Alq_(3)/HAT-CN叠层电致发光器件的激子调控机制探究

Exciton regulation mechanism of Alq_(3)/HAT-CN tandem electroluminescent devices

叠层有机电致发光器件(organic electroluminescent device,OLED)因寿命长和电流效率高等优点引起广泛关注.本文利用Alq_(3)/HAT-CN作为中间连接层制备了双发光单元叠层OLED,对其光电性能和激子调控机制探究.结果表明,在80 mA/cm^(2)电流密度下,叠层OLED的亮度(11189.86 cd/m^(2))和效率(13.85 cd/A)达到了单发光单元OLED(亮度和效率分别为4007.14 cd/m^(2)和5.00 cd/A)的2.7倍.在室温下,磁场诱导极化子对发生系间窜越(intersystem crossing,ISC),增加三重态激子(triplet exciton,T_(1))浓度,促进电荷散射,使磁电致发光(magneto-electroluminescence,MEL)低磁场快速增加和高磁场缓慢上升.当固定电流,Alq_(3)/HATCN器件中未复合的电荷较少,导致T_(1)与电荷湮灭(triplet-charge annihilation,TQA)减弱,致使MEL上升幅度最小.随电流增大,T_(1)浓度升高使TQA增强而ISC减弱.降低温度,电荷减弱,T_(1)浓度增大,使T_(1)湮灭(triplet-triplet annihilation,TTA)增强.因此,通过改变注入电流和温度可调控T_(1)的浓度,进而影响ISC,TQA和TTA的强弱,导致直接发光的单重态激子数量的增加,最终引起叠层OLED发光效率的提高.总之,本工作有助于深入理解小分子叠层OLED发光机制,对探明其光电性能提高的机理具有重要意义.

Tandem organic electroluminescent devices(OLEDs)have attracted widespread attention due to their long lifetime and high current efficiency.In this study,a double-emitting unit tandem OLED is fabricated by using Alq_(3)/HAT-CN as an interconnect layer.Its photovoltaic properties and exciton regulation mechanism are investigated.The results show that the luminance(11189.86 cd/m^(2))and efficiency(13.85 cd/A)of the tandem OLED reaches 2.7 times that of the single electroluminescent(EL)unit OLED(luminance and efficiency of 4007.14 cd/m^(2) and 5.00 cd/A,respectively)at a current density of 80 mA/cm^(2).This proves that the Alq_(3)/HAT-CN is an efficient interconnect layer.At room temperature,the polaron pair undergoes intersystem crossing(ISC)due to hyperfine interaction(HFI)when a magnetic field is applied to the device.This increases the concentration of the triplet excitons(T_(1)),thus promoting the charge scattering.The result is a rapid increase in the low magnetic field and a slow increase in the high magnetic field of the MEL.When the injection current strength is constant,there is less uncompounded charge in the Alq_(3)/HAT-CN device than in other connected layer devices.Triplet-charge annihilation(TQA)is weak,resulting in a relative increase in the value of T_(1),which is not involved in the TQA.This suppresses the ISC and leads to a minimal increase in the MEL.As the current strength increases,the T_(1)value increases,causing TQA to increase and ISC to decrease.Since the TQA is related to charge and T_(1)value,lowering the temperature reduces the carrier mobility in the device,resulting in the relative decreasing of charge concentration and the weakening of TQA.Lowering the temperature reduces the quenching of thermal phonons and increases the T_(1)value while extending its lifetime,resulting in the enhancement of triplet-triplet annihilation(TTA).At low temperatures,the high magnetic field shape of the MEL changes from slowly increasing to rapidly decreasing.Therefore,the T_(1)value can be regulated by varying the current strength and temperature,which further affects the strength of ISC,TQA and TTA,and the luminescence and efficiency of the device can be effectively improved by reducing TQA and ISC.This work is of great significance in understanding the luminescence mechanism of small molecule tandem devices and studying the mechanism for improving their photovoltaic properties.