Hole Transport Materials in Conventional Structural (n-i-p) Perovskite Solar Cells: From Past to the Future

With the application of organic-inorganic hybrid perovskites to liquid-type solar cells, the unprecedented development of perovskite solar cells (Per-SCs) has been boosted by the introduction of solid-state hole transport materials (HTMs). The removal of liquid electrolyte has lead to improved efficiency and stability. Supported by high-quality perovskite films, the certified efficiency of Per-SCs has reached 25.2%. For Per-SCs assembled in a conventional structure (n-i-p), the hole transport layer (HTL) plays an extra role in preventing the perovskite layer from external stimuli. In summary, the successful design and fabrication of the HTL must meet various requirements in terms of solubility, hole transport, recombination prevention, stability, and reproducibility, to name but a few. Many research strategies are focused on the development of high-performance HTMs to meet such requirements. Such strategies for the development of HTMs employed in conventional n-i-p solar cells are reviewed herein. A vision of the future HTMs is proposed in this review based on the already proposed solutions and current trends.