三重态激子浓度对激子-电荷反应中散射和解离过程的调控

Triplet induced competition between scattering and dissociation process in exciton-charge reaction

制备了4-(dicyanomethylene)-2-methyl-6-(4-dimethylaminostyryl)-4H-pyran(DCM)掺杂4,4′-N,N′-dicarbazolebiphenyl(CBP)的有机发光二极管器件,并利用有机磁电导(magneto-conductance,MC)作为一种灵敏的探测工具,研究了器件的载流子传输特性.发现器件的MC随注入电流、温度和掺杂浓度的变化呈现出正、负磁电导效应,正磁电导和负磁电导分别由三重激发态与电荷反应(triplet-charge interaction,TQI)中的散射过程与解离过程所引起.研究表明,器件中TQI的散射和解离过程共存时,注入电流、工作温度和掺杂浓度都是通过改变三重态(triplet,T)激子的浓度来调节磁电导使其发生正负转变,即T激子浓度对TQI中的散射过程和解离过程有不同的作用:T激子浓度越大,TQI中载流子的散射通道越易占主导作用,此时器件呈现出正磁电导效应;反之,T激子浓度越小,TQI中三重态激子的解离通道越易占主导作用,此时器件表现出负磁电导效应.本工作为有机磁电导效应的有效调控提供了一条新途径,也加深了对有机光电器件中电荷与激发态间相互作用的理解.

Magneto-conductance （MC） refers to the relative change rate of conduction current with and without external magnetic field B and is used as the sensitive detection tools to study the carrier transport characteristics in organic semiconductor devices. This provides a non-contact approach to tune conduction current properties because the cartier transport shows strong dependence on magnetic field. In general, the magnetic phenomenon where the values of conduction current of device are growing at constant bias upon the application of external magnetic field is named positive MC effect. Inversely, when the B-induced current value is decreased it is named negative MC effect. Recently, researchers had found that the scattering and dissociation processes of triplet-charge interaction （TQI） respectively caused the positive and negative MC in organic polymer materials, the charge transfer states （CTS） leaded to dissociation process of TQI, and the exciton states （ES） leaded to scattering processes, and the rivalry between the two states could tune the MC between positive and negative. However, it remains further study whether the micro mechanism of positive and negative MC in small molecule materials is similar to that in polymer materials. In this work, organic light-emitting diodes （OLEDs） that based on small molecule materials had been fabricated through doping the 4-（Dicyanomethylene）-2-methyl-6-（4- dimethylaminostyryl）-4H-pyran （DCM） into 4,40-N,N0-dicarbazole-biphenyl （CBP） as an active layer. Here, we demonstrate that the MC of the device could be changed between positive and negative which were caused by the scattering and dissociation processes of TQI by fitting the MC curves with the combination of two non-Lorentzian empirical equations. The electroluminescence spectrum of CBP：x%DCM shows that small molecule materials were difficult to form CTS, although it had similar theory with polymer materials. As a result, both the scattering process and dissociation process in TQI had been caused by ES in CBP：x%DCM, and it was necessary to study the transformation mechanism between these two processes in ES. Therefore, it had been found that changing the injection currents, the temperature and the doped concentration could alter the concentration of triplet excitons, consequently leading to the positive and negative MC. This means that the scattering process and dissociation process in TQI have opposite dependences on the concentration of triplet excitons in CBP：x%DCM. The scattering process plays a major role when the concentrations of triplet excitons are large enough, yielding positive MC. On the other hand, the dissociation process plays an important role when the concentrations of triplet excitons are low enough, producing negative MC. This finding provides a new pathway to efficiently tune the organic magneto-conductance, and further to understand the microscopic mechanism of the interaction between charges and excite states in organic optoelectronic devices.