Cyanine dye J-aggregate films are a class of absorbing and luminescent materials which have been extensively applied in the polariton-based research. Here we systematically study the DEDOC cyanine dyes J-aggregate films made by layer-by-layer assembly and spin-coating processes to establish a clear correlation between the film structure and the absorption and luminescence properties. From detailed analyses of morphology, optical spectra, and light-emitting diode characteristics, we demonstrate that layer-by-layer assembled film has higher degrees of homogeneity and molecular packing quality than spin-coated film, leading to a higher absorption coefficient, more uniform luminescence, and a greater electroluminescence quantum efficiency with maximized thickness.