Enabling Flexible All-Perovskite Tandem Solar Cells

Multijunction all-perovskite solar cells offer a route toward efficiencies of III-V materials at low cost by combining the advantages of low thermalization loss in multijunction architectures with the beneficial properties of perovskitesnamely, low processing cost, high-throughput fabrication, and compatibility with flexible substrates. However, there are two main challenges for enabling high-efficiency tandems: (1) design of a recombination layer to efficiently combine two perovskite subcells while also preventing bottom cell damage during top cell processing and (2) achieving high open-circuit voltage of the widegap subcell. Herein, we overcome both of these challenges. First, we demonstrate a nucleation layer consisting of an ultra-thin polymer with nucleophilic hydroxyl and amine functional groups for nucleating a conformal, low-conductivity aluminum zinc oxide layer by atomic layer deposition (ALD). This method enables ALD-grown recombination layers that reduce shunting as well as solvent degradation from solution processing on top of existing perovskite active layers. Next, we demonstrate a band-gap tuning strategy based on A-site cations of mismatched size (dimethylammonium and cesium) to enable a 1.7 eV perovskite with high, stable voltages. By combining these advances, we fabricate two-terminal all-perovskite tandem solar cells with 23.1% power conversion efficiency on rigid substrates and 21.3% on flexible plastic substrates.