Controlling the morphology of metal halide perovskite layers during processing is critical for the manufacturing of optoelectronics. Here, a strategy to control the microstructure of solution-processed layered Ruddlesden–Popper-phase perovskite films based on phenethylammonium lead bromide ((PEA)₂PbBr₄) is reported. The method relies on the addition of the organic semiconductor 2,7-dioctyl[1]benzothieno[3,2-b]benzothiophene (C₈-BTBT) into the perovskite formulation, where it facilitates the formation of large, near-single-crystalline-quality platelet-like (PEA)₂PbBr₄ domains overlaid by a ≈5-nm-thin C₈-BTBT layer. Transistors with (PEA)₂PbBr₄/C₈-BTBT channels exhibit an unexpectedly large hysteresis window between forward and return bias sweeps. Material and device analysis combined with theoretical calculations suggest that the C₈-BTBT-rich phase acts as the hole-transporting channel, while the quantum wells in (PEA)₂PbBr₄ act as the charge storage element where carriers from the channel are injected, stored, or extracted via tunneling. When tested as a non-volatile memory, the devices exhibit a record memory window (>180 V), a high erase/write channel current ratio (10⁴), good data retention, and high endurance (>10⁴ cycles). The results here highlight a new memory device concept for application in large-area electronics, while the growth technique can potentially be exploited for the development of other optoelectronic devices including solar cells, photodetectors, and light-emitting diodes.