Identification of the loss mechanisms in TiO2 and ZnO solar cells based on blue, piperidinyl-substituted, mono-anhydride perylene dyes

Abstract

While perylene dyes have suitable properties for dye-sensitized solar cells (DSSCs), the performance has been limited by various factors that are not fully understood yet. We prepared DSSCs based on mesoporous TiO2 and ZnO with three blue piperidinyl-substituted perylene derivatives and three different electrolyte solutions. The perylene dyes have different non-conjugated side units and HOMO levels, and the anchoring group is a mono-anhydride moiety that would provide better compatibility with ZnO. Using current - voltage characteristics, incident and absorbed photon-to-electron conversion efficiency measurements (IPCE/APCE), stepped light induced transient measurements, and transient absorption spectroscopy (TAS), the limiting processes governing the solar cell performance are analyzed. With TiO2, weak electronic coupling between the dye and TiO2 appears to limit the injection efficiency, while for ZnO, the coupling seems stronger resulting in higher injection yield and higher APCE. The HOMO levels and structural differences of the dyes have only a minor impact on the reduction kinetics of oxidized dyes. DFT calculations illustrate that the HOMO and LUMO of the dye are spatially not well separated. This probably leads to a short-excited state lifetime, resulting in the relatively low APCE values. The TA measurements of the cells with and without redox couple show similar decays, suggesting the oxidized dyes in the solar cells are reduced not only by redox couple but also by electrons from TiO2 and ZnO, further decreasing the conversion efficiency. These results suggest the design rules to be followed for the successful implementation of all-organic perylene-based dyes for DSSCs.