The focus on solar cells as a source of photovoltaic energy is rapidly increasing nowadays. The amount of sun's energy entering earth surface in one hour is more than the world consume in one year. The photovoltaic market has been increasing by more than 20% annually since 2002. Improving solar cells aims at increasing the power conversion efficiency and reducing manufacturing costs. Crystalline silicon is the most commonly used material in making solar cells with more than 90% market use. Solar cells manufactured in 2011 are capable or converting about 15% of its absorbed light energy into output electricity. Solar cells panels that employ optical concentrators can convert up to 30% of absorbed light into electricity. The remaining 70% of absorbed energy is turned into heat inside the solar cell. The continuous decrease in solar cells size coupled with the increase in their energy absorption generates more heat inside the solar cells and raise their temperature. The increasing the solar cell temperature negatively affect its power conversion efficiency and can damage it. Therefore, it is important to control the solar cell temperature and effectively remove the unwanted heat. Most solar cells utilize passive cooling technologies. A basic heat sink can reduce the solar cell temperature by about 15°C, which increases the output power by 6%. 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 heat sink. This resistance is a result of the small air gaps in between the joined surfaces and caused by the surfaces imperfections. Since air is a poor thermal conductor, we need to fill these gaps with a material that has better thermal conductivity. Thermal interface materials (TIMs) do this task and improves the interfacial contact by decreasing the thermal resistance. The goal of this research is to achieve a better thermal management of solar cells by improving the interface contact between the solar cell and a passively cooled heat sink.

The interest to photovoltaic solar cells as a source of energy for a variety of applications has been rapidly increasing in recent years. Solar cells panels that employ optical concentrators can convert more than 30% of absorbed light into electricity. Most of the remaining 70% of absorbed energy is turned into heat inside the solar cell. The increase in the photovoltaic cell temperature negatively affects its power conversion efficiency and lifetime. In this dissertation research I investigated a feasibility of using graphene fillers in thermal interface materials for improving thermal management of multi-junction concentrator solar cells. Graphene and few-layer graphene fillers, produced by a scalable environmentally-friendly liquid-phase exfoliation technique, were incorporated into conventional thermal interface materials. Characteristics of the composites have been examined with Raman spectroscopy, optical microscopy and thermal conductivity measurements. Graphene-enhanced thermal interface materials have been applied between a solar cell and heat sink to improve heat dissipation. The performance of the single and multi-junction solar cells has been tested using an industry-standard solar simulator under the 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 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 and multi-junction photovoltaic solar cells.