Beyond PCBM: methoxylated 1,4-bisbenzyl[60]fullerene adducts for efficient organic solar cells

Organic solar cells have been based mostly on conjugated polymers and the classic fullerene derivative PCBM and are characterized by modest open circuit voltages (V-oc). Increasing V-oc requires fullerene acceptors with higher LUMOs than PCBM. To date, most fullerene derivatives synthesized for this purpose either do not achieve the high photocurrent afforded by PCBM or show relatively poor compatibility with the next-generation low bandgap conjugated polymers used in high-efficiency organic solar cells. Here, we report the facile synthesis of methoxylated 1,4-bisbenzyl fullerene adducts and their application as efficient electron acceptors in conjugated polymer-based solar cells. The methoxy groups are found to be essential to increasing the LUMO levels, and accordingly the V-oc, of the devices compared to the parent 1,4-bisbenzyl fullerene, and more importantly, to PCBM. The best fullerene 1,4-bisadduct provides a similar to 20% enhancement in power conversion efficiency over PCBM when used with the classic crystalline polymer P3HT. When used in combination with a higher-performance low bandgap polymer, PTB7, the bisadduct both increases the device open-circuit voltage and maintains the high photocurrent provided by the more traditional PCBM. We also examine 10 different 1,4-fullerene bisadducts and show that the photovoltaic device performance is strongly influenced by the number and relative position of the methoxy substituents on the benzyl addends: moving a single methoxy substituent by one position on the benzyl rings can change the device efficiency by over a factor of 2.