Intensity Dependence of the Back Reaction and Transport of Electrons in Dye-Sensitized Nanocrystalline TiO2Solar Cells

Abstract

The lifetime τ n and diffusion coefficient D n of photoinjected electrons have been measured in a dye-sensitized nanocrystalline TiO 2 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 I 3 - 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.