Solar photovoltaics (PV) continues to be the leader among renewable generation technologies as governments around the world work on building an environmentally friendly, low-carbon society. A major factor behind the increasing popularity of PV systems is the consistent progress made toward raising their conversion efficiency and wattage output. Improvements in the performances of PV systems have also brought down their costs significantly. With regard to the latest technological advances in the solar industry, recent research collaboration between Aalto University in Finland and Michigan Technological University (MTU) in the US has yielded solutions that can lower the cost of producing black silicon (black-Si) PV cells. Black-Si cells are crystalline silicon PV cells with surfaces covered with micrometer- or nanometer-sized needle-shaped structures. This feature results in lower reflectivity and higher light absorption, thus increasing the conversion efficiency of the PV cell. In sum, the solutions devised by the researchers from Aalto University and MTU are expected to reduce the cost per unit power of black-Si cells by about 10.8% versus that of conventional PV cells.
The two main types of crystalline silicon PV cells that are on the market are monocrystalline (mono-Si) and multicrystalline (multi-Si). Products of the former category offer much higher conversion efficiency rates at higher prices, whereas products of the latter category offer relatively lower conversion efficiency rates at more economical prices. Both mono-Si and multi-Si cells have been etched to have surface textures consisting of tiny bumps or needles in order to capture more light rays and preventing them from reflecting away. Hence, the colors of the surfaces of PV cells are usually darker to lighter shades of blue because much of the visible light has been absorbed except the part that contains the blue spectrums.
The black-Si treatment is about making the surface of the cell as dark as possible, since black is best at absorbing light. To create products featuring the black-Si surface in the multi-Si category, the silicon wafers have to be sliced by diamond-wire saws and have their surfaces modified through an etching process. Wafer and cell manufacturers have developed different types of etching processes. Their goal is to raise the conversion efficiency creating optimal surface textures that can capture much of the visible and infrared light.
Nano-texturing of the wafer surface through dry etching forms millions of silicon needles that cover the wafer surface like a forest from a magnified view. Joshua Pearce, professor of material sciences and electrical engineering at MTU, said that nano-texturing creates better black-Si cells since the nanometer-sized needles on the surface of the cell can effectively trap light rays and prevent them from bouncing off.
The manufacturing process of black-Si cells can produce surface defects that can lower the conversion efficiency. However, researchers from Aalto University and MTU have also found that this problem can be mitigated with another treatment – the passivation coating of the cell’s surface by atomic layer deposition (ALD).
Although researchers are optimistic that their findings will make black-Si cells a more viable option in the market for high-efficiency PV technologies, the combination of ALD and black-Si treatments is estimated to raise the total cell production cost by 15.8-25.1% compared with the average production cost of conventional PV cells. On the other hand, the two treatments are projected to reduce the cost per unit power of the cell by 10.8% on average compared with that of conventional PV cells. Hence, the efficiency gain may be able to offset the production cost in certain situations for the customers in the downstream market.
Pearce pointed out that the positive effects of lowering cost per unit power for PV cells will amplify along the downstream of the supply chain. Pearce also noted that the demand growth for solar PV has been rapid because it has become cost-competitive against conventional forms of electricity. So, another 10% reduction in the cost per unit power could further accelerate the adoption of solar energy worldwide. The joint findings by the researchers from Aalto University and MTU have been published in the academic journal Energies this September. Solar companies around the world can consider these solutions as they develop their high-efficiency products.
(The above article is an English translation of a Chinese article written by Daisy Chuang. The credit of the top image goes to Aalto University.)