ELSEVIER Synthetic Metals 88 (1997) 57-63 Characterization of Safranine 0 based thin-film sandwich devices by analysing their electrical and photoelectrical behaviour G.D. Shama a7* , MS. Roy b, S.K. Gupta a a Department of Physics, J.N.V. University, Jodhpur, Rajastan, India b CarnoufIage Division, Defence Laboratory, Jodhpur, Rujastan, India Received 22 November 1996; revised 3 February 1997; accepted 3 February 1997 Abstract Safranine 0 (SAF), chemically known as 3,7-diamino-2,8-dimethyl-5phenyl-phenazinium chloride,was employed in the form of thin film for the fabrication of In/SAF/ITO and Al/SAF/ITO (IT0 is indium-tinoxide) sandwich devices. In these devices, at low voltages, the dark current in forward bias condition corresponds to a positive voltage at ITO, which varies exponentiaIIy with applied voltage. The square law dependence at higher voltage region indicates that conduction is limited by adiscrete trapping level above the conduction band edge. The analysis of the 1/C2 versusVplots, for low frequencies,reveals that the SAF behaves as a semiconductor. At higher frequencies, thedevices exhibit voltage-independent capacitances, whichis explained in terms of theextremelyslow kinetics of space charges, The photogeneration process in these devices is explained in terms of the exciton dissociation into free carriers at the Schottky barrier (AI/SW andIn/SAF interfaces). Various electrica and photovoltaic parameters are calculated and discussed in detail. Keywords: SaGanine 0; Interfaces; Electrical behaviour; Photoelectrical behaviour; Films; Schottky junction devices 1. Introduction Over the years, organic materialsin the form of thin films have been used extensively in the field of photography and electrophotography . Each of these processes involves the col- lection and use of light energy.The production andmovement of chargeasan integral part of these processes suggest aclose similarity to the photovoltaic process. Using organic thin layers in the electronics and optoelectronics application is a subject of current interest of many researchers [l-7]. This is mainly due to the low cost, simplicity of the fabrication of large areas and their interesting electronic and optica prop- erties. Safranine 0 (SW), chemically known as 3,7-dia- mino-2,8-dimethyl-5phenyl-phenazinium chloride, belongs to the family of phenazine dyes, having a phenazinium nucleus in the molecular ring system of the pigments. The effective electronic transition in the molecular system is an n-rr* transition. The absorption spectra of SAF were taken between 200 and 800 nm, and show maximum absorption at 530 nm in the visible region. The present communication deals with the evaluation of the electrical and photoelectrical properties of SAF thin films, sandwiched between metal (In, Al) andIT0 * Corresponding author. 0379-6779/97/$17.00 0 1997 Elsevier Science S.A. All rights reserved I’IISO379-6779 (97) 03826-5 (indium-tin oxide). The dark conduction mechanism and photogeneration processes have been discussed by the detailed analysis of current-voltage (J-V) characteristics in the dark, as well as under illumination, capacitance-voltage (C-V) characteristicsand comparison of the action spectra of the devices with the absorption spectra of the SAF layer. 2. Experimental detail The SAF was procured from Aldrich Chem. Co. andchem- ically recrystallized. Absorption spectra of SAF were recorded with an ECL spectrophotometer, between 200 and 800 nm region using dimethyl formamide (DMF) as a sol- vent. Thin films of SAF were made by the spin-coating tech- nique on IT0 substrates using DMF as solvent. The metal electrodes (In or Al) were deposited on the top of the SAF layer by the vacuum evaporation method. The effective area of the device was about 0.5 cm’ and the thickness of the SAP layer was around 700 nm, The J-V characteristics were measured using a Keithley model 610 C electrometer and a stabilized power supply. The C-V characteristics were recorded with a Keithley model 3330 LCZ meter at different frequencies. The C-V and J-V characteristics were also measured at different temperatures