Transition metal oxide as hole transport layer for crystalline silicon solar cells: Progress and prospects

High-efficiency crystalline silicon (c-Si) solar cells, including silicon heterojunction (SHJ) and tunnel oxide passivating contact (TOPCon), are hampered by their capital-intensive preparation process, which necessitate the use of flammable and toxic gasses. Moreover, the parasitic absorption of the doped amorphous silicon within SHJ cells, particularly in the short wavelength range, inevitably leads to a decline in device performance. Given these drawbacks, silicon compound heterojunction (SCH) solar cells have garnered significant attention owing to their reduced parasitic absorption, lower production costs, and simpler preparation procedures. The electron transport layer (ETL) and hole transport layer (HTL) ensure efficient extraction and transport of electrons and holes, and play a decisive role in device performance. The HTL materials include transition metal oxide materials (TMOs), organic materials, low-dimensional semiconductor materials, etc. This comprehensive review summarizes the research progress of SCH solar cells, with a focus on TMOs as HTL. The review primarily delves into the transport mechanism of TMOs as HTL, the factors influencing device performance, and the corresponding mitigation strategies. This review aims to foster a deeper understanding of the performance-impacting factors and offer insights for the future development of high-efficiency solar cells.