Spectroscopic study of white organic light-emitting devices with various thicknesses of emissive layer
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abstract
White light-emitting devices based on a donor-acceptor structure were fabricated in order to investigate the dependence of the optical properties of white light emission on the thickness variance (15, 20, 25, and 30 nm) of the emissive layer. The emissive layer has a donor-acceptor system with the host 4,4′,4′′-tris(carbasol-l-nyl)triphenylamine molecule doped with 4,4′-bis(9-ethyl-3-carbazovinylene)-1,1′-biphenyl (BCzVBi) and 5,6,11,12-tetraphenylnaphtacene (Rubrene) molecules for blue and yellowish-green light activators, respectively. The characteristics of current density were analyzed by using a power function of applied field, J=σlEl+1 and the characteristic exponential function, J=J0(e(V-Vd)/V0-1). Through spectroscopic analysis, we obtained three physical quantities governing the device performance: 1) an effective conductivity, 2) a threshold potential, and 3) a characteristic potential barrier, which are associated with the trap-charge limited concentration in the bulk layer, the energy gap of the organic materials, and the barrier energy at the contact of electrodes, respectively. The electroluminescent spectra were studied quantitatively using a multi-peak fit with a Gaussian distribution for each electromagnetic transition. By this approach, we deduced the energy levels of the BCzVBi and Rubrene molecules that give leading information on the light emission mechanism and the energy transfer in the host-dopant system.
