Can morphology tailoring improve the open circuit voltage of organic solar cells?

While the effect of interfacial morphology on the short circuit current (I-SC) of organic photovoltaic devices (OPVs) is well known, its impact on open circuit voltage (V-OC) and fill-factor (FF) are less clear. Since the output power of a solar cell P-out = ISCVOCFF, such understanding is critical for designing high-performance, morphology-engineered OPVs. In this letter, we provide an explicit analytical proof that any effort to radically improve V-OC by tailoring bulk heterojunction morphology is futile, because any increase in I-SC due to larger interface area is counterbalanced by corresponding increase in recombination current, so that the upper limit of V-OC(BHJ) cannot exceed that of the corresponding planar heterojunction devices, i.e., V-OC(BHJ) <= V-OC(PHJ). We discuss the implication of this V-OC-constraint on the efficiency optimization of organic solar cells. (C) 2012 American Institute of Physics. [doi:10.1063/1.3672221]