Study Of Charge Carrier Trapping Effect In Doped OLED

Charge trapping play important role in doped organic light-emitting device. It important for device improve device brightness, efficiecy, charge injection balance, etc. Energy transfer and trapping should take place together. Many methods had found for decide whether trapping take place or not, but contribution of trapping in organic light-emitting device is not clear.Chapter two of this paper discussed two methods which can judge the percents of contribution of trapping in OLED. The first method is compare PL spectra of organic film and EL spectra of OLED. In devices based on these complexes, it is expected that both singlet and triplet excitons generated by electrical injection and charge recombination can be utilized for EL emission through F?rster-type singlet-singlet exciton transfer or Dexter-type triplet-triplet exciton transfer. Several groups observed the existence of triplet–triplet energy transfer from polymer hosts to phosphorescence dopants by a time-resolved electroluminescence (EL) experiment and phosphorescence decay dynamics at low temperature. However, Lane et al. could not observe the triplet-triplet energy transfer from poly (9,9-dioctylfluorene) (PFO) to PtOEP by investigating the modulation frequency dependence of photoinduced absorption (PA) signal and Gong et al. made the same conclusion by the investigation of the Ir-complex doped poly(vinylcarbazole)(PVK) system. Without consider Dexter energy transfer, By now we can depart EL spectra to two parts: one is from energy transfer, the other is from trapping. We can calculate the constribution percent of trapping.Chapter tree of this paper discussed the factor which can infection efficiency of trapping, which may include charge balance, doping concentration and depth of carrier trap. By adjust the concentration of electron in polymer film, we found that charge balance is an important factor for trapping. Depth of charge trap is an important factor too. Deep charge trap (>0.5eV) can trap carrier firmly, and can improve trapping efficiency.Chapter four of this paper we made a white OLED, for most of exciton formed by trapping process, this device have high efficiency and stable color with different voltage. The device max efficiency is 2.6cd/A, the max brightness is 1800cd/m2, CIE coordinates is (0.33, 0.38).