Thermal Annealing Effect on the Thermal and Electrical Properties of Organic Semiconductor Thin Films
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Thermal Annealing Effect on the Thermal and Electrical Properties of Organic Semiconductor Thin Films Xinyu Wang, Boyu Peng and Paddy Chan Department of Mechanical Engineering, The University of Hong Kong, Hong Kong ABSTRACT The thermal and electrical properties of organic semiconductor are playing critical roles in the device applications especially on the devices with large area. Although the effect may be minor in a single device like field effect transistors, the unwanted waste heat would cause much more severe problems in large-scale devices as the power density will go up significantly. The waste heat would lead to performance degradation or even failure of the devices, and thus a more detailed study on the thermal conductivity and carrier mobility of the organic thin film would be beneficial to predict the limits of the device or design a thermally stable device. Here we explore the thermal annealing effect on the thermal and electrical properties of the small molecule organic semiconductor, dinaphtho[2,3-b:2’,3’-f]thieno[3,2-b]thiophene (DNTT). After the post deposition thermal annealing, the grain size of the film increases and in-plane crystallinity improves while cross-plane crystallinity keeps relatively constant. We demonstrated the crossplane thermal conductivity is independent of the thermal annealing temperature and high annealing temperature will reduce the space-charge-limited current (SCLC) mobility. When the annealing temperature increase from 24 oC to 140 oC, the field effect mobility shows a gradual increase while the threshold voltage shifts from positive to negative. The different dependence of the SCLC mobility and field effect mobility on the annealing temperature suggest the improvement of the film crystallinity after thermal annealing is not the only dominating effect. Our investigation provides the constructive information to tune the thermal and electrical properties of organic semiconductors. INTRODUCTION Due to the highly flexible and stretchable properties of the active layers and the substrates, high-performance organic electronics are believed to be a high potential candidate for the next generation flexible electronics. Researches on improving the thermal and electrical properties of organic semiconductors have been drawing a lot of attentions. Since the organic semiconductors are holding together by van der Waals force, they usually exist in the form of amorphous to polycrystalline structure. The crystallinity, grain size, roughness, dislocation of organic semiconductor thin films significantly rely on the growth of organic semiconductors, which would further determine the performance of devices1. Up to now, different techniques, including self-assembled monolayers (SAMs)2, elevated substrate deposition temperature3, and plasma treatment4, can be used to modify the properties of organic semiconductor thin films. Thermal annealing, a post processing treatment on the thin films, is a facile and direct method to optimize the properties of thin films. Some previous work have found th
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