Doping effect of viologen on rectification, charge transport processes and photovoltaic properties of furazano (3,4-b)piperazine thin film device

The electrical and photoelectrical properties of a sandwich junction device based on allyl viologen (AV)-doped furazano (3,4-b)piperazine (FP) having structure ln/AV : FP/ITO have been reported. A significant enhancement in the rectification, dark conductivity and photovoltaic response has been observed in AV-doped FP devices compared with undoped FP devices. The present communication deals with the charge transport mechanism and photogeneration process in ITO/AV doped FP/ln Schottky devices. The J–V characteristics recorded in the dark show a rectification effect due to the formation of a barrier at the AV-doped FP/ln interface. Impedance spectroscopy has been used to study the charge transport mechanism for AV-doped FP and its interface with ln. The bulk and junction resistance along with capacitance were determined by analyzing their contribution at an individual level. Doping imparts an improvement in photoresponse of FP as well as an abundance of photoexcited species at the interface. © 2001 Kluwer Academic Publishers     

Effect of 1,1′‐dibenzyl‐4,4′‐bipyridyl dichloride (DBD) on charge‐conduction process and photovoltaic response of a polypyrrole (PPy) thin‐film device

The present communication deals with analysing the effect of 1,1′‐dibenzyl‐4,4′‐bipyridyl dichloride (DBD) substitution at the N‐position of 2,5‐polypyrrole (PPy), on electrical, impedance and photovoltaic properties. The thin‐film device was fabricated by sandwiching DBD‐substituted PPy between indium tin oxide (ITO) and aluminium (Al) electrodes. The formation of a Schottky barrier with Al and ohmic contact with ITO are explained in terms of p‐type semiconducting behaviour of DBD‐substituted PPy. In the low‐voltage region, Ohm's law is followed, while in the high‐voltage region, a space‐charge‐limited conduction (SCLC) controlled by the exponential‐trap distribution was observed. DBD substitution causes shifting of the Fermi level towards the valence‐band edge and an increase in charge‐carrier mobility. A remarkable change in dark electrical conductivity of the order of five has been observed in DBD‐substituted PPy. The electrical and impedance measurements of an ITO/PPy:DBD/Al device confirms the formation of a Schottky barrier at the DBD‐substituted PPy/Al interface. Additionally, it can be modelled by a simple equivalent circuit of two resistance–capacitance (RC) elements in series representing the bulk and a junction‐region. At low frequency, the device capacitance follows a pronounced voltage dependence. From a detailed analysis of the J–V and C–V characteristics, the ionized acceptor concentration (N_a), width of depletion layer (W) and potential barrier height (?_b) have been evaluated. We observed a significant enhancement in photocurrent on DBD substitution. The increase in photocurrent is explained by the efficient charge separation induced by the intermolecular transfer of photo‐excited electrons from PPy to DBD. The substitution also causes a reduction in the trapping centres in the material. © 2002 Society of Chemical Industry