Citation: Baranwal, A.K.; Hayase, S. Recent Advancements in Tin Halide Perovskite-Based Solar Cells and Thermoelectric Devices. Nanomaterials 2022, 12, 4055. https://doi.org/10.3390/ nano12224055 Academic Editor: Eric Wei-Guang Diau Received: 20 October 2022 Accepted: 14 November 2022 Published: 17 November 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). nanomaterials Review Recent Advancements in Tin Halide Perovskite-Based Solar Cells and Thermoelectric Devices Ajay Kumar Baranwal * and Shuzi Hayase Info-Powered Energy System Research Center, The University of Electro-Communications, Tokyo 182-8585, Japan * Correspondence: ajaybaranwal@uec.ac.jp Abstract: The excellent optoelectronic properties of tin halide perovskites (Sn-PVKs) have made them a promising candidate for replacing toxic Pb counterparts. Concurrently, their enormous potential in photon harvesting and thermoelectricity applications has attracted increasing attention. The optoelectronic properties of Sn-PVKs are governed by the flexible nature of SnI 6 octahedra, and they exhibit extremely low thermal conductivity. Due to these diverse applications, this review first analyzes the structural properties, optoelectronic properties, defect physics, and thermoelectric properties of Sn-PVKs. Then, recent techniques developed to solve limitations with Sn-PVK-based devices to improve their photoelectric and thermoelectric performance are discussed in detail. Finally, the challenges and prospects for further development of Sn-PVK-based devices are discussed. Keywords: tin halide; perovskite; solar cells; thermoelectric devices 1. Introduction Lead halide perovskite (Pb-PVK) has emerged as a versatile semiconducting mate- rial with extensive applications in photovoltaics [1], thermoelectricity [2], light-emitting diodes [3], laser detectors [4], piezoelectricity [5], radiation detectors [6], and optical fibers [7]. Pb-PVKs have a 3D crystal structure with ABX 3 formulae, where A is an or- ganic or inorganic monoatomic element (methyl ammonium (CH 3 NH 3 /MA), formamidine (CH 3 NH 2 NH/FA), or Cesium/Cs), B is a bivalent metal ion (Pb 2+ , Sn 2+ , or Ge 2+ ), and X is a halide ion (I - , Br - , or Cl - ). The B metal ion of the PVK crystal is surrounded by corner-dividing BX 6 octahedra, which are flexible enough to allow such diverse electronic applications. PVK semiconductor thin films can be coated at low temperatures using a solu- tion printing method, which makes the entire fabrication process economical and promising for mass production. The excellent optical and electronic properties of Pb-PVK have made possible a jump in the photoconversion efficiency of solar cells from an initial 3.1% to a record-breaking 25.7% within a short time of research [8,9], surpassing crystalline indium phosphide crystalline solar cells (24.20%), cadmium-free copper indium gallium selenide solar cells (23.35%), and close to crystalline Si solar cells (26.70%) [10], because of the excep- tional optical-electrical properties of ambipolar, high defect tolerance, small exciton binding energy, long carrier diffusion length, and high absorption coefficient [11–13]. However, despite bearing such promising optoelectronic properties, the notorious toxicity of Pb has raised concerns about commercial implementations. Consequently, various studies have attempted to substitute Pb with other abundant, robust, and biocompatible metals, such as Germanium (Ge), Antimony (Sb), Bismuth (Bi), Titanium (Ti), Copper (Cu), and Tin (Sn) [14–16]. Replacing Pb with other metals has obvious implications for the desirable optoelectrical properties relevant to highly efficient solar cells or stable materials. The use of Bi 3+ or Sb 3+ results in limitations in the charge transport due to the formed layered vacant structure. Ge 2+ -based halide perovskite materials exhibit poor chemical stability and poor solubility in polar solvents [17]. Cu 2+ -based halide perovskite has shown maximum efficiency of 0.99% to date due to the limitations of low absorption coefficient and high Nanomaterials 2022, 12, 4055. https://doi.org/10.3390/nano12224055 https://www.mdpi.com/journal/nanomaterials