Optical Properties and Defect Structures of Double Perovskite Cs2SnGeI6
Koichi Yamashita a, Azusa Muraoka b, Masanori Kaneko a, Giacomo Giorgi c
a Graduate School of Nanobioscience,, Yokohama City University
b Graduate School of Science, Japan Women's University
c The University of Perugia
Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics
Proceedings of Asia-Pacific International Conference on Perovskite, Organic Photovoltaics and Optoelectronics (IPEROP24)
Tokyo, Japan, 2024 January 21st - 23rd
Organizers: Qing Shen and James Ryan
Invited Speaker Session, Koichi Yamashita, presentation 098
DOI: https://doi.org/10.29363/nanoge.iperop.2024.098
Publication date: 18th October 2023

Solar cells using methylammonium lead perovskite as a solar cell light absorbing material have achieved an astounding 25.5% conversion efficiency improvement, equivalent to silicon solar cells as of 2023. There are concerns about the toxic effects of lead in perovskite solar cell materials on human health and the environment, and there is an urgent need to completely replace lead with a more inert metal. In this study, optical properties and defect structures of double perovskite solar-cell materials in which lead is replaced by tin and germanium are analyzed by first-principles calculations to evaluate and design novel lead-free perovskite materials.

First-principles calculations were performed using the Vienna ab initio package (VASP) with PBE and HSE06 for the functional, a plane wave cutoff energy of 520 eV, and a cell size of 2 × 2 × 2. In order to predict what kind of defects are formed in perovskite crystals, the formation energy of the defect a in charge state was calculated for a series of possible defects, such as vacancies, interstitials and antisite occupations.

In the level diagram of defects of Cs2SnGeI6, many defect levels appeared in the band gap, but the formation energies of many of them were found to be high and difficult to generate. The VSn(-/0) and VGe(-/0) defects appearing near the VBM and CBM are considered to be defects that trap photogenerated carriers and reduce the conversion efficiency (VSn(-/0) refers to defect levels that ionize at the Sn vacancy site). The formation energies of these defects are strongly dependent on the chemical potentials of the constituent elements (Sn, Ge, and I), and defect formation can be controlled by changing the crystal growth conditions (chemical potentials). Optical properties of Cs2SnGeI6 will be discussed in the poster.

We acknowledge financial support from NEDO project (“Development of materials for Pb free perovskite tandem solar cells”) on international joint study. We would like to thank to Prof. Hayase of The University of Electro-Communications for fruitful discussions based on experiments of perovskite solar cell devices.

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