Density Functional Theory Study On The Doped Carrier Transport Materials

In recent years, improving the performance of the carrier transport layer for enhancing the luminous efficiency of organic light-emitting device has become the hot spot. Now, many experimental studies have been done on alkali metal-doped electron transport layer, and high work function metal oxides or organic compound doped hole transport layer. But only depending upon experiment method, the mechanism for improving carrier injection and transport can’t be fully verified. Therefore, it would be of quite significance to investigate them through the methods of quantum chemistry in theory. In this paper, molecular geometries, molecular orbital energy levels and distribution, charge transfer property of the n-type doped electron transport materials and p-type doped hole transport materials have been studied by density functional theory. And the mechanism of carrier transport improvement were analyzed in theory.1. For n-type doped electron transport materials, Li atom doped Alq3molecules has been studied. The results show that in Li-doped Alq3system, Li-N and Li-O bonds were formed and Li-Alq3electron transfer complexes were obtained. The incomplete electron transfer from Li atoms to the pyridine side of Alq3result in shallow donor levels in the band gap of Alq3, a typical n-type doping formation, which can improve efficiently electron transport efficiency. But when Alq3combines with Li, Al-O bond in Alq3molecules becomes longer. The heavy Li doping can induce the rupture of the Al-0bond and the dissociation of Alq3, which would affect the packing between the Alq3molecules. So that the π-π interactions between molecules becomes weak, and electron transport ability will drop, when Li:Alq3is1:3, Alq3molecules can no longer maintain its original configuration. So when the Li:Alq3doping ratio is about2:1, the Li-doped Alq3layer will have the maximum electron transport efficiency.2. For p-type doped hole transport materials, we study the hole transport materials NPB and CBP, respectively, with molybdenum oxide, tungsten oxide and F4-TCNQ as dopant. The results show that metal oxides mainly exist in the form of Mo3O9and W3O9, while a small amount of MoO3and WO3. However, because MO3and WO3combine with the hole transport materials strongly. The band structure has no essential difference between the charge-transfer complex and host materials. So it would not improve the hole injection and transport. On the contrary, when Mo3O9and W3O9compound with host organic molecules, there is only the weak interaction between Mo3O9, W3O9clusters and organic molecules. The very low unoccupied molecular orbital energy levels of their complexes are formed, which are equivalent to deep acceptor levels in the band gap of NPB or CBP, and a typical p-type doping formation. These deep acceptor levels can improve the hole injection and transport. As lowest unoccupied molecular orbits energy level of W3O9is higher than Mo3O9, it makes organic-W3O9complexes form large energy gap, that is relative shallow acceptor levels in the band gap of NPB or CBP. And electronic transition will become difficult relatively. So Mo3O9is a more excellent p-type dopant than W3O9. The F4-TCNQ has a weak interaction with host molecules like Mo3O9and W3O9, and a lower unoccupied molecular orbital energy levels relative to metal oxide. So it can get more electronics from hole transport materials, and the F4-TCNQ doped hole transport layers should be more advantageous to the hole migration. We also compared the hole transport materials NPB and CBP, when they are doped. Because NPB have higher occupied molecular orbital energy level than CBP, the transfer of electrons from NPB to the dopants is easier than CBP, thus more holes will creat in p-doped NPB. So the doped NPB enhances the hole transport more highly compared with the doped CBP.