Boosting the absorption of slim solar cells (with a simple trap)

What if solar cells were ten times thinner than today's standard?

In principal, we would deploy ten times more solar power with the same amount of absorber material: Ten times thinner solar cells could increase solar electricity production, contribute to a more rapid expansion of photovoltaics (PV) and help to reduce our carbon footprints. In fact, as refining the silicon raw material is an energy-intensive process, ten times thinner silicon cells would not only reduce the need for refinieries but also cost less, hence empowering our green transition in multiple aspects.

Last year, roughly 0.2 million tons of solar-grade silicon led to a newly installed PV capacity of ca. 0.2 PWh. While this matches the electricity needs of Argentina, ten times thinner wafers would already meet the annual electricity consumption of India (2 PWh). In ten years time, a thin solar technology thus could in principle cover half the world's electricity needs (15 PWh), just by assuming a steady silicon production of 0.2 MT/year. [While such technology may seem unrealistic today, it is useful as a thought experiment.]

However, thinner material layers absorb less sunlight. Most of the near-infrared light will simply pass through a thin silicon slab (Si) as if it were a transparent layer of glass.

Therefore, using less material will not solve world's energy problems, unless the absorption of a thinner layer can be magically enhanced.

But this magic trick is well known in the photonics community, as exemplified by every optical fiber: If the incident light propagates through the plane of a slab (instead of traversing it), its thickness becomes irrelevant. Light would simply be trapped inside the absorber layer by bouncing up and down between its top and bottom.

The question then is, how to redirect the incident light into the absorber plane? How to keep the sunlight long enough trapped, such that it gets eventually absorbed and trigger a photovoltaic effect? In the last decade, R&D has dived into this research question and one promising approach is emerging: Adding a surface structure to the absorber layer.

As surface structures can guide the incident sunlight into the plane of the absorber layer, the race began in finding the structure that can do so the best. Scientists came up with more and more complicated and evermore efficient patterns -- at the expense of their complexity, as every magic comes with a price. For example, one of the top best light-trapping structures appears quite randomly, comes with very small feature sizes (< 50 nm) and stems from intensive computational efforts, because its design must be fine-tuned to the absorber's physical properties.

We found this approach extremely enlightening but not really practical, and hence came up with a simpler idea. Instead of repeating a complicated and random-looking texture periodicially in space, we repeat a simple and symmetric-looking texture more or less randomly in space. Every anechoic chamber might stand exemplary for such a pattern. So, if anechoic patterns work for sound waves, why not for light waves, too?

In fact, our investigations show that our idea actually rivals the absorption enhancement of more sophisticated designs -- while also absorbing more light deep in the plane and less light near the surface structure itself.

This has quite important implications, because the closer light is absorbed to an interface, the more unlikely it will be converted into electricity. Furthermore, our pattern can not only be derived from back-of-the-envelope calculations. Its large features (> 200 nm) facilitate its manufacturing and are thus more forgiving of fabrication imperfections.

In overall, we found a simple trick for boosting the absorption of slim solar cells. Our open-access study just got published in Optica (today) !! Optica is the flagship journal of the Optical Society of America, and, as such, a highly-selective journal for pioneering research. Happy reading!