Correlating reduced fill factor in polymer solar cells to contact effects

A probable limiting factor for efficiency and fill factors of organic solar cells originates from the cathode-polymer interface. We utilize various forms of cathode layer such as Al, Ca, oxidized Ca, and low melting point alloys in model systems to emphasize this aspect in our studies. The current-voltage (JV) response in the fourth quadrant indicates a general trend of convex shaped JV characteristics (d2J∕dV2>0) for illuminated devices with good cathode-polymer interfaces and linear or concave JV responses (d2J∕dV2<0) for inefficient cathode-polymer interfaces.