Having explored the phase emergence behaviors at the top surfaces, we assess the phase emergence behaviors at the bottom-surface to investigate the vertical phase homogeneities in each quasi-2D HP combinatorial library (Figure ) -a key factor determining optoelectronic functionalities. For RP-type HP systems, it is known that, in contrast to the top surfaces where 3D-like phases are abundant, the low-n (e.g., n=1 and 2) 2D phases are dominant at the bottom of the matrices (Figure ). This is likely due to the imbalanced crystallization kinetics of each HP phase, where the crystallization of the 3D-like HP phase majorly takes place at the solution-air interface initially . The precursor ions of (quasi-)2D HPs reside longer time in the solution and are concentrated at the bottom surface, rather than crystallizing at the top surface -attributed to the stronger solvent-solute interactions due to the larger polarity of the spacer cations , which thereby undergo crystallization. Such distinct phase inhomogeneity along the vertical direction can hamper appropriate charge transport, thus significantly restricting the PV performance. In contrast, for Cs-BDA quasi-2D HPs -a representative DJ-type HP system in this study, similar phase emergence behaviors are observed at both the top and bottom surfaces (Figure ). This suggest that a more homogenized phase distributions are manifested along the vertical direction of the DJ-based quasi-2d HP film matrices, rendering a smoother energetic landscape that allows more efficient vertical charge transport in PV applications. Note that such a difference also implies that the crystallization mechanism of DJ-based quasi-2d HPs does not follow that of RP-type counterparts.