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

Abstract

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 carrier 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.