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 (V-T) 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, V-T 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 V-T shift is attributed to the increase of drain current in the sandwich device. In order to explain the V-T shift, a model was assumed in the linear region of thin film transistor operation and the V-T shift agrees with a tan(-1) function of film thickness. The total charge density (Q(o)) of 1.53 x 10(-7) C/cm(2) (9.56 x 10(11) electrons or holes/cm(2)) was obtained. Furthermore, the V-T shift and Q(o) could be adjusted by selecting a p-type semiconductor. Crown Copyright (C) 2013 Published by Elsevier B. V. All rights reserved.