ITO-free perovskite solar cells: From past milestones to future prospects

Indium tin oxide (ITO) has long served as the dominant transparent conducting electrode employed in both rigid and flexible perovskite solar cells (PSCs). However, the strong dependence on ITO, which is plagued by the scarcity of indium, its intrinsic brittleness, and high-temperature processing constraints, has undermined the sustainable development and scalable manufacturing of next-generation photovoltaic devices. Herein, we comprehensively investigate viable alternatives that are potentially capable of replacing ITO in PSCs. This review systematically examines the optoelectronic and mechanical parameters required for ideal transparent conductive electrodes on both rigid and flexible substrates and evaluates the full spectrum of emerging transparent front electrode materials, including ceramics (such as oxides, nitrides, and chalcogenides), metallic nanostructures (e.g., metal nanowires, meshes, and ultrathin films), carbon-based nanomaterials (e.g., graphene and carbon nanotubes), and conducting polymers. By clarifying the design principles, interfacial engineering strategies, and performance requirements of these materials across various device architectures, this review lays the foundation for developing more sustainable, scalable, and commercially viable perovskite photovoltaic technologies in the future. Progress in this direction is expected to significantly enhance the viability, affordability, and broader adoption of emerging optoelectronic devices, including rigid and flexible next-generation PSCs.