Entering the 27% Era: Practical Design Rules for Single-Junction Perovskite Solar Cells

sı Supporting Information R ecent reports of single-junction perovskite solar cells exceeding 27% power conversion efficiency (PCE) mark a consolidation phase in device physics and materials engineering, reflecting convergence in materials choice and device design.Across independent demonstrations reported between late 2025 and early 2026, the perovskite composition, deposition routes, and device architectures are strikingly similar.This convergence indicates that further efficiency gains are now dictated by how precisely ionic species, interfaces, and contacts are controlled, particularly under operational conditions.This Viewpoint examines representative >27% single-junction perovskite solar cells and distills the practical design rules that underpin both their record efficiencies and their simultaneously improved operational stability.The discussion is framed to focus on quantitative performance metrics, interfacial physics, and mechanisms relevant to operational stability rather than record-centric narratives.