Experimental method and static absorption spectrum of P3HT. a Experimental setup for time-resolved soft X-ray spectroscopy. A 15 fs visible pump pulse excites the π→π* transition in P3HT, and time-delayed attosecond soft X-ray pulses probe the absorption edges of carbon and sulfur. b Visible absorption spectrum of the P3HT sample used in this work, and the spectrum of the pump pulse centered on the maximum of the π→π* resonance in P3HT. c Typical soft X-ray spectrum extending to ~330 eV (black line). The red line is the X-ray absorption spectrum of a P3HT sample, with absorption features from the sulfur L1,2,3 and carbon K edges resolved simultaneously. Credit: Nature Communications (2022). DOI: 10.1038/s41467-022-31008-w
The researchers used ultrafast lasers and X-rays to trigger the reaction and then measured the changes it caused within femtoseconds (trillionths of a second). They tracked the first fractions of a second after light hit the solar cells, giving insight into how they generate electricity. Exploring the initial moments of the process that converts light into electricity could help researchers improve new solar cells so they can generate energy more efficiently.
The researchers have now used the technique to study organic photovoltaic (OPV) materials, which can harvest sunlight to generate energy or split water.
OPV materials are being intensively studied because they could provide cheaper, renewable energy. However, many of the materials currently used are unstable or inefficient due to the complex interactions of electrons excited by light.
Further investigations of these electrons' rapid interactions, such as the paper published in Nature Communications, which combines fast time resolution with measurements localized to atoms, provide valuable insights into ways to improve solar cells and catalysts.
More efficient devices
“OPVs are a cheap and flexible alternative to silicon-based photovoltaics, and therefore hold attractive promise for use in future solar power infrastructure,” said Professor Jon Marangos of Imperial's Department of Physics.
“This work demonstrates the power of our new time-resolved X-ray technique, which can now be applied to a wider range of materials and may provide the understanding needed to make more efficient OPV devices.”
The team explored the first step in solar energy conversion - the reaction of a material caused by exposure to light. They first fired laser pulses lasting 15 femtoseconds into the material to stimulate the reaction. They then used X-ray pulses with a duration of just attoseconds (less than a billionth of a second) to measure the resulting changes in the material.
Fast Evolution of States
For the first time, the team observed a direct X-ray signature of the material's initial state when an electron is knocked out of position. This creates an electron and 'hole' pair that can travel through the material.
This initial state rapidly evolves into a new, more stable state within 50 femtoseconds. Calculations by Professor Tom Penfold of Newcastle University agreed closely with the observations, suggesting that the initial state depends on the distances between the molecular chains in the material.
Dr Artem Bakulin of Imperial's Department of Chemistry said: "The sensitivity of time-resolved X-ray methods to the initial electron dynamics occurring directly after photoexcitation paves the way for a deeper understanding of the photophysics of a wide range of organic optoelectronics and other materials. ”
The team now plans to explore ultrafast charge dynamics in other organic semiconductor materials, including recently discovered materials that use different molecules as electron acceptors and have shown enhanced OPV efficiency.
