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2D hybrid perovskites are attractive for optoelectronic devices. In thin films, the color of optical emission and the texture of crystalline domains are often difficult to control. Here, a method for extinguishing or enhancing different emission features is demonstrated for the family of 2D Ruddlesden–Popper perovskites (EA_1−xFA_x)₄Pb₃Br₁₀(EA = ethylammonium, FA = formamidinium). When grown from aqueous hydrobromic acid, crystals of (EA_1−xFA_x)₄Pb₃Br₁₀retain all the emission features of their parent compound, (EA)₄Pb₃Br₁₀. Surprisingly, when grown from dimethylformamide (DMF), an emission feature, likely self‐trapped exciton (STE), near 2.7 eV is missing. Extinction of this feature is correlated with DMF being incorporated between the 2D Pb‐Br sheets, forming (EA_1−xFA_x)₄Pb₃Br₁₀∙(DMF)_y. Without FA, films grown from DMF form (EA)₄Pb₃Br₁₀, retain little solvent, and have strong emission near 2.7 eV. Slowing the kinetics of film growth strengthens a different emission feature, likely a different type of STE, which is much broader and present in all compositions. Films of (EA_1−xFA_x)₄Pb₃Br₁₀∙(DMF)_yhave large, micron‐sized domains and homogeneous orientation of the semiconducting sheets, resulting in low electronic disorder near the absorption edge. The ability to selectively strengthen or extinguish different emission features in films of (EA_1−xFA_x)₄Pb₃Br₁₀∙(DMF)_yreveals a pathway to tune the emission color in these compounds.

Hybrid halide 2D perovskites deserve special attention because they exhibit superior environmental stability compared with their 3D analogs. The closer interlayer distance discovered in 2D Dion–Jacobson (DJ) type of halide perovskites relative to 2D Ruddlesden–Popper (RP) perovskites implies better carrier charge transport and superior performance in solar cells. Here, the structure and properties of 2D DJ perovskites employing 3‐(aminomethyl)piperidinium (3AMP²⁺) as the spacing cation and a mixture of methylammonium (MA⁺) and formamidinium (FA⁺) cations in the perovskite cages are presented. Using single‐crystal X‐ray crystallography, it is found that the mixed‐cation (3AMP)(MA_0.75FA_0.25)₃Pb₄I₁₃perovskite has a narrower bandgap, less distorted inorganic framework, and larger PbIPb angles than the single‐cation (3AMP)(MA)₃Pb₄I₁₃. Furthermore, the (3AMP)(MA_0.75FA_0.25)₃Pb₄I₁₃films made by a solvent‐engineering method with a small amount of hydriodic acid have a much better film morphology and crystalline quality and more preferred perpendicular orientation. As a result, the (3AMP)(MA_0.75FA_0.25)₃Pb₄I₁₃‐based solar cells exhibit a champion power conversion efficiency of 12.04% with a high fill factor of 81.04% and a 50% average efficiency improvement compared to the pristine (3AMP)(MA)₃Pb₄I₁₃cells. Most importantly, the 2D DJ 3AMP‐based perovskite films and devices show better air and light stability than the 2D RP butylammonium‐based perovskites and their 3D analogs.