ИСТИНА

Organic field-effect transistors (OFET) are key basic elements for low-cost, light-weight and large-area electronics. The most important parameter that characterizes the OFET performance is the charge carrier mobility. The common method of mobility measurement in OFET consists in approximation of the OFET transfer characteristics by the Shockley equations in the linear and saturation regimes. However, this method can lead to incorrect estimation of charge mobility in organic semiconductor. In the common staggered device architecture, e.g. top-contacts and bottom-gate OFET geometry, the current needs to pass through the thickness of organic semiconductor layer under the source and drain contacts, and the voltage can drop across the layer due to space charge limited current (SCLC). This can significantly decrease the source-drain current and consequently the apparent (effective) OFET mobility µeff. -- Fig. 1. (a) OFET scheme with areas of current flow. (b) Apparent mobility in the saturation regime vs the active layer thickness H for different mobility ratios µ⊥/µ||. In this work, we study how the SCLC under the source and drain electrodes affects the apparent mobility determined from the OFET transfer characteristics. We developed both numerical and analytical OFET models with SCLC under the contacts and found how the fitted OFET mobility changes with the active layer thickness. In the models, the current flow is divided into five areas (Fig. 1a): areas I and V with vertical SCLC, areas II and IV with horizontal increasing/decreasing current and OFET channel III. The modeling showed that the apparent OFET mobility drops with increasing the active layer thickness H and decreasing the intrinsic transverse charge mobility µ⊥ (across the active layer) relatively to intrinsic “longitudinal” mobility µ|| (along the channel length). These findings allow more accurate evaluation of the intrinsic OFET mobility and provide guidelines for further improvement of OFET performance. This work was supported by Russian Science Foundation (project № 15-12-30031).