A new series of organic light emitting copolymers, poly(dioctyl fluorene - co - diphenyl oxadiazole)s P(DOF-DPO), are reported in this paper. We found organic polymer light emitting devices with comparable device efficiency and more saturated blue color based on this new series of copolymers with less than 10% DPO moiety when compared with poly(dioctyl fluorene) P(DOF) devices. The device structures used in this study were indium tin oxide (ITO) anode/ PEDOT-PSS layer/emissive layer/calcium (Ca)/aluminum (Al) cathode, while the emissive layer was P(DOF) or P(DOF-DPO) copolymer with different DOF and DPO ratio. The AFM data shows that the emissive layer has a smooth surface with RMS roughness about 0.5 ± 0.2 nm over a 2 μm by 2 μm area. The photo-luminescence quantum efficiency of the P(DOF-DPO) copolymer decreases with increasing DPO moiety. Both the photo-luminescence spectra and electro-luminescence spectra are slightly blue shifted when adding less than 10% DPO moiety into P(DOF). Both the emission efficiency (~0.3-0.6 cd/A at 100 cd/m2) and power efficiency (~0.1-0.2 lm/W at 100 cd/m2) of the P(DOF-DPO) based PLEDs with less than 10% DPO moiety are comparable to the P(DOF) based PLEDs.
A Monte Carlo method for modeling the light transport phenomena in organic polymer light-emitting devices (PLEDs) is reported. In this simulation we assumed a point light source having photon emission spectrum represented by the photoluminescence (PL) spectrum of the organic polymers. This method describes the fate of photons through multiple scattering events determined by the wavelength-dependent material optical properties in a 3-D Cartesian geometry, thus considering the effects of refraction at different interfaces, back-reflection from the cathode, interference effect in the ITO thin film, and absorption within the polymer layers. We apply this method to analyze the wavelength output distribution and extraction efficiency. We found that the simulated light emission spectra of the green and red light-emitting devices are very similar to the measured PL spectra, suggesting that the light transport phenomena do not change the energy distribution significantly. We also established that the calculated extraction efficiency for the red (ηext = 19.5%) and green (ηext = 19.9%) PLEDs are approximately the same. We further investigated the light emission angular distribution of the PLEDs, and found that the simulated angular distribution shows better agreement with the experimental data than previously used models that rely on standard refraction theory at one interface.
Organic polymer red light-emitting devices (OPLEDs) with the double layer structure have been fabricated on flexible plastic substrates. Dow red emissive polymer and poly(3,4- ethylenedioxythiophene)/poly(styrene)(PEDOT/PSS) have been used as an emissive and a hole injection polymer, respectively. The spin coating technique was used to deposit different polymers. The absorption and the cyclic voltammetry spectra have been used to construct the band diagram of our OPLEDs. The following electrical and optical properties have been obtained for our OPLEDs: turn-on voltage, defined at 1 cd/m2=~3.0V; voltage and current density defined at 100 cd/m2=~6.5V and ~34mA/cm2; maximum emission efficiency =~0.25 cd/A; and maximum luminous efficiency =~0.1m/W. The extrapolated lifetime of unpackaged OPLEDs on flexible plastic substrate of about 1160 min for initial brightness of 100 cd/m2 has been obtained.
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