Proceedings of MATSUS Spring 2024 Conference (MATSUS24)
Publication date: 18th December 2023
Organic semiconducting materials are attracting considerable attention due to their mechanical flexibility, easy processing, potentially low cost, recyclability and easy tuning of their optoelectronic properties, making them interesting for applications in scalable electronics e.g. solar cells, light emitting diodes, and photodetectors [1]. The understanding of the correlation between the material structure and its optical and electrical properties, is a vital requirement for performance modeling as well as for design optimization of optoelectronic devices. The most important optical parameter that governs the behaviour of optoelectronic devices is the refractive index of the materials involved and thus an accurate determination of this parameter is crucial for the development of technology. Precisely, spectroscopic ellipsometry (SE) is a non-invasive and highly sensitive technique for the determination of optical constants and thicknesses of thin films [2].
In this study, we implemented variable-angle spectroscopic (1.2 – 5.4 eV) ellipsometry (VASE) to determine the refractive index (n), extinction coefficient (k), and thickness of different thin films of organic materials deposited by blade coating onto glass substrates. We selected 6 wide bandgap materials, interesting for indoor photovoltaics and for tandem solar cells. The ellipsometric data was analyzed using an electronic model based on the Tauc-Lorentz (TL) oscillator, as it leads to accurate fits of the optical functions of our materials. The thicknesses deduced by these fits are in the range of ca. 16 to 146 nm, which is in good agreement with those obtained using surface profilometry, and the calculated absorption spectra using the optical constants match those measured by spectrophotometry. The aim of this study is to demonstrate the potential of ellipsometry towards the determination of the optical properties of organic semiconductors, that will be followed by numerical simulations in the future, allowing us to model appropriately the performance of optoelectronic devices based on these materials.
We acknowledge "The Consejo Superior de Investigaciones Científicas (CSIC)" for funding the project No. 242343 (MONDA). The Spanish "Ministerio de Ciencia e Innovación (MICINN)" for the grant No. CEX2019-000917-S (FUNFUTURE) and the AEI/FEDER(UE) grants PID2021-128924OB-I00 (ISOSCELLES) and PDC2022-134001-I00 (SOLS-PV proof of concept). The authors also thank the Catalan agency AGAUR for grant 2021-SGR-00444. MCV acknowledges a FPI fellowship (PRE2019-089855) from MICINN cofinanced by the European Social Fund. M.C.V. and A.Q. also thank the Ph.D. program in Materials Science from Universitat Autònoma de Barcelona in which both were enrolled. Dr. Alfonsina A. A. Torimtubun is acknowledged for her helpful discussions on device fabrication.
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