Deep defects and the attempt to escape frequency in organic photovoltaic materials

摘要

Trap states are well-known to plague organic photovoltaic devices and their characterization is essential for continued progress. This letter reports on both the deep trap profiles and kinetics of trap emission, studied through temperature dependent capacitance measurements. Three polymer based systems relevant to photovoltaics, namely, P3HT:PC_(60)BM, PTB7:PC_(70)BM, and PCDTBT:PC_(70)BM were investigated. Each polymer showed a markedly different deep trap profile, varying in shape from a nearly constant density of states to a sharp Gaussian. In contrast, the frequency of trap emission was similar for each-ca. 10~8 - 10~9 Hz-indicating a universal value and similar trapping mechanisms despite the differences in energetic distribution. The latter result is important in the light of range of conflicting values reported, or higher value (10~(12)Hz) typically borrowed from crystalline inorganic materials.