Solution-based processing of optoelectronic active layers promises to drive down the manufacturing costs of emerging technologies, such as light-weight, large-area solar panels. While significant effort has focused on the molecular tuning of soluble semiconductors to improve optical and electronic properties, their performance ultimately depends on the extent and manner of crystallization as solvent rapidly evaporates during film deposition. Confining crystallization to the sub-micron length scale during solution deposition presents a powerful strategy to select for preferred polymorphs, crystal orientations and sizes that promote efficient optoelectronic processes. In small-molecule organic semiconductor systems, we take advantage of crystal growth dynamics to preferentially orient crystals to align the fast charge transport direction with device current using porous scaffolds. In metal halide perovskite systems, we have found nanoconfinement to shift both the thermodynamics and kinetics of solid-state phase transitions. Recently, we have exploited the dramatically enhanced stability of nanoconfined MHPs to study fundamental optoelectronic properties these materials using temperature-dependent photoluminescence and low-frequency Raman spectroscopy.