Electron Trapping in Higher Adduct Fullerene-Based Solar Cells

Here, the performance of bulk-heterojunction solar cells based on a series of bisadduct analogues of commonly used derivatives of C-60 and C-70, such PCBMs and their thienyl versions, is investigated. Due to their highest lowest unoccupied molecular orbital an increase in open-circuit voltage and thus performance is expected. It is shown that the occurrence of a multitude of different isomers results in a decrease in the electron transport for some of the materials. Surprisingly, the solar-cell characteristics are very similar for all materials. This apparent discrepancy is explained by a significant amount of shallow trapping occurring in the fullerene phase that does not hamper the solar cell performance due the filling of these shallow traps during illumination. Furthermore, the trisadduct analogue of [60]PCBM has been investigated, which, despite an even further increase in open-circuit voltage, results in a significantly reduced device performance due to a strong deterioration of the electron mobility in the fullerene phase.