Organic bulk heterojunction solar cells enhanced by spin interaction

One of the major losses in organic photovoltaic (OPV) devices has been the recombination of polaron pairs (PP) at the donor–acceptor (D–A) domain interfaces that prevent photogeneration of free charges. Here we report our study on a new method, namely doping the device active layer with spin ½ radical galvinoxyl, to suppress PP recombination at the D–A domain interfaces, and thereby improve the OPV solar cell efficiency. We demonstrate that at an optimal doping level of 3 wt.%, the efficiency of a standard regio-regular poly(3-hexylthiophene) (P3HT) and 1-[3-(methoxycarbonyl)propyl]-1-1-phenyl)[6,6]C61 (PCBM) solar cell improves by 18%. We found that this enhancement is not due to morphology change upon galvinoxyl doping, but rather is directly related to the spin degree of freedom of the radical, by reducing PP recombination rate at the P3HT/PCBM interfaces via spin flipping interaction. The enhancement is more significant in OPV systems that are PCBM-rich. We also conclude that the existence of D–A interface is crucial for the enhancement to occur. In addition, we also found that the galvinoxyl radical additives improve the efficiency of solar cells based on other D–A blends with of other donor polymers, but only with the acceptor fullerene PCBM. However, we believe this method may work with other D–A blends using other spin ½ radical additives.