Three-dimensional (3D) halide perovskite (HP) solar cells have been thriving as promising postsilicon photovoltaic systems. However, despite the decency of efficiency, they suffer from poor stability. Partial dimensionality reduction from 3D to 2D was found to significantly meliorate the instability, thus mixed-dimensional 2D/3D HP solar cells have been expected to combine favorable durability and high efficiency. Nevertheless, their power conversion efficiency (PCE) does not live up to the expectation, hardly exceeding 19%, in sharp contrast with the ∼26% benchmark for pure 3D HP solar cells. The low PCE primarily arises from the restricted charge transport of the mixed-phasic 2D/3D HP layer. Understanding its photophysical dynamics, including its nanoscopic phase distribution and interphase carrier transfer kinetics, is essential for fathoming the underlying restriction mechanism. This Account outlines the three historical photophysical models of the mixed-phasic 2D/3D HP layer (denoted as models I, II, and III hereafter). Model I opines (i) a gradual dimensionality transition in the axial direction and (ii) a type II band alignment between 2D and 3D HP phases, hence favorably driving global carrier separation. Model II takes the view that (i) 2D HP fragments are interspersed in the 3D HP matrix with a macroscopic concentration variation in the axial direction and (ii) 2D and 3D HP phases instead form a type I band alignment. Photoexcitations would rapidly transfer from wide-band-gap 2D HPs to narrow-band-gap 3D HPs, which then serve as the charge transport network. Model II is currently the most widely accepted. We are one of the earliest groups to unveil the ultrafast interphase energy-transfer process. Recently, we further amended the photophysical model to consider also (i) an interspersing pattern of phase distribution but (ii) the 2D/3D HP heterojunction to be a p-n heterojunction with built-in potential. Anomalously, the built-in potential of the 2D/3D HP heterojunction increases upon photoexcitation. Therefore, local 3D/2D/3D misalignments would severely impede charge transport due to carrier blocking or trapping. Contrary to models I and II which hold 2D HP fragments as the culprit, model III rather suspects the 2D/3D HP interface for blunting the charge transport. This insight also rationalizes the distinct photovoltaic performances of the mixed-dimensional 2D/3D configuration and the 2D-on-3D bilayer configuration. To extinguish the detrimental 2D/3D HP interface, our group also developed an approach to alloy the multiphasic 2D/3D HP assembly into phase-pure intermediates. The accompanying challenges that are coming are also discussed.