Studies On The Performance Of Highly Efficient Tandem OLED Based On Charge Generation Layer Of HAT-CN/CuPc Heterojunction

Tandem OLEDs(organic light-emitting diodes)have great potential applications in outdoor lighting,display and other high luminance scenes because they offer high luminance and high efficiency at low current density.Based on HAT-CN/CuPc heterojunction as a charge generation layer(CGL),high efficiency tandem OLEDs were fabricated.The valence electron structures,carrier dynamics,energy band structure and photoelectric performance of the devices were analyzed to clarify the CGL carrier dynamics process and the working mechanism of the corresponding OLEDs.Firstly,the effectiveness of the CGL based on HAT-CN/CuPc heterojunction was studied.The maximum current efficiency and external quantum efficiency(EQE)of the tandem OLED based on this CGL were 46.3 cd/A and 15.1%,respectively,which were 120%and 110%higher than that of the single OLED device.At the same time,tandem OLED exhibited better stability.The current efficiency and EQE roll-off of tandem OLED from maximum to at a luminance of 1000 cd/m2 were 7.3%and 7.9%,respectively,which were much lower than that of single OLED(20.9%and 21.9%).The experiments also showed that the electroluminescent(EL)performance of tandem OLED devices was weakly dependent on the thickness of CuPc.The reason was that the charge injection ability of CGL increased with the increase of the thickness of CuPc,while the transmittance of the CGL decreases.Hence,the thickness of CuPc had little effect on the overall performance of the CGL.According to the modified F-N tunneling model,the J-V characteristic curves of CGL devices were fitted.The good linear relationship between ln(J/U2)and 1/U suggested that the charge injection in HAT-CN/CuPc was tunnel injection.In addition,when HAT-CN/CuPc was used as the hole injection layer of OLED,the driving voltage and luminescence of the device were improved.When the current density was 20 mA/cm2,the driving voltage of the device with HAT-CN/CuPc as the hole injection layer was 24.2 V,lower than that of the control device(25.3 V).Secondly,the interfacial energy levels of HAT-CN/CuPc were characterized by UPS and XPS spectra.According to the UPS spectra,the highest occupied molecular orbital(HOMO)of HAT-CN and CuPc layers are-8.99 eV and-5.94 eV,respectively.The XPS spectrum showed that the depth of band bending at the interface of HAT-CN and CuPc was 0.7 eV.Then,the bandgap of HAT-CN and CuPc were determined to be 3.44 eV and 1.62 eV,respectively,according to the absorption spectra fitted by the Mott-Schottky relation.The detailed energy level structure of HAT-CN/CuPc was further analyzed.The results delivered that HAT-CN and CuPc have high compatibility,and the charge injection could be enhanced due to the large depth of HOMO energy level bending of CuPc at the interface.Therefore,HAT-CN/CuPc has high efficiency charge generation and separation ability.Finally,p-i-n and p-i type CGLs based on HAT-CN and CuPc was constructed.The EL characteristics curves of the tandem OLEDs showed that the optimal doping ratio of the i layer in the p-i-n type CGL based on HAT-CN/HAT-CN:CuPc/CuPc was 2:1.At high driving voltage,the non-doping CGL tandem OLED device offered higher luminous efficiency compared with the tandem device with p-i-n CGL because the depletion caused by the formation of electron-hole pairs increased with carrier concentration in the doping layer.According to the fitting curves of the C-V characteristic curves of HAT-CN/CuPc and HATCN/HAT-CN:CuPc/CuPc structure,both of the two CGLs provided high carrier concentration,which ensured a high tunneling current density.In addition,the results showed that the optimal doping ratio of CGLs based on HAT-CN/HAT-CN:CuPc structure was 1:2.