Development Of Tandem Near-ultraviolet Organic Light-emitting Diodes

With the study of organic light-emitting diodes(OLEDs)in visible wavelenths becoming mature,more and more researchers are extending the study of OLEDs to near-infrared or near-ultraviolet wavelenths.Because of its ability to emit high energy photon,near-ultraviolet organic light-emitting diodes(NUV-OLEDs)can be widely used in the fields of disinfection,anti-counterfeiting detection,and lighting display.Compared to ordinary visible light emitting materials,organic NUV luminescent materials usually have a wide band gap(>3.5 e V),leading to large difficulty to improve the luminous efficiency of related devices.In order to realize the application of NUV-OLEDs,the basic commercialized requirements of luminescent power greater than 10 m W/cm~2 and external quantum efficiency greater than 1%under long working conditions are also need to be meet.In this thesis,in order to enable the device to emit a large radiation power,tandems NUV-OLEDs were fabricated by using wide band gap organic materials.Two ultraviolet luminescence units were connected by using a charge generation layer(CGL).This method is expected to realize high emission power of NUV light and improve the long-term emission stability of devices.The specific research process is as follows:Firstly,HAT-CN/TAPC organic heterojunctions with excellent charge generation function was prepared as CGL,and the thickness of these organic heterojunctions was compared and optimized.The experimental results show that the thickness of heterojunctions is exactly the same as the thickness of the space charge regions when the thickness of HAT-CN and TAPC were set as 10 nm.In this case the HAT-CN/TAPC heterojunctions had the best charge generation performance.Then,the CGL is applied to NUV-OLEDs with tandem structres,and single and stacked NUV-OLEDs based on BCPO and TAZ were developed,respectively.The peak wavelength of electroluminescence of BCPO device is 385 nm,and the proportion of NUV light in the emission spectrum is 56.8%.And the peak wavelength of electroluminescence of TAZ device is 380 nm,and the proportion of NUV light in the emission spectrum is 79.4%.The measurement results of BCPO-based OLEDs show:for single BCPO device,the maximum current density is 632.8 m A/cm~2,the maximum radiance power is 35.4m W/cm~2,and the maximum external quantum efficiency is 2.94%;for stacked BCPO device,the maximum current density is 303.2 m A/cm~2,the maximum radiance power is48.1 m W/cm~2,and the maximum external quantum efficiency is 5.68%.The experimental results show that the electroluminescence performances of tandem BCPO device is better than that of single BCPO device.The measurement results of TAZ-based OLEDs show:for single TAZ device,the maximum current density is 227.8 m A/cm~2,the maximum radiance power is 13.7 m W/cm~2,and the maximum external quantum efficiency is 3.73%;for stacked TAZ device,the maximum current density is 136.1 m A/cm~2,the maximum radiance power is 22.1 m W/cm~2,and the maximum external quantum efficiency is 7.25%.Once again,the experimental results show that the electroluminescence performances of tandem TAZ device is better than that of single TAZ device.It’s indicated by experimental results that the HAT-CN/TAPC heterojunction can be effectively used as CGL to connected two NUV luminescence units,making the carriers transportation in two luminous units more balanced.The luminescence efficiency and emitting stability of tandem devices are higher than single device.The above-mentioned basic requirements for commercialization of NUV-OLEDs are met.