Study On Carrier Control And Performance Of Organic Light-emitting Diodes

Organic light-emitting diodes(OLEDs) exhibit many advantages, such as various materials, full solid self-emission, high efficiency, flexible, ultra-thin, large area and multi-function application. OLEDs have already been developed as a hot-point technology of information display and gradually realized large scale commercial production. Over the past three decades, with the development of new materials, structures, processes and in-depth study of luminescence mechanism, the significant progresses of basic research and application have been made in OLEDs. However, the efficiency, stability, yield rate and cost of OLEDs are still the main problems, which need to be overcome. The key point is focused on basic research, especially the design and optimization of novel device architectures, and the further study of carrier control on device performance. In this dissertation, aiming at solving the above problems, several novel structures are proposed. Carrier control method has been used to improve the charge carrier balance in OLEDs with dual emissive layers. Bipolar fluorescent material works as carrier control layer and green light-emitting layer in white organic light-emitting diodes(WOLEDs). The mechanism study of organic bulk heterojunction charge generation layers on performance of tandem OLEDs. Moreover, the carrier control mechanism is applied in inverted polymer solar cells(IPSCs) and Zinc oxide(Zn O) is employed as cathode buffer layer to fabricated high performance device. The analysis of carrier control structure and mechanism on device performance promotes the development of the OLEDs commercial application. The main contents of this dissertation are as follow.1. Phosphorescent OLEDs(PhOLEDs) have been fabricated with carrier control structure and multi-emissive layer. A hole transport material, N, N′-bis(naphthalen-1-yl)-N,N′-bis(phenyl)-benzidine(NPB) has been introduced as hole transport layer and host of the first emissive layer. A bipolar material 4,4′-bis(carbazol-9-yl) biphenyl(CBP) and an orange phosphorescent dopant of bis[2-(4-tert-butylphenyl)- benzothiazolato-N,C2’]iridium(acetylacetonate)(t-bt)2Ir(acac) were employed as host of the second emissive layer and guest in dual emissive layer, respectively. The influence and mechanism of carrier control structure on carrier balance and expansion of exciton formation zone has been investigated in PhOLEDs. The turn-on voltage and maximum luminance of dual emissive device were 3.3 V and 30898 cd/m2, which exhibited 36.5% lower and 174% higher compared with single emissive device. Moreover, the roll-off of power efficiency in dual emissive device was restricted. It was found that the carrier control structure was beneficial for the elimination of highest occupied molecular orbital(HOMO) energy barrier between hole transport layer and host of the first emissive layer, which decreased the charge carrier accumulation of organic interface. In addition, the optimized multi channels of electrons and holes were favorable for carrier balance. The effective emission and device stability of OLEDs were attributed to the expansion of charge carrier recombination zone. tris(8-hydroxy-quinoline) aluminum(Alq3), 1,1-bis[(di-4-tolylamino)phenyl] cyclohexane(TAPC), CBP and N,N’-dicarbazolyl-3,5-benzene(mCP) were used as spacers to fabricate white Ph OLEDs, respectively. The influence of different spacers on device performance, charge transport and energy transfer was systematically investigated. When the spacer is Alq3, a fairly pure WOLEDs with Commission Internationale De L’Eclairage(CIE) coordinates of(0.35, 0.34) and lowest turn-on voltage of 6.2 V were obtained.2. A bipolar green fluorescent material 2-{4-[bis(9,9-dimethylfluorenyl)amino] plenyl}-5-(dimesityl boryl)thiophene(FlAMB-1T) was employed as carrier control and green emissive layer in nondoped three-primary color WOLEDs. An ultrathin 3-(dicyanomethylene)-5, 5-dimethyl-1-(4-dimethylamino-styryl) cyclohexene(DCDDC) was used as red emitting layer and 4, 4’-bis(2,2’-diphenyvinyl)-1,1’-dipenyl(DPVBi) was employed as blue emitting layer. The thickness influence of FlAMB-1T on charge device performance and charge carrier recombination zone was investigated. The results showed that with the increasing of thickness, the luminance and maximum power efficiency of devices were remarkably enhanced by 218% and 330%. The charge carrier balance in WOLEDs was derived from the carrier control structure of F1AMB-1T, which was beneficial for both electron and hole transport. Meanwhile, the construction of FlAMB-1T/DCDDC/FlAMB-1T charge carrier trapping realized the direct carrier trapping, which easily controlled the carrier recombination center. When the thickness of FlAMB-1T is 10 nm, a fairly pure WOLEDs with CIE coordinates of(0.33, 0.36) was obtained. In addition, with the increasing of driven voltage, the charge carrier recombination centers of WOLEDs were limited in three stable regions, which demonstrated that the FlAMB-1T could effective control the exciton formation region.3. High-performance tandem OLEDs based on two kinds of organic bulk heterojuntion charge generation layers(CGL) consisted of boron subphthalocyanine chloride(SubPc):fullerene(C60) and cobalt(II) phthalocyanine(CoPc):C60 were fabricated. The effect of carrier control ability of CGL on device performance was systematically studied. It was found that the optimization of CGL was beneficial for the improvement of the hetero-inface, faciliting more interface dipole and improving the charge generation ability of CGL. Additionaly, the optimized device performance and carrier balance was mainly contributed from the improved charge carrier pathways. With the optimizing of SubPc and CoPc weight ratio, the maximum current efficiencies of 63.6 cd/A and 50.2 cd/A, respectively, were achieved for tandem OLEDs.4. High-performance IPSCs were obtained by using low temperature annealing zinc oxide(ZnO) as the cathode buffer layer. It was found that the low temperature dynamic vacuum annealing was beneficial for improve the surface morphology of Zn O film. The obvious decreased surface roughness was contributed from the distribution of heat flux, valatilization process of solvent and thermal decomposition process of ZnO precursor. The results showed that IPSCs with dynamic vacuum annealing ZnO exhibited 15.8% enhancement within power-conversion efficiency(PCE) of 4.01%, compare to the IPSCs with in situ annealing process based ZnO films. The light intensity dependence and impedance spectra of IPSCs were investigated. It was found that the performance enhancement of IPSCs was due to decreased contact resistance between ITO and active layer, suppression of leakage current, effective control of carrier recombination, suppression of leakage current, transport and collection enhancement of electron.In summary, we have designed several kinds of device architecture with carrier control capability, and this work provides theory basic and theory guidance to achieve highly efficient OLEDs. Also, the functional layer of ZnO with carrier control capability was used to improve the device performance of IPSC, indicating that carrier control is an effective way to realize high performance organic optoelectronic device.