Achieving fill factor above 80% in organic solar cells by charged interface

Poor carrier mobility in organic semiconductors and high free carrier recombination at the polymer interfaces hinder efficient collection of photogenerated carriers in organic photovoltaic (OPV) devices. This inefficiency in charge collection is reflected in the low fill factor (FF) of the OPV cells (FF^OPV ∼ 65%) compared with its inorganic counterparts (FF ∼ 85%). In this paper, we show that the FF of the OPV devices made of the conventional low-mobility materials can be radically improved (>80%) by introducing a “fixed charge layer” at the donor-acceptor interfaces. We find that the fixed charges prevent free-carrier accumulation at the interface and, hence, minimize charge loss due to interfacial recombination. We use detailed device simulation to estimate the fixed charge density required for significant performance gain in typical P3HT:PCBM cells. We conclude by suggesting several strategies for introducing such charged interfaces within the conventional OPV device structure.