Axel Palmstrom1 Tomas Leijtens1 Giles Eperon1 Rohit Prasanna2 Sanjini Nanayakkara1 Steven Christensen1 Kai Zhu1 Michael McGehee3 David Moore1 Joseph Berry1

1, National Renewable Energy Laboratory, Lakewood, Colorado, United States
2, Stanford University, Stanford, California, United States
3, University of Colorado Boulder, Boulder, Colorado, United States

The emergence of metal halide perovskites as high efficiency, low-cost photovoltaic materials with a tunable band-gap has led to significant interest for perovskite-based tandems. Of existing perovskite-based tandem technologies, pairing wide-gap and low-gap perovskites in a monolithic all-perovskite tandem arguably offers the greatest potential by enabling efficiencies beyond the single-junction Shockley-Queisser limit with low-cost solution processing on a flexible, lightweight substrate. As of yet, efficiency of all-perovskite monolithic tandems has lagged behind perovskites paired with silicon or CIGS despite significant improvement to low-gap perovskite materials. The state-of-the-art all-perovskite 2T tandems have been limited by several main factors: 1) relatively low fill factors, 2) shunting due to the use of a thick (100nm) and conductive recombination layer of indium tin oxide allowing lateral connectivity of shunt pathways in either subcell, and 3) large voltage losses in the wide gap subcell due to photoinduced halide segregation, resulting in lower than ideal voltages for the tandem overall. Here, we develop strategies to overcome all of these issues and attain record-efficiency all-perovskite tandem devices.

We have substantially improved the recombination layer with an ultra-thin nucleophilic surface modification to C60for improved atomic layer deposition (ALD), enhancing the solvent/sputter barrier properties and mechanical stability of ALD-grown recombination layers. This strategy enables the fabrication of recombination layers that are thinner and less laterally conductive (reduced shunting) than currently reported strategies. Secondly, we improve the voltage of our wide-gap cell through A-site cation bandgap tuning. A combination of large and small A-site cations are used to tilt the metal halide octahedral. This increases the bandgap and therefore reduces the amount of bromine needed to attain a certain bandgap. We achieved a band gap of 1.7eV with a solution bromine concentration of only 20%. We find that this strategy results in higher voltages and efficiencies that are stable under operation. Combining our robust recombination layer and high-Voc wide-gap perovskites, we demonstrate two-terminal all-perovskite tandems with efficiencies over 23% for rigid devices and 21% for flexible devices.