Overcoming degradation in organic photovoltaics: Illuminating the role of fullerene functionalization

Photobleaching rates are investigated for thin films of poly(3-hexylthiophene) (P3HT) blends employing either an indene-C₆₀ bisadduct (ICBA) or [6,6]-phenyl-C₆₁-butyric acid methyl ester (PCBM) as the electron acceptor. Relative to the bisindene, PCBM significantly enhances resistance to photobleaching of the P3HT donor polymer. We tentatively attribute a decrease in the charge transfer rate as the mechanism responsible for the more rapid photobleaching in the sample containing the bisindene adduct. In order to elucidate the influence of the photobleaching rate on the initial performance of unencapsulated devices, we also monitored the time-dependent behavior for P3HT:fullerene inverted devices. Under conditions of constant illumination, we observe essentially identical behavior in device performance parameters regardless of the energy levels of the electron acceptor. We conclude that over the time frame measured for these devices, the primary degradation mechanism of the active layer is independent of the electron acceptor, despite the enhanced tolerance to photobleaching it may impart to the donor material.