Electronic structure of Ba(OH)2 interface in inverted organic photovoltaics: Improved electron transport by charged state of C60

Abstract Lowering the work function (WF) of the cathode in inverted organic photovoltaics (OPVs) is critically important to obtaining a high power conversion efficiency (PCE). The insertion of functional interlayers between the cathode and acceptor is a widely employed strategy to lower the WF. Among these functional materials, Ba(OH) 2 is known to be an efficient solution-processable cathode buffer layer that improves the electron transport in organic optoelectronic devices. Despite several reports of device performance enhancement with the use of a Ba(OH) 2 layer, its interfacial energetics is yet to be clearly understood. In this study, we investigated the electronic structure of Ba(OH) 2 interfaces and the improvement in the PCE of inverted small molecule OPVs with the use of a Ba(OH) 2 layer. On implementing the optimum thickness of the Ba(OH) 2 layer, the PCE of the OPVs was significantly enhanced from 1.29% to 3.41%, and the S-shaped kink in the current-density–voltage curve was eliminated. To elucidate the underlying mechanism of this phenomenon, we explored the interfacial electronic structures of C 60 /indium tin oxide (ITO) and C 60 /Ba(OH) 2 /ITO using in situ photoelectron spectroscopy. The spin-coated Ba(OH) 2 was physisorbed onto the ITO, which significantly reduced its WF. Owing to the Ba(OH) 2 , the reduced WF of the ITO is lower than the electron affinity of C 60 . Thus, a charge transfer is induced from Ba(OH) 2 /ITO to C 60 , and the charged states of C 60 are observed within the monolayer. These charged states result in significant band bending in the C 60 layer, such that its lowest unoccupied molecular orbital (LUMO) level shifts toward the Fermi level ( E F ) of the Ba(OH) 2 /ITO. As a result, the energy offset between the C 60 LUMO level and the cathode E F is substantially reduced from 0.45 eV to 0.15 eV with the use of the Ba(OH) 2 layer. This is the origin of the enhanced device performance of OPVs.