The role of work function and thermodynamic selectivity of hole collecting contacts on the origin of open circuit voltage (VOC) 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 VOC, 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, RSH) and improved VOC. Based on these observations, we provide a set of contact design criteria for tuning the VOC 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 VOC; 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.
- work function
ASJC Scopus subject areas
- Renewable Energy, Sustainability and the Environment
- Materials Science(all)