energies Review Integrating Photovoltaic (PV) Solar Cells and Supercapacitors for Sustainable Energy Devices: A Review Noor Afeefah Nordin 1, *, Mohamed Nainar Mohamed Ansari 1,2, *, Saifuddin M. Nomanbhay 3 , Nasri A. Hamid 2 , Nadia M. L. Tan 4,5 , Zainudin Yahya 2 and Izhan Abdullah 1   Citation: Nordin, N.A.; Ansari, M.N.M.; Nomanbhay, S.M.; Hamid, N.A.; Tan, N.M.L.; Yahya, Z.; Abdullah, I. Integrating Photovoltaic (PV) Solar Cells and Supercapacitors for Sustainable Energy Devices: A Review. Energies 2021, 14, 7211. https://doi.org/10.3390/en14217211 Academic Editor: Antonino Laudani Received: 27 July 2021 Accepted: 6 October 2021 Published: 2 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 Institute of Power Engineering, Universiti Tenaga Nasional, Kajang 43000, Selangor, Malaysia; izhan.abdullah@uniten.edu.my 2 Department of Mechanical Engineering, College of Engineering, Universiti Tenaga Nasional, Kajang 43000, Selangor, Malaysia; nasri@uniten.edu.my (N.A.H.); zai@uniten.edu.my (Z.Y.) 3 Institute of Sustainable Energy, Universiti Tenaga Nasional, Kajang 43000, Selangor, Malaysia; saifuddin@uniten.edu.my 4 Department of Electrical and Electronic Engineering, College of Engineering, Universiti Tenaga Nasional, Kajang 43000, Selangor, Malaysia; nadia@uniten.edu.my 5 Power Electronics, Machines and Controls Research Group, University of Nottingham Ningbo China, Ningbo 315100, China * Correspondence: noor.afeefah@uniten.edu.my (N.A.N.); ansari@uniten.edu.my (M.N.M.A.); Tel.: +60-135943435 (N.A.N.); +60-164975068 (M.N.M.A.) Abstract: Hybrid systems have gained significant attention among researchers and scientists world- wide due to their ability to integrate solar cells and supercapacitors. Subsequently, this has led to rising demands for green energy, miniaturization and mini-electronic wearable devices. These hybrid devices will lead to sustainable energy becoming viable and fossil-fuel-based sources of energy gradually being replaced. A solar photovoltaic (SPV) system is an electronic device that mainly functions to convert photon energy to electrical energy using a solar power source. It has been widely used in developed countries given that they have advanced photovoltaic (PV) technology that reduces dependence on fossil fuels for energy generation. Furthermore, a supercapacitor is an alternative solution for replacing heavy batteries and it is a system with a prominent high power density and a long life cycle. Its unique properties of high capacitance with low voltage limits lead to this highly in-demand material being incorporated into goods and services that are produced by the electrical and electronics industries. It is another option for grid-based power or large batteries. Since supercapacitors have the ability to store huge amounts of energy, they allow for a novel system that integrates supercapacitors with solar cells in which energy generation and energy storage are combined into one system. This paper explores the common materials that are used for solar cells and supercapacitors, the working mechanisms, the effectiveness of the integrated device and the technical challenges that are encountered when refining this device. Hence, this review serves as a guide for choosing the right materials and methods in order to produce an integrated PV solar cell–energy storage device for various applications. Keywords: PV solar cell; supercapacitor; energy storage; integrated device; power conversion efficiency; levelized cost of energy (LCOE) 1. Introduction The conventional supercapacitor-charging method using photovoltaic (PV) was origi- nally designed using a solar cell and supercapacitor to operate as two independent units that are connected by wires. Despite being able to simultaneously generate and store en- ergy, the system faces some technical challenges, such as being bulky, inflexible, expensive and, in particular, it loses energy through the external wires connecting the two units [1]. Such disadvantages can be overcome by integrating the supercapacitor and PV cell into one device so that it is compact, flexible, modular and minimizes energy loss, as no wires Energies 2021, 14, 7211. https://doi.org/10.3390/en14217211 https://www.mdpi.com/journal/energies