Measurement of electron injection barriers in OS/Au (OS=phthalocyanine and pentacene) using accumulated charge measurement

Abstract

Accumulated charge measurement (ACM) is an experimental technique employed for studying the charge injection and extraction process in a capacitor composed of metal 1 (M1), insulator, organic semiconductor (OS), and metal 2 (M2). In this method, the change in the accumulated charge (Qacc) is estimated by integrating the displacement current when the applied voltage changes from Voff to Voff + Va. From the observed data set of Qacc and Va, ΔQ and VOS are derived, where ΔQ is the degree of charge injection and VOS is the voltage drop within the OS layer caused by Va. The injection barrier at the OS/M2 interface can be obtained from the plot of ΔQ as a function of VOS. This study presents a new ACM scheme and discusses its application to pentacene/Au and metal-free phthalocyanine (H2Pc)/Au junctions. In this scheme, a large negative Voff is applied and Va is swept towards the positive direction. Based on this scheme, the band gaps of OSs and the electron injection barriers at the OS/M2 interfaces are determined. In addition, a new measurement technique that directly determines the electron injection barrier from the displacement current measurement is developed. The observed values are generally consistent with those determined by photoelectron spectroscopies for non-oriented films. The existence of the mobility edge due to disorder in H2Pc film is proposed.