Charge conduction process and photoelectrical properties of Schottky barrier device based on sulphonated nickel phthalocyanine

In this paper, the optical and electrochemical properties of sulphonated nickel phthalocyanine (NiPcS_mix) were investigated. The ground state of spectra of NiPcS_mix show splitting of Q band in DMF, but the fluorescence spectra have only one band, suggesting that only some component of the sulphonated NiPc fluoresce. Since, the organic materials are described on the basis of molecular orbital energies, i.e. highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) instead of valence band and conduction band. Such energies were estimated from cyclic voltammetry and optical absorption measurements. The dark current–voltage characteristics of ITO/NiPcS_mix/Ag device in dark at room temperature shows a rectification behavior. At low voltages, current in forward direction was found to obey the diode equation, i.e. exponential increase in current with applied voltage and the conduction was controlled by thermionic emission mechanism. For relatively higher voltages, the conduction was dominated by a space charge limited conduction mechanism with single trap level of 0.34 eV. The J–V characteristics in dark and the comparison of photoaction spectra of the device with the optical absorption spectra of NiPcS_mix indicates the formation of Schottky barrier and Ohmic contact at Ag-NiPcS_mix and ITO-NiPcS_mix, respectively, in ITO/NiPcS_mix/Ag device. The junction parameters such as built in potential, potential barrier height, carrier concentration and width of depletion layer were estimated from the capacitance–voltage (C–V) measurement. The J–V characteristic under illumination shows photovoltaic effect. The power conversion efficiency of the device has been improved upon thermal annealing attributed to the enhancement in the both crystallinity of NiPcS_mix and charge carrier mobility due to the thermal treatment. We have also investigated the effect of PEDOT:PSS buffer layer in between ITO and NiPcS_mix on electrical and photovoltaic properties of the device.