(Invited) Tin-Based Perovskite Solar Cells and Development to All-Perovskite Tandem Solar Cells
Authors: Hayase Shuzi 
Published: December 2024

The efficiency of Lead perovskite solar cells (Pb-PVK-PV) is now 26.1 % which is close to that of silicon solar cells (26.8%) (1). The research has been shifted to the fabrication of the large modules and the module stability. After the research of the Pb-PVK-PV, Tin perovskite solar cells (Sn-PVK-PV) are attracting attentions. The bandgap of MAPbI3 and MASnI3 is about 1.55 eV and 1.35 eV, respectively. When the Pb-PVK is mixed with the Sn-PVK, the bandgap becomes narrower (1.2 eV) than that of each solar cell. The SnPb alloyed perovskite (SnPb-PVK-PV) covers the bandgap from 1.5 eV to 1.0 eV, which contracts to the Pb-PVK having light absorption of visible region. The tandem structure is composed of the top cell with the bandgap of 1.6-1.8 eV and the bottom cell with the bandgap:1.1-1.3 eV. The SnPb-PVK fulfills the requirement of the bottom cell. The efficiency of the perovskite/Si tandem solar cells is now 33.9% (1) and that of all-perovskite tandem solar cells consisting of Sn-PVK-PV as the top cell and SnPb-PVK-PV as the bottom cells is 29.1 %. This presentation focusses on the latter tandem solar cells. Roughly speaking, to enhance the efficiency of the tandem cell, the current matching of the bottom cell and the top cell, the efficiency-enhancement of the top and bottom cell, and the optimization of charge recombination rate at the interconnecting layer. Before talking about SnPb-PVK-PV, the efficiency-enhancement of Pb-free Sn PVK-PV are discussed. For both of the Sn-PVK-PV and the SnPb-PVK-PV, Sn4+ in the perovskite layer decreases the efficiency. Sn4+ may be contained in the row material and is formed in the perovskite inks which contains DMSO working as oxidant. Sn metal deposition at both side of the Sn-PVK layer enhance the efficiency(2). In addition, we have reported the addition of Ge ions, the passivation of the Sn-PVK layer with Lewis-base such as ethylenediamine, and the band-offset optimization between the Sn-PVK and the electron transport layer (ETL) are effective to enhance the efficiency (3). 15.3 % efficiency is reported. In the same way, the efficiency of the SnPb-PVK-PV was enhanced to 23.3 %(4). In addition, the thermal stability (85 ℃, 1000 h) of the SnPb-PVK-PV was significantly improved by adding Ge ion to the perovskite ink and inserting ion-migration-blocking layer. The interconnecting layer must be designed so that the holes formed in the bottom layer and the electrons formed in the top layer could be effectively recombined at the interconnection layer. For examples, Bottom/GO/ALD SnOx/Top, Bottom/AZO/ALD SnOx/Top, Bottom/Au/ALD-SnOx/Top, Bottom/ITO/ALD-SnOx/Top, Bottom/ALD -SnOx/Top have been reported as the interconnection layer. We report the IZO as the interconnection layer prepared by optimizing the sputtering condition (5). The IZO was designed so that hole-injection rate from the bottom cell to the IZO is close to the electro injection from the top cell to the IZO. The efficiency of 26.8 % is reported. To aim at the efficiency over 30%, the development on the four items listed before is further needed.

Reference: [1] Martin A. Green, eta al., Prog. Photovolt. Res. Appl, 2023, 31, 651-663, Efficiency Table 62. [2] L. Wang, S. Hayase, et al., Angewandte, 2023, 135, e202307228. [3] Hayase, Shuzi, Sn-based Halide Perovskite Solar Cells, Chapter 10, 293-319, 2021, Perovskite Photovoltaics and Optoelectronics, Edited by Miyasaka, T., Eiley-VCH. [4] K. Gaurav and S. Hayase, et al., ACS Energy Lett., 2022, 7, 3, 966–974. [5] b. Fuan and S. Hayase, et.al., ACS Energy Letters, 2023, 8, 3852-3859.