Study On Efficient Organic Light-Emitting Field Effect Transistors

Organic light-emitting field effect transistors?OLEFETs?are a novel class of multifunctional optoelectronic devices that combine the light emission function of an organic light-emitting diode?OLED?with the switching function of a field-effect transistor.This multifunctional property ensures their wide applications such as flat-pannel displays,integrated optoelectronics and electrical-pumped organic lasers.Although OLEFETs have the advantages of integrated function,high current density and low exciton loss,they have strict requirements on carrier mobility,photoluminescence quantum yield and energy level alignment making the current devices suffer from unbalanced carrier injection and transport,low exciton formation,weak light-emission and low efficiency.In this thesis,we concentrated on improving carrier transport ability,exciton utilization and developing novel luminescent materials to improve the light emission intensity and efficiency of the OLEFET.The charge transport ability and exciton formation can be promoted by the introduction of charge generation layer and electron injection layer.The triplet excitons can also be efficiently harvested due to the application of thermally activated delayed fluorescent materials.The key research work and results are as follows:1.Fabrication of efficient OLEFET based on MoO_x charge generation layer.The transition metal oxide MoO_x was introduced between the hole transport layer Pentacene and emission layer as the charge generation layer.The hole mobility and the concentration were significantly enhanced by virtue of the extra carriers generated from charge transfer between MoO_x and Pentacene.Simultaneously,we took advantage of the TPBI to lower the electron injection barrier and improve the carrier balance in the emission layer,and thus the exciton formation was greatly increased resulting in the promoted light emission intensity of the OLEFET.The optimized OLEFET achieved the mobility of 0.58 cm²V^-1s^-1,on/off ratio of 10²,brightness of238 cdm^-2 and external quantum efficiency?EQE?of 0.16%.Different sizes?25/50/100/200/400?m?of pixeled OLEFETs were fabricated by restricting the pixel length via narrowing the width of the gate electrode.2.Fabrication of efficient OLEFET based on organic heterojunction charge generation layer.We implemented the organic heterojunctions constructed by NPB and HAT-CN as charge transfer layer with the purpose of improving the carrier mobility and lowering the energy barrier between Pentacene and emission layer.The influence of both HAT-CN/NPB junction?npJ?and NPB/HAT-CN junction?pnJ?on the optoelectronic performance of OLEFETs were conscientiously investigated.The charge transfer between HAT-CN and Pentacene beneath source electrode improved the transport ability of holes and the charge transfer between HAT-CN and NPB under drain electrode facilitated the hole injection into emission layer for the npJ OLEFET.Consequently,the npJ devices exhibited higher EQE.Owing to the charge transfer between HAT-CN and Pentacene beneath source electrode in the pnJ OLEFET,the device represented superior hole transport ability.Considering the advantages of each OLEFET,we further fabricated OLEFET utilizing the heterojunction of HAT-CN/NPB/HAT-CN with the optimal performance of brightness of 8350 cdm^-2,EQE of 4.7%,hole mobility of 0.66 cm²V^-1s^-1 and on/off ratio of 10⁵.Efficient red OLEFET employing Alq₃:DCJTI as emission layer and blue OLEFET employing Bepp₂:BCzVBi as emission layer were also demonstrated based on the organic heterojunction for the realization of colorful OLEFET.3.Fabrication of highly efficient exciplex thermal activated delayed fluorescence?TADF?OLEFET.We first incorporated the exciplex TADF emitters in heterostructured OLEFETs to harvest triplet excitons under electrical excitation.Two exciplex TADF OLEFETs based on TCTA:B3PYMPM and m-MTDATA:OXD-7were fabricated.It is found that energy barrier is an important factor on the optoelectrical performance.The m-MTDATA:OXD-7?1:1 molar ratio?OLEFET presented optimal performance of brightness 1890 cdm^-2,EQE of 3.76%thanks to the high triplet utilization,small hole energy barrier between Pentacene and m-MTDATA and balanced electron and hole transport in the emission layer.Moreover,the OLEFET represented better“efficiency roll-off”than corresponding OLED with same emission layer at high brightness(>1000 cdm^-2).Our results demonstrate that the exciplex TADF emitters can be promising candidates to develop OLEFETs with high performance.4.Fabrication of efficient inorganic CsPbBr₃ perovskite light-emitting device?PeLED?.The light-emitting field effect transistors based on novel perovskite materials would not only be helpful to realize the flexible solution processed OLEFET with large area but also conducive to the study of the transport mechanism for perovskite materials.High quality CsPbBr₃ films assisted by polymer PEG with low non-radiative defects and high fluorescence quantum efficiency were formed.We also fabricated efficient PEG:CsPbBr₃ PeLED with brightness of 36600 cdm^-2,peak current efficiency of 19 cdA^-1 and maximum EQE of 5.34%and studied its emission mechanism.This work would lay the foundation for preparation of efficient OLEFET in the future.