrode. The pores are filled with an electrolyte containing a redox couple, typically I -3 /I -. Dye sensitization at distributed TiO2/redox electrolyte interfaces provides an efficient means of light harvesting and charge separation: the photoexcited dye molecule injects an electron into the conduction band of TiO2 and a hole into the redox electrolyte solution. Owing to the screening of the macroscopic electric fields by the high ionic strength electrolyte, electron transport in TiO2 occurs by diffusion [4]. Diffusion of photocarriers is ambipolar [5], involving the electrostatic coupling of the electron and ion motion. At light intensities up to 1 sun, the injected electron density in TiO2 is orders of magnitude lower then the ion density in the electrolyte such that the ambipolar diffusion coefficient essentially equals the electron diffusion coefficient [5]. Recombination in the conventional dye-sensitized solar cell has been accounted for in terms of the following mechanism [6]: K1
→ I 2 +I I -3 ←
(1)
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k2
I 2 + e - → I•-2
(2)
2I•− 2 → I3 + I
(3)
Triiodide ions in solution are assumed to be in equilibrium with molecular iodine on the TiO2 surface (reaction 1) [7], which reacts with photoinjected electrons (reaction 2). The resulting iodine radical anion ( I•2- ) undergoes dismutation (reaction 3) [6]. An alternative to reaction 3 has been proposed [8] but will not be discussed in this paper. The main focus of the present work is on reaction 2. Taking the equilibrium constant of reaction 1 to be 10-10 mol/cm3 [8], the density of I2 is estimated
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