Effect of layer thickness on the electrical parameters and conduction mechanisms of conjugated polymer-based heterojunction diode

Abstract

In this study, thickness-dependent current density–voltage (J–V) characteristics obtained for poly{(9,9-dioctylfluorene)?2,7-diyl-(4,7-bis(thien-2-yl) 2-dodecyl-benzo[1,2,3] triazole)} (PFTBT) conjugated copolymer based heterojunction diode fabricated on ITO were investigated in terms of electrical characteristics. In order to analyze J–V plots with ITO/PEDOT:PSS/PFTBT:PC61BM/LiF/Al configuration, the thickness-dependent J–V measurements were applied in the thickness range between 90 and 200 nm. The effect of PFTBT:PC61BM layer thickness on the forward J–V characteristics were investigated by evaluating electrical parameters such as zero bias barrier height (?Bo), ideality factor (n), shunt resistance (Rsh), series resistance (Rs), the interface states density (Nss), and space-charge limited mobility. The results show that at PFTBT:PC61BM layer thickness of 90 and 200 nm, ideality factor for ITO/PEDOT:PSS/PFTBT:PC61BM/LiF/Al heterojunction diodes ranged from 2.726 to 3.121 and the thermionic emission over the heterojunction diodes is crucial at low current densities and the intrinsic thermally generated charge carriers controlled the forward current this region of the heterojunction diode. At relatively higher voltage, the current mechanism of ITO/PFTBT:PC61BM/PEDOT:PSS/LiF/Al heterojunction diodes were found to obey a space charge limited (SCLC) conduction mechanism. The values of Nss and Rs in heterojunction diodes increase with increasing PFTBT:PC61BM layer thickness and effect the main electrical parameters of diodes. In addition, the leakage current of heterojunction diodes are taken and interpreted via Poole-Frenkel emission and Schottky emission. The leakage current was controlled in ITO/PEDOT:PSS/PFTBT:PC61BM/LiF/Al heterojunction diodes by Poole-Frenkel emission above 140 nm and by Schottky emission under 140 nm. © 2017 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2017, 134, 44817. © 2017 Wiley Periodicals, Inc.