Characterization of Interfacial Modifiers for Hybrid Solar Cells

Abstract

This study systematically investigates the influence of different TiO2 surface modifiers on the device properties of TiO2–poly(3-hexylthiophene) (P3HT) hybrid solar cells to infer design rules for interfaces in hybrid solar cells. Bare TiO2 is compared to TiO2 modified with a Ru(II) dye (Z907), phenyl-C61-butyric acid (PCBA), the carboxylated polymer poly[3-(5-carboxypentyl)thiophene-2,5-diyl] (P3HT-COOH), and Sb2S3, respectively. Self-assembled monolayers are investigated for the former three modifiers, whereas a thin coating of only a few nanometers is used in the case of Sb2S3. Photoluminescence quenching analysis is performed for the different TiO2–P3HT interfaces to gain insight into the mechanism of charge separation. For further analysis, the different modifiers are tested in solar cells. We focus on bilayered devices with a well-defined near-planar TiO2–P3HT interface for easier data analysis. Absorption of light by the modifiers can thus be neglected. In addition, detailed current density–voltage curve analysis, photovoltage, and photocurrent decay measurements reveal mechanisms of charge carrier recombination and extraction. Our study underlines the importance of recombination control and matching energy levels as well as proper alignment of P3HT at the interface.