Studies On Performances And Its Mechanism Of Nodoped Phosphorescent Organic Light-emitting Devices With Semi-quantum-well Structure

Phosphorescent organic light-emitting devices (PHOLEDs) have attracted widespread attention because they can break through the efficiency limitation in traditonal fluroscent counterparts by harvesting radiatively both singlet and triplet excitons. However, high-performance PHOLEDs are generally fabricated by doping the emitters into a florescent host matrix in a rather low and narrow concention rage. Consistent control of the doping process to keep a constant, low dopant contration (<10%) is difficult for the reproducibility of mass production. Besides, phase separation in a dopant system is also a potential problem for performance degradation in dopant devices. Therefore, designing high performance PHOLEDs with non-doped emitting layers is of much importance and interest.Nondoped PHOLEDs (NPOLEDs) of semi-quantum-well with various well numbers are fabricated using CBP as potential barrier layer (PBL) and [(F-BT)2Ir(acac)] as potential well layer and emiting layer (PWL/EML), respectively. The NPHOLED with well numbers of four demonstrates the best performance, surpassing that of dopant courterpart. We attribute the improvement to the efficienct carrier and exciton confinement and the suppression of triplet-triplet annihilation by single-step long-range (F?rster-type) energy transfer between excited molecules. Nevertheless, the power efficiency of NPHOLED with well numbers of 4 is still unsatisfactory due to the high turn-on voltage. The NPHOLED with well numbers of 2 shows the low turn-on voltage, whereas the current efficiency (CE) is low owing to carriers and excitons owerflowing out of PWL/EML.The simple method to reduce turn-on voltage is to reduce well numbers, while the CE may be enhanced by confining carriers and excitons within PWL/EML, which demands the search for optimal PBL materials. Therefore, we fabricate NPOLEDs with well numbers of 2 using TPD, TCTA, CBP and TPBi as PBL, respectively. The experimental results imply that NPOLED with TPBi as PBL indicate the highest power efficiency at lowest voltage. Besides the carrier and exciton confinement in PWL/EML, the self-balancing effect caused by high hole energy barrier at the interface of m-MTDATA/TPBi as well as electron energy barrier at TPBi/[(F-BT)2Ir(acac)] is responsible for the low turn-on voltage.The relation ofΔE∝1 /l2 deduced from inorganic semiconductor theory is unfit for organic semiconductor device by investigating the electroluminescence (EL) spectra changing with well thickness. There exits a threshold, that is, the blue shift of EL spectra will happen if the well thickness is no more than the threshold, or else, the blue shift will disappear. This phenomenon will be relevant to the electron structure of organic molecular, which demands further work to understand the emitting mechanism of organic light-emitting with quantum well structure.