Spectroscopic observation of the field induced carrier doping effect in ultrathin layers of organic semiconductors

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In order to provide a basis for understanding the field-induced carrier doping (FICD) phenomenon in ultrathin organic semiconductor layers, firstly, we have adopted a theoretical approach. In this approach, a metal/insulator/semiconductor/metal four-layer model has been considered. Introduction of several physical approximations has simplified the model and allowed us to solve analytically the simultaneous derivative equation describing the model according to the Fermi distribution and the Gauss law. The behavior of the obtained solution has been examined by setting the parameters to the cases in which the insulator and semiconductor layers are made by Langmuir-Blodgett (LB) technique. This approach has shown the next two points : i) Though the FICD phenomenon becomes nonlinear, the thickness of the accumulation layer is kept much smaller than the thickness of the semiconductor layer (ds) even when both of ds and the density of states in that layer are very small ; ii) In the other cas … More es the FICD phenomenon is essentially the same as that in the inorganic devices.As an experimental approach, we have planned to apply modified infrared reflection-absorption spectroscopy (IR-RAS) techniques to metal/insulator/semiconductor/metal four-layer devices made of LB films. Two types of the modified IR-RAS technique have been prepared. The low-frequency type switches on and off the U.C. bias voltage applied to the device after each Fourier-transform (FT) scan completes so that the spectrometer records the difference between the RAS signals with and without the bias voltage. The high-frequency type applies A.C. (1-100 kHz) bias voltage to the device and inserts a phase-sensitive detector between the IR detector-signal preamplifier and the analogue-to-digital converter so that the spectrometer records the difference between the RAS signals with and without the bias voltage, too.The low-frequency type has been applied to four-layer devices with LB films of alkylTCNQ (as the semiconductor layer), LB films of fatty acid (as the insulator layer), LB films of the BO-fatty acid mixture (as one of the metal layer), and evaporated gold film (as the other metal layer) and changes in the RAS signal induced by the bias voltage have been detected. Unfortunately, however, the observed changes are not fully reversible and then we have concluded that some electrochemical effect occurs. At present, we are trying to eliminate the electrochemical effect and to observe the pure FICD effect by optimizing the sample structure, the magnitude and the frequency of the bias voltage, etc. Less