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Optical transitions in hybrid perovskite solar cells: Ellipsometry, density functional theory, and quantum efficiency analyses for CH3NH3PbI3

We report artifact-free CH3NH3PbI3 optical constants extracted from ultra-smooth perovskite layers without air exposure and assign all the optical transitions in the visible/ultraviolet region unambiguously based on density functional theory (DFT) analysis that assumes a simple pseudo-cubic crystal structure. From the self-consistent spectroscopic ellipsometry analysis of the ultra-smooth 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 CuInGaSe2 and CdTe, and high alpha values reported in earlier studies are overestimated seriously by extensive surface roughness of CH3NH3PbI3 layers. The polarization-dependent DFT calculations show that CH3NH3+ interacts strongly with the PbI3- cage, modifying the CH3NH3PbI3 dielectric function in the visible region rather significantly. When the effect of CH3NH3+ on the optical transition is eliminated in the DFT calculation, CH3NH3PbI3 dielectric function deduced from DFT shows excellent agreement with the experimental result. As a result, distinct optical transitions observed at E0 (Eg) = 1.61 eV, E1 = 2.53 eV, and E2 = 3.24 eV in CH3NH3PbI3 are attributed to the direct semiconductor-type transitions at the R, M, and X points in the pseudo-cubic Brillouin zone, respectively. We further perform the quantum efficiency (QE) analysis for a standard hybrid-perovskite solar cell incorporating a mesoporous TiO2 layer and demonstrate that the QE spectrum can be reproduced almost perfectly when the revised CH3NH3PbI3 optical constants are employed. Depth-resolved QE simulations confirm that Jsc 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.