Radiation induced recombination centers in organic solar cells

Abstract

Prolonged x-ray exposure of bulk heterojunction organic solar cells induces deep trap states that are observed in measurements of the photocurrent spectral response. The density of induced trap states is proportional to the density of recombination centers as measured by the voltage dependence of the photocurrent, therefore identifying the traps as primary recombination centers. The states are reversible by thermal annealing to about 100C, which implies a metastable structural change with binding energy 1-1.2 eV. However, the annealing kinetics reveal three different annealing processes, although for defect states with essentially the same electronic character. Analysis of the radiation damage indicates that defects are formed by hydrogen released from C-H bonds due to interaction with energetic secondary electron-hole pairs. Theoretical structure calculations of possible hydrogen related defects find specific defect states that match the experimental observations, and provide values for hydrogen migration energies. The effects of prolonged white light exposure are very similar to x-ray exposure, although the annealing properties are significantly different. Measurements of the spectral response with bias illumination provide further insight into the properties of the localized states.