Efficient and Thermally Stable All-Perovskite Tandem Solar Cells Using All-FA Narrow-Bandgap Perovskite and Metal-oxide-based Tunnel Junction

Commercialization of all-perovskite tandem solar cells requires thermally stable narrow-bandgap (NBG) perovskites and tunnel junction. However, the high content of methylammonium (MA) and organic hole transport layer used in NBG perovskite subcell undermine the thermal stability of all-perovskite tandems. Here, thermally stable mixed lead-tin NBG perovskite solar cells (PSCs) are developed by using only formamidinium (FA) for the A-site cation. Solution-processed indium tin oxide nanocrystals (ITO NCs) are deployed further to replace the conventional organic charge transport layer. Meanwhile, the ITO NCs layer simultaneously functions as a recombination layer in the tunnel junction, which simplifies the architecture of all-perovskite tandem devices. The thermally stable all-FA Pb-Sn PSCs achieve a high power conversion efficiency (PCE) of 21.0%. With the thermally stable all-FA NBG perovskite and optimized tunnel junction, a stabilized PCE of 26.3% is further obtained in all-perovskite tandems. The unencapsulated tandem devices maintain >90% of their initial efficiencies after 212 h aging at 85 degrees C in the N-2 atmosphere. The strategies herein offer a crucial step toward efficient and thermally stable all-perovskite tandem solar cells.