Investigating the Influence of Interfacial Contact Properties on Open Circuit Voltages in Organic Photovoltaic Performance: Work Function Versus Selectivity

The role of work function and thermodynamic selectivity of hole collecting contacts on the origin of open circuit voltage (V OC ) in bulk heterojunction organic photovoltaics is examined for poly(N‐9′‐heptadecanyl‐2,7‐carbazole‐alt‐5,5‐(4′,7′‐di‐2‐thienyl‐2′,1′,3′‐benzothiadiazole) (PCDTBT) and [6,6]‐phenyl‐C 71 butyric acid methyl ester (PC 71 BM) solar cells. In the absence of a charge selective, electron blocking contact, systematic variation of the work function of the contact directly dictates the V OC , as defined by the energetic separation between the relative Fermi levels for holes and electrons, with little change in the observed dark saturation current, J 0 . Improving the charge selectivity of the contact through an increased barrier to electron injection from the fullerene in the blend into the hole contact results in a decreased reverse saturation current (decreased J 0 and increased shunt resistance, R SH ) and improved V OC . Based on these observations, we provide a set of contact design criteria for tuning the V OC in bulk heterojunction organic photovoltaics.