Layer‐by‐Layer J‐Aggregate Thin Films with a Peak Absorption Constant of 106 cm–1

Layer-by-layer assembly is used to produce thin films of J-aggregates of cyanine dye molecules with an absorption constant of 106 cm–1. The films display high reflection and absorption at resonance (see Figure, ∼50 nm films) and nanoscale features that make them well suited for use in strong-coupling optoelectronic applications, enabling the creation of exciton–polariton optoelectronic devices that operate at room temperature.