Investigation On OLED Device Capable Of Storage With PMMA By Dynamic Measurement

With the rapid development in organic materials and design of devices,Organic light-emitting diodes(OLEDs)have made great progress in performance.However,the stored charges(accumulated charges,trapped charges,etc.)cannot be ignored as the impacting on performance.In this thesis,the behaviors of charges stored in OLEDs are investigated together with the mechanism underlying.Additionally,kinds of methods are applied to reuse these wasted charges,which will provide' a way to further understand the mechanism of stored charges and to make full use of them.First,the devices capable of stored charges with PMMA were prepared,whose performance was achieved through I-V-L characteristics.With respect to control device,the performance of the devices with PMMA was remarkably enhanced,which is attributed to the confinement of electrons by PMMA and its effective recombination with holes.Among these devices,device with 6nm PMMA performed better,which is due to the moderate thickness of PMMA reconciling both the injection and recombination.Based on I-V-L characteristics,the further investigation shows the different behaviors between current and luminance profile.To control device,they almost matched with each other.As to device with PMMA,however,the luminance first detached from current and then crossed,which gave rise to an enclosed region.Its area increased with the thickness of PMMA increased,which can reflect the storage of charges since stored charges have participated both in current and luminance.Thus,it provides a new way to find stored charges in simple.By transient EL measurement,the rationality of the method we proposed is certificated by the presence of EL spike,which derived from the stored charges and enhanced with the increase of PMMA.Then by applying different pulses,the different phenomena brought indicate the stored charges consist of the charges accumulated in interface and that confined in shallow trap and deep trap.By superposing different pulses,the reuse of stored charges was realized.As it shows,an enhanced EL intensity of stored charges is more than 3 times as against to steady-state.With respect to the electric memory device,the device based on PMMA,to some extent,can also be capable of memory.Nevertheless,it is volatile memory instead of nonvolatile memory.Then,the device with PMMA was driven by different alternating-current(AC)stress,which also brought various transient EL.As to sinusoidal AC stress,it appeared two peaks in transient EL.The first one took place in forward semi-period corresponding to recombination of injected carriers.The increase of second peak originated at 4.3 V and reached the maximum at-0.1V and then decreased.By transient EL measurement,it is certificated to correspond to emitting from stored charges,which consists of surface and bulk storage.Thus,when operating under AC stress,the reusing of stored charges can be realized.Then,other AC stress as saw tooth wave and triangle wave were applied to drive the device with PMMA.The various EL shapes indicate distinct behaviors of stored charges,which can facilitate to the investigation of mechanism.Then,the stored charges in device with PMMA were further investigated by C-V characteristics,especially the capacitance under illumination.When operated under dark,the presence of two capacitance peak took place in C-V characteristics.The first one is depletion capacitance,which decreased and shifted to high voltage with the increase of PMMA.It can attribute to the overload of the voltage drop.The second peak is induced by injected carriers and blocked electrons by PMMA,which increased and broadened with the increase of PMMA.As to the device with PMMA but without LiF,a great change took place in capacitance under illumination,whose first peak was enhanced remarkably together with the shifting to low voltage.With the PMMA increased,the shifting increased along with the decrease of minimum capacitance,even presence of new peak occurred.Combined with G-V characteristics,it is revealed that the first peak derives from the storage of photo-carriers also with interfacial charges.By tuning illumination of power,the mechanism is deeply understood.Additionally,the linear relation between peak value and power indicates its application in optical detection.As to the device capable of storage with LiF,the first capacitance under illumination gradually behaved unlike a peak together with broadening.By G-V characteristics and frequency-dependent characteristics,it can be explained by the reversed field brought by the charges blocked by PMMA and LiF.Thus,it will be beneficial to investigate the mechanism of charges in OLEDs with C/G-V characteristics and its frequency-dependent characteristics.