Determination of electrical and solar cell parameters of FTO/CuPc/Al Schottky devices
Introduction
Solid-state solar cells based on organic semiconductors have attracted much interest because of the motivation for developing inexpensive, efficient and renewable energy sources [1], [2]. Phthalocyanines (Pc) are organic semiconductors that have attracted lots of attention for applications in various organic electronic devices, such as solar cells [3], light-emitting diodes [4], gas sensors [5] and transistors [6]. In solar cell applications, Pc materials can be either used alone in a Schottky barrier structure [7], [8] or used together with other n-type materials in a multilayer structure [9]. Although multilayer solar cells show more promising performance in terms of higher power conversion efficiency, Schottky barrier solar cells are convenient for investigation of the influence of materials properties, active layer thickness, and electrode properties on the solar cells performance. The Schottky cell is a single layer device, which exhibits optimal performance when one contact is ohmic while the other is a barrier contact. Many Pc's show interesting photoconductive and photovoltaic responses and hence have been widely used in Schottky barrier cells [10], [11], [12], [13]. Among these Copper phthalocyanine (CuPc) is a promising organic semiconductor for photovoltaic applications. High work function materials such as gold and ITO, have been used by various workers to make ohmic contact with Pc's [13], [14], [15], [16]. CuPc is a p-type organic semiconductor with direct band gap and high optical density. Aluminium is usually used as the blocking contact in Pc based Schottky devices [8], [13], [17], [18]. The band gap and ionization potential can be tuned to the desired energies by modifying the chemical structure and solar cells made from organic semiconductors are commonly lightweight and flexible. From the available literature it is seen that not much work has been performed on the junction properties of CuPc thin films using FTO and aluminium contacts. FTO is a commonly used substrate for polymeric solar cells [19], [20], since it is more resistant to the exposure to temperatures above compared to ITO [21]. In this communication we report the basic electrical and solar cell parameters of FTO/CuPc/Al devices. This work may give a general idea of the role of FTO on the electrical property of organic thin film devices; thereby we can establish that it can play as competitor for ITO.
Section snippets
Experimental details
The CuPc powder obtained from Aldrich Inc. USA is used as the source material. Thoroughly cleaned FTO glass substrates with surface sheet resistance of are used as the substrates. For optical and structural studies pre-cleaned micro glass slides are kept adjacent to FTO substrates. Thin film samples are prepared by thermal evaporation at a base pressure of . The thickness of the films are measured using the Tolansky's multiple beam interference technique [22]. Above CuPc film,
Results and discussion
The absorption spectrum of CuPc thin film deposited at room temperature is given in Fig. 1. The distinct characterized peaks in the visible region are generally been interpreted in terms of – excitation between bonding and antibonding molecular orbitals. The absorption peaks at higher energy region and lower energy region results from B (Sorret) band and Q-band, respectively.
Both Q and B bands arise from to transitions [24]. The absorption is related to
Conclusions
Current–voltage measurements on FTO/CuPc/Al structure show characteristics of typical Schottky-barrier devices. The rectifying behavior of the device is explained using thermionic emission theory, and the basic diode parameters are determined. From the dependence of capacitance C on the reciprocal film thickness the permittivity of the samples are calculated. The reverse bias curve is interpreted in terms of Schottky emission. In this work we have performed the solar cell characterization of
Acknowledgments
The authors would like to thank Prof. K.P. Vijaya Kumar and Dr. Tenny Theresa John, Department of Physics, Cochin University of Science and Technology, India for providing experimental facilities for the photovoltaic measurements.
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