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 degrees C leads to lower V-oc compare to analogues 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 replaces. This observation is thus a strong indication that such changes in V-oc with annealing sequence are manifestation of changes at the top electrode-photoactive layer interface, and not structural changes in the bulk of the photoactive layer electronis 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 contracts to stem from an interfacial chemical reaction between the photoactives lasers and the metal electrodes. this chemically generated interfacial layer is removed upon electrode delamination, effectively reverting the Voc to its original value prior to thermal annealing when the top electrodes are replaced.