On the response time of dye-sensitized solar cells to pulsed monochromatic illumination

It is common knowledge that the slow photocurrent response time of dye-sensitized solar cells (DSSCs) is due to trap-limited diffusion of photogenerated carriers in the mesoporous photoanode; therefore this quantity should depend on the carrier generation profile and on the carrier density, which are determined by the light wavelength and intensity, respectively. However, the dependence of the response time of DSSCs on these two parameters has scarcely been investigated. Here, we systematically measure the response time of two devices with different photoanode thickness to square-wave modulated illumination as a function of incident wavelength and of the intensity of the background bias light and we show that a simple transport model in the small-modulation approximation can account for the observed dependence. This information is relevant for the characterization of carrier transport in the device and for the design of correct photon conversion efficiency measurement setups when using lock-in techniques.