激基复合物有机发光二极管中平衡载流子增强电荷转移态的反向系间窜越过程

Enhanced reverse inter-system crossing process of charge-transfer stated induced by carrier balance in exciplex-type OLEDs

电荷转移(charge transfer,CT^(1)和CT^(3))态的反向系间窜越(reverse inter-system crossing,RISC,CT^(1)←CT^(3))过程是提高激子利用率的有效途径,精准利用该过程对于制备高效率激基复合物型(exciplex-type)有机发光二极管(organic light-emitting diodes,OLEDs)具有重要科学价值和应用前景.基于m-MTDATA:Bphen的典型激基复合物由于其内部高的RISC速率而受到广泛关注.但到目前为止,在实验上仅从瞬态光致发光谱中推测存在该RISC过程,这不利于全面认识并运用该过程设计高性能的光电器件.本文通过精确调控发光层(x m-MTDATA:y Bphen,x,y为质量分数)中给体与受体的共混比例和流过器件的载流子密度,获得了载流子平衡与非平衡的激基复合物器件,采用特征磁电导(magneto-conductance,MC)响应曲线可视化了平衡激基复合物器件中CT态间的RISC过程,且相比于非平衡器件,该器件具有更高的电致发光效率.本工作不仅能加深对于激基复合物器件中给体/受体共混比例影响载流子平衡的理解,还为最优利用RISC过程制备高效率光电器件提供理论依据和实验基础.

The reverse inter-system crossing(RISC,CT^(3)→CT^(1))process in charge transfer(CT^(1) and CT^(3))states is an effective approach to improving the energy utilization rate of excited states,and precise control and full use of the RISC process have important scientific significance and application prospect for fabricating and realizing the efficient exciplex-type organic light-emitting diodes(OLEDs).The conventional exciplex-type OLEDs based on m-MTDATA:Bphen have received extensive attention among researchers owing to the fact that the energy difference between CT^(1) and CT^(3) around zero promotes the efficient occurrence of RISC process.But up to now,only transient photoluminescence can infer the existence of RISC process in experiment,which is quite unfavorable for the comprehensive understanding and application of this process to design high-performance OLEDs.Fortunately,in this paper,a series of balanced and unbalanced exciplex-based devices are prepared by changing the donor-acceptor blending ratio in the emitting layer(x%m-MTDATA:y% Bphen;x%,y% is the weight percent)and the carrier density flowing through the device.The RISC process of CT states is directly observed via analyzing fingerprint magneto-conductance(MC)traces of the balanced device at room temperature,and the balanced device has higher electroluminescence(EL)efficiency than the unbalanced device.Specifically,the low-field MC curves of unbalanced device only show an inter-system crossing(ISC)line shape,whereas those from the balanced exciplex device present an RISC line shape at low bias-current and the conversion into an ISC line shape with the further increase of bias current.The line shape transition from RISC to ISC is attributed to the triplet-charge annihilation(TQA)process caused by excessive charge carries under high bias current.Combining the physical microscopic mechanism of device,the above-mentioned MC curves of various exciplex devices can be explained as follows:under the same bias current,extra holes or electrons are generated in the emitter layer of unbalanced devices due to the mismatch of donor-acceptor molecular concentrations.These superfluous holes or electrons will react with the CT^(3) state,which aggravates the TQA process in the device and weakens the RISC process in which the CT^(3) state participates.That is to say,there are strong TQA process and weak RISC process in unbalanced exciplex device.Contrarily,the strong RISC process and weak TQA process in the balanced exciplex device are beneficial to the occurrence of delayed fluorescence,resulting in its EL efficiency higher than that of the unbalanced device.This work not only deepens the physical understanding of the influence of donor-acceptor blending ratio on the carrier balance in exciplex devices,but also paves the way for designing highly efficient OLED by fully employing the RISC process of balanced device.