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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.
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Keywords:
- organic light-emitting diodes /
- half-band-gap turn-on characteristics /
- electron mobility /
- triplet-triplet annihilation
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