Breaking News on Highly Efficient Inorganic Tin-Lead Perovskite Solar Cells

To date, hybrid lead halide perovskite (LHP) solar cells have achieved great advances on a PCE from 3.8%¹ to an impressive certified value of 26.1%² within fourteen years. Despite the high efficiency of state-of-the-art lead-based perovskite solar cells is approaching the market-dominant silicon photovoltaic technology, they still face two concerns, e.g. poisonous Pb and non-ideal bandgaps for single-junction solar cells. To addressing this issue, according to Shockley-Queisser (S-Q) limit theory³, alloying tin (Sn) into B site is more effective to obtain less toxicity and preferable bandgap for a single-junction solar cell, which can yield the maximum efficiency with bandgap energies ranging from 1.2 to1.4 eV, at which the solar spectrum utilization and the photocarrier relaxation would be optimized and balanced. Among these Pb-Sn alloyed perovskites, inorganic tin-lead perovskites are attractive candidates owing to their additional merit of inherent compositional stability.<br/>Inorganic tin-lead perovskites have been investigated with low exciton binding energy, high charge mobility, high absorption coefficients and thermal stability. Encouraged by these outstanding properties, lead-rich CsPb_0.9Sn_0.1IBr₂ PSCs with a wide bandgap of 1.79 eV were first reported in 2017⁴, which obtained an efficiency of 11.33%; up to 2023, Zhang et al. reported a record PCE of 17.19% based on low-bandgap (1.34 eV) CsPb_0.7Sn_0.3I₃ PSCs via a post treatment method⁵. As seen from the simple development of inorganic Pb-Sn PSCs progress, the PCE still lags far behind their Pb-based analogs and Pb-Sn hybrid counterparts and the origin remains unclear. In addition to the common challenges of easy oxidation of Sn²⁺, crystallization regulation has been crucial yet less explored in low-bandgap CsPb_xSn_1-xI₃ PSCs. This could be ascribed to the lower enthalpy of the Cs⁺ in comparison with MA⁺ or FA⁺, which endows the crystallization kinetics quite different from their hybrid counterparts. It is thus imperative to acquire in-depth understanding of the unique crystallization dynamics of cesium tin-lead perovskites, which would provide guidance of the solidification process of cesium tin-lead perovskites and therefore high optoelectronic films and devices.<br/>In this breaking news, for the first time, Shbin Li and colleagues reveal the B-site initiated asynchronous crystallization of inorganic tin-lead halide perovskites by comprehensive in-situ and ex-situ characterizations as well as molecular dynamics calculations⁶. Then, a novel synchronous alloying process is developed using a novel Sn-perovskite-targeted crystallization regulator. The regulator suppresses the asynchronous crystallization by enhancing the formation barrier energy of CsSnI₃, endowing a synchronous Sn and Pb alloying reaction in precursor film, achieving a homogeneous film after the annealing process. Finally, a record-high PCE of 17.55% is obtained for modified CsPb_0.7Sn_0.3I₃ solar cells, in a sharp contrast to the control device which shows a PCE of 3.6%. More importantly, B-site synchronous alloyed target devices show impressive stability with negligible degradation either at 65 ^○C over 3500 h for the net films or under continuous LED illumination for 700 h.<br/>Research on Pb-Sn alloyed perovskites mostly focuses on the anti-oxidation of tin and is still at a relatively early stage, but the work of Shibin Li and colleagues provides a valuable B-site initiated synchronous crystallization approach to create highly efficient PSCs. Their proposed approach would be applicable to other high-performance and stable optoelectronic devices based on Pb-Sn alloyed perovskite.<br/><b>Reference</b><br/>¹ <i>J Am Chem Soc</i> <b>2009</b>, <i>131</i>(17): 6050-6051.<br/>² National Renewable Energy Laboratory, “Photovoltaic World Records. NREL,” can be found under https://www.nrel.gov/pv/interactive-cell-efficiency.html <b>2023</b>.<br/>³ <i>Nat Energy</i> <b>2018</b>, <i>3</i>(10): 828-838.<br/>⁴ <i>J Am Chem Soc</i> <b>2017</b>, <i>139</i>(40):14009-14012.<br/>⁵ <i>Chem Eng J</i> <b>2024</b>, 479:147554.<br/>⁶ <i>Appl Phys Rev</i> <b>2023</b>, <i>10</i>(4).