Rohit Prasanna1 Tomas Leijtens1 Aryeh Gold-Parker1 3 Bert Conings2 Aslihan Babayigit2 Ravichandran Shivanna4 Alan Bowman4 Hans-Gerd Boyen2 Samuel Stranks4 Michael Toney3 Michael McGehee1

1, Stanford Univ, Stanford, California, United States
3, SLAC National Accelerator Laboratory, Menlo Park, California, United States
2, Hasselt University, Diepenbeek, , Belgium
4, University of Cambridge, Cambridge, , United Kingdom

All-perovskite tandem solar cells offer the exciting possibility of surpassing limits on single junction solar cells. Low band gap ABX3 perovskites with mixtures of tin and lead at the B-site have recently had breakthrough success, but face unique challenges. Our work maps oxidation stability and optoelectronic properties - band gap, carrier lifetime and photoluminescence quantum efficiency - across a wide compositional space, varying tin-lead ratio at the B-site and varying the A-site cation, to identify the optimal composition for the bottom cell in a tandem.

Band gap tuning is critical for subcells in monolithic tandems to absorb complementary spectra. We map band gap across a wide range of A-site and B-site cation compositions and identify mechanisms by which the A-site cation alters the band gap both by lattice contraction and octahedral tilting. The smallest band gaps result from an A-site cation that produces a contracted cubic lattice with no octahedral tilting.

While pure Sn perovskites are unstable to oxidation, we show that mixing Sn and Pb at the B-site drastically improves stability because mixed Sn-Pb perovskites are forced to go through a less favourable mechanism for oxidation. Our results suggest that the low-gap perovskites most likely to succeed will have tin contents of 30 - 50%. While these compounds have slightly larger band gaps than ideal, the small price in band gap will likely be worth large gains in stability. We also study how the A-site cation can improve oxidation stability. We demonstrate tin-lead perovskite solar cells that maintain 80% of their efficiency over 10 hours of maximum power tracking in air with no encapsulation. The drop in performance is reversible in the dark, showing that there is little irreversible oxidation of the perovskite.

Due to lower absorption cross-sections of tin-lead perovskites, thick layers are needed to absorb most above-gap light, which necessitates long carrier diffusion lengths. While early low gap perovskites had short carrier lifetimes (< 1 ns), compositional tuning and improved processing has raised the liftetime to over 250 ns. Further, tin-lead perovskite solar cells have already attained impressive open circuit voltage relative to their band gap despite having much lower radiative efficiency compared with pure lead perovskites. Improving the photoluminescence quantum efficiency (PLQE) can lead to even higher voltages. We map PL lifetime and PLQE across compositions and study the reasons for voltage loss across compositions.

Taken together, our results form a framework for rational selection of compositions and processing methods of low band gap tin lead perovskites for efficient and stable all-perovskite tandem solar cells. With a 1.68 eV perovskite top cell over one such low band gap perovskite, we demonstrate a record 21.4% efficient mechanically stacked all-perovskite tandem.