Refractive index and extinction coefficient of CH_3NH_3PbI_3 studied by spectroscopic ellipsometry

Research concerning CH3NH3PbI3 solar cells (SCs) has attracted great attention. However, the CH3NH3PbI3 material’s critical dispersion relationships, i.e. the refractive index and the extinction coefficient, n(λ) and k(λ), as functions of λ, have been little studied. Without this knowledge, it will be difficult to quantitively investigate the optical properties of the CH3NH3PbI3 SCs. We studied n(λ) and k(λ) of CH3NH3PbI3 with spectroscopic ellipsometry. The CH3NH3PbI3 film was fabricated by dual-source evaporation, and the surface roughness was investigated to facilitate SE modeling. With the acquired n(λ) and k(λ), we applied the finite difference time domain method to calculate the ultimate efficiency, η(d), without considering carrier recombination, of the planar CH3NH3PbI3 SC as a function of the film thickness, d, from 31.25 nm to 2 μm, and compared with those of GaAs, c-Si, and a-Si:H(10%H) SCs. It is demonstrated that, η(d) for CH3NH3PbI3 SC is a little smaller than, but very close to that for the GaAs SC, however, much larger than that for the c-Si SC, for all d calculated; and much larger than that for the a-Si:H(10%H) SC when d > 100 nm. Apart from an appropriate band gap near 1.5 eV, the larger k(λ) and smaller n(λ) of CH3NH3PbI3 explain why the CH3NH3PbI3 SC has high efficiency.