2, National Renewable Energy Laboratory, Golden, Colorado, United States
In recent years perovskite photovoltaics have reached impressive research lab scale power conversion efficiencies (PCE) >22%. Although the efficiency of perovskite photovoltaics is competitive with other photovoltaic technologies, commercialization is impeded by their low stability in ambient conditions and elevated application relevant conditions. The hole-transporting layer (HTL) within these devices is crucial for high performance. Typically the HTL in high performing perovskite devices is Spiro-OMeTAD and a lithium-based dopant. Spiro-OMeTAD takes multiple challenging synthetic steps, therefore it is expensive, and the lithium dopant reduces device stability. Initial work using a synthetically simple carbazole-based HTL, EH44, has shown similar PCE to Spiro-OMeTAD and superior stability in ambient conditions over 1000 hrs without encapsulation. We have developed a series of novel carbazole-based hole transporting materials (HTMs) with increased conjugation and preoxidized salts of these HTMs were used as a lithium-free dopant when preparing the HTL to increase hole mobility and conductivity within the HTL. We have also demonstrated a crosslinkable HTM to enhance the device long-term stability. The hole mobility of these HTMs were measured using space charge limited current (SCLC). The device stability was characterized under ambient moisture and oxygen conditions, as well as at elevated temperatures of 80°C. The enhanced stability of these HTMs and simplified synthetic methods should make the commercialization of perovskite photovoltaics easier.