Enhanced thermal stability of organic solar cells by using photolinkable end-capped polythiophenes

The use of poly(3-hexylthiophene) (P3HT) end-capped with anthracene (P3HT-A) in a blend with [6,6]-phenyl C-61 butyric acid methyl ester (PCBM) is demonstrated to physically stabilize bulk-heterojunction photovoltaic solar cells. Bulk heterojunction-based devices are known to undergo phase separation of donor and acceptor materials during operation resulting in the formation of large (mu m scale) PCBM crystals that dramatically decrease the photovoltaic characteristics of the cell. By way of a facile UV-curing step, the P3HT-A chain most likely reacts with PCBM fullerene via a [2+2] cyclo-addition to stabilize the blend. Photovoltaic devices based on P3HT-A and PCBM have been optimised in terms of thermal annealing to obtain initial devices to determine the UV-curing protocols. UV-exposure was found to improve device stability while simultaneously having a minimal effect on device efficiency. Optical microscopy demonstrates that the few reactions of P3HT-A with PCBM are efficient enough to prevent the formation of micro-sized PCBM crystals responsible for the failure of solar cells.