Intensity Dependence of the Back Reaction and Transport of Electrons in Dye-Sensitized Nanocrystalline TiO2 Solar Cells

• Published in: The journal of physical chemistry. B 2000-02, Vol.104 (5), p.949-958
• Main Authors: Fisher, A. C, Peter, L. M, Ponomarev, E. A, Walker, A. B, Wijayantha, K. G. U
• Format: Article
• Language: English
• ISSN: 1520-6106; 1520-5207
• DOI: 10.1021/jp993220b

The lifetime τ n and diffusion coefficient D n of photoinjected electrons have been measured in a dye-sensitized nanocrystalline TiO2 solar cell over 5 orders of magnitude of illumination intensity using intensity-modulated photovoltage and photocurrent spectroscopies. τ n was found to be inversely proportional to the square root of the steady-state light intensity, I 0, whereas D n varied with I 0 0.68. The intensity dependence of τ n is interpreted as evidence that the back reaction of electrons with I3 - may be second order in electron density. The intensity dependence of D n is attributed to an exponential trap density distribution of the form N t(E) ∝ exp[−β(E − E c)/(k B T)] with β ≈ 0.6. Since τ n and D n vary with intensity in opposite senses, the calculated electron diffusion length L n = (D n τ n )1/2 falls by less than a factor of 5 when the intensity is reduced by 5 orders of magnitude. The incident photon to current efficiency (IPCE) is predicted to decrease by less than 10% over the same range of illumination intensity, and the experimental results confirm this prediction.