A Light Soaking Free Solution Processable Metal Oxide Cathode Interfacial Layer Enables High Efficiency in Bulk Heterojunction Polymer Solar Cells

The light soaking process is a well-known technique to mitigate defects on the surface and the energy band mismatch of the TiO2-based cathode interfacial layer (CIL) in organic solar cells (OSCs). However, the excess energy consumption by the light soaking process strongly hampers the commercially viable fabrication of OSC devices. In this study, without adopting the light soaking treatment, we improved the power conversion efficiency (PCE) of organic solar cells based on the TiO2 CIL by heteroatom (N and S) passivation and graphene incorporation. The N, S doping passivates the positively charged trap states (oxygen vacancy), and graphene incorporation enhances the carrier transportation of the TiO2 CIL. Interestingly, without light soaking treatment, similar to 100 times enhanced electron mobility and similar to 5 times shorter charge carrier lifetime are achieved with the heteroatom passivated (N and S) and graphene incorporated TiO2 CIL (TUG-TiO2), in contrast to the bare TiO2 CIL-based devices. The improvement in the carrier mobility of the TUG-TiO2 CIL is examined by ultraviolet photoelectron spectroscopy and Kelvin probe measurement. Moreover, the OSC device fabricated with TUG-TiO2 CIL along with PTB7-Th:PC7-1BM (ITO/TUG-TiO2/PTB7-Th:PC71BM/MoO3/Ag) as an active layer exhibits PCE up to 10.72% than its counter device (ITO/TiO2/PTB7-Th:PC71BM/MoO3/Ag and PCE is 6.18%), without compromising the stability. This work spotlights the salient feature of defect passivation on TiO2 for the enhancement in PCE and the stability of organic solar cells and also highlights an alternative strategy to avoid the light soaking treatment.