Optical Properties and Modeling of 2D Perovskite Solar Cells

The optical constants (complex refractive indices) of 2D (BA)₂(MA)_m-1Pb_mI_3m+1 (BA = CH₃(CH₂)₃NH₃ ⁺, MA = CH₃NH₃ ⁺, m = 2, 3, 4, and 5) perovskite thin films at room temperature are investigated by spectroscopic ellipsometry and UV-Vis spectrophotometry, and compared with those of 3D MAPbI₃ (m = ∞). High-frequency dielectric constants (ϵ_∞) derived from ellipsometric fitting for the members of the homologous series increase with increasing thickness of perovskite [(MA)_m−1Pb_mI_3m+1]²⁻ slab (m), which qualitatively agrees with the results predicted by a simple quantum well model. Using the optical transfer matrix formalism with the knowledge of optical constants, we model the absorption and optimize the 2D perovskite film thickness for planar-structure solar cells. Moreover, a modified drift-diffusion model is proposed to study the effect of charge-carrier generation and recombination. For 2D perovskite solar cells, the model gives suitable estimates of the charge recombination rate constants, which are found to be one order of magnitude larger than that in the 3D parent compound. Solar cells are fabricated by varying the 2D perovskite layer thickness, and the experimental results verify key aspects of our modeling, enabling the optimization of the device design via optical cavity-tuned absorption enhancement, accounting for charge-carrier recombination.