Research Direction toward Theoretical Efficiency in Perovskite Solar Cells

The recently certified efficiency of 22.7% makes perovskite solar cells (PSCs) rise to the top among the thin film technologies of photovoltaics. The research activities of PSCs have been triggered by the ground-breaking report on a 9.7% efficient and 500 h-stable solid-state perovskite solar cell employing methylammonium lead iodide adsorbed on mesoporous TiO2 film and an organic hole conducting layer in 2012. However, PSCs are facing issues on stability, current–voltage hysteresis, ion migration, and so on, which should be solved for commercialization. In addition, further improvement in power conversion efficiency is still needed for PSCs. In this Perspective, the Shockley–Queisser (S-Q) limit in PSCs is investigated, where the best performing state-of-the-art PSC is used for this study. Short-circuit photocurrent density (Jsc) is found to approach the S-Q limit, while open-circuit voltage (Voc) and fill factor (FF) are far below their S-Q limits. Thus, toward an S-Q limit efficiency of ∼30% for PSCs with a light absorber having a band gap of 1.6 eV, a strategy of reducing nonradiative recombination and interface recombination to achieve a theoretical Voc and FF is more important than finding a method to improve Jsc. To this end, types of defects should be sophisticatedly characterized and engineered, although organic–inorganic halide perovskites are known to be defect-tolerant and have a benign grain boundary.