Less is More: Dopant-Free Hole Transporting Materials for High-Efficiency Perovskite Solar Cells

Perovskite solar cells have delivered power conversion efficiency beyond 22% in less than seven years, implying the potential for the paradigm shift of lowcost photovoltaics with high efficiency and low embedded energy. Besides the perovskite fever, the development of new hole transport materials (HTM), especially dopant-free HTMs, is another research hotspot. This is because the currently used HTMs, such as spiro-OMeTAD derivatives, require additional chemical doping process to ensure sufficient conductivity and proper ionic potential level for efficient hole transport and collection. However, the commonly used dopants are volatile and hygroscopic which not only increase the complexity and cost of device fabrication but also deteriorate the device stability. So far, there have been several reviews on new HTMs, but review or analysis on dopant-free HTMs is scarce. In this review, all reported dopant-free HTMs are categorized into four primary different types and lessons will be learned during the separate discussions. The stability test behavior of all the intrinsic HTMs will be evaluated directly. In the end, the correlations between the properties of the intrinsic HTMs and parameters of the devices will be plotted to shed light on the future direction of development of this field.