The same bias in variable selection is observed in the distribution of HTL layers, where MoOx and PEDOT:PSS make up the vast majority of interlayers (Figure ). The choice of ETL is only slightly more varied with Ca, PDINO, ZnO, and PFN-Br making up the vast majority of ETLs. As with solvents and solvent additives, the consequence of these selections is that relatively little of the interlayer or energy level range has been explored. However, this is likely due to the relative difficulty of finding appropriate interlayer materials compared to solvents and additives. Reporting of interlayer thicknesses is sparse (360 for HTL, 256 for ETL) and biased toward round numbers (10, 20, 30 nm) as well.