Towards optimization of device performance in organic light emitting diodes and organic photovoltaics via architecture modification

Abstract

Organic light emitting diodes (OLEDs) and organic photovoltaic devices (OPVs) have particularly attracted enormous attention over other organic electronics due to their potential for displays and solid-state lighting as well as to provide renewable and clean energy. However, their relative low power efficiencies and short lifetime hamper their applications for commercialization.

This thesis focuses on the optimization of OLEDs and OPVs through novel device structures. Particular attention is paid to three aspects: i) the employment of different interlayers and utilization of mixed triplet host systems for improving the performance of blue-emitting phosphorescent OLEDs (PHOLEDs); ii) systematic studies on the effects of various non-absorbing organic materials or fluorescent dyes as donors on the photovoltaic responses of OPV devices with modified bulk heterojunction; and iii) the influences of different exciton blocking layers (EBLs) or anode buffer layers on the photovoltaic responses of OPV devices.

Firstly, a simple means for improving device performance of blue-emitting PHOLEDs by inserting an interlayer between hole-transporting layer and emissive layer was demonstrated. It was found that the insertion of interlayers with higher triplet energies could effectively enhance the efficiencies of blue-emitting PHOLEDs. High external quantum efficiencies (EQE) of 7.6% and 10.9% could be achieved for devices with 1,3-bis(carbazol-9-yl)benzene (mCP) and 4,4’,4”-tris(N-carbazolyl)triphenylamine (TCTA) interlayers, higher than that without the interlayer (~4.5%). On the other hand, device performance deteriorated with the insertion of 4,4’-bis(carbazol-9- yl)biphenyl (CBP) interlayer. The findings suggested the importance of triplet energy and matched highest occupied molecular orbital (HOMO) levels for achieving highly efficient blue-emitting PHOLEDs.

Utilization of mixed triplet host systems for improving efficiencies and reducing power consumption of blue-emitting PHOLEDs was attempted. Device efficiency was found to be dependent on both energy levels and the composition of triplet host materials in the emissive layer. The introduction of hole-transporting TCTA yielded high current and power efficiencies of 47.4 cd/A and 37.3 lm/W. Doping of electrontransporting 3-(4-biphenyl)-4-phenyl-5-tert-butylphenyl)-1,2,4-triazole (TAZ) into mixed host system resulted in a poor device performance. The transient phosphorescence decays studies suggested a shorter lifetime of triplet excitons in the TAZ-doped thin films may be the reason for the relatively low device efficiencies.

The influences of different non-absorbing organic materials or fluorescent dyes as donors on the photovoltaic responses of OPV devices with modified bulk heterojunction were studied. It was surprising that the employment of very low donor concentration of ~5% leaded to a dramatic change on the photovoltaic responses. A three-fold increase in the ISC and a large VOC of up to 0.95 V could be obtained when a hole-transporting material or fluorescent dye was doped into fullerene matrix. A better charge carrier mobility in EBL might account for the performance improvement.

Finally, the interplay of different EBLs and anode buffer layers were systematically investigated. Photovoltaic responses could be significantly improved by using EBLs with higher electron mobilities or using anode buffer layer. In addition, a linear dependence between HOMO levels of EBLs and VOC of the devices was derived, in which the linearity parameter increased with increasing electron mobility in EBLs.