In-situ optical spectroscopy of the organic semiconductor/electrolyte dielectric interface.

Abstract

The implementation of organic thin film transistors into microelectronic devices hinges upon the development of organic semiconductors and gate dielectric materials. In a working device, the place where the two materials meet is critical to device performance. This buried interface between the organic semiconductor and the gate dielectric is notoriously difficult to characterize. One way to probe this interface is through the use of attenuated total internal reflection Fourier transform infrared and near infrared spectroscopy (ATR-FTIR). This method allows one to do optical spectroscopy on a working device and gain insight into the physical processes which occur at the semiconductor/dielectric interface during the application of voltage. One example of such a process is the induction of charge in the organic semiconductor layer. Charging of an organic semiconductor gives rise to distinct spectroscopic signatures which can be used to characterize properties intrinsic to the semiconductor. For example, high charge carrier density can give rise to unique spectroscopic signatures which may be related to the Mott insulator to metal transition. Crystallinity affects the spectral signatures of charge carriers, and these effects can give insight into sources of energetic disorder in the solid. Examining semiconductor charging also gives insight into the operating mechanisms of the dielectric material responsible for inducing charge carriers. Dielectric materials using mobile ions have become attractive for use in organic thin film transistors because they allow low voltage transistor operation. The physical mechanisms for charge induction are distinguishable when in-situ optical spectroscopy is applied. For example, the mobile ions in a dielectric material can penetrate the bulk of the semiconductor film or stop at the semiconductor/dielectric interface depending on the size of the ion. The rate of charging can be analyzed and used to estimate a material specific maximum operating speed of an electrochemical transistor. Using optical spectroscopy to examine the organic semiconductor/electrolyte dielectric interface gives insight into many aspects of device operation, many of which are critical to making organic thin film transistors a viable technology.