Light intensity, temperature, and thickness dependence of the open-circuit voltage in solid-state dye-sensitized solar cells

Abstract

We present an analytical and experimental investigation into the origin of the open-circuit voltage in the solid-state dye-sensitized solar cell. Through Kelvin probe microscopy, we demonstrate that a macroscopically uniform electric field exists throughout the nanocomposite between the electrodes. Considering a balance between drift and diffusion currents, and between charge generation and recombination, we develop an analytical expression for the open-circuit voltage which accurately follows experimental data. We find the open-circuit voltage increases with light intensity as $1.7\phantom{\rule{0.2em}{0ex}}kT∕q$, where $T$ is absolute temperature, however it decreases with increasing temperature and device thickness. The intensity dependence arises from the charge generation rate increasing more strongly with intensity than the recombination rate constant, resulting in increased chemical potential within the device. We find that the temperature dependence arises from a reduction in the charge lifetime and not from increased charge diffusion and mobility. The thickness dependence is found to derive from two factors; first, charge recombination sites are distributed throughout the film, enabling more charges to recombine in thicker films before influencing the potential at the electrodes, and second, the average optical power density within the film reduces with increasing film thickness.