By using an ultrathin dopant dye layer deposited on the top of host materials, the influence of concentration of three fluorescent dyes, dimethylquinacridone (DMQA), 4-(dicyanomethylene)-2-t-butyl-6-(1,1,7,7-tetramethyljulolidyl-9-enyl)-4H-pyran (DCJTB), and 5,6,11,12-tetraphenylnaphthacene (rubrene), on the luminescence spectra of OLEDs was studied. The characteristic of the brightness-efficiency-bias voltage performance was investigated. The results showed that compared to the conventional doping devices, the devices consisting of ultrathin dye layer exhibited a weak peak originating from host matrix, and the more obvious concentration quenching was existent. The degree of concentration quenching for the three ultrathin doping dyes from high to low is in the order of DMQA, DCJTB and rubrene. Meanwhile, the authors used the photoluminescence spectra of these three dopants dilute solution to testify the relationship between EL properties of OLEDs and concentration quenching of these dyes.