Harnessing Thermally De-Doped Surface for Pb-free Sn Perovskite Solar Cells

Perovskite solar cells (PSCs) have emerged as a promising photovoltaic technology for integration into the future urban environment to power buildings and internet of things (IoT)
devices. However, state-of-the-art PSCs invariably use perovskite semiconductor materials containing Pb as a core element, and Pb toxicity has become one of the most challenging issues
for the commercialization of this technology. This calls for the development of highperformance PSCs without the Pb involvement. In this context, this project proposes to fabricate
and study a rationally designed surface microstructure, namely thermally de-doped surface (TD²S), for achieving Pb-free Sn PSCs with simultaneously enhanced power conversion
efficiencies (PCEs) and stability.

Sn perovskite has been recognized as one of the most promising Pb-free alternatives for PSCs. Unlike Pb, Sn is relatively chemically active in its +2 oxidation state and subject to easy oxidation to +4. This creates a barrier for achieving high performance in Sn PSCs, because Sn(II) is favored not only for forming the standard perovskite structure but also for offering lone-pair electrons to unlock benign defect properties. Conventional methods mitigating this issue include grain boundary engineering, reductive processing, and crystallographic engineering. But fundamental insights into the correlation of film microstructure with Sn
oxidation have been rare, and therefore, there is still a lack of adequate, effective materials tailoring strategies.

This project aims to harness the beneficial properties and structural versatility of TD²S that is formed via thermal reaction between post-deposited organic halide precursors and Sn
perovskites. The TD²S formation reaction captures and de-dopes Sn(IV) from bulk Snperovskite films. The overall approach of this project consists of three interrelated tasks: (I) unravelling the atomic-scale details and formation mechanisms of TD²S structures and their correlations to optoelectronic properties and chemical stability via advanced characterizations; (ii) realizing the design of passivated TD²S (P-TD²S) based on rationally controlled thermal reactions to enable enhanced optoelectronic properties and stability in Sn perovskite films; (iii) optimizing PCEs and long-term stability of Pb-free Sn PSCs based on P-TD²S.

The outcome of this project will impart visible technological impacts, since a conceptually new method for highly efficient, stable Pb-free PSCs will be demonstrated. Meanwhile, the fundamental materials and physical sciences, as well as the new methodology for studying them, will have overarching influence on the development of numerous other semiconductor (opto-)electronics, including but not limited to light-emitting devices, memristors, and sensors.