Origin of Stark Signals Induced by Continuous Photoirradiation for Working Dye-Sensitized Solar Cells Revealed by Photoinduced Absorption Measurements

It has recently been reported that characteristic signals resulting from a Stark effect are included in the spectra of photoinduced absorption (PIA) measurements for dye-sensitized solar cells (DSCs), and the origin of the Stark effect has been discussed in several papers. Here, we explore the origin of the Stark signal observed under continuous photoirradiation from results of continuous wave (cw)-PIA experiments for several control DSCs of ruthenium-based sensitizers (N719). Typical line shape resulting from the Stark effect was observed in the quadrature phase of cw-PIA spectra under short-circuit conditions. The Stark signals are found to arise in relation to stable photocurrent and be independent of the presence of particular electrolyte ions. Time-resolved PIA measurements demonstrate that the Stark signals rise and decay in time scales much slower than the response time of injected electrons in TiO2. We conclude that redox actions of a pair of I-/I-3(-) are responsible for the Stark effect under continuous photoirradiation and conjecture that the observed Stark signals are due to local electric fields acting on neutral dyes given by complexes consisting of dyes and iodine molecules or iodine-based ions. We propose to use Stark signals to monitor electrostatic environments around neutral dyes in working DSCs.