Preparation Of Flexible OLED And Factors Influencing Luminous Properites Of Flexible OLED

At present,flexible OLED technology,as an important branch of the OLED field,has achieved little success with everyone’s efforts,and some flexible OLED products have appeared on the market.However,some problems are difficult to solve such as the high roughness of the flexible substrate,the low melting point,and the imbalance of carrier injection inside the device,which results in the poor luminous performance of flexible OLEDs.This article focuses on the above issues such as hole injection layer,substrate,hole transport layer,and cathode to study its effect on the light efficiency of flexible OLEDs.The aim is to improve the brightness,luminous efficiency,power efficiency and other parameters of flexible devices by using new hole injection layers,introducing new flexible substrates,changing the thickness of hole injection/transport layers,and developing new cathodes.The main research of this article are summarized as follows:(1)The effect of different hole injection layers on the light efficiency of PET/ITO flexible substrates was experimentally investigated.HAT-CN,PEDOT:PSS and PEDOT:PSS/HAT-CN composite films were used as hole injection layers to prepare flexible OLED devices.PEDOT:PSS was prepared by spin coating,and the remaining organic layers and cathode were prepared by vacuum evaporation.The results show that the device using PEDOT:PSS/HAT-CN composite film as hole injection layer has the best performance.The device’s maximum current efficiency and maximum power efficiency are 84 cd/A and 76 lm/W,respectively.The reason is that the surface of the flexible substrate modified by PEDOT:PSS is more continuous and smooth,and it is not easy to cause leakage and short circuit of the device.The PET/ITO/PEDOT:PSS/HAT-CN composite film has a high transmittance in the green light band,which can improve the light output,in addition,the double hole injection structure makes the injection of carriers in a dynamic equilibrium state,increasing the recombination probability of electrons and hole carriers.(2)The effect of the thickness of hole-injection layer HAT-CN on the light efficiency of the device with a microcavity structure using a semi-transparent Al film anode was studied.The results show that in a flexible OLED device of this structure,due to the influence of the electric field strength,the thickness of the HAT-CN is not as thick as possible.When the thickness of the hole injection layer is too thick,the hole injection ability of the device will decrease sharply,thus affects the light-emitting performance of flexible OLED devices.When the hole injection layer thickness is 4 nm,the performance of the device is optimal.At a current density of 50 mA/cm2,the driving voltage is 4.94 V,which is 1.77 V lower than the 8 nm HAT-CN device.The maximum current efficiency and power efficiency are 58.4 cd/A and 59.3 lm/W,which are 1.3 times that of the 8 nm HAT-CN device.Moreover,the increase of HAT-CN thickness caused a change of cavity length in microcavity structure,resulting in a red shift in the emission spectrum of the device.(3)The effect of substrates on the light efficiency of the flexible OLED was studied.A new COP substrate was introduced into the flexible OLED device and compared with the commonly used PET substrate.The results show that the performance of flexible OLED devices prepared by COP substrates with lower roughness and higher transmittance in the visible range is better.The maximum current density and power efficiency are 79 cd/A and 78 lm/W,respectively,which is about 1.1 times that of PET substrate based device.When the voltage is 5 V,the device has a brightness of 12794 cd/m2,which is about twice that of PET substrate based device.Moreover,the change of substrates will not affect the light emission spectrum of the flexible OLED device.It is fully proved that COP is a good alternative flexible substrate.(4)The effect of the thickness of hole transport layer TAPC on the light efficiency of COP substrate based flexible OLED device was studied.The results show that by changing the thickness of hole transport layer TAPC,the number of holes injected into the light emitting layer can be well adjusted.As the thickness of hole transport layer becomes thicker and thicker,the hole injection ability becomes weaker and weaker.The number of holes injected into light-emitting layer will decrease;when the thickness of hole injection layer is appropriate,the number of holes and electrons injected into the light emitting layer will be more balanced,and the probability of holes and electrons participating in recombination to form excitons will increase.As a result,various luminous indexes can be improved.When the thickness of TAPC is 70 nm,the performance of the device is optimal.At a current density of 50 mA/cm2,the device’s luminous brightness is 31103 cd/m2,which is 2127 cd/m2,582 cd/m2,and 2583 cd/m2 higher than the other three groups of devices.The current efficiency of this device is 62 cd/A,which is 4.4 cd/A,1 cd/A,and 5 cd/A higher than the other three groups of devices,respectively.(5)The effect of cathode on the light efficiency of the flexible OLED was studied.Flexible OLED devices were prepared by using different content of Al:Yb alloys as cathodes,and compared with pristine Al-based device.The results show that the doping of Yb can effectively improve the electron injection ability of cathode,reduce the driving voltage of the device,and make the brightness significantly increase.When Yb content is 9%,the device performance is optimal.The driving voltage of this device is 4.6 V,which is 20%lower than that of pristine Al-based device.At a driving voltage of 4.6 V,the luminous brightness is 22497 cd/m2,which is 1.8 times higher than that of pristine Al-based device.However,the doping of Yb will cause the cathode reflectivity to decrease.When the devices emit the same light intensity to the outside,the light emitting layer of Al:Yb based device needs to synthesize more photons,which affects the performance of the device.Therefore,when Yb contene is 9%,compared with pristine Al-based device,the power efficiency is slightly improved,but the current efficiency is not significantly improved.