Efficient All-Perovskite Tandems Enabled by Surface Anchoring of Bifunctional Long-Chain Amphiphilic Ligands

Perovskite solar cells (PSCs) in the stability-enhancing pin structure are limited by nonradiative recombination at the materials interface with the electron transport layer (ETL). This recombination is particularly acute in narrow-bandgap (~ 1.2 eV) mixed Pb-Sn PSCs, a crucial active layer in emergent all-perovskite tandem solar cells: these suffer higher <i>V</i>_OC deficits than do their Pb-based counterparts due to Sn-related surface oxidation and consequent detrimental p-doping. Our photoluminescence quantum yield studies herein indicated that state-of-the-art ethane-1,2-diammonium (EDA) passivation only partially alleviates harmful perovskite/ETL quasi fermi level splitting losses. We pursued physical separation at the defect-rich perovskite:ETL interface to reduce non-radiative losses: our target was to unite chemical surface coordination of Sn sites with physical interlayer separation, an approach we implemented by introducing amphiphilic long-chain carboxylic acid ligands at the perovskite surface. Treatment with oleic acid (OA), a long-chain carboxylic acid, led to carrier dynamics and surface chemistry indicative of reduced carrier recombination at the perovskite/ETL interface, and evidence of coordination with surface Sn²⁺. The addition of OA reduces the <i>V</i>_OC deficit to 0.34 V, resulting in a 0.89 V <i>V</i>_OC and PCE of 23.0% (22.4% stabilized) for 1.23 eV Pb-Sn PSCs. Incorporating the OA-treated Pb-Sn layer into a monolithic all-perovskite tandem, we report 27.3% PCE (26.4% certified) having a record <i>V</i>_OC of 2.21 V.