基于Alq3的有机发光器件的发光特性

以Alq₃作为发光层,在OLED串联型制作系统上成功制备出ITO/TPD/Alq₃/LiF/Al结构的有机发光器件,并建立了一套OLED电流(J)、电压(V)、亮度(B)自动测试系统,在氮气和空气环境下测试并分析了OLED的发光特性。结果表明,基于Alq₃的双层OLED的正向J-V特性可以用陷阱电荷限制流来描述;反向工作时,低压下的反偏电流可能是针孔产生的漏电流,高压下反偏OLED的J-V特性应满足F-N隧穿机制。随着电流进入快速增长阶段,B-J曲线近似地呈线性关系;在低场下,发光效率随电压升高而增大,在高场下,发光效率随电压升高而减小。实验中,还观察到了在电压V=4V左右时,器件具有明显的负阻特性(NDR),进一步的分析表明,由针孔引起的丝状电流可能是负阻特性的成因。

Luminescence Properties of Alq3-based OLEDs

Organic light emitting diodes(OLEDs) based on aluminum tris(8-hydroxyquinoline)(Alq₃) as an active luminescent material have shown tremendous growth since its inception.Indeed,numerous experiments have provided an insight into mechanisms and processes such as the formation of the metal/organic interface,injection processes,charge transport,the effects of doping,transport,and recombination near organic/organic interfaces.However,even with a relatively good knowledge of these particular processes,it is still not an easy task to predict the properties of a multiplayer organic device without recourse to experiment.We therefore focused our effort on the detailed study of the luminescence characteristics of Alq₃-based OLED.The OLEDs with the structure of ITO/TPD/Alq₃/LiF/Al were fabricated in the in-line 5-chamber deposition system.The J-V-B characteristics were tested in N₂ atmosphere glove-box and in the air using the home-made system.The J-V characteristics in the forward direction,when ITO is positively biased,appears to be trapped-charged limited current(TCLC) which can be described by power laws J∝V^m.The reverse current may be governed by tunneling under high electric field.For low reverse bias,the current may be the leakage caused by pinhole.At high current densities,the brightness-current relationship tends to a linear function.The luminescence efficiency rises as the voltage rising at the low bias voltage region and decrease as the voltage rising at the high bias voltage region.In case of this typical bilayer structure,which is often called "model device",a strong influence of ambient atmosphere on the electrical properties was observed.The J-V characteristics in vacuum exhibited anomalous behaviour,including regions with negative differential resistance(NDR).In air,the anomalous behaviour disappeared.Such J-V characteristics were also observed by other researchers in some OLEDs based on polymers and dye doped Alq₃ during the last few years. But no reasonable explanation has been provided for this phenomenon.Especially,the NDR of the "model device" at the low voltage region was not discussed.The test results showed that the negative differential resistance is observed at the voltage of 4 V.Futher analysis indicates that this phenomenon may be attributed to the current filament caused by the pinhole.