applied sciences Article Thermal Management of Concentrated Multi-Junction Solar Cells with Graphene-Enhanced Thermal Interface Materials Mohammed Saadah 1,2, Edward Hernandez 2,3 and Alexander A. Balandin 1,2,3,* 1 Nano-Device Laboratory (NDL), Department of Electrical and Computer Engineering, University of California, Riverside, CA 92521, USA; [email protected] 2 Phonon Optimized Engineered Materials (POEM) Center, Bourns College of Engineering, University of California, Riverside, CA 92521, USA; [email protected] 3 Materials Science and Engineering Program, University of California, Riverside, CA 92521, USA * Correspondence: [email protected]; Tel.: +1-951-827-2351 Academic Editor: Philippe Lambin Received: 20 May 2017; Accepted: 3 June 2017; Published: 7 June 2017 Abstract: We report results of experimental investigation of temperature rise in concentrated multi-junction photovoltaic solar cells with graphene-enhanced thermal interface materials. Graphene and few-layer graphene fillers, produced by a scalable environmentally-friendly liquid-phase exfoliation technique, were incorporated into conventional thermal interface materials. Graphene-enhanced thermal interface materials have been applied between a solar cell and heat sink to improve heat dissipation. The performance of the multi-junction solar cells has been tested using an industry-standard solar simulator under a light concentration of up to 2000 suns. It was found that the application of graphene-enhanced thermal interface materials allows one to reduce the solar cell temperature and increase the open-circuit voltage. We demonstrated that the use of graphene helps in recovering a significant amount of the power loss due to solar cell overheating. The obtained results are important for the development of new technologies for thermal management of concentrated photovoltaic solar cells. Keywords: graphene; thermal interface materials; solar cells; thermal management 1. Introduction The interest to photovoltaic (PV) solar cells as a source of energy for a variety of applications has been rapidly increasing in recent years [1–10]. Improving solar cell performance is an important issue, and the efforts have been mostly aimed at increasing power conversion efficiency and reducing manufacturing costs. Crystalline silicon (Si) is the most commonly used material in manufacturing solar cells, occupying more than 90% of the market [10,11]. Conventional solar cells, manufactured using two-decades-old technology, are capable of converting about 20% of absorbed light energy into electricity [11,12]. Solar cell panels that employ optical concentrators can convert more than ~30% of absorbed light into electricity [13]. Most of the remaining 70% of absorbed energy is turned into heat inside the solar cell [14]. While Si PV cells remain the most common and affordable for commercial use for power generation, there is a strong motivation for development of higher efficiency multi-junction PV cells with concentrators. The concentrator multi-junction solar cells can find applications in aerospace and other technologies where efficiency and smaller size are more important considerations than cost [15]. One of the problems in developing the concentrator multi-junction PV technology is thermal management of the solar cells. Concentration of solar light into a small area, increase in the energy absorption, and layered structure of the multi-junction cells result in significant temperature increase Appl. Sci. 2017, 7, 589; doi:10.3390/app7060589 www.mdpi.com/journal/applsci Appl. Sci. 2017, 7, 589 2 of 13 Appl. Sci. 2017, 7, 589 2 of 13 during the cell’s operation [16]. [16]. The The increase in th thee PV cell temperature negatively affects its power conversion efficiency,efficiency, and and it it can can damage damage the the solar solar cell overcell over time time [17]. Therefore,[17]. Therefore, it is important it is important to control to controlsolar cell solar temperature cell temperature by effectively by effectively removing remo the unwantedving the heat.unwanted Temperature heat. 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Most passive solar cooling cells utilize technologies passive cooling [28,29]. technologiesA basic heat sink[28,29]. can A reduce basic heat the temperature sink can reduce of a standardthe temperature Si solar of cell, a standard under one Si sunsolar illumination, cell, under oneby aboutsun illumination, 15 ◦C, which by increasesabout 15 °C, the which output increases power bythe6% output [30]. powerWhen by a heat6% [30].sink When is attached a heat sink to a issolar attached cell, a to thermal a solar cell, resistance a thermal between resistance the two between interfaces the two can interfaces limit the can amount limit ofthe heat amount transferred of heat transferredbetween the between solar cell the and solar the cell heat and sink the [31 heat]. This sink resistance [31]. 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