Ir(PPY)3掺杂PVK的电致发光机理

近几年来发展起来的电致磷光(electrophosphorescence)是有机发光二极管(OLED)研究的新生长点。对电致磷光发光机理的研究随即得到了人们普遍的关注。比较了不同正向偏压条件下Ir(PPY)₃掺杂聚乙烯基咔唑(PVK)的光致发光(PL)和电致发光(EL)光谱。研究结果显示在电场和注入电流的共同作用下,PL光谱中基质PVK发光的相对强度并没有发生显著的变化。电场或注入载流子不会影响PVK向Ir(PPY)₃的能量传递。磷光掺杂聚合物EL主要是由于载流子在掺杂磷光分子上的直接复合,而不是由基质向磷光掺杂分子的能量传递。

Electroluminescent Mechanism of Ir(PPY)3 Doped PVK

With the increasing development of organic opto-electronic devices, interest in the mechanisms of charge carrier photogeneration, separation, transport and recombination continues to grow. Electromodulation of photoluminescence has been used as an efficient probe to investigate evolution of primary excitation in an electric field. This method can provide useful information on carrier photogeneration, the formation and dissociation of excitons, energy transfer, and exciton recombination in the presence of an electric field. Operation of organic light emitting diodes(OLED) brings electrons and holes from opposite electrodes and generates singlet and triplet excitons. However, triplet excitons are wasted because a radiative transition from triplets is spin-forbidden. Spin statistics predicts that singlet-to-triplet ratio is 1:3 in organic semiconductors. One way to harvest light from triplet excitons is to use phosphorescent materials. These materials incorporate a heavy metal atom to mix singlet and triplet states by the strong spin-orbit coupling. As a result, a spin forbidden transition may occur allowing an enhanced triplet emission. Among phosphorescent materials, factris(2-phenylpyridine) iridium [Ir(PPY)₃] has attracted attention because of its short triplet lifetime to minimize the triplet-triplet annihilation. High quantum efficiencies were obtained by doping Ir(PPY)₃ in organic molecules and in polymers. The mechanisms of electrophosphorescence has been a interesting topic since high efficient electrophosphorescence was reported. In this paper, we investigated the PL spectrum under the operation condition of OLED based on phosphorescent materials doped polymer. The photoluminescence under the modulation of an electric field and electroluminescence from thin films of poly(N-vinylcarbazol) (PVK) doped by Ir(PPY)₃ are measured. The PL spectra were gotten by reducing EL from whole emission spectra when the bias is above the threshold of EL. For different biases, no very obvious difference in the profiles of photoluminescent spectra from Ir(PPY)₃ doped PVK. The relative intensity of PVK emission peak does not decrease while increasing the bias, which indicates the energy transfer rate from PVK to Ir(PPY)₃ does not change with increasing bias. Both PL spectra in the prescence and absence of electric field are nearly the same but different from their EL spectra. For EL spectra, the emission from PVK host is hardly found. We can deduce that the main EL emissions don’t come from PVK chains to Ir(PPY)₃ molecules which obey Fster energy transfer rule, but come from the recombination of injected carriers at Ir(PPY)₃ molecules acting as both the carrier-trap and recombination centers.