Lead-free halide double perovskites (A2B1B2X6), are derived from ABX3 single perovskites through cation engineering in divalent B site where B-site Pb2+ is replaced by a monovalent B1 and heterovalent B2-cation forming two structurally distinct corner-sharing octahedra of [B1X6]n- and [B2X6]n- with B1 and B2 present in octahedra formed by halide ions (Cl-, Br-, I-) and A-site occupying the cuboctahedral cavity in a three-dimensional crystal structure. Perovskite halides have long been known for their excellent optoelectronic properties and light harvesting applications such as solar cells, photodetectors, x-ray detectors, photocatalysis, photoelectrocatalysis and light emitting diodes.
The structural and optical tunability, stability in ambient condition and the ease of device fabrication make this class of lead-free halide perovskites an attractive choice for different optoelectronic applications. However, there is still a considerable research gap when it comes to the repeatability of device performance owing to different factors such trap states formation, defects engineering, quantum confinement effect etc. which comes into picture due to variation in synthesis conditions (as time, temperature, pressure, concentration plays a crucial role in the properties of these materials), different protocol for device fabrication and testing and a lack of clear guideline that can act as an rulebook to move forward the research in this field.
In this work, we present an in-depth study on lead free halide perovskite where we started with Cs2AgB’X6 (B’= In3+, Bi3+ and Sb3+) where we demonstrated the structural tunability in lead free halide double perovskite with mixed vacancies of trivalent cation. For our studies we synthesized a range of halide double perovskite with general formula of Cs2AgInxBi1-xCl6, Cs2AgInxSb1-xCl6 and Cs2AgBixSb1-xCl6 with x= 0, 0.2, 0.4, 0.6, 0.8 and 1. With the in-depth study of all these materials, we successfully demonstrated the effect of trivalent cation engineering and octahedral distortion on morphological, light absorption, photoluminescence and excited state lifetime properties. The single crystal x-rays diffraction with FT-far-IR and Raman spectroscopy was used as a tool to understand the structural distortion and P-E loop studies were also done for the same. To understand the electronic behaviour as a result of structural distortion, density functional theory (DFT) was used to calculate the band positions and band behaviour in these materials.
In the later stage of our work, we tried to lay a generalized map correlating the distortion in lead free halide perovskite with their structural and optoelectronic properties and have successfully synthesizes material with different order of inter and intra octahedral distortion which also led to change in dimensionality in some cases. Finally, from our preliminary studies we observed the potential of these materials for different set of photovoltaic and solar fuels applications and for this purpose we extended our understanding of these materials and used them for solar cell, solar water splitting and white light emission applications