Interface Engineering with Aromatic Diammonium Acetates for Amplifying Hole Extraction in Tin Perovskite Solar Cells

Perovskite solar cells have emerged as one of the most promising photovoltaic technologies because of their high efficiency, solution processability, and mechanical flexibility, which enables ubiquitous energy harvesting. From device perspective, interfaces play critical roles in charge transport and collection, which turns to be particularly important for tin-based perovskite solar cells. Tin perovskite solar cells (TPSCs) stand at the forefront as toxicity-lean technology, featuring compelling properties such as the large light absorption coefficient, small exciton binding energy, ideal bandgap, slow hot-carrier cooling, and high charge carrier mobility. Especially, tin perovskites are adjustable to a bandgap of ~1.34 eV being optimal for single-junction solar cells according to the Shockley-Queisser limit. Beyond merely a toxicity issue, such superb properties drive TPSCs to an efficiency of power conversion (PCE) close to 15%. To close a gap with that (~26%) of lead counterparts, more attentions have been attracted to tailor tin perovskites to mitigate fast crystallization and Sn(II) oxidation, and to modify their interfaces to attain efficient and stable charge collection. Currently, the inverted p-i-n planar architecture is adopted widely for TPSCs because charge transport is more efficient due to the nature of p-type tin perovskites. Poly(3,4-ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) has been used as hole transport layer (HTL) due to the high-performance and reproducibility in an inverted planar architecture. PEDOT:PSS benefits from the demanding properties for TPSCs including a suitable work function (WF: ~5.0–5.2 eV), decent conductivity, and dopant-free merits. However, ambient annealed PEDOT:PSS thin films are terminated with a thin layer of PSS, which has acidity and is unfavorite for hole collection. We tackle this challenge via interface engineering. Specifically, we treated the surface of PEDOT:PSS using aromatic diammonium acetate salts dissolved in a highly volatile but interactive solvent, which not only modifies PEDOT:PSS itself but also its interface with tin perovskite. The salts are embedded into PEDOT:PSS to bridge and ameliorate its interface with tin perovskite and hence to amplify its hole extraction characteristics. Consequently, we attain a high device efficiency as 12.1% and impressive stability without encapsulation for ~2800 h. The materials and methodologies of our development are extendable to other perovskites- and PEDOT:PSS-based bio, energy, and electronic applications. Moreover, they can expand on surface and interface engineering to gain broader scopes, thereby lying a critical bridging stone on paths to diverse applications.