Electronic Trap States in Methanofullerenes

TL;DR

This study investigates trap states in three fullerene derivatives using thermally stimulated current measurements, revealing trap densities, energy distributions, and their potential impact on organic solar cell performance.

Contribution

It provides detailed characterization of trap states in methanofullerenes, highlighting differences among derivatives and implications for device efficiency.

Findings

Trap densities are around 10^22 m^-3 for all studied fullerenes.

Trap energy distributions vary, with broad ranges from 15 meV to 270 meV.

Higher trap energies in PC71BM and bisPC61BM suggest different charge trapping behaviors.

Abstract

The trap states in three fullerene derivatives, namely PC61BM ([6,6]-phenyl C61 butyric acid methyl ester), bisPC61BM (bis[6,6]-phenyl C61 butyric acid methyl ester) and PC71BM ([6,6]-phenyl C71 butyric acid methyl ester), are investigated by thermally stimulated current measurements (TSC). Thereby, the lower limit of the trap densities for all studied methanofullerenes exhibits values in the order of 10^22 m^-3 with the highest trap density in bisPC61BM and the lowest in PC61BM. Fractional TSC measurements on PC61BM reveal a broad trap distribution instead of discrete trap levels with activation energies ranging from 15 meV to 270 meV and the maximum at about 75 meV. The activation energies of the most prominent traps in the other two fullerene derivatives are significantly higher, being at 96 meV and 223 meV for PC71BM and 184 meV for bisPC61BM, respectively. The influence of these findings on the performance of organic solar cells is discussed.