All‐Inorganic Perovskite Photovoltaics

As the perovskite solar cells (PSCs) continue to make progress on their power conversion efficiency (PCE), the apprehension about their long-term stability that presently falls short of commercial requirements grows bigger. A fair amount of evidence reveals that the organic part of organic–inorganic hybrid perovskite crystals is a weaker link to thermal and environmental stability. Therefore, all-inorganic perovskites that do not include any organic cation in the structure have received a lot of attention in the recent years. It is quite encouraging that the PCE of some all-organic perovskites (CsPbI 3 ) has reached 19% in a relatively short time of development. As expected, in comparison with organic–inorganic hybrid perovskites, all-inorganic perovskites demonstrate significantly greater thermal stability by not undergoing any chemical change even up to 400 °C in certain cases. However, the latter faces a different challenge, which is crystallization/stabilization of its high-temperature photoactive phases at lower temperatures. Therefore, several methods such as the use of additives, metal doping, surface treatment, etc. have been followed to stabilize the photoactive phase of CsPbI 3 at lower temperatures. Besides, compositional engineering, that is, mixing different halides (I and Br) in the structure, and interfacial modifications have also helped to improve the structural stability and cell efficiency. In this chapter, we discuss different strategies used in the development of PSCs based on all-inorganic perovskites of different compositions like cesium lead iodide, cesium lead bromide, and cesium lead mixed-halide perovskites. As Pb toxicity remains a concern in general, several Sn-based all-inorganic perovskites like CsSnX 3 and Cs 2 SnX 6 have also gained interest owing to their excellent optoelectronic properties. However, the major challenge that remains with CsSnI 3 is easy oxidation of the Sn 2+ to Sn 4+ . Different approaches have been employed to restrict/slow down the oxidation of Sn 2+ and thereby improve the stability of CsSnI 3 . The vacancy-ordered double perovskite, Cs 2 SnI 6 with a bandgap close to 1.4 eV, looks promising, but it suffers from some intrinsic undesirable properties. The potential of these Sn-based all-inorganic perovskites and the challenges involved in their development are also discussed in this chapter. Recently, all-inorganic lead-free double perovskites like Cs 2 AgBiBr 6 and silver-bismuth rudorffites, such as AgBiI 4 , Ag 2 BiI 5 ,AgBiI 6 , AgBi 2 I 7 , have also drawn a lot of attention. Although efficiency of these non-toxic PSCs are still low some of these materials hold great potential. The chapter discusses recent developments and the standing challenges of such Pb-free Ag-Bi compounds.