What breakthroughs are needed for solar cells?
Clean energy has become a major trend in future development, and solar
cells are one of them, but its conversion efficiency has always been a difficult
problem to solve. Now, researchers have discovered the main reasons that affect
its conversion efficiency. Through improvement, the service life of the battery
will be greatly improved, which will make an important contribution to our
protection of the environment. Let us appreciate the glory of the leader
silicon!
As low-carbon energy becomes a major trend in future world development,
large-scale solar power generation will become an important measure to mitigate
climate change by the middle of this century. Climate scientists believe that by
2030, the world will need more than 10 trillion watts (TW) of solar power
generation, which is no less than 50 times the current power generation. At the
MIT Photovoltaics Research Laboratory (PVLab), teams are working to explore new
technologies and help make this possible. Our job is to find economically and
environmentally sustainable ways to reach more than 10TW of solar power through
technological innovation. said Tonio Buonassisi, associate professor of
mechanical engineering and laboratory director.
This is a huge challenge. First, they calculated the growth rate needed to
achieve 10TW of solar power by 2030, and the minimum price at which that growth
could be achieved without the help of subsidies. Current technology is clearly
not up to the task. "It would require $1 trillion to $4 trillion in additional
debt," Buonassisi said, "and that's just bringing existing technology to market
to do the job, which is hard." So, are there any other methods?
Using a model that combined technical and economic variables, the
researchers determined that three changes were needed: reducing the cost of the
module by 50 percent, increasing the module's conversion efficiency (i.e., the
percentage of solar energy converted into electricity) by 50 percent, and
increasing the The cost of building a new factory is reduced by 70%. These three
changes need to be completed as soon as possible, within five years, and will
require near-term policies to incentivize deployment and a strong push for
technological innovation to reduce costs so that government support can be
reduced over time.
Make strides in efficiency
At MITPVLab and around the world, significant progress has been made in the
conversion efficiency of solar energy. One particularly promising technology is
Passivated Emitter Rear Cell (PERC), which is based on low-cost crystalline
silicon but has a special structure that captures more solar energy than
conventional silicon cells. While costs must come down, the technology promises
to increase efficiency by 7%, and many experts predict widespread adoption.
But there is still a problem that needs to be solved. In field tests, some
modules containing PERC cells degraded in sunlight, with conversion efficiency
dropping by 10% in the first three months. Ashley Morishige, a Ph.D. in
mechanical engineering, said: These modules were thought to last 25 years, but
within just a few weeks to a few months, their power generation dropped to 90%
of the original design. This situation is very confusing because manufacturers
have thoroughly tested the efficiency of their products before releasing them.
In addition, not all modules will have problems, and not all companies will
encounter this problem. Interestingly, it took several years for companies to
mutually realize that other companies had the same problem. Manufacturers have
come up with various solutions to deal with it, but its exact cause remains
unknown, and there are fears that it may reappear at some point, which could
affect the architecture of next-generation batteries.
For Buonassisi, this seemed like an opportunity. His lab generally focuses
on basic research on wafers and battery materials, but researchers can equally
apply their equipment and expertise to modules and systems. By defining the
problem, they can support the adoption of this energy-efficient technology and
help reduce material and labor costs per watt of electricity consumed.
The MIT team worked closely with industrial solar cell manufacturers to
conduct a root cause analysis to explore the source of the problem. The company
has helped them analyze unexpected degradation of PERC modules and reported some
unusual trends. In the test, after being placed in the sun for 60 days, the PERC
module in the closed circuit will no longer have a significantly higher
efficiency than traditional solar cells; under the same storage conditions, the
efficiency of the module in the open circuit will drop more significantly.
Furthermore, modules made from different silicon ingots showed different power
loss behaviors, with battery modules manufactured at a peak temperature of 960°C
losing significantly faster efficiency than cells fired at 860°C.
subatomic misconduct
Explaining how defects affect conversion efficiency requires understanding
how solar cells work at a basic level. In photosensitive materials, electrons
can be in two different energy levels: electrons in the valence band are bound;
while electrons in the conduction band can move freely. When light strikes a
material, electrons can absorb enough energy to jump from the valence band to
the conduction band, leaving behind vacancies called holes. An electron making a
transition like this, before losing this extra energy and falling back to the
valence band, will move around the conduction band creating an electric
current.
Typically, an electron or hole must gain or lose a certain amount of energy
to jump between energy levels. Although holes are defined as electron defects,
physicists consider both electrons and holes to be carriers within a
semiconductor. Metalization or structural defects in silicon introduce defect
energy levels in the forbidden band, and electrons and holes transition to
intermediate energy levels, allowing electron transitions to achieve less energy
gain or loss. If both electrons and holes move, electron-hole recombination will
occur, and at this time, the open circuit voltage will drop significantly.
PVLab researchers quantified this behavior as the average time an electron
remains in an excited state before recombining with a hole. Lifetime severely
affects a solar cell's energy conversion efficiency, which is very sensitive to
the presence of defects, Buonassisi said.
To measure lifetime, a team led by Morishige and mechanical engineering
graduate student Mallory Jensen used spectroscopy: shining light on or heating
the sample and monitoring conductivity during and immediately after. When the
current rises, electrons are excited by external energy and jump into the
conduction band; when the current falls, they lose energy and fall into the
valence band. The change in conductivity over time reflects the average lifetime
of electrons in the sample.
Localization and defect characterization
To address PERC solar cell performance issues, researchers need to figure
out where the main defects in the module are, including the silicon surface,
aluminum backing and various interfaces between the materials. But the MIT team
believes the defect is most likely to be in the silicon wafer itself.
To test this hypothesis, they used solar cells manufactured at 750°C and
950°C, and set up two storage environments: light and dark room. Afterwards, the
top and bottom layers of the battery are chemically removed, leaving only the
silicon wafer, which is then tested for electron life. At low temperatures, the
lifespans of samples in the two storage environments are roughly the same; at
high temperatures, the lifespan of samples stored in light is significantly
lower than those in darkrooms.
These findings confirm that efficiency degradation is primarily
attributable to defects in the silicon, which affect the lifetime of electrons
in the cell, causing a significant decrease in efficiency. In subsequent tests,
the researchers found that heating and degrading the sample at 200°C for an hour
could restore the lifespan, but it would still fall back when exposed to light
again.
So how do these defects interfere with conversion efficiency, and what
types of contaminants might be involved in their formation? Two characteristics
of the defect will help researchers answer these questions. The first is that
the defect energy level is between the valence band and the conduction band; the
second is the capture cross section - a defect at a specific location can trap
electrons and holes (the volume of electrons may be different from the volume of
the defect).
While these properties are not easily measured directly in samples,
researchers can infer them based on empirical equations using lifetimes at
different irradiation intensities and test temperatures. Lifetime spectroscopy
experiments were performed under different test conditions using samples fired
at 950°C and then exposed to light. These data are used to calculate the energy
levels and primary capture cross-sections leading to electron-hole
recombination. Prioritized candidates for reduced sample conversion efficiency
were enumerated by reviewing the literature to see which elements had been found
to possess these properties.
The Morishige team has tried their best to narrow down the list. At least
one agrees with most of what we observe. she says. In this case, metal
contaminants caused during manufacturing create defects in the silicon's crystal
lattice, and hydrogen atoms bond with the metal atoms, leaving it electrically
neutral and therefore unavailable as sites for electron-hole recombination. But
under special conditions, especially when the electron density is high, hydrogen
atoms dissociate from the metal, making the defects extremely recombinantly
active.
This explanation is consistent with the company's preliminary reports on
its module. Cells fired at higher temperatures will be more susceptible to
light-induced damage because the silicon in them typically contains more
impurities and less hydrogen, and their performance varies from batch to batch.
Contains varying concentrations of pollutants as well as hydrogen. As the
researchers found, baking silicon wafers at 200°C can cause hydrogen atoms to
recombine with the metal to create inert defects.
Based on this hypothesized mechanism, the researchers offer two
recommendations to manufacturers. First, try to adjust the manufacturing process
so that they can perform the firing step at a lower temperature; second, ensure
that the concentration of those suspect-listed metals in their silicon wafers is
minimized.
unexpected result
The high efficiency of PERC technology comes from the special structure
that effectively captures solar energy, which reveals problems inherent in
manufacturing materials. Cell Man did everything right, he said. If the key to
the problem lies in too high a density of excited electrons interacting with
defects in the silicon wafer, then finding effective strategies to solve it will
be especially important as next-generation device designs and Wafer thinning
will result in higher electron density.
This work requires cooperation among experts in various fields, and he
advocated communication among all participants, including private companies and
research institutions, and experts in various fields, from raw materials to
wafers, cells and modules, to system integration and module installation. Our
lab is taking a series of steps to bring together stakeholder groups to create a
new R&D platform. It is hoped that this will allow us to solve technical
challenges more quickly and help achieve the goal of 10TW photovoltaics by 2030.
So said Buonassisi.
The research was funded by the National Science Foundation, the U.S.
Department of Energy, and the National Research Foundation of Singapore through
the Singapore-MIT Research and Technology Alliance.
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