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半带隙开启特性是有机发光二极管独有的一种光电属性,它在研制低驱动电压器件方面具有优势,但电子注入层(electron injection layer, EIL)如何影响半带隙开启的研究还未见报道. 本文选取电子迁移率按数量级依次降低的EIL材料制备了三组控制器件,测量了器件室温下的I-B-V曲线,发现随EIL电子迁移率的依次降低, 器件的开启电压分别呈现出半带隙(half-band-gap)开启、亚带隙(sub-band-gap)开启和正常开启的物理现象.器件发光的特征磁效应(magneto-electroluminescence, MEL)结果显示: EIL电子迁移率高的器件实现半带隙开启主要归因于三重态-三重态湮灭(triplet-triplet annihilation, TTA, T1, Rb + T1, Rb→ S1, Rb + S0)过程有效降低了器件的开启电压(1.1 V). 但在EIL电子迁移率较低的器件中,为注入更多电子需要在低电子迁移率的EIL上施加更高的电压,从而抵消了TTA过程降低的开启电压,因此随着EIL电子迁移率依次降低其开启电压表现为亚带隙开启(2.1 V)和正常开启(4.1 V). 此外, 尽管三组器件中都观测到TTA过程,但高EIL电子迁移率的器件中TTA更强,亮度更高,这是由于EIL迁移率高的器件在Rubrene发光层中形成了更多数量的三重激基复合物EX3态及其更强Dexter能量传递过程(EX3
® T1, Rb). 而且,通过给这三组器件降温,都降低的载流子迁移率导致I-V曲线开启电压因EIL迁移率的不同引起其增幅明显不同,它们低温下的MEL曲线因载流子迁移率降低和激子寿命延长的共同作用呈现不同的温度依赖行为.本工作进一步加深了对Rubrene/C60型器件中电子迁移率对开启电压影响的理解以及相关物理微观机制的认识. Half-band-gap turn-on characteristic is a unique photoelectric property of organic light-emitting diodes (OLEDs), which has advantage in the development of low power consumption devices. But the physical mechanism that the electron injection layer (EIL) affects the half-band-gap turn-on characteristics has not been reported. Herein, we found that the change from half-band-gap turn-on electroluminescence (EL) to sub-band-gap turn-on EL to normal turn-on EL is observed by tuning the electron mobility of EIL in Rubrene/C60 based devices. Three sets of devices were fabricated by using BCP (~10-3 cm2·V–1·s–1, Dev.1), Bphen (~10-4 cm2·V–1·s–1, Dev.2) and TPBi (~10-5 cm2·V–1·s–1, Dev.3) as EIL materials. By measuring the I-B-V curves of devices at room temperature, we found that the turn-on voltage of devices obviously increases with the decreases of electron mobility of EIL by an order of magnitude. Specifically, the turn-on voltage of Dev.1, Dev.2, and Dev.3 exhibit the physical phenomena of half-band-gap turn-on (1.1 V), sub-band-gap turn-on (2.1 V) and normal turn-on (4.1 V) properties, respectively. The magneto-electroluminescence (MEL) results show that the half-band-gap turn-on characteristic of high EIL electron mobility (Dev.1) is attributed to the triplet-triplet annihilation (TTA, T1, Rb + T1, Rb → S1, Rb + S0) process which can effectively reduce the turn-on voltage. However, the half-band-gap turn-on characteristic is not observed in the devices (Dev.2 and Dev.3) with low carrier mobility, which can be reasonably explained by a higher voltage is applied to the EIL with low electron mobility in order to inject more electrons. The higher voltage counteracts the reduced turn-on voltage of the TTA process, resulting in Dev.2 and Dev.3 with sub-band-gap turn-on and normal turn-on, respectively. In addition, although the TTA process was observed in all three devices, the TTA process was stronger and the EL was higher in Dev.1 with high EIL electron mobility. This is because a large number of triplet Rubrene/C60 exciplex states (EX3) was formed at the Rubrene/C60 interface, enhancing the Dexter energy transfer (DET, EX3 → T1, Rb) process from EX3 to triplet exciton of Rubrene (T1, Rb). That is, Dev.1 exhibits stronger TTA process and higher EL due to the presence of a large number of T1, Rb exciton formed by DET process as compared to Dev.2 and Dev.3. Furthermore, by measuring the I-V curves of devices acquired at low temperature, it was found that the reduced carrier mobility caused by lowering operational temperature increases the turn-on voltages of these three devices. The significantly different increases in the turn-on voltage of Dev.1-3 at the same low temperature is due to the different influences of temperature on the electron mobility of EIL. The tradeoff between the decrease of carrier mobility and the extension of exciton lifetime makes the MEL curves present different temperature-dependent behavior. Obviously, this work further deepens the understanding for the influence of EIL electron mobility on the turn-on voltage and the related physical microscopic mechanism in Rubrene/C60 devices.-
Keywords:
- organic light-emitting diodes /
- half-band-gap turn-on characteristics /
- electron mobility /
- triplet-triplet annihilation
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