Impurity-related degradation in a prototype organic photovoltaic material: A first-principles study

The phenyl-C61-butyric acid methyl ester (PCBM) fullerene derivative is among the most widely used acceptor materials in organic photovoltaics (OPVs). Similar to the case of several other organic semiconductors and systems, experimental data show that the performance of OPV blends of PCBM with a polythiophene polymer degrades when the system is exposed to ambient molecules, such as O-2 and H2O. Here we use first-principles calculations to identify physical mechanisms that can give rise to this type of degradation. We find that oxygen impurities can be incorporated in PCBM crystals and form several different types of configurations that range from intact molecules in crystalline voids to oxidized PCBM molecules. A number of O-related impurity structures generate states within the energy band gap of the PCBM crystal, creating thus shallow and deep carrier traps. Likewise, incorporation of H2O molecules gives rise to a shallow acceptor-like trap. The results are consistent with pertinent experimental observations of degradation paths for air-exposed PCBM samples and can aid in the optimization of PCBM-based OPV blends. (C) 2013 Elsevier B. V. All rights reserved.