While a ≥0.1 change in n and k for any material can result in significant optical device design discrepancy challenges, the Δk = 0.35 below the band edge in Figure is of specific concern for these representative MAPbBr3 and expected PSC solar radiation absorption. Given a bandgap of ~539 nm, it is expected that k should rapidly approach zero at this wavelength and below. However, this is not the case for most reported values of k, except for Choi and coworkers. Here, again, the large Δk for MAPbBr3 is likely due to differences in material surface quality (e.g., oxidation, doping, and/or surface roughness) in addition to inadequate optical dispersion data analysis to derive the reported values. Furthermore, the differences in the crystallographic face orientation responses reported by Tao and coworkers are likewise unexpected given MAPbBr3 is not considered a birefringent material (exhibiting neither uniaxial or biaxial optical anisotropy). When solar radiation interacts with a planar thin-film solar cell, the response of the light through a given device architecture is determined by the optical properties and thicknesses of each layer in the PSC stack. Here, we use an illustrative single junction planar solar cell architecture (five layers excluding the substrate) to demonstrate the impact of variable active layer perovskite n and k inputs on the modeled PSC optical performance: (i) encapsulating substrate, (ii) front contact, (iii) electron transport layer, (iv) active absorbing layer, (v) hole transport layer, and (vi) back contact. For simplicity, we use a similar device architecture as reported by Ball et al. (see Figure ) with MAPbBr3 as the active layer. Here, the substrate is glass (infinite modeled thickness), the front contact is fluorine-doped tin oxide (FTO, 420 nm thick), the electron transport layer is titanium dioxide (TiO2, 41 nm thick), the active absorbing layer is MAPbBr3 (492 nm thick), the hole transport layer is 2,2',7,7'-tetrakis-(N,N-di-4-methoxyphenylamino)-9,9'- spirobifluorene (Spiro-OMeTAD, 253 nm thick), and the back contact is gold (Au, 30 nm thick).