The Resonances of Light
The world remains dependent on fossil fuels. The majority of our energy needs are still being met by coal, oil and gas. Renewable forms of energy, such as solar power, could bring this unsustainable dependency to an end. However, the most widely used semiconductor-based solar cells convert only about 30% of the incident light into electricity. Aurora Manzi, an Italian doctoral student in the Department of Photonics and Optoelectronics, is helping to change that. In the Nano-Institute‘s laboratories, she is working on perovskite nanocrystals with the aim of enhancing the light conversion efficiency in solar cells. The term ‘perovskite’ refers to a specific type of crystal structure. Irradiation of such nanocrystals generates photo-generated charge carriers only if the wavelength of the light lies above the material’s ‘bandgap’. However, Manzi discovered that it is possible to strongly improve the light conversion efficiency of these nanocrystals when using arrays of them even when the excitation energy lays below the bandgap. At first, the researchers were unable to account for this result. “It was a complete surprise,” Manzi admits.
Further investigation revealed the observed effect was due to simultaneous absorption of multiple photons in association with a phenomenon like that associated with the resonant interaction between overtones in music. Together, they make it possible for the absorbing layer to capture light quanta with energies that lie below the bandgap – the spectral region usually cannot be converted into electricity by solar cells. Manzi draws an analogy with strumming a guitar. “If one thinks of the incoming light as inducing oscillations of the first string, and the second string as reflecting those of an exciton that lies within the bandgap, acoustics tells us that the two strings will resonate when an overtone of the first string corresponds in frequency to a different overtone of the second.”
In 2015, Manzi received the Center for NanoScience‘s Innovation Award for her Master’s thesis. At that stage, her research was devoted to a different form of transformation – how best to transform CO2 into methane with help of solar energy, essentially converting a waste product into a fuel. In both instances her motivation is the same – using basic research as a means to bring a future without fossil fuels closer to reality. And the Nano-Institute provides the best conditions for realizing this goal. “In our new laboratories, we can perform our experiments precisely as planned, which makes them more informative and more efficient.”