Influence of the Film Thickness on the Optical and Electrical Properties of ITO
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1212-S11-13
Influence of the film thickness on the optical and electrical properties of ITO M. K. Fung1, K. Y. Cheung1, Y. C. Sun1, A. B. Djurišić1 and W. K. Chan2 1 Department of Physics, The University of Hong Kong, Hong Kong, People's Republic of China 2 Department of Chemistry, The University of Hong Kong, Hong Kong, People's Republic of China
ABSTRACT Indium tin oxide (ITO) is widely used for opto-electronic products such as organic lightemitting diodes, organic photovoltaic devices and liquid crystal displays due to its high transparency and electrical conductivity. Since there is a trade-off between the conductivity and transparency of ITO, it is necessary to optimize performances of opto-electronic products by balancing the sheet resistance and transmittance. Both sheet resistance and transmittance are affected by a number of factors such as working temperature, working pressure, oxygen-to-argon ratio during the fabricating process, and thickness. In our study, ITO thin films were deposited on glass substrates by dc sputtering. Effects of ITO with different thicknesses, sheet resistances, and transmission spectra on the performance of bulk heterojunction photovoltaic devices were investigated. INTRODUCTION Polymer solar cells based on bulk heterojunction blend films have attracted increasing attention due to their potential for low-cost and large area solar cells. Recently, the reported polymer solar cell has achieved efficiencies around 5-6 % [1-4]. These high efficiency values have been achieved due to the development of new conjugated polymers [4,5], studies of thermal treatment of the polymer blend films [1-2, 6] and incorporation of inorganic materials [7]. All these different devices have one factor in common, and that is the use of indium tin oxide (ITO) as one of the electrodes. Due to the variation in the properties of ITO from different suppliers [8], it is important to establish the influence of ITO properties in solar cell’s performance. The solar cell performance can be enhanced when ITO with low resistivity and high transmittance is employed. However, whether the optical or electrical property will play a dominant role is still needed to be investigated since there is a trade-off between both properties. Herein, we reported the effects of film thickness on the optical and electrical properties of ITO, and their effect on photovoltaic performance of a polymer solar cell. Since the annealed poly(3hexylthiophene): [6,6]-phenyl-C61-butyric acid methyl ester (P3HT: PCBM) bulk heterojunction photovoltaic devices have achieved high efficiencies in the range 2.5-5 % [1,2,3,9], this material combination has been chosen for our study. In this work, the influence of different parameters of ITO on the performance P3HT: PCBM bulk heterojunction photovoltaic devices has been studied by the J-V characteristics measurements in the dark and under simulated solar illumination (AM 1.5, 100 mW/cm2). It has been found that thicker ITO films result in lower sheet resistance but also lower transmittance. The device
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