7.1 Introduction

In recent years, organic devices including OTFT and OLED have found important application in large-area, low performance and low-cost integrated circuits. Such applications include driving devices for active matrix flat panel displays, light identification tags, sensors, etc. The key traits distinguishing devices with organic active layer from conventional FETs are their potential for low-cost and low-temperature processing, and their compatibility with flexible substrates. As organic device applications increase, a more accurate and yet simple model of device characteristics is necessary for understanding, improving, and applying these devices. Up to now, many of the numerical or analytical organic device models available in commercial devices simulators use the same expressions as used for crystalline devices. However, organic devices show several differences with respect to crystalline devices because of the low conductivity of organic semiconductors. Furthermore, OTFTs are primarily operated as accumulation field effect transistors as opposed to the usual inversion mode of crystalline MOSFETs. OTFTs are normally conducting at zero gate voltage, and the field-effect mobility usually increases with the gate voltage [130].

At the same time, different parameters such as barrier height, mobility and device length affect the current of OLEDs, so it is useful to consider organic diode structures in which single carrier type dominates the current flow in order to clarify the device operation in a relatively simple situation. Such unbipolar devices can be easily fabricated by choosing the contact so that the energy barrier for one carrier type is much larger than that for the other.