The Rise of Colloidal Lead Halide Perovskite Quantum Dot Solar Cells

CONSPECTUS: The colloidal synthesis of low-dimensional metal halide perovskite quantum dots (PQDs) has endowed the emerging semiconductors with new peculiarities in optical and electrical properties, such as quantum confinement-induced size tunability, separated perovskite crystallization from film deposition, layer-by-layer processing, improved structural stability, etc., excavating more potential of the perovskites as a promising candidate for next-generation photovoltaics. The first PQD solar cell was first reported in 2016 by Luther and co-workers, with a high open circuit voltage of 1.23 V and efficiency of 10.77%, as well as excellent air stability, showing great competitiveness relative to the polycrystalline thinfilm analogues. Benefiting from the outstanding properties of perovskites themselves and rational surface manipulation strategies of colloidal PQDs, the PQD solar cells have achieved a record efficiency of 13.4% in 2017 that surpassed all reported QD solar cells. Since then, the PQD solar cells have attracted increasing attention and study and reached a highest certified efficiency of 16.6% to date, indicating its great potential toward stable and efficient perovskite solar cells. However, the insufficient carrier transport between the PQDs because of the remaining long capping ligands has tremendously hindered the performance of the PQD solar cells and their further development. Paradoxically, removing all of the binding ligands of PQDs will break the intact perovskite structure. Balancing the efficient carrier transport and stable crystal structure of PQDs is a crucial and challenging task that needs to be addressed to pursuit a new generation of low-cost photovoltaic technology. This Account summarizes the typical strategies reported from our group to improve the photovoltaic efficiency, as well as long-term stability of the PQD solar cells during the past several years in term of synthesis, composition, and surface chemistry regulation of PQDs and device architecture engineering. We highlight the unique properties of PQDs and corresponding integrated solar cells that distinguish them from their polycrystalline counterparts, including the surface chemistry, structure stability, and fabrication techniques, and meanwhile solve the puzzle of why PQDs can be applied in solar cells. In the end, we discuss the challenges for current PQD solar cells and propose possible solutions to them, and we point out some directions for the future development of PQD solar cells, such as direct synthesis of conductive PQDs, lead-free tandem devices, scalable fabrication, and extended optoelectronic application. We expect that this Account would open up more thorough insights into PQD solar cells and propel this field further.