Selenium-Rich Conjugated Polymer Additives Enable Enhanced Stability in Perovskite Solar Cells

Perovskite solar cells (PSCs) have garnered significant attention in recent years due to their remarkable optoelectronic properties and the unprecedented performance gains achieved in an unprecedentedly short timeframe since their discovery. Among the various strategies to enhance PSC efficiency and stability, polymer additives have proven effective, either by incorporation into the perovskite bulk or by deposition on the surface via antisolvent engineering. Diketopyrrolopyrrole (DPP)-based conjugated polymers, widely studied in organic field-effect transistors (OFETs) and organic photovoltaics (OPVs), represent a promising but underexplored class of additives for PSCs. In this study, we developed stable n-i-p-based PSCs by introducing DPP-based polymers with biselenophene (poly-ADD1) and thienothiophene (poly-ADD2) donor units via antisolvent treatment. The resulting PSCs achieved PCEs of 21.0% (poly-ADD1) and 20.6% (poly-ADD2), both slightly surpassing that of the reference PSC (20.5%). More importantly, poly-ADD1 significantly enhanced thermal stability: after 1600 h of thermal aging at 85 °C in ambient air, PSCs retained 67.0% of their initial PCE, compared with 29.0 and 27.0% for the reference and poly-ADD2 cells, respectively. Furthermore, under ambient storage conditions with exposure to oxygen and humidity, poly-ADD1-treated PSCs showed no measurable degradation over 1300 h. In contrast, the reference cell retained only 79.0% of its initial efficiency, demonstrating the exceptional stability of treated cells. Time-of-flight secondary ion mass spectrometry (ToF-SIMS) provides mechanistic insight, revealing that the selenophene-containing polymer poly-ADD1 forms a uniform and stable interfacial layer that effectively restricts Li+ migration and Au diffusion, suppresses AuI formation within the bulk, and thereby enhances long-term cell stability (under thermal and ambient stress); collectively, these findings underscore the potential of DPP-based polymers, particularly poly-ADD1, as universal interfacial modifiers that form conformal layers on perovskite surfaces, effectively mitigating Au+ and Li+ ion diffusion and enabling broadly applicable stability improvements in perovskite solar cells incorporating Au- and Li-containing components.