Enhanced photovoltaic performance of polymer solar cells by adding fullerene end-capped polyethylene glycol

A fullerene end-capped polyethylene glycol has been synthesized and used as an additive in organic solar cells based on poly(3-hexylthiophene) and phenyl-C61-butyric acid methyl ester (PCBM). It was found that an appropriate amount of the additive can dramatically improve both the photovoltaic performance and the lifetime of the devices, which have been attributed to the successful morphological control of PCBM by the additive. Under transmission electron microscopy, uniformly distributed and stable PCBM crystallites with an average size of only several nanometres in the active layer were observed. We speculate that the immobile fullerenes bonded to polyethylene glycol chains act as nucleation centers for PCBM crystallites and thus result in a stable and uniform PCBM distribution in the active layer, which is essential for high performance organic solar cells. Since PCBM is used as a universal acceptor in polymer solar cells, these results suggest that such an additive can be used in many other different polymer solar cells to control the morphology of PCBM and improve the device performance.