Non-Metallic Back Contacts for Indoor Perovskite Solar Cells: Material Criteria, Recent Progress, and Future Outlook.

Perovskite solar cells (PSCs) have emerged as a leading technology for indoor photovoltaics due to their high-power conversion efficiency, tunable bandgaps, and cost-effective fabrication. While significant efforts are made in optimizing perovskite absorbers and charge extraction layers, the role of back contacts (BCs), particularly non-metallic alternatives, remains largely underexplored in indoor PSCs. BCs critically influence charge collection, device stability, and overall performance under low-intensity indoor illumination, yet most studies have historically prioritized metallic electrodes, overlooking cost-effective, stable, and flexible non-metallic options. This review provides a comprehensive analysis of BC materials in indoor PSCs, with a particular focus on non-metallic electrodes, including carbon-based electrodes and transparent conductive electrodes. It begins with an overview of indoor PSCs, covering indoor light sources, perovskite materials and bandgaps, and fundamental roles of BCs, followed by recent developments in non-metallic BCs. Key challenges related to performance and energy output density, processability and scalability, mechanical flexibility and durability, as well as different types of BC materials, are discussed along with promising strategies for interface engineering, low-temperature processing, and material innovation. By highlighting this critical research gap, the review offers actionable insights into advancing efficient, stable, and scalable indoor PSCs for self-powered electronics and IoT applications.