Traps induced memory effect in rubrene single crystal phototransistor

Abstract

In this report, phototransistors based on rubrene single crystals have been fabricated using octyltrichlorosilane (OTS) treated SiO2 as a substrate and memory effect has been observed and studied. Memory writing realized by the combined stimulation of applied gate voltage and light illumination was observed to be strongly dependent on the applied gate voltage. When the applied gate voltage, Vg = 100 V, only 500 ms writing time was found to be sufficient to achieve the memory window of 25 V. Long retention time of over 10 000 s as well as stable writing-reading-erasing-circle were also observed in the rubrene single crystal based phototransistor device. Even at high temperature of 100 °C, the device demonstrated stable memory effect. Thus we attributed the excellent memory effect to the deep electron traps. To clarify further the origin of persistent photocurrent, strongly hydrophobic fluorinated polyimide (FPI) was used as a dielectric layer to exclude the hydroxyl groups at the semiconductor/insulator interface. Compared to the OTS treated SiO2 based device, the photocurrent was saturated in a shorter time in FPI based counterpart, regardless of the light intensity and persistent photoconductivity was also observed. X-ray photoelectron spectroscopy analysis of rubrene single crystal revealed the existence of characteristic oxidation states, both in the high resolution C 1s and O 1s spectra, which behave as electron traps. In conclusion, we attribute the photo memory effect in rubrene single crystal based device to the oxygen related defects as well as to the hydroxyl groups at the dielectric/single crystal interface.