Light-induced photocarrier generation is an essential process in all solar cells, including organic-inorganic hybrid (${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$) solar cells, which exhibit a high short-circuit current density ($J_{\mathrm{sc}}$) of approximately $20\mathrm{mA}/{\mathrm{cm}}^2$. Although the high $J_{\mathrm{sc}}$ 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 ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ 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 ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ layers, we find that the absorption coefficients of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ ($\alpha =3.8\times {10}^4{\mathrm{cm}}^{-1}$ at 2.0 eV) are comparable to those of ${\mathrm{CuInGaSe}}_2$ and CdTe, and high $\alpha$ values reported in earlier studies are overestimated seriously by the extensive surface roughness of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ layers. The polarization-dependent DFT calculations show that ${\mathrm{CH}}_3{{\mathrm{NH}}_3}^{+}$ interacts strongly with the ${{\mathrm{PbI}}_3}^{-}$ cage, modifying the ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ dielectric function in the visible region rather significantly. In particular, the transition matrix element of ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ varies, depending on the position of ${\mathrm{CH}}_3{{\mathrm{NH}}_3}^{+}$ within the Pb—I network. When the effect of ${\mathrm{CH}}_3{{\mathrm{NH}}_3}^{+}$ on the optical transition is eliminated in the DFT calculation, the ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ dielectric function deduced from DFT shows an excellent agreement with the experimental result. As a result, distinct optical transitions observed at $E_0(E_g)=1.61\mathrm{eV}$, $E_1=2.53\mathrm{eV}$, and $E_2=3.24\mathrm{eV}$ in ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ 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 ${\mathrm{TiO}}_2$ layer and demonstrate that the QE spectrum can be reproduced almost perfectly when the revised ${\mathrm{CH}}_3{\mathrm{NH}}_3{\mathrm{PbI}}_3$ optical constants are employed. Depth-resolved QE simulations confirm that $J_{\mathrm{sc}}$ 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.

• Received 30 July 2015

DOI:https://doi.org/10.1103/PhysRevApplied.5.014012