Photoelectric behavior of n-GaAs/orange dye, vinyl-ethynyl-trimethyl-piperidole/conductive glass sensor M.M. Ahmed a, ⁎ , Kh.S. Karimov b , S.A. Moiz a a Department of Electronic Engineering, Mohammad Ali Jinnah University, Blue Area, Islamabad, Pakistan b Physical Technical Institute of Academy of Sciences, Rudaki Ave, 33, Dushanbe, 734025, Tajikistan, Pakistan Received 13 February 2007; received in revised form 2 March 2008; accepted 10 April 2008 Available online 22 April 2008 Abstract In this article, the photoelectric behavior of sandwich type n-GaAs/orange dye, vinyl-ethynyl-trimethyl-piperidole/conductive glass sensors was investigated. Devices were fabricated by employing electrodes of heavily doped (2 × 10 18 cm - 3 ) n-type GaAs and conductive glass of In 2 O 3 , whereas the electrolyte was a mixture of orange dye (OD) and vinyl-ethynyl-trimethyl-piperidole (VETP). Dark current voltage (I–V) characteristics of the devices exhibited small rectification behavior with zero offset voltage, which shows the presence of electrochemical effects in the devices. Photo-induced alternating and direct open-circuit voltages and short-circuit currents were investigated by using infra-red, red, green and blue light sources, and it was observed that the devices were sensitive to the wavelengths ranging 550 nm–700 nm. Based on the observed optical spectra of OD-VETP electrolyte and GaAs a plausible band diagram of the device was developed to explain the observed I–V characteristics and to draw its equivalent circuit diagram. © 2008 Elsevier B.V. All rights reserved. Keywords: Organic photelectric behavior; Organic semiconductor; Organic–inorganic heterojunction; Electrochemical effects 1. Introduction Inherent ease associated with the fabrication of organic semiconductor devices and dependent of their electrical proper- ties on the ambient conditions made them very promising for the development of various type of sensors to evaluate humidity, temperature, light, radiation, strain, etc. [1–9]. In these sensors, unlike solar cells, the power conversion efficiency does not play a principal role. These sensors, nevertheless, are characterized by their spectral sensitivity, frequency response and current– voltage (I–V) change upon being exposed to light. There are a number of organic semiconductors based photo- electric sensors which have been studied and reported [10–17]. Historical background, present status and development pro- spects for new generation of photo-electrochemical sensors, including the dye–sensitized nanocrystaline TiO 2 films, were reviewed by Gratzel [15]. Whereas organic-inorganic Ag/n- GaAs/p-CuPc/Ag photoelectric sensors sensitive to UV–visible- IR spectra (200 nm–1000 nm) were studied by Karimov et al. [5]. In a photoelectric sensor, the wavelength dependent photo induced response shows a substantial increase in the device current. The electric field distribution in these devices, especially at organic–inorganic interface, controls the I–V characteristics of the device. When a device is irradiated, free carriers are generated and more mobile positive charges (holes) drift/diffuse in the crossection of the device contributing to the total current. Whereas the negative charges (electrons) get localized in the trap sites and thus their contribution in the terminal current is negligible [18]. The carriers are generated only for those wavelengths where the inter or the intraband transitions take place. Thus, in these sensors the photocurrent, in principle, should follow the absorption spectrum [19]. Upon photo excitation, a significant change in the I–V response of a photo sensitive device is observed, followed by a slow non exponential relaxation on termination of illumination [18]. Slow relaxation of photo-induced charge carriers through recombina- tion process is a well defined feature in conjugate polymers. However, increase in carrier concentration on irradiating a photo Available online at www.sciencedirect.com Thin Solid Films 516 (2008) 7822 – 7827 www.elsevier.com/locate/tsf ⁎ Corresponding author. E-mail address: mansoor@jinnah.edu.pk (M.M. Ahmed). 0040-6090/$ - see front matter © 2008 Elsevier B.V. All rights reserved. doi:10.1016/j.tsf.2008.04.084