Since the commercial application of solar energy as a renewable source of alternate energy has gained grounds, researchers across the world are working on the ways and methods to reduce the solar energy costs. This will further help in mass scaling of solar energy production and usage, reducing burden on the conventional forms of energy. Solar panel is the major cost centre in this entire chain and hence reduction in the prices of solar cells (which is the major component of solar panel) need to be focused on.
Apart from the advancements that have been made in solar cell technology and manufacturing plants, currently there are only two ways in which costs can be further reduced:
1) Reduction in the prices of raw material for solar cells
2) Reduction in the number of process steps involved in a manufacturing of solar cells.
Reduction in the prices of raw material for solar cells:
There are two major researches that seem plausible to achieve this result:
One study was jointly conducted by MIT and NREL researchers. The study focused on reducing the cost of solar panels. They found that this is possible by using thinner solar cells, a major part of the solar panel.
This method was tried almost a decade ago when the silicon prices peaked. But because at that time, the manufacturing processes and handling equipment were not so advanced, the silicon wafers produced were brittle and fragile; which resulted in breakage and wastage of raw material; thus increasing the costs.
But now due to better manufacturing processes & handling equipment and advance solar cell architecture, all those issues can be mitigated.
Today 90% of the solar panels are made of crystalline silicon and the whole industry is dependent on this as raw material. The current systems and equipments have the capability to produce silicon wafers of 160 micrometer thickness. But with some improvement in handling equipments and manufacturing processes, it is possible to take this thickness further down to 100 micrometer. Any further advancement in technology can help produce silicon wafers with thickness as low as 40 micrometer. That would require almost 1/4th of silicon for a given size of a panel.
The research involved studying the efficiency levels of 4 different variants of the solar cell architecture at different thickness levels. It was observed that solar wafer efficiency doesn’t decline much for thickness levels up to 40 micrometer, if improved manufacturing processes are used.
This would result in reduction in the cost of solar panels and would enable rapid expansion of plants producing silicon crystal cylinders. This would further lead to expansion of solar cell manufacturing plants, where these silicon cylinders are sliced into silicon wafers of required thickness.
In future, new technologies are expected to be developed that would directly grow thin silicon wafers instead of the current method of slicing from big silicon cylinders. When this happens, the silicon wafer thickness can further go down to 15 micrometers without compromising the efficiency.
All this would eventually lead to reduction in cost. But usage of these technologies and methods on commercial scale would require huge capital expenditure. So the companies in this field who are looking to expand must do a cost benefit analysis with respect to changing dynamics of the industry and future technologies.
Another study that focused on reducing the cost of the panels, involved finding alternate material as replacement for silicon crystals. Researchers found that a family of crystals – Perovskite – is actually good at absorbing light and have power conversion efficiency up to 22%, which is equivalent to that of silicon crystals.
Perovskite crystals were also tested less than a decade back, but found to have certain inherent problems. Perovskite crystals are not as stable as silicon crystals are and hence tend to dissolve quickly in humid conditions. To avoid this, these crystals need to be protected in sealed glass plates. While it was possible to achieve this in lab controlled conditions, but not on large commercial scale. Besides, there are no factories to build Perovskite commercially. The startups and research bodies, who are researching Perovskites, can’t compete with the well funded companies who are researching silicon crystals.
Therefore, researchers found a new way that seemed viable. They proposed using “Tandem” solar cells instead of pure silicon solar cells. Tandem solar cells are produced by placing a layer of Perovskite on top of silicon. Since Perovskite is semi transparent in nature, it tends to capture certain wavelengths in visible spectrum of light, allowing other wavelengths to pass and captured by the silicon layer below.
These tandem cells are 10% more efficient than pure silicon cells. Also Perovskite crystals are produced at low temperatures as compared to silicon crystals that are produced at high temperature, which require more heat and hence are costlier.
Reduction in the number of process steps involved in manufacturing of solar cells.
Higher cell efficiency is the key to reduce the no. of process steps and thus lower manufacturing cost.
Key requirement of a solar cell is to have high level of efficiency. This can be achieved by minimizing the surface reflectance and maximizing the absorption of photons. This would result into maximum conversion of light to electric energy.
In order to reduce the surface reflectance, Anti Reflection (AR) coatings are used. But conventional AR coatings have limitations because the reduction of reflection occurs for narrow range of wavelength of light and incident angle.
A good replacement to these conventional ARs is Black Silicon. Black Silicon is a type of porous silicon that reduces light reflection (low reflectivity – approx 1%) and high light absorption levels.
Out of all Black Silicon fabrication methods, MACE (Metal assisted chemical etching) is the most efficient. It involves deposition of metal – Au, Ag, Pt as nano particles on Silicon surface.
These nano particles attract electrons from the Silicon surface and make it conducive for oxidation to SiO2. SiO2 corrodes the silicon surface and a pit is formed under each nano particle. The remaining part of silicon substrate forms Black Silicon which is highly porous in nature.
This whole method eliminates few steps from the manufacturing process, removes complexities and hence reduces costs.
(Sources: Financial Times, Materials Today, MIT)