The Influence of Particle Sizes on the Optical Characteristics of Nanocrystalline TiO2 Films for Dye-Sensitized Solar Ce
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The Influence of Particle Sizes on the Optical Characteristics of Nanocrystalline TiO2 Films for Dye-Sensitized Solar Cells Peter Chao-Yu Chen, Guido Rothenberger, and Michael Grätzel Institut des Sciences et Ingenierie Chimique, Ecole Polytechnique Federale de Lausanne, EPFL SB ISIC LPI, Station 6, Lausanne, CH 1015, Switzerland ABSTRACT Increasing the light harvesting efficiency (LHE) of photovoltaic devices is important for improving the conversion efficiency of solar light into electricity. The optical properties of mesoporous TiO2 films for Dye-Sensitized Solar Cells (DSCs) were investigated by varying the size of the particles (20 – 150 nm). A four–flux optical model was used to describe the light absorption and scattering within the film. Reflectance and transmittance spectra were recorded by a spectrophotometer equipped with an integrating sphere. The wavelengthdependent absorption coefficient k(λ) and scattering coefficient s(λ) were determined for different films. The absorptance G(λ) (absorbed light flux) for films made of different particle sizes and of varying thicknesses can be calculated from these optical parameters. This study helps in improving the optical design of dye-sensitized solar cells. INTRODUCTION As the economical and ecological concerns raised by fossil-based energy supplies increase, sustainable and renewable energy sources, such as photovoltaic solar energy, have drawn an increased attention. Providing a cost effective and affordable solar energy conversion technology is still a challenge nowadays [1]. Dye-sensitized solar cells, due to their low cost and low energy consumption in the manufacturing process, have a potential to become an alternative for the semiconductor-based solar cells [2]. A good light harvesting efficiency is the first step for achieving efficient solar energy devices. This requires an optical path length long enough to absorb a large fraction of the solar light. On the other hand, the charge diffusion length limits the thickness of the absorbing layer. To describe the optical properties for TiO2 porous films used in DSCs, an optical model [3] that considers 2 collimated and 2 diffuse light fluxes has been proposed. The comparison of the model with reflectance and transmittance spectra of scattering and absorbing films allows for the determination of the absorption and scattering coefficients k(λ), s(λ), and of the equivalent path length coefficient ε and of the forward scattering ratio coefficient ζ for each film. In this work, we study the light absorption and scattering in dye-sensitized porous nanocrystalline TiO2 films and focus on the effects of different particle sizes.
EXPERIMENT The colloidal TiO2 pastes of 30 and 60 nm. Ø particles are prepared by procedures described in reference [4]. The 20 nm. Ø paste is prepared according to reference [5]. Films are doctor bladed onto glass slides to form a thin film around 2µm controlled with scotch tapes. After sintering, the films are treated with TiCl4 (40mM in water) solution for 6 hours and sinte
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