Annealing sequence dependent open-circuit voltage of inverted polymer solar cells attributable to interfacial chemical reaction between top electrodes and photoactive layers

The ability to laminate and delaminate top metal contacts during the processing and testing of inverted polymer solar cells has led us to uncover the peculiar dependence of their open-circuit voltage (V_oc) on the annealing sequence. Specifically, thermally annealing inverted polymer solar cells having bulk-heterojunction photoactive layers after top electrode deposition above 100 °C leads to lower V_oc compared to analogous devices with unannealed photoactive layers or photoactive layers that have been annealed prior to metal electrode deposition. This reduction in V_oc, however, can be reversed when the top electrodes are replaced. This observation is thus a strong indication that such changes in V_oc with annealing sequence are manifestations of changes at the top electrode-photoactive layer interface, and not structural changes in the bulk of the photoactive layer. Electronic characterization conducted on the photoactive layers and metal contacts after dissection of the polymer solar cells via delamination suggests the reduction of V_oc on thermal annealing in the presence of the metal top contacts to stem from an interfacial chemical reaction between the photoactive layers and the metal electrodes. This chemically generated interfacial layer is removed upon electrode delamination, effectively reverting the V_oc to its original value prior to thermal annealing when the top electrodes are replaced.