Monday, May 18, 2015

ALD boosts efficiency to 22.1% for nano structured Black Silicon solar cells

As reported by The researchers from Finland's Aalto University and Universitat Politècnica de Catalunya have obtained the record-breaking efficiency of 22.1% on nanostructured silicon solar cells as certified by Fraunhofer ISE CalLab. 

An almost 4% absolute increase to their previous record is achieved by applying a thin passivating film on the nanostructures by Atomic Layer Deposition, and by integrating all metal contacts on the back side of the cell.

The surface area of the best cells in the study was already 9 cm2. This is a good starting point for upscaling the results to full wafers and all the way to the industrial scale (Aalto University).

The results were published online 18.5.2015 in Nature Nanotechnology.

Black silicon solar cells with interdigitated back-contacts achieve 22.1% efficiency

Hele Savin, Päivikki Repo, Guillaume von Gastrow, Pablo Ortega, Eric Calle, Moises Garín
& Ramon Alcubilla

Nature Nanotechnology (2015) doi:10.1038/nnano.2015.89

Figure 1: Structure and reflectance of b-Si. a, Scanning electron microscopy (SEM) image (cross-sectional view) of a b-Si surface. Typical height of a silicon pillar, ∼800 nm; diameter at the bottom of the pillar, ∼200 nm. The 20 nm Al2O3 layer can be seen as a brighter layer on t…

The nanostructuring of silicon surfaces—known as black silicon—is a promising approach to eliminate front-surface reflection in photovoltaic devices without the need for a conventional antireflection coating. This might lead to both an increase in efficiency and a reduction in the manufacturing costs of solar cells. However, all previous attempts to integrate black silicon into solar cells have resulted in cell efficiencies well below 20% due to the increased charge carrier recombination at the nanostructured surface. Here, we show that a conformal alumina film can solve the issue of surface recombination in black silicon solar cells by providing excellent chemical and electrical passivation. We demonstrate that efficiencies above 22% can be reached, even in thick interdigitated back-contacted cells, where carrier transport is very sensitive to front surface passivation. This means that the surface recombination issue has truly been solved and black silicon solar cells have real potential for industrial production. Furthermore, we show that the use of black silicon can result in a 3% increase in daily energy production when compared with a reference cell with the same efficiency, due to its better angular acceptance.