Compositionally engineered vacancy-ordered double-perovskite nanocrystals for photovoltaic application

Lead-free double perovskites, in particular tin-based perovskites, have emerged to be a promising photo-absorber in solar cells for attaining protruding efficiency and stability. However, the tin-based double perovskite nanocrystals are yet to be investigated in photovoltaics. There are several advantages of nano-crystals over their bulk counterpart, including tunable band gap, higher fluorescence, and higher carrier lifetime, to name a few. In this work, we synthesized vacancy-ordered Cs₂SnI₆ nanocrystals (NC) and explored their potential in solar cell applications as a photo-absorber for the first time. Due to the 4+ oxidation state of Sn, this perovskite has a high stability against oxidation and hydrolysis during manufacturing and device operation. We also incorporated alkali-metal ion Rb⁺ in the Cs₂SnI₆ NCs for a possible enhancement of efficiency and stability in photovoltaic cells. Better carrier extraction was found through this incorporation as the optical band gap of the NCs is narrowed down to 1.39 eV. The Density Functional Theory (DFT) study revealed the change in the electronic band gap and density of states (DOS) of the NCs upon Rb-doping. The photovoltaic device fabricated with the FTO/TiO₂/(Cs_1−xRb_x)₂SnI₆ NC/CuI/Au architecture has achieved a maximum efficiency of 0.71% with an inspiring short-circuit current (J_sc) of ∼5 mA/cm². The champion cell has also shown remarkable stability after ∼40 days in an ambient atmosphere. This work, therefore, provides new insights into the alkali-metal ion regulated lead-free fully inorganic perovskite NCs for stable photovoltaic application.