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

摘要

We report artifact-free CH3NH3PbI3 optical constants extracted fromultra-smooth perovskite layers without air exposure and assign all the opticaltransitions in the visible/ultraviolet region unambiguously based on densityfunctional theory (DFT) analysis that assumes a simple pseudo-cubic crystalstructure. From the self-consistent spectroscopic ellipsometry analysis of theultra-smooth CH3NH3PbI3 layers, we find that the absorption coefficients ofCH3NH3PbI3 (alpha = 3.8 x 10^4 cm-1 at 2.0 eV) are comparable to those ofCuInGaSe2 and CdTe, and high alpha values reported in earlier studies areoverestimated seriously by extensive surface roughness of CH3NH3PbI3 layers.The polarization-dependent DFT calculations show that CH3NH3+ interactsstrongly with the PbI3- cage, modifying the CH3NH3PbI3 dielectric function inthe visible region rather significantly. When the effect of CH3NH3+ on theoptical transition is eliminated in the DFT calculation, CH3NH3PbI3 dielectricfunction deduced from DFT shows excellent agreement with the experimentalresult. As a result, distinct optical transitions observed at E0 (Eg) = 1.61eV, E1 = 2.53 eV, and E2 = 3.24 eV in CH3NH3PbI3 are attributed to the directsemiconductor-type transitions at the R, M, and X points in the pseudo-cubicBrillouin zone, respectively. We further perform the quantum efficiency (QE)analysis for a standard hybrid-perovskite solar cell incorporating a mesoporousTiO2 layer and demonstrate that the QE spectrum can be reproduced almostperfectly when the revised CH3NH3PbI3 optical constants are employed.Depth-resolved QE simulations confirm that Jsc is limited by the material'slonger wavelength response and indicate the importance of optical confinementand long carrier diffusion lengths in hybrid perovskite solar cells.