Optical Transitions in Hybrid Perovskite Solar Cells: Ellipsometry, Density Functional Theory, and Quantum Efficiency Analyses for CH3NH3PbI3

Light-induced photocarrier generation is an essential process in all solar cells, including organic inorganic hybrid (CH3NH3PbI3) solar cells, which exhibit a high short-circuit current density (J(sc)) of approximately 20 mA/cm(2). Although the high J observed in the hybrid solar cells relies on strong electron-photon interaction, the optical transitions in the perovskite material remain unclear. Here, we report artifact-free CH3NH3PbI3 optical constants extracted from ultrasmooth perovskite layers without air exposure and assign all of the optical transitions in the visible and ultraviolet region unambiguously, based on density-functional theory (DFT) analysis that assumes a simple pseudocubic crystal structure. From the self-consistent spectroscopic ellipsometry analysis of the ultrasmooth CH3NH3PbI3 layers, we find that the absorption coefficients of CH3NH3PbI3 (alpha = 3.8 x 10(4) cm(-1) at 2.0 eV) are comparable to those of CuInGaSe, and CdTe, and high a values reported in earlier studies are overestimated seriously by the extensive surface roughness of CH3NE3PbI3 layers. The polarization-dependent DFT calculations show that CH3NE3 interacts strongly with the PbI3-cage, modifying the CH3NE3PbI3 dielectric function in the visible region rather significantly. In particular, the transition matrix element of CH3NH3Pb11 varies, depending on the position of CH3NH3 within the Pb-1 network. When the effect of CH3NH3 on the optical transition is eliminated in the D1-1 calculation, the CH3NH3PbI3 dielectric function deduced from DFT shows an excellent agreement with the experimental result. As a result, distinct optical transitions observed at E-o(E-g) = 1.61 eV, El = 2.53 eV, and E-2, = 3.24 eV in CH3NH3PbI3 are attributed to the direct semiconductor-type transitions at the R, M, and X points in the pseudocubic Brillouin zone, respectively. We further perform the quantum efficiency (QE) analysis for a standard hybrid-perovskite solar cell incorporating a mesoporous TiO, layer and demonstrate that the QE spectrum can be reproduced almost perfectly when the revised CELNH3PbL optical constants are employed. Depth-resolved QE simulations confirm that J(sc) is limited by the material's longer wavelength response and indicate the importance of optical confinement and long carrier-diffusion lengths in hybrid perovskite solar cells.