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-C₇₁ butyric acid methyl ester (PC₇₁BM) 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, J₀. 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 J₀ 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. Contact work function and thermodynamic selectivity both play key roles in determining the open circuit voltage in PCDTBT:PC₇₁BM OPVs. Systematically increasing the work function of the contact increases the V_OC; adding a selective contact with the same work function further increases the V _OC due to a change in the space charge region at the active layer/contact interface.