Growth of CH₃ NH₃ PbI₃ cuboids with controlled size for high-efficiency perovskite solar cells

Perovskite solar cells with submicrometre-thick CH₃ NH₃ PbI₃ or CH₃ NH ₃ PbI _{3- x} Cl _x active layers show a power conversion efficiency as high as 15%. However, compared to the best-performing device, the average efficiency was as low as 12%, with a large standard deviation (s.d.). Here, we report perovskite solar cells with an average efficiency exceeding 16% and best efficiency of 17%. This was enabled by the growth of CH₃ NH₃ PbI₃ cuboids with a controlled size via a two-step spin-coating procedure. Spin-coating of a solution of CH 3 NH 3 I with different concentrations follows the spin-coating of PbI 2, and the cuboid size of CH₃ NH₃ PbI₃ is found to strongly depend on the concentration of CH₃ NH₃ I. Light-harvesting efficiency and charge-carrier extraction are significantly affected by the cuboid size. Under simulated one-sun illumination, average efficiencies of 16.4% (s.d.±0.35), 16.3% (s.d.±0.44) and 13.5% (s.d.±0.34) are obtained from solutions of CH₃ NH₃ I with concentrations of 0.038M, 0.050 M and 0.063 M, respectively. By controlling the size of the cuboids of CH₃ NH₃ PbI₃ during their growth, we achieved the best efficiency of 17.01% with a photocurrent density of 21.64mA^-2, open-circuit photovoltage of 1.056V and fill factor of 0.741.