Increase the Long-Wavelength Absorption for Carbon-Electrode Based Hole-Conductor-Free Perovskite Solar Cells by Thicker Perovskite Films.

Weak reflectivity of carbon-electrode (CE) limits light harvesting of perovskite solar cells that are based on CEs (CPSCs), especially for long-wavelength region. To solve this problem, herein the crystallization of perovskite (PVSK) is regulated by tuning the concentration of PbI₂ precursor during the two-step growth method. As concentration increases from 1.0 to 1.7 mol L^-1, film thickness of PVSK rises from ≈360 to ≈850 nm, and the average grain/crystallite size of PVSK enlarges from 0.70 to 1.14 µm, and from 54.2 to 67.5 nm, respectively. Due to the upgraded crystallization, Urbach energy drops from 98 to 41 meV, lifetime of charge carriers increases obviously, meanwhile the light harvesting of PVSK is improved during the long-wavelength regions (600-810 nm). External quantum efficiency test on the CPSCs shows that the integrated short circuit current density (J_SC) increases by 15.6% during the long-wavelength region. Accordingly, J_SC improves from 20.27 (±0.36) to 22.58 (±0.27) mA cm^-2. Besides, leakage is retarded, and open-circuit voltage is improved. These merits help elevate the power conversion efficiency from 15.37 (±0.42) to 16.37 (±0.68) % (optimized at 17.69%). Prolonging spin-time of PbI₂ further improves PVSK crystallization and light harvest, which optimizes device efficiency to 19.17%.