Here, l was sample thickness and reflection losses (R) were calculated from R=(n-1) 2 /(n+1) , where n are the perovskite refractive indices. Absorption losses, on the other hand, were estimated from perovskite absorption coefficients (=4k/, where k are the MAPbX3 extinction coefficients. Perovskite n and k values were derived from spectroscopic ellipsometry measurements of the single crystals depicted in Figure . Ellipsometry data for our MAPbBr3 and MAPbCl3 single crystals will be published in a separate research article currently in preparation, but prior complementary results can be used for reference. Archimedes' Density Measurements Archimedes density measurements were performed using an Excellence XS Analytical Balance (Mettler Toledo, sensitivity down to ±0.001 g cm -3 ). Anhydrous hexane was used for liquid mass measurements at temperatures of ~21±2 C. Reported single crystal densities represent the average from the values of five different MAPbBr3 and MAPbCl3 single crystals grown from solution. Averaged single crystal densities (standard deviation provided) of 3.823±0.031 g cm -3 (MAPbBr3) and 3.115±0.020 g cm -3 (MAPbCl3) were used to derive the relative density values in Table .