Research On Alternating Current Driven Organic Light-emitting Devices

Organic light-emitting device(OLED)is a surface light source with unique advantages of self-illumination,all solid-state display,thin and environmental protection,large viewing angle,fast response speed,high image refresh speed,great temperature characteristics and flexibility.It has shown great commercial value and application potential in the field of lighting and display.Unlike home power supply systems,which use 50 Hz alternating current(AC),traditional OLEDs are typically driven by direct current,requiring back-end circuits to convert and increasing energy consumption.To solve this problem,design and fabricate AC driven OLED(AC-OLED)is the ideal method.The use of AC shows incomparable advantages compared with direct current,such as more adapt to people's household life.AC is helpful to suppress the device internal charge accumulation under negative half cycle.Using the frequency,amplitude,duty ratio and related characteristics of AC,OLED will be more controllable and suitable for intelligent lighting and other applications.At present,the research and application on AC-OLED is rarely reported,there is a gap compared with the development of direct current driven OLED.Thus,in this paper,we designed new device structures to achieve a series of high performance AC-OLED.The working mechanism of the devices were analyzed deeply,and its applications in intelligent lighting were discussed.(1)Tandem AC-OLED based on a new intermediate connection layer(Ag/Bphen)_nwas proposed.The optical and electrical properties of the intermediate connection layer was verified,showing high transmittance and excellent capacity of carrier generation and injection,which can meet the requirements of the intermediate connection layer in tandem devices.Then,we connected the stacked forward and inverted light emitting units based on the intermediate connection layer and successfully prepared the tandem AC-OLED.The tandem AC-OLED can work normally under the positive and negative voltages,emitting green light and yellow light respectively.This proves that the intermediate connection layer(Ag/Bphen)_n can be used in tandem AC-OLED.Furthermore,we used this intermediate connection layer to prepare the tandem white AC-OLED,which emits blue and yellow light,respectively,under the positive and negative half cycles.Through the transient electroluminescence response characteristic,we found that two light emitting units of the tandem white AC-OLED responded to the positive and negative half periods of the AC signal independently.Then the tandem white AC-OLED could adjust from blue,cold white,white,warm white to yellow by adjusting the amplitude of the positive and negative half periods of AC signal.(2)AC-OLED based on the in-planar electrodes was proposed to simplify the preparation process and improve the device performance.Firstly,in order to verify the feasibility of coplanar electrodes in OLED,we prepared a bottom-emitting green OLED based on coplanar electrodes.Experimental results show that the two units were connected in series by the intermediate connecting layer.In the unit which emits light,the injected holes and injected induction electrons form excitons in the light-emitting layer to radiative recombination.At the same time,in the unit which doesn't emit light,the induction holes generated by the intermediate connection layer are neutralized at the ITO interface,thus achieving electrical neutrality.Thus,the coplanar electrodes can be used into AC-OLED.Then,by using blue and yellow light emitting unit,we prepared in-planar electrode type bottom emitting white AC-OLED,which emits blue light under positive half cycle of AC and yellow light under negative half cycle of AC signal.We proposed scattering device and miniaturized light emitting units to achieve white light,respectively.Since two light emitting units respond independently to the positive and negative half periods of AC signal,they can be adjusted by the amplitude and duty cycle of the positive and negative half periods to realize the adjustment from blue to white and yellow light.In addition,we also discussed the application of the in-planar electrodes type bottom emitting white AC-OLED in intelligent lighting system,such as mine.(3)Flexible top-emitting white AC-OLED based on dual microcavity technology and in-planar electrodes was proposed to realize top-emitting white color with stable spectra.Through optical simulation and calculation,the reason that a single microcavity cannot achieve high quality top-emitting white OLED is demonstrated.Then the dual microcavity technology was proposed.By adjusting the length of the two microcavities,the microcavity effect of white light complementary color was optimized respectively,and the angle characteristics of the microcavities were improved by using the light extraction layer.Then,an efficient flexible top-emitting white AC-OLED was fabricated based on in-planar electrodes,which emits blue and yellow light under the positive and negative half cycles of AC,respectively.Moreover,the performance of the device is significantly improved by using the microcavity effect.Because the two light emitting units respond independently to the positive and negative half cycles,the AC driving mode makes it also adjustable in brightness and color temperature.By adjusting the intrinsic parameters of the AC signal,the change from blue through cold white,white warm white to yellow can be realized.In addition,the flexible top-emitting white AC-OLED can be prepared on a variety of flexible and opaque substrates,such as paper substrates,breaking the limitations of substrates.