Top electrode materials for semi-transparent perovskite solar cells: A review

The rising demand for renewable energy solutions has accelerated interest in semi-transparent solar cells (STSCs) for emerging applications such as building-integrated photovoltaic, automotive systems, and wearable electronics. Perovskite solar cells (PSCs) show considerable promise as STSCs due to their high performance, cost-effectiveness, solution processability, compatibility with flexible substrates, and transparency of perovskite films. Collaborative efforts have been directed towards developing transparent top electrodes (TTEs) and device architectures for PSCs to enhance the performance and transparency. The choice of top electrode materials significantly influences the performance and transparency of semi-transparent perovskite solar cells (STPSCs). Various materials such as dielectric/metal/dielectric (DMD) layers, metal thin film, metal nanowires, transparent conducting oxide (TCO), conductive polymers (e.g., PEDOT: PSS), graphene, and carbon nanotubes have been identified as potential TTEs. TCO, DMD, and metal thin film electrodes typically require sputtering or thermal deposition methods; others are solution-processable. The material selection and thickness of the top electrode play crucial roles in improving both the efficiency and transparency of PSC devices, posing challenges in optimising device performance while maintaining high transparency. This review comprehensively covers the essential material characteristics required for top electrodes in STPSCs; surveys reported top electrode materials and discusses their characterisation, stability, scalability, current challenges, and prospects.