Femtosecond SRS Lets Scientists Look into Solar Panel Molecular Mechanics
8 July 2014
Scientists from the University of Montreal have discovered how light excites the molecules in plastic solar panels and generates electric current in them, with the help of femtosecond stimulated Raman spectroscopy (SRS).
In their experiment, the researchers targeted three separate beams of light at panels based on blends of polymeric semiconductors and fullerene derivatives. The first green laser pulse was absorbed by the polymer in a way similar to how light is absorbed in a solar cell, after which an infra-red pulse and a white pulse probed the excited, vibrating polymer.
Then the femtosecond SRS gathered information about those vibrations, revealing that electrons that move away from a positive center cause their parent molecules to rearrange themselves very quickly - within 300 femtoseconds (a femtosecond is one millionth of one billionth of a second). Because of that speed, the positive and negative charges can't recombine, while all the processes of relaxation and molecular reorganization remain extremely small.
The efficiency of solar devices is determined by how many molecules recombine and how many charges stay separated - the latter case being the more effective one. According to University of Montreal associate professor Carlos Silva, the team's findings open avenues to determine the difference between material systems that produce efficient solar cells and systems that should do the same, but for some reason they don't.
The research, carried out in conjunction with the UK's Science and Technology Facilities Council and Imperial College London and the University of Cyprus, provides a basis for a fundamental mechanistic understanding of all solar conversion systems. Ultimately, it could lead to developing much more efficient solar panels and broadening their use.