Thin film organic polymer solar cells offer promising advantages in cost and utility, but have not been able to touch crystalline silicon cells in solar cell efficiency.
Attempts to optimize the organic solar cells by varying the thickness of the polymer layers has achieved mixed results, leading researchers down a different path: if photons of light can be trapped for a longer time at the polymer layer, more of them can be converted into energy.
Trapping Light by Stabbing in the Dark
In the most recent development, scientists at Northwestern University's McCormick School of Engineering and Applied Science let evolution inspire them to a new light trapping record.
Before their Eureka moment, trapping light was a bit of a stab in the dark, you could say. The many different geometries possible when developing light scattering layers creates challenges to finding the optimal solution. Competing effects of reflection and diffraction make an intuitive solution improbable (non-geek speak: you can't invent this stuff by trial and error). Cheng Sun, assistant professor of mechanical engineering, notes:
With so many possibilities, it’s difficult to know where to start, so we looked to laws of natural selection to guide us.
Darwin to the Rescue
The researchers used a mathematical algorithm based on the laws of natural selection, the theory to which Darwin owes his greatest fame. Under natural selection, random mutations and fortuitous crossing of genes may produce a superior result. The advantages that enable species to pass on the new, superior capabilities by surviving and breeding are "naturally selected" while inferior genes do not survive.
The mathematical algorithm operates similarly: a set of rules applies to calculate the relative superiority of the scattering results against the desired effects. The patterns with the best results then combine or mutate to create a "next generation" of scattering patterns. Repeating this process achieves consistently better scattering results.
By the end of the process, the team achieved light trapping capabilities that are three times better than the maximum theoretical limit (the Yablonovitch Limit) for how long a photon can be trapped in a semiconductor. (How is that possible you ask? It has to do with the fact that the Yablonovitch limit applies to light landing on layers that are large compared to the wavelength of the light, whereas the new thin film layers can be much thinner than solar light wavelengths.)
The paper “Highly Efficient Light-Trapping Structure Design Inspired by Natural Evolution,” appeared January 3 in Scientific Reports, a publication of Nature.
So only one question remains: can we teach this in the schools?