Stacking Matters – A Spectroscopic Overview from SMAs to PSMAs in Organic Solar Cells

Abstract Non‐fullerene acceptors (NFAs), especially Y‐series small‐molecule acceptors (SMAs), have propelled organic solar cells (OSCs) to power conversion efficiencies (PCEs) surpassing 20%. Oligomeric small‐molecule acceptors (OSMAs) provide a balanced solution by preserving crystallinity and performance, addressing the poor stability of SMAs caused by molecular diffusion and the low charge mobility of polymeric small‐molecule acceptors (PSMAs). Despite their potential, the interplay between molecular structure, aggregation behavior, and photovoltaic performance remains poorly understood, limiting the ability to rationally design materials with optimal stability and efficiency. To address this gap, absorption edges of Y‐series acceptors from the UV–vis spectra are summarized to systematically analyze how structural modifications influence J/H‐type aggregation. By linking the structural features to photovoltaic parameters with absorption edges, this review clarifies how molecular engineering governs packing behavior and phase separation, and the impacts on photovoltaic parameters, providing actionable guidelines for next‐generation OSC materials.