Narrow-band photoluminescence (PL) together with high quantum efficiency from organic molecules is essential for high color-purity emitters. Supramolecular assemblies like J-aggregates are promising materials due to their narrow PL signal with full-width at half maximum <20 nm. However, their microcrystalline nature and coherent exciton migration results in strong nonradiative exciton recombination at the grain boundaries that diminish the photoluminescence quantum yield (PLQY), and possibilities for improving the crystallinity by tuning the growth mechanism are limited. Here, two distinct routes to grow different J-aggregate morphologies like platelets and lamellar crystals with improved crystallinity by surface-guided molecular assembly are demonstrated, thereby suppressing nonradiative decay and improving PLQY (up to 5% at room temperature). Both platelets and lamellar crystals show similar absorbance at room temperature. However, temperature dependent PL studies show sevenfold (twofold) higher PLQY for lamellar films compared to platelets at 6 K (300 K). Using time-resolved PL spectroscopy, different nonradiative decay pathways are identified.1 Revisiting the J-aggregates formation in solution, we demonstrate that cyanine J-aggregates can reach an order of magnitude higher photoluminescence quantum yield (increase from 5% to 60%) in blend solutions of water and alkylamines at room temperature.2 By means of time-resolved photoluminescence (TRPL) studies we show an increase in the exciton lifetime as a result of the suppression of non-radiative processes. Small-angle neutron scattering studies suggest a necessary condition for the formation of such highly emissive J-aggregates: the presence of a sharp water/amine interface for J-aggregate assembly and the coexistence of nanoscale-sized water and amine domains to restrict the J-aggregate size and solubilize monomers, respectively. References 1. Anantharaman, S. B., Stöferle, T., Nüesch, F. A., Mahrt, R. F. & Heier, J. Exciton Dynamics and Effects of Structural Order in Morphology-Controlled J-Aggregate Assemblies. Adv. Funct. Mater. 29, 1806997 (2019). 2. Anantharaman, S. B. et al. Enhanced Room Temperature Photoluminescence Quantum Yield in Morphology-controlled Jaggregates. Under Revision. (2020).