###### 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. graphene; thermal interface materials; solar cells; thermal management +
Footnote †: journal: Applied Physics Letters  
1  
[PERSON], [PERSON], [PERSON], [PERSON]. during the cell's operation [16]. The increase in the PV cell temperature negatively affects its power conversion efficiency, and it can damage the solar cell over time [17]. Therefore, it is important to control solar cell temperature by effectively removing the unwanted heat. Temperature effects on performance of conventional solar cells have been the subject of many reported studies [18; 19; 20; 21; 22; 23; 24; 25; 26; 27]. One can distinguish two main methods of thermal management of solar cells: active cooling and passive cooling. The active cooling involves a cooling medium, e.g., air or water, and uses fans or water pumps to push the medium through the heated surfaces. Passive cooling uses a heat sink that dissipates heat without pushing a cooling medium through it. Most solar cells utilize passive cooling technologies [28; 29]. A basic heat sink can reduce the temperature of a standard Si solar cell, under one sun illumination, by about 15 \({}^{\circ}\)C, which increases the output power by 6% [30]. When a heat sink is attached to a solar cell, a thermal resistance between the two interfaces can limit the amount of heat transferred between the solar cell and the heat sink [31]. This resistance is a result of small air gaps between the two joined surfaces, which are caused by the surfaces microscopic imperfections [32]. Since air is a poor thermal conductor, it must be replaced by a material that has better thermal conductivity (see Figure 1). The thickness of this material, referred to as bond line thickness (BLT), should be kept small to minimize the overall thermal resistance of the connected surfaces. One should note that, even if the joined surfaces are polished to perfection, the thermal boundary resistance (TBR) will still exist at the interface of two materials owing to the mismatch in their acoustic phonon properties. Thermal interface materials (TIMs) or thermal phase change materials (PCMs) perform the task of reducing thermal resistance between two surfaces and facilitating heat transfer between the heat generating device and a heat sink [33; 34; 35; 36; 37; 38; 39; 40]. In this paper, we demonstrate that the thermal management of concentrator multi-junction solar cells can be substantially improved by enhancing properties of TIMs via incorporation of graphene [41]. Conventional TIMs are typically made of polymeric or grease base material loaded with conductive materials such as silver or ceramic particles [42]. The amount of various fillers in the commercial TIMs can be high, reaching the loading volume fractions \(f\sim\)70% [43]. The approach of increasing the filler loading fraction \(f\) for higher thermal conductivity of the resulting composite has limitations.