Enhancing Chemical Stability and Suppressing Ion Migration in CH₃NH₃PbI₃ Perovskite Solar Cells <i>via</i> Direct Backbone Attachment of Polyesters on Grain Boundaries

Abstract

Organic–inorganic halide perovskites feature excellent optoelectronic properties but poor chemical stability. While passivating perovskite grain boundary (GB) by polymers shows prospects on long-term performance of perovskite solar cells (PSCs), its detailed impact on the ion migration phenomenon, which largely deteriorates the PSC stability, remains less probed. Here, we introduce a new polar polymer, polycaprolactone (PCL), to passivate GBs of methylammonium lead triiodide (MAPbI3) perovskite with only 1–2 polymer monolayers via direct backbone attachment. The PSCs with passivated MAPbI3, using a classic but less stable Spiro-OMeTAD (2,2′,7,7′-tetrakis[N,N-di(4-methoxyphenyl)amino]-9,9′-spirobifluorene) hole transport layer (HTL), exhibit improved power conversion efficiencies up to 20.1%, with 90% of the initial PCE being preserved after 400 h ambient storage, and 80% even after 100 h, 85 °C aging. The improved PSC stability indicates critical roles of PCL GB passivation in retarding moisture-induced decomposition and suppressing ion migration within the perovskite. Time-of-flight secondary ion mass spectrometry reveals that I– ions can actively migrate into the electrode, HTL, and their interface in nonpassivated PSCs, even without an externally applied electric field, while such migration is significantly mitigated in PCL-passivated PSCs. This effective GB passivation by PCL suggests an important potential of polymer additives toward the development of stable high-performance PSCs.