Ellipsometric studies on thin film CuPC: C60 blends for solar cell applications

Organic solar cells with copper pthalocyanine (CuPC) and fullerene (C60) blends with efficiency around 5% have already been demonstrated. However, the data of reliable optical constants of these organic materials and their blends in thin films are not readily available. In this paper, we discuss thin films of CuPC and C60 fabricated using multiple evaporation cells at a base pressure < 10(-8) torr are studied using spectroscopic ellipsometry. Ellipsometric data were obtained in the spectral range of 0.75 eV to 4.5 eV for determining the optical constants. We model CuPC and C60 using Tauc-Lorentz dispersion relation. The blend at the composition ratio of 1: 1 were modelled by Maxwell-Garnett and Bruggeman effective medium approximations. In addition, a new approach is taken to model the blend by adding separately all the oscillators for each energy transition for both the composites and were found to fit the raw data better than the Maxwell-Gamett and Bruggeman effective medium approximations. The obtained refractive index data by this approach is then presented. We find that the Q-band and B-band of CuPC and the broad absorption of C60 around 3.56 eV still maintain their optical identity in the blend. (c) 2007 Elsevier B.V. All rights reserved.