energies Article Density Functional Theory Calculations of Pinus brutia Derivatives and Its Response to Light in a Au/n-Si Device Mehmet Yilmaz 1,2 , Yasar Demir 3 , Sakir Aydogan 1,4 and Maria Luisa Grilli 5, *   Citation: Yilmaz, M.; Demir, Y.; Aydogan, S.; Grilli, M.L. Density Functional Theory Calculations of Pinus brutia Derivatives and Its Response to Light in a Au/n-Si Device. Energies 2021, 14, 7983. https://doi.org/10.3390/en14237983 Academic Editor: Anastasia Soultati Received: 19 September 2021 Accepted: 24 November 2021 Published: 29 November 2021 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2021 by the authors. Licensee MDPI, Basel, Switzerland. This article is an open access article distributed under the terms and conditions of the Creative Commons Attribution (CC BY) license (https:// creativecommons.org/licenses/by/ 4.0/). 1 Advanced Materials Research Laboratory, Department of Nanoscience and Nanoengineering, Graduate School of Natural and Applied Sciences, Ataturk University, Erzurum 25240, Turkey; mehmetyilmaz@atauni.edu.tr (M.Y.); saydogan@atauni.edu.tr (S.A.) 2 Department of Science Teaching, Kazım Karabekir Faculty of Education, Atatürk University, Erzurum 25240, Turkey 3 Department of Chemistry, Science Faculty, Mugla Sitki Kocman University, Kotekli/Mugla 48000, Turkey; yasdemir@mu.edu.tr 4 Department of Physics, Science Faculty, Atatürk University, Erzurum 25240, Turkey 5 Casaccia Research Centre, Energy Technologies and Renewable Sources Department, ENEA—Italian National Agency for New Technologies, Energy and Sustainable Economic Development, 00123 Rome, Italy * Correspondence: marialuisa.grilli@enea.it Abstract: In this study, the performance of an organic dye obtained from the bark of the red pine (Pinus brutia) tree growing in Mu˘ gla/Turkey as an interface layer in the Au/n-Si Schottky diode (SD) structure was evaluated. For this purpose, at first, the optimized molecular structure, the highest occupied molecular orbital (HOMO), and the lowest unoccupied molecular orbital (LUMO) simulations of the organic dye were calculated by the Gauss program and it was theoretically proven that the dye exhibits semiconducting properties. Then, the electrical and photodiode variables such as ideality factor, effective barrier height, series resistance, interface states density distribution, photosensitivity, and photo responsivity were evaluated employing current-voltage measurements under dark and different illumination densities. Additionally, C-V measurements were used to demonstrate that the fabricated device has capacitive features and this capability varies as a function of the frequency. Under these measurements, the possible conduction mechanism for the organic dye-based Au/n-Si device was investigated and the results showed that Au/Pinus brutia/ n-Si may be a good candidate for optoelectronic applications. Keywords: photodiode; Pinus brutia; organic dye; density functional theory; spray pyrolysis 1. Introduction Metal-semiconductor (MS) and metal-interface layer-semiconductor type Schottky barrier diodes (SBDs) have been extensively investigated over the years due to their unique properties for electronic applications, such as fast response, low resistance and low transi- tion reverse current during switching [1,2]. The performance of Schottky barrier diodes is also known to be dependent on the characteristics of the metal/semiconductor junction [3]. Especially in recent years, improvement of interfaces in metal/semiconductor SBDs and novel alternative materials for the interface have been searched for intensively [4,5]. Con- sidering their chemical stability as well as their optical properties, it can be said that organic semiconductors are good candidates for optoelectronic and photonic applications. Indeed, in a previously published study [6], some of the authors of this manuscript emphasized that organic substances are primarily composed of carbon-based molecules and that a wide range of biological materials suitable for electronic applications can be obtained due to these molecules’ ability to form extended polymeric chains. Namely, two major classifications can be made for organic semiconductors as low molecular weight mate- rial and polymers. Both have a conjugated π-electron system created by the p z -orbitals of the sp 2 -hybridized C-atoms of the molecules [7]. Compared to σ-bonds, π-bonds are Energies 2021, 14, 7983. https://doi.org/10.3390/en14237983 https://www.mdpi.com/journal/energies