In-situ study of pn-heterojunction interface states in organic thin film transistors

• A threshold voltage was in-situ measured in an organic sandwich thin film transistor.
• Density of pn-heterojunction interface states by evaluating the threshold voltage shift.
• The threshold voltage shift attributes to the increase of drain current.
• In order to explain the threshold voltage shift, a model was assumed.

In this paper, we have investigated the density of pn-heterojunction interface states by evaluating the threshold voltage shift with in-situ measurement of electrical characteristics of a sandwich fluorinated copper phthalocyanine/pentacene thin film transistor with various thicknesses of pentacene thin films. A threshold voltage (VT) undergoes a significant shift from + 20.6 to + 0.53 V with increasing the thickness of pentacene. When the thickness of pentacene is more than a critical thickness of 15 nm, VT undergoes hardly any shift. On the other hand, the value of mobility is lightly decreased with increasing the thickness of pentacene due to the effect of the bulk current. Thus the VT shift is attributed to the increase of drain current in the sandwich device. In order to explain the VT shift, a model was assumed in the linear region of thin film transistor operation and the VT shift agrees with a tan− 1 function of film thickness. The total charge density (Q0) of 1.53 × 10− 7 C/cm2 (9.56 × 1011 electrons or holes/cm2) was obtained. Furthermore, the VT shift and Q0 could be adjusted by selecting a p-type semiconductor.