Multifunctional quantum dot materials for perovskite solar cells: Charge transport, efficiency and stability

Perovskite solar cells (PSCs) have recently emerged as an ideal candidate for next-generation photovoltaic applications. While promising, many challenges stand between PSCs and widespread application, including moisture, thermal and UV stability, photocurrent hysteresis behavior, flexibility, and large-scale productions. Meanwhile, quantum dot materials have attracted intensive research interest within past decades owing to their fantastic optical, electrical and optoelectrical properties, such as size-dependent energy band gaps derived from quantum confinement, high photon absorption coefficient, and multiple exciton generation. Their facile solution synthesis, tunable energy-level structures, and variable surface chemistry via ligand engineering make quantum dots (QDs) attractive for a variety of significant functions in PSCs. In this review, we summarize how a variety of QD materials (e.g., carbon, graphene, metal oxides, metal sulfides, metal selenides, metal tellurides, black phosphorus, organic/inorganic halide perovskites, etc.) can be applied in PSCs. We detail that QDs can play diverse roles in PSCs, including light harvesters, electron and hole transporters, additives into perovskite and charge transport layers, and interfacial modifiers. Particularly, the introduction of QD materials into PSCs enables the growth of high-quality perovskite films with larger grain sizes and reduced trap-state density due to the strong chemical interaction between QDs and perovskites, yielding high efficiency of stable PSCs. The size-dependent energy band gaps of QDs enable enhanced energy-level alignment for efficient charge transfer in PSCs. Moreover, the incorporation of QDs capped with highly hydrophobic ligands can enhance the long-term moisture stability of PSCs. Additionally, the photoluminescence property of QDs can be used to convert UV-radiation into harvestable visible light to improve the photocurrent and photostability of PSCs. The different characteristics and functions of QDs in PSCs are then discussed. Finally, insight into the further development of QD materials in PSCs is outlined.