Study On The Excitons Dissociaton Of Organic Quantum Well Stuctures Induced By Reverse Bias Voltage

Since 1994, Tokito S et al reported the super crystal lattice structure made by organic small molecule material Alq₃ and inorganic material MgF, and indicated the structure and its optical properties. They observed the quantum confinement effect.The researches about organic/inorganic structure of super crystal lattice and quantum wells made by other materials were also reported.In this thesis, we fabricate organic devices with quantum well structures which have different periods by PBD and Alq₃.We study the energy transfer of OLEDs with quantum well structures and the exciton dissociation induced by reverse bias voltage,and then get some significant results.1 .There is a large overlap between the absorption spectrum of Alq₃ with single layer and the the photoluminescence(PL) spectrum of PBD,so there is a big Forster energy transfer between these two materials. Therefore, under 330nm (PBD best excitation wavelength) optical excitation, PBD can transfered more energy to Alq₃. So the emission of Alq₃ is predominant and the emission of PBD is very weak.2. When make the reverse bias voltage on the OLEDs with quantum well structures excited by 330nm, the emission intensity of Alq₃ decreases with the increasing of reverse bias, and the current of the devices excited by light is larger than non-excited. First of all, the device generates excited exciton states by light exciting, and then the exciton separation takes place under the reverse electric field. The dissociation of exciton results in the quench of luminescence. The free charge carriers yielded by the dissociation can tunnel to the electrodes. This process increases the current through the devices.3. For the PBD/Alq₃ OMQWs with different periods, the optimum period number is 3 with the maximum energy transfer rate. Comparing the luminescence of different period devices under 12V reverse bias and no reverse bias, we find that with the increasing of the periods, the luminescence quenching improved. According to the I-V curves of the devices with different periods excited by light under reverse bias, it can be obtained that the current increases when the periods increases. The well layers and barrier layers of the device with a few period are thicker. So the free charge carriers produced by the exciton dissociation are bound in the well layers more easily. Under the Coulomb attraction the free charge carriers would capture each other and form excitons. This process reduces the quenching. When there are more periods, the well layers and barrier layers are thinner. The free charge carriers would tunnel to the electrodes and quench there. This process increases the current throughout the devices.