Earth-abundant non-toxic perovskite nanocrystals for solution processed solar cells

Semiconductor nanocrystals, used in quantum dot solar cells, are interesting materials for photovoltaics because they can be obtained in solution and can be composed of abundant elements. Moreover, as compared to other photovoltaic materials, nanomaterials show unique features due to their novel size- and shape-dependent properties such as band gap tuning, multiple exciton generation, and modulation of n- or p-type behaviour by doping or by modifying the ligands on the surface of the nanocrystals. Quantum dot solar cells, together with perovskite solar cells, are the latest incorporation to photovoltaic technologies and have already shown impressive progress in efficiencies and great promise as alternatives to commercial solar cells. However, in all cases, the highest efficiencies are obtained with materials that contain lead in their composition. To solve the problem of toxicity, several materials have been proposed as substitutes. In this review, we summarize some of the non-toxic alternatives that have been synthesized as nanocrystals and incorporated in photovoltaic solar cells, specifically: tin (Sn), germanium (Ge), bismuth (Bi), and antimony (Sb)-based materials. Our findings show that this field has been scarcely covered; there are very few reports on non-toxic perovskite nanocrystals incorporated in solar cells and in general, the efficiencies are still modest. However, this area deserves more attention since some nanocrystal-based solar cells already outperform bulk counterparts. For each case, we also discuss factors limiting efficiency, the approaches followed to overcome these limitations, and the possible solutions to improve efficiency.