Competition between transport and recombination in dye solar cells at low light intensity

Dye‐sensitized solar cells (DSCs) are being considered as a promising technology for indoor photovoltaics due to their exceptional performance in ambient environments and under low illumination intensity conditions. The efficiency of electron collection in the photoanode of a DSC is determined by a delicate balance between electronic transport within the mesoporous metal oxide layer and recombination losses caused by electron transfer to dye cations and electron acceptors in the electrolyte. While state‐of‐the‐art DSCs have demonstrated quantitative electron collection under standard 1‐sun conditions, the nonlinear nature of the trapping/detrapping dynamics in the metal oxide photoanode with respect to the stored electronic charge can result in suboptimal performance under the low illumination intensity conditions commonly found in indoor environments. In this study, we thoroughly examine the key factors influencing electron collection in relation to light intensity using impedance spectroscopy and numerical analysis. Impedance analysis of test devices reveals that the electron diffusion length tends to decrease as the quasi‐Fermi level is lowered, approaching the critical limit (L n /d ∼ 1) at intensities characteristic of indoor illumination. We tested a range of TBP concentrations, to tune the position of the semiconductor band edge, and different thermal and chemical treatments of the TiO2 layer, showing that this low intensity limitation is inherent to the multiple‐trapping mechanism that governs the functioning of a DSC. We conclude that relatively high ideality factors, non‐optimal electrolyte compositions or small variations in the quality of the TiO2 layer can result in electron collection limitations that are not apparent under 1 sun but become noticeable at low light intensity levels.This article is protected by copyright. All rights reserved.