Solar power is the cheapest, and the moment of the light bulb literally shows us that we can cut costs and emissions even further
Recent extreme weather events have underscored the need to reduce CO₂ emissions that increase global temperatures. This requires a rapid transition from an energy economy to renewable energy sources, the cheapest of which is solar photovoltaic (PV). And our recently published research shows how we can lower shift costs even further using a cheaper form of silicon for highly efficient solar panels.
Australia has taken the lead with solar PV installations, but our solar energy journey has only just begun. This year, humanity reached the milestone of 1 terawatt (TW) – 1 million × 1 million watts – installed solar capacity. However, experts predict 70TW of solar PV may be needed by 2050 to power all sectors of the economy.
To help drive this rapid uptake of solar PV, we need solar panels with high efficiency and low cost. Over the past ten years, several new solar cell designs have resulted in record high efficiency. The problem is that this design also requires higher quality materials, which cost more.
Our recent research suggests that we may be able to rethink the type of silicon needed to make these high-efficiency solar cells.
Read more: Australia is the runaway global leader in building new renewable energy
Not all silicones are the same
More than 95% of solar panels are made using silicon. The silicon used to make solar cells is similar to that used in computer chips. This is effectively very pure sand.
To make a solar cell work, we need to form an electric field so that the current produced can all flow in one direction. This is done by adding impurity atoms to the silicon, a process known as “doping”.
In the manufacture of commercial panels, the most commonly used type of silicone is “p-type” silicone. This material is doped with atoms that have one electron less than silicon, such as boron or more recently gallium.
We can then introduce a very thin layer on the surface filled with atoms with one extra electron relative to silicon, which is called “n-type” silicon. Putting these two types of silicon together forms what is called a “pn junction”. The large difference in the number of electrons between the p-type region and the n-type region forces the electrons to move rapidly, creating an electric field that drives the current in our solar cells.
Conventional solar panels on Australian roofs today are mostly made using p-type silicon, as they are about 10% cheaper than alternative “n-type” silicon, which is coated with phosphor.
Read more: The sun’s rays that power solar panels also damage them. ‘Galium doping’ provides a solution
Higher efficiency comes at a cost
Researchers continue to push to improve the efficiency of solar panels so they can produce more power for consumers. In 2017, a record efficiency of 26.7% was achieved for silicon solar cells. Last month, LONGi Solar announced an efficiency of 26.5% – very close to a world record – for the same type of solar cell made in a manufacturing environment, not in a laboratory.
This type of solar cell is called “silicon heterojunction”. A special element of silicon heterojunction solar cells is that their surface is covered with a very thin layer – about 1,000 times thinner than a human hair – of amorphous silicon. This thin layer smoothes the surface and reduces much of the energy loss.
Sanyo developed this cell design in the 1990s. At that time, high-quality n-type silicon wafers were used to make silicon heterojunction cells, although these wafers were more expensive.
The main reason for this is that sunlight degrades the cheaper p-type wafers. However, our understanding of this phenomenon and how to treat it has come a long way since the 1990s.
Our light bulb moment
Over the past 30 years, all silicon heterojunction solar cells, including record-breaking cells, have been fabricated using n-type silicon wafers. In our research project, we wanted to test whether cheaper p-type wafers could also be used.
Through comprehensive testing, we found that heterojunction solar cells made with p-type silicon do not perform well. We are confused by this. But one day we had a literal light bulb moment.
We noticed that accidental exposure to room lighting for only ten seconds prior to testing reduced the voltage of a p-type cell by 30mV, which could reduce its efficiency by a percentage point (ie from 22% to 21%). This causes our cells to work much worse than expected. Just as someone with a severe allergy is more sensitive to pollen in the spring, we realized that high-efficiency silicon heterojunction solar cells made with p-type wafers are much more sensitive to light-induced degradation.
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Problem identified, we now have the solution
We believe this observation is the reason why high-efficiency cells were previously only explored using expensive silicon. Previous researchers were not aware of the sensitivity of p-type wafers to degradation and lacked the knowledge to overcome them.
Fortunately, we now know that unwanted bonding of boron and oxygen in silicon wafers causes this degradation. Treatment with high-intensity lasers has been shown to stabilize cells within seconds.
Laser illumination can make hydrogen, already floating in silicon, more mobile to move and “passive” unwanted boron-oxygen defects. Exactly how hydrogen does this is still an active area of research, but we know it solves the problem. Our research confirms short laser treatment can stabilize the performance of p-type silicon heterojunction solar cells.
Armed with this new knowledge, we were able to further develop high-efficiency technologies with cheaper raw materials. This will reduce the cost of each watt of solar electricity generated. In March this year, solar panel manufacturer LONGi Solar announced a 25.47% efficiency for silicon heterojunction solar cells made using p-type wafers.
To see manufacturers making high-efficiency solar cells that are potentially cheaper means our findings have a real impact on the industry. Reducing the cost of solar cells will provide millions of consumers with cheaper electricity while tackling climate change.
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