Interface passivation and electron transport improvement via employing calcium fluoride for polymer solar cells

To enhance the performance of inverted structure polymer solar cells (PSCs), interfacial engineering considered as an effective and straightforward method was employed. In this study, to overcome the surface traps and energy level mismatches of the electron transport layer, a means of interface passivation by evaporating an ultrathin CaF₂ layer above ZnO thin film as the electron transport layer was successfully adopted. We display that CaF₂ layer could passivate the surface traps of ZnO thin film and decrease the interfacial barrier between PC₆₁BM and ZnO, so that electron transfer efficiency is facilitated, the recombination of electrons and holes is inhibited at the contact interface, and the series resistance is reduced. After the introduction of the CaF₂ layer, the short-circuit current and the fill factor was greatly improved, also the power conversion efficiency (PCE) was increased from 3.21% of the reference device without the CaF₂ layer to 4.22% in the inverted PSCs based on P3HT:PC₆₁BM bulk heterojunction photoactive layer. These results could have special guiding significance for high-efficiency PSCs and also great potential for applications of photovoltaic devices in the future.