Light-harvesting boosted by 'forests' of nanostructures on thin silicon films

Thin films of silicon are attractive for use in solar cells because of their low material cost and suitability for large-scale fabrication, but their power-conversion efficiency has so far been lacking. The efficiency of thin-film-based devices, however, could rival that of bulk silicon solar cells if the surface of the thin film is engineered on the nanoscale using the specifications suggested in a theoretical study by Junshuai Li and co-workers at the Institute of Microelectronics, A*STAR, Singapore1.

Trapping light with nanostructures on the surface of thin-film-based solar cells can boost the solar-to-electrical power-conversion efficiency, explains co-author Patrick (Guo-Qiang) Lo. Constructing arrays of nanopillars on the film, for instance, prolongs the path traveled by the light, allowing for more scattering and therefore increasing light absorption, he notes.

Before commencing their design, the researchers also had to consider that once absorbed, photons should efficiently generate electron–hole pairs that exist long enough to be separated in the electric field—generated in the standard p–n junction setup of solar cells—to give rise to a photocurrent. This meant that their careful and precise design involved a trade-off between the absorption of solar radiation and the efficient collection of the photo-generated carriers, which is sensitive to the detailed topographical variations of the patterned film.

“Enhancing the power-conversion efficiency is a balancing act,” says Lo, “because the solar-radiation spectrum is composed of many different wavelengths with varying power intensities.” To provide a practical guideline for their design, Li and his co-workers systematically investigated the performance of nanopatterned thin-film solar cell devices, containing arrays of nanopillars and nanocones, using a combination of electromagnetic, quantum and electron transport theory.

From the simulations, the researchers were able to determine optimized structural parameters, such as diameters, heights and periodicities of pillars and cones, giving the most efficient light-harvesting and collection of photo-generated carriers.

Low-cost, large-scale production of the optimally designed solar cells is feasible because the nanopatterned devices are much thinner than their bulk counterparts and can be fabricated using standard silicon processing techniques. Furthermore, the simulations show them to be remarkably efficient.

“Today’s thin-film efficiencies are typically around 10–12% whereas those of bulk silicon are 15–20%, with a record of 25%,” notes Lo. The cells simulated by the team, however, displayed efficiencies in excess of 25%. According to Lo, the researchers expect to be able to boost these values further by applying more advanced nanotechnological concepts, such as plasmonics.

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