Structure Design And Performance Study Of OLEDs Based On Organic Heterojunction

With the application of OLEDs in the fields of smart phones,tablet computers,TV displays and green lighting,the requirements for the efficiency and stability have become higher,which is urgent to develop high efficiency and long-lifetime OLEDs.For the development of high efficiency OLEDs,on the one hand,it can start with the new materials,and design,synthesize high efficiency materials of OLEDs;on the other hand,designing more efficient OLEDs device structure to further improve the efficiency of devices.In the structure design of device,organic heterojunctions exhibit superior performance.Studies have shown that organic heterojunctions possess highly efficient charge generation characteristics and high conductivity,which act as charge injection layer,achieving efficient and balanced injection ofcharge;As an intermediate connection layer of tandem OLEDs,not only increase the brightness and current efficiency exponentially,but also improve their power efficiency significantly,sloving the problem that the power efficiency can't be improved in normal tandem OLEDs.Therefore,it is meaningful to deeply study the charge generation mechanism of organic heterojunctions and fabricate the high performance OLEDs.In this thesis,the structure and performance of OLEDs based on organic heterojunction were investigated and the charge generation mechanism of organic heterojunctions was systematically explored.And high Efficiency,long-life stacked OLEDs and color-adjustable AC OLEDs base on the organic heterojunctions as charge injection layer and intermediate connection layer have been fabricated successfully.The main work of this thesis is summarized as follows:1.The charge generation mechanism of organic heterojunction was studied.By comparing the positive and negative I-V characteristics of organic heterojunction devices composed of N-type materials of HAT-CN and P-type materials of m M,TAPC,TCTA,NPB,and 4P-NPB at different P-type layer thicknesses,confirming the highly efficient charge generation characteristics of organic heterojunctions.The I-V characteristics analysis of HAT-CN/TAPC heterojunction devices at different temperatures showed that the efficient charge generation process of organic heterojunctions is based on an electron tunneling mechanism;based on the Mott-Shockley model,the free charge density in the HAT-CN/TAPC heterojunction was calculated as high as 4.5×10¹⁷ cm^-3,which ensures the high conductivity characteristics of the heterojunction.These results laid a physical foundation for the application of organic heterojunctions.2.The structure and performance of single and tandem OLEDs based on HAT-CN/TAPC heterojunction as charge injection layer and charge generation layer were studied.Studies have shown that the organic heterojunctions possess good charge injection characteristics and as the charge injection layer,the injection balance of electrons and holes was improved,and the efficiency and stability of the device were improved.They are used as both charge injection layer and an intermediate connection layer,not only the quantum efficiency and current efficiency have been doubled,but also the power efficiency and stability have been significantly improved in the fabricated tandem OLEDs.The maximum EQE,CE and PE of the resulting blue,green,orange and red tandem OLEDs reached 48.5%,99.2 cd A^-1 and 59.8 lm W^-1?42.0%,159.5 cd A^-1 and 92.7 lm W^-1?47.2%,161.9 cd A^-1 and93.2 lm W^-1 and 48.1%,59.0 cd A^-1 and 33.8 lm W^-1,and the half lifetime of the green tandem device at brightness of 1000 cd m^-2reached 1350 hours.These results fully demonstrated the superiority of organic heterojunctions for the fabrication of high performance OLEDs.3.The structure and properties of AC OLEDs based on TAPC/HAT-CN/TAPC heterojunction as an intermediate connection layer were studied.Using highly efficient charge generation characteristics of three-layer TAPC/HAT-CN/TAPC heterojunction under forward and reverse voltage,two bicolor AC-OLEDs,blue-orange and green-orange,were successfully fabricated.By changing the voltage value of the AC forward and reverse which not only realized the continuous conversion between blue and orange,green and orange,but also obtained good white light emission,which fully demonstrated the working characteristics of AC devices.These results provide a new direction for the application of organic heterojunctions.