Characterisation of electron transport and back reaction in dye-sensitised nanocrystalline solar cells by small amplitude laser pulse excitation

The characterisation of the transport and interfacial reaction of electrons in dye-sensitised nanocrystalline solar cells is complicated by the non-linearity of these processes. This problem has been overcome by superimposing small amplitude pulsed laser excitation on steady background illumination. The laser perturbation of the photostationary state is sufficiently smell that the photocurrent and photovoltage responses can be fitted using constant values of the electron diffusion coefficient D-n and electron lifetime tau(n). Analytical and finite difference solutions of the continuity equation have been used to analyse the experimental photocurrent, photocharge and photovoltage transients, and the intensity dependence of D-n and of tau(n) has been established by varying the bias illumination level, and hence the de photocurrent density, j(dc). The intensity dependence of D-n (D(n)alpha j(dc)(0.68)) is attributed to trapping/detrapping involving a distribution of trapping levels. The intensity dependence of tau(n) (tau(n)alpha j(dc)(-0.62)) may indicate that the back reaction of electrons with I-3(-) is not first order in electron concentration. Other possible explanations are that the interfacial electron transfer rate constant depends on trap occupancy or on thr rate of surface or bulk electron diffusion. (C) 2000 Elsevier Science S.A. All rights reserved.