Citation: Lazau, C.; Nicolaescu, M.; Orha, C.; ¸ Serban, V.; Bandas, C. Self-Powered Photodetector Based on FTO/n-TiO 2 /p-CuMnO 2 Transparent Thin Films. Materials 2022, 15, 5229. https://doi.org/10.3390/ ma15155229 Academic Editors: Adrien Carretero- Genevrier and César Magén Received: 5 July 2022 Accepted: 26 July 2022 Published: 28 July 2022 Publisher’s Note: MDPI stays neutral with regard to jurisdictional claims in published maps and institutional affil- iations. Copyright: © 2022 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/). materials Article Self-Powered Photodetector Based on FTO/n-TiO 2 /p-CuMnO 2 Transparent Thin Films Carmen Lazau 1,† , Mircea Nicolaescu 1,2,† , Corina Orha 1,† , Viorel ¸ Serban 2,3 and Cornelia Bandas 1, * 1 National Institute for Research and Development in Electrochemistry and Condensed Matter Timisoara, 300569 Timisoara, Romania; carmen.lazau@gmail.com (C.L.); nicolaescu.mircea13@yahoo.com (M.N.); orha.corina@gmail.com (C.O.) 2 Department of Materials and Manufacturing Engineering, Faculty of Mechanical Engineering, University Politehnica of Timisoara, 300222 Timisoara, Romania; viorel.serban@upt.ro 3 Romanian Academy of Technical Sciences, 300223 Timisoara, Romania * Correspondence: cornelia.bandas@gmail.com † These authors contributed equally to this work. Abstract: A self-powered photodetector with the FTO/n-TiO 2 /p-CuMnO 2 configuration, represent- ing the novelty of the work, was successfully achieved for the first time and presumes two steps: deposition of the n-type semiconductor (TiO 2 ) by the doctor blade method and of the p-type semi- conductor (CuMnO 2 ) by the spin coating technique, respectively. Investigation techniques of the structural and morphological characteristics of the as-synthesized heterostructures, such as XRD, UV-VIS analysis, and SEM/EDX and AFM morphologies, were used. The I-t measurements of the photodetector showed that the responsivity in the self-powered mode was 2.84 × 10 7 AW −1 cm 2 and in the 1 V bias mode it was 1.82 × 10 6 AW −1 cm 2 . Additionally, a self-powered current of 14.2 nA was generated under UV illumination with an intensity of 0.1 mW/cm 2 . Furthermore, under illumination conditions, the response time (t res ) and the recovery time (t rec ) of the sensor exhibited a good response; thus, t res = 7.30 s and t rec = 0.4 s for the self-powered mode, and in the 1 V bias mode, these were t res = 15.16 s and t rec = 2.18 s. The above results show that the transparent heterojunction device of n-TiO 2 /p-CuMnO 2 exhibited a self-powered ultraviolet photodetector with high sensitivity. Keywords: self-powered photodetector; n-TiO 2 /p-CuMnO 2 heterojunction; thin film 1. Introduction Self-powered ultraviolet photodetectors (SPVs) have been widely studied lately be- cause they have a great advantage; namely, they do not need any external power sources, can operate continuously and independently and do not require high costs to produce them [1]. The operating principle of photodetectors (PD) is very well known, being based on the conversion of photon energy into an electrical signal by photogenerated carriers [2–4]. The PD performance is mainly dependent on the properties of the semicon- ductor materials used, regarding their abilities to transform optical signals into electrical signals that are determined by a process of electron-hole pair generation and recombina- tion within semiconductor materials [5]. Self-powered operation happens because of the photovoltaic effect, i.e., the photocurrent generated without applied bias, and requires a heterojunction to be formed in the device. In most cases, the dark current has high values, and thus power losses appear, this being a major disadvantage, leading to unwanted power losses [6]. In this way, to remove the mentioned inconveniences, UV photodetectors have been studied and developed that have the ability to detect UV radiation and produce an electric field, so they can be self-powered. Over time, it was shown that metal oxide heterojunctions can facilitate photovoltaics for self-powered operation due to their built-in potential, low cost, non-toxicity and easy Materials 2022, 15, 5229. https://doi.org/10.3390/ma15155229 https://www.mdpi.com/journal/materials