Probing the free electron density and diffusion length in dye-sensitized solar cells

Abstract

Highly time-resolved laser induced photovoltage transients have been obtained for a range of steady state background photovoltages on a 4 m N719sensitised DSC. The technique, first proposed by O’Regan, allows the determination of the apparent electron diffusion coefficient from the time taken for the photovoltage to rise from a steady state value as a consequence of a small extra pulse of light. We have extended the technique to measure the apparent electron lifetime and diffusion coefficient under the same operating conditions, at open circuit. This has allowed computation of the diffusion length without the need to make assumptions about the average electron density throughout the TiO2 under short circuit conditions. The diffusion length, which was found to increase from approximately 40 to 70 microns over the range studied, was in reasonable agreement with the results of IMVS and IMPS, once the latter had been corrected for the equivalent quasi Fermi level position under open circuit conditions.

A microwave reflectivity technique, designed to probe the free electron density by measuring the conductivity in TiO2, was developed and applied to a working DSC for the first time. This allowed simultaneous measurement of the photovoltage and conductivity as a function of light intensity. In order to cross check the validity of these measurements, modelling based on the Fresnel reflection coefficients was carried out and the conductivity was also measured directly across a ca. 250 m insulating gap between two FTO electrodes. Assuming a constant electron mobility, these conductivity measurements provide an estimate of the free electron concentration, which is a key parameter in DSC modelling and characterisation, and yet has not been measured until now. These measurements have allowed us to begin to unlock some deeprooted unknowns of DSC function, namely the well documented, but poorly understood nonideality in the relationship between the photovoltage and the light intensity. Finally, the microwave reflectivity technique was adapted to make preliminary measurements of the free electron density at short circuit, and frequencyresolved measurements of the apparent electron lifetime which were in good agreement with that measured by IMVS.

Date of Award	1 Dec 2009
Original language	English
Awarding Institution	University of Bath
Supervisor	Laurie Peter (Supervisor)