New layer raises efficiency of heterojunction solar cells

Researchers at the Netherlands’ Delft University of Technology have used highly transparent, hydrogenated nanocrystalline silicon oxide (nc‐SiO_x:H) layers to increase the efficiency of crystalline silicon heterojunction solar cells.

The Delft group said the layers offer an alternative to commonly-used contact stack structures such as hydrogenated amorphous silicon ((i)a‐Si:H). The latter have limited thickness but suffer significant parasitic losses when placed on the sunny side of cells. Scientists have tried higher doping to address such losses but observed the change resulted in a rise in defect density and deterioration of carrier collection.

The Dutch group claim the superior electrical properties of the nc‐Si:H layers improved optoelectrical parameters in the contact stacks of heterojunction silicon cells when applied to carrier‐selective contacts.

“The material consists of small nanocrystals embedded in the amorphous matrix, giving confirmed anisotropy properties in the growth direction,” the researchers wrote in the paper Doped hydrogenated nanocrystalline silicon oxide layers for high‐efficiency c‐Si heterojunction solar cells, published in Progress in Photovoltaics. Anisotropy relates to the magnetic properties of an object differing depending on direction.

n-type better

The solar cells produced at lab level by the Delft group measured 7.84cm² with 12.5% metal coverage. “For both n and p‐type layers, highly conductive nc‐Si:H layers are obtained when CO₂ is missing during the deposition,” the researchers stated.

The group observed the n‐type nc‐SiO_x:H layers demonstrated higher conductivity than p‐type alternatives in the experiment. Regardless of the kind of doping, however, the scientists claimed the tunable optoelectrical properties of doped nc‐SiO_x:H layers in general provided the flexibility necessary to tailor selective transport and enhance cell performance.

Improved fill factor

The researchers added, they were able to improve the fill factor of the cells by using interface treatments on the contact stacks, which led to an improvement from 65.6% in a rear-junction cell to 77.9% in a front-junction device.

“Cells with [an] area of 3.92cm² featuring 4.4% metal coverage, demonstrate improvement of FF [fill factor] from 77.9% to 79.1%, and 75.3% to 78.6%, thus η from 18.4% to 21.8% and 18.3% to 22% are achieved for front and rear-junction cells, respectively, with perspective of further improvements by optimizing the front grid; [bulking] thickness; and using more transparent conductive oxides,” stated the paper.

In other words, the Delft group claim to have achieved cell efficiencies of 21.8% and 22% for front and rear-junction configurations, respectively. “The optical effectiveness of contact stacks based on nc‐SiO_x:H is demonstrated by, [on average], 1.5 mA/cm²-higher short‐circuit current density, thus, nearly 1%_abs higher cell efficiency as compared with the (n)a‐Si:H,” stated the paper.