Thermal Management of Concentrated Multi-Junction Solar Cells with Graphene-Enhanced Thermal Interface Materials

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. Temperature effects on performance effects on performanceof conventional of solarconventional cells have solar been cells the have subject been ofmany the subject reported of ma studiesny reported [18–27]. studies One can [18–27]. distinguish One cantwo distinguish main methods two of main thermal methods management of thermal of solar manage cells:ment active of coolingsolar cells: and active passive cooling cooling. and The passive active cooling.cooling involvesThe active a coolingcooling medium,involves a e.g., cooling air or medi water,um, and e.g., uses air fansor water, or water and pumpsuses fans to pushor water the pumpsmedium to through push the the medium heated surfaces.through Passivethe heated cooling surfaces. uses aPassive heat sink cooling that dissipates uses a heat heat sink without that dissipatespushing a heat cooling without medium pushing through a cooling it. Most medium solar th cellsrough utilize it. 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]. This is resistance a result of is small a result air gapsof small between air gaps the betweentwo joined the surfaces, two joined which surfaces, are caused which by are the caused surfaces by the microscopic surfaces microscopic imperfections imperfections [32]. Since air[32]. is Sincea poor air thermal is a poor conductor, thermal it mustconductor, be replaced it must by be a materialreplaced that by hasa material better thermal that has conductivity better thermal (see conductivityFigure1). The (see thickness Figure of 1). this The material, thickness referred of this to material, as bond linereferred thickness to as (BLT),bond shouldline thickness be kept (BLT), small shouldto minimize be kept the small overall to minimize thermal resistancethe overall oftherma the connectedl resistance surfaces. of the connected One should surfaces. note that,One should even if notethe joined that, even surfaces if the are joined polished surfaces to perfection, are polished the to thermal perfection, boundary the thermal resistance boundary (TBR) resistance will still exist (TBR) at willthe interfacestill exist of at two the materials interface owingof two to materials the mismatch owing in to their the acoustic mismatch phonon in their properties. acoustic Thermalphonon properties.interface materials Thermal (TIMs) interface or thermalmaterials phase (TIMs) change or thermal materials phase (PCMs) change perform materials the (PCMs) task of reducingperform thethermal task of resistance reducing between thermal two resistance surfaces between and facilitating two surfaces heat and transfer facilitating between heat the transfer heat generating between thedevice heat and generating a heat sink device [33– and40]. a heat sink [33–40]. Figure 1. IllustrationIllustration of of the the function function of of the the thermal thermal interface interface materials materials for for heat heat removal removal from from the the solar solar cell under concentrated light. InIn this this paper, paper, we demonstrate demonstrate that that the the thermal thermal management management of of concentrat concentratoror multi-junction multi-junction solar cells can bebe substantiallysubstantially improved improved by by enhancing enhancing properties properties of TIMsof TIMs via via incorporation incorporation of graphene of graphene [41]. [41].Conventional Conventional TIMs TIMs are typically are typically made ofmade polymeric of polymeric or grease or base grease material base loaded material with loaded conductive with conductivematerials such materials as silver such or as ceramic silver particlesor ceramic [42 pa].rticles The amount [42]. The of variousamount fillersof various in the fillers commercial in the commercialTIMs can be TIMs high, can reaching be high, the reaching loading the volume loading fractions volumef ~70% fractions [43]. f ~70% The approach [43]. The ofapproach increasing of increasingthe filler loading the filler fraction loadingf for fraction higher f thermalfor higher conductivity thermal conductivity of the resulting of the composite resulting hascomposite limitations. has limitations.Higher loadings Higher may loadings result in may the result uneven in dispersionthe uneven of dispersion the fillers, of their the agglomeration, fillers, their agglomeration, air gaps, and airincreases gaps, inand the increases TIM cost in [44 the,45 ].TIM Our cost results [44,45]. indicate Our that results the use indicate of low loadingthat the fractionsuse of low of graphene loading fractionsas an additional of graphene filler toas commercialan additional TIMs filler can to be commercial an effective TIMs strategy can forbe decreasingan effective the strategy operation for decreasingtemperature the of operation multi-junction

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