Glowing graphene dots
doi:10.1038/nindia.2011.104 Published online 14 July 2011
Researchers have designed a new type of polymer-modified graphene quantum dot that could be useful for enhancing the efficiency of solar cells and producing better organic light-emitting diode (OLED) devices.
Quantum dots are interesting because they have a size-dependent optical response, exhibit efficient charge transport and are cheap to produce. However, cadmium-based quantum dots are toxic and hazardous, which prevents their use in large-scale device applications.
To find safe nanomaterials with similar properties as cadmium-based quantum dots, the researchers looked at graphene — a material that also facilitates good charge transport. They prepared graphene quantum dots (GQDs) from graphene sheets. The dots were of 1–2 layers and about 5–15 nm n size. Then they modified the dots with polymers to form three distinct materials: pure GQDs, aniline-modified GQDs (ANI-GQDs) and methylene blue-modified GQDs (MB-GQDs).
Upon irradiation with ultraviolet and visible light, the polymer-modified dots showed significant change in luminescence properties. The ANI-GQDs exhibited blue–white luminescence, whereas the MB-GQDs exhibited green luminescence.
The ANI-GQDs showed a decrease in resistance to charge transport, which improved the material's efficiency to harness solar energy. The study also found that the MB-GQDs dispersed in MEH-PPV saw an increase in charge carrier density, which provides a lower turn-on voltage and much higher efficiency than existing photovoltaic materials. Dispersing MB-GQDs in MEH-PPV provides more electrical transport paths, which enhances charge injection.
"Polymer-modified GQDs can be a cost-effective, environmentally friendly and more stable material for photovoltaics than current organic materials," says lead researcher Vinay Gupta.
- Gupta, V. et al. Luminescent graphene quantum dots for organic photovoltaic devices. J. Am. Chem. Soc. 133, 9960-9963 (2011) | Article |