Achieving Thermal, Atmospheric, and Operational Stability in Low Band Gap Tin-Lead Halide Perovskite Solar Cells
Rohit Prasanna1, Tomas Leijtens1,2,3, Sean P. Dunfield2,4, James A. Raiford5, Eli J. Wolf1,2, Simon A. Swifter1, Giles E. Eperon2, Camila de Paula5, Axel F. Palmstrom2, Maikel F. A. M. van Hest2, Stacey F. Bent5, Glenn Teeter2, Joseph J. Berry2, Michael D. McGehee2,4,6
1Stanford University Materials Science and Engineering, Stanford, CA, United States
/2National Renewable Energy Lab, Golden, CO, United States
/3Swift Solar, Golden, CO, United States
/4University of Colorado Boulder Materials Science Program, Boulder, CO, United States
/5Stanford University Chemical Engineering, Stanford, CA, United States
/6University of Colorado Chemical and Biological Engineering, Boulder, CO, United States

Tin-lead perovskites have ideal band gaps for the rear cells of monolithic two-junction tandem solar cells, which offer a promising route to high-efficiency low-cost photovoltaics. However, tin-containing perovskites are perceived to be unstable due to the tendency of tin to be oxidized, and good long-term stability has yet to be demonstrated in photovoltaic cells based on tin-lead perovskites. We study the effects of perovskite composition, film morphology, and device architecture on solar cell stability. By fabricating a  FA..75 Cs.25 Sn.4 Pb.6 I3 solar cell that uses band bending instead of a dedicated hole transport layer to selectively extract holes, developing a processing method that results in a compact large-grained morphology, and capping the device with a sputtered IZO protective top electrode, we demonstrate low gap perovskite solar cells that maintain on average 95% of its initial efficiency after 1000 hours at 85 C in air (without encapsulation) and in damp heat (with glass-on-glass encapsulation). The device also maintains performance under continuous operation in the light in nitrogen, actually improving in efficiency over 1000 hours. This set of results proves the viability of tin-lead perovskite solar cells, and takes efficient all-perovskite tandems a crucial step closer to successful implementation.