2, National Renewable Energy Laboratory, Golden, Colorado, United States
3, Colorado School of Mines, Golden, Colorado, United States
Spiro-OMeTAD is currently the dominant hole-transport material (HTM) in perovskite solar cells. However, commonly employed dopants, such as LiTFSI, severely impact stability and lifetime. For instance, due to its hygroscopic nature, the use of LiTFSI can lead to water absorption that ultimately decomposes the perovskite. Additionally, lithium ions have been shown to diffuse from the HTL through the perovskite to the electron-transport layer. Recently, the use of a cost-effective carbazole-cored hole-transport material, EH44, and its pre-oxidized TFSI salt as a dopant has been shown to maintain up to 94% of its power conversion efficiency unencapsulated over 1,000 hours in ambient conditions. This outperforms Spiro-OMeTAD and analogous pre-oxidized TFSI salt as dopant. To this end, we have prepared a series of cross-linkable carbazole-cored hole-transport materials (HTMs) with varying levels of conjugation for improved hole mobility and conductivity within the HTL. Pre-oxidized salts for each HTM have been synthesized for use as lithium-free dopant in devices. Thermal properties of new HTMs are superior to EH44 while maintaining proper energetic alignment with the perovskite. These improvements will facilitate the commercialization of cost-effective, efficient, and stable perovskite solar cells in the imminent future. This is collaborative work with the National Renewable Energy Lab (NREL).