Improvement of organic solar cells with ammonium salt, tetrabutylammonium tetraphenylborate, as cathode buffer layer

A solution-processible ammonium salt, tetrabutylammonium tetraphenylborate (TBATPB) was investigated as cathode buffer layer in organic solar cells (OSCs). Using this buffer layer the power conversion efficiencies of OSCs were improved greatly with Al as cathode, compared to those without buffer layer. These devices performed even better than those with Ca/Al as cathode. Explored by UPS, it was found that after TBATPB spin-coated onto the surface of Al, the work function of Al was pulled down. This suggested that there existed dipoles between TBATPB and Al. As a result the built-in potential was enhanced and it was beneficial to the extraction of charge carriers. Furthermore our results proved that TBATPB could be a very useful cathode buffer layer in three different donor:acceptor systems, namely P3HT:PC61BM, PBDTTT-C:PC71BM and PTB7:PC71BM, indicating its importance in fabricating high-performance OSCs.