Solution-processable thin-film solar cells can be competitive with silicon-based ones in terms of electricity output/cost ratio and therefore have great potential in solar energy utilization. Due to the requirement for efficient light harvesting, however, so far the most successful low-cost thin-film solar cells require materials containing either rare or toxic metals. In my talk I will discuss our efforts in making graphene, which is primarily made of carbon, an active component for solar energy conversion. Our work is based on the synthesis of solution-processable colloidal graphene quantum dots with tunable size and bandgap. Our spectroscopic studies have shown that the graphene quantum dots have some unique electro-optical properties, which could potentially enable us to overcome the Shockley-Queisser limit imposed on single-junction solar cells.