Abstract
In this thesis, we report the electrical characteristics of several organic field-effect transistors using hydrated protein as gate dielectric, and the mechanism of mobile ions in hydrated proteins is proposed. With the assistance of 2nm pentacene interlayer, the moblilty(μFE) of C60 OFET using silk fibroin as gate dielectric is improved from 0.014 cm2V-1s-1 to 1 cm2V-1s-1 in vacuum. The μFE value of the C60 OFET is further enhanced to 10 cm2V–1s–1 when the OFET is exposed to air in a relative humidity of 55 %. The enhancement of μFE is due to the generation of mobile and immobile charged ions in silk fibroin in air ambient, and additional charge carriers are accumulated in the conduction channel in OFETs operation. Generation of mobile ions in hydrated proteins may influence the chararteristics of OFETs. The characteristics of pentacene OFETs using silk fibroin as gate dielectric maybe changed at different relative humidity. At a relative humidity of 70%, the amount of mobile ions in silk fiborin is large, the dielectric capacitance and threshold voltage are 100 nF/cm2和-0.9V, respectively. The dielectric capacitance is reduced to 15 nF/cm2 and threshold voltiag is increased to -4V when the relative humidity is reduced to 40%. The hysteresis of pentacene OFETs is affected by the migration of ions in the silk fibroin. The voltage shift between forward and backward sweeps of gate voltage is 4V at a relative humidity of 40%, and the voltage shift is respectively reduced to 1.7V and 1.3V when the relative humidity is increased to 50% and 70%. Since the ions migration in the silk fibroin is a time dependent effect, OFETs operated at different gate voltage sweep rates exhibit different characteristics. At the high sweep rate, the threshold voltage and hysteresis shift are -1.7V and 2.6V, respectively. When the sweep rate is further reduced to 0.08V/s, the threshold voltage and hysteresis shift are reduced to -0.6V and 0.5V. Using tetratetracontane(TTC) layer inserted into the silk fibroin and pentacene bilayer structure, the effect of mobile ions in OFETs opreration may be reduced. The device characteristics of pentacene OFETs using gelatin as gate dielectric at different sweep rates have the same tendency as that using silk fibroin as gate dielectric. The main reson to obtain similar results is due to the ion migration in the hydrated proteins. The C60/pentacene ambipolar OFETs using silk fibroin as gate dielectric exhibit excellent performance in the air ambient. Stacking sequence of semiconductor is another factor for good performance of ambipolar OFETs. In our transistor structure, pentacene is deposited on silk fibroin and C60 is deposited on pentacen layer. The sequence of semiconductors helps to get well crystalized pentacene and C60 and the μFE values of holes and electrons are increased. The electron and hole mobility are 10 cm2V-1s-1 and 11.5 cm2V-1s-1, respectively, for a bilayer of 20nm C60 /10nm pentacene and the deposition temperature is 70℃for C60 and 25℃ for pentacene. An ambipolar inverter made with two C60/pentacene ambipolar OFETs exhibits normal swithing characteristics in both the first and third quadrants.