PhD-Postdoc series

Ellipsometric measurements in organic semiconducting
materials for photovoltaics

Authors: Arianna Quesada-Ramirez,a Miquel Casademont-Viñas,a Alfonsina A. A. Torimtubun,a Gregor Trimmel,b Matiss Reinfelds,b Alejando R. Goñi, ac Mariano Campoy-Quilesa*
a Institut de Ciència de Materials de Barcelona (ICMAB-CSIC), Bellaterra, Spain
b Institute for Chemistry and Technology of Materials, NAWI Graz, Graz University of Technology, Graz, Austria
c Institució Catalana de Recerca i Estudis Avançats (ICREA), Barcelona, Spain

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 six wide bandgap materials that are 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, which will be followed by numerical simulations in the future, allowing us to appropriately model the performance of optoelectronic devices based on these materials.

References

[1] Y. Zhou and C. Wang, “Organic optoelectronics creating new opportunities for science and applications”, Frontiers of Optoelectronics 15, 51 (2022).

[2] M.I. Alonso and M. Campoy-Quiles, “Conjugated Polymers: Relationship Between Morphology and Optical Properties” in Ellipsometry of Functional Organic Surfaces and Films. K. Hinrichs, KJ Eichhorn, eds. (Springer Series in Surface Sciences, vol 52., Springer, Cham., 2018).