Optical stability of small-molecule thin-films determined by Photothermal Deflection Spectroscopy
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Optical stability of small-molecule thin-films determined by Photothermal Deflection Spectroscopy M. Stella1, M.Della Pirriera2, J. Puigdollers2, J. Bertomeu1, C.Voz2, J. Andreu1, R. Alcubilla2. 1. Dep. Física Aplicada i Òptica. Universitat de Barcelona 2. Micro and Nanotechnology Group. Dept Enginyeria Electrònica. Universitat Politècnica Catalunya
ABSTRACT In this paper the optical absorption properties of n-type C60 and PTCDA, and p-type CuPc small molecule semiconductors are investigated by optical transmission and Photothermal Deflection Spectroscopy (PDS). The results show the usual absorption bands related to HOMOLUMO transitions in the high absorption region of the transmission spectra. PDS measurements also evidences exponential absorption shoulders with different characteristic energies. In addition, broad bands in the low absorption level are observed for C60 and PTCDA thin-films. These bands have been attributed to contamination due to air exposure. In order to get deeper understanding of the degradation mechanisms single and co-evaporated thin-films have been characterized by PDS. The dependence of the optical coefficient on exposure to light and air have been studied and correlated to the structural properties of the films (as measured by X-Ray Diffraction Spectroscopy). The results show that CuPc and PTCDA are quite stable against light and air exposure, while C60 shows important changes in its absorption coefficient. The bulk heterojunctions show stability in agreement with what observed for single layers, since the absorption coefficient of CuPc:PTCDA is almost not altered after the degradation treatments, while CuPc:C60 shows changes for low energy values. INTRODUCTION Organic semiconductors represent a new interesting class of materials for several electronic applications. Organic solar cells performance have improved significantly in the last few years thanks to the optimization of the solar cell structure and, specially, to the ability to process new organic semiconductors with optimised properties. The conventional transmission and reflection spectroscopies are not suitable for detecting low level absorptions because their sensibility is in the order of αd = 10−2, so Photothermal Deflection Spectroscopy (PDS) has been used in this work. PDS is a technique known for its highly sensitive and non-destructive capability of determining the optical absorption of solids, liquids or gases by employing the photothermal effect. P-type semiconductor materials, as like as copper phtalocyanine (CuPc) and n-type ones, as like as fullerene (C60) and perylene-3, 4, 9, 10-tetracarboxylic-3,4,9,10-dianhydride (PTCDA), are organic semiconductors that are commonly used for organic electronic applications [1-4]. The combination of these materials in coevaporated thin films is used for photovoltaic studies [5, 6]. PDS technique was used to know the optical properties of these materials [7] and X-Ray Diffraction spectroscopy (XRD) was used to analyze the amorphous and crystalline phase in the films [8].
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