APbI₃ (A = CH₃NH₃ and HC(NH₂)₂) perovskite solar cells: From sensitization to planar heterojunction

Since the first report on long-term stable solid-state perovskite solar cell with a power conversion efficiency (PCE) of 9.7 % based on methylammonium lead iodide in 2012, a certified PCE of 21 % was demonstrated in 2015. Methylammonium lead halide perovskite materials were first attempted as sensitizers in a liquid electrolyte based on dye-sensitized solar cell structure in 2009, showing PCE as low as ~4 %, which was almost doubled in performance by intensifying loading concentration of perovskite in the thinner TiO₂ film in 2011. Lead halide perovskites employing organic ammonium cations such as methylammonium and formamidinium are undoubtedly promising photovoltaic materials. In this chapter, emergency and structural evolutions of perovskite solar cell are described. Basic fundamentals of perovskite materials are described in terms of absorption coefficient, refractive index, dielectric constant, and carrier mobility. Since the perovskite film quality is directly related to its photovoltaic performance, effective methods for high PCE solar cells are described based on two-step spin-coating and adduct approach. Lower band gap perovskite with formamidinium is one of the promising materials because of higher photocurrent without losing photovoltage than methylammonium case. High-quality formamidinium lead iodide films can be prepared by two-step or adduct methods, which is related to photovoltaic performance. Stability issues such as photo-, moisture, and thermal-stabilities are mentioned and methodologies to solve the underlying instability problems are proposed.