Solar has become the lowest cost electricity source in more and more locations globally. According to the latest report by the International Renewable Energy Agency (IRENA), the levelized cost of energy (LCOE) generated by large scale solar plants is around $0.068 per kWh, compared to $0.378 per kWh ten years ago. Between 2018 and 2019 alone, the price dropped by 13.1 percent.
This has been made possible due to the emergence of high efficiency,
low cost technologies such as PERC, PERT and bifacial modules. Of these, PERC
is a mature technology with a relatively simple process and therefore benefits
from low cost of ownership. With PERC technology, a record cell efficiency of 24.06 percent
was reached by LONGi (January 2019). While record efficiencies are good, what counts more are
conversion efficiency averages in volume production and efficiency stability
over time. Technology experts have often pointed out the challenge that PERC
technology faces in early days with regard to the potential degradation
effects. LONGi understood the challenge early on and started research and
testing to address the issue of Light Induced Degradation (LID) in PERC cells and
modules quite early on in order to prevent degradation issues and offer the
best quality modules to its customers.
Figure 1: Impact of Ga-doping and low oxygen on cell efficiency
With process optimization at ingot pulling and cell manufacturing, solar
cells made with Ga doped wafers demonstrated efficiency improvement of 0.06-0.12%
(abs.) comparing to B doped wafers.
In the last few years, another solar cell/module efficiency
degradation phenomenon has caught everyone’s attention, light and elevated
temperature induced degradation, aka LeTID.
LeTID is believed to be caused by interaction between metal impurity and
hydrogen in wafers. With Ga doped
wafers, it is easier to control LeTID on solar cells, as there is no need to
introduce excessive hydrogen in cell processing to mitigate LID as required for
B doped wafers.
Through thorough research and testing, LONGi’s technology experts concluded
that LID and LeTID problems could be effectively solved by using gallium-doped
monocrystalline silicon wafers in combination with cell process control,
without the need for regeneration (light injection or electrical injection)
treatment. Compared with boron-doped silicon wafer, gallium-doped silicon wafer
can improve the efficiency of PERC cells to some extent. There is no
boron-oxygen complex in gallium-doped PERC cells, so there is not the usual
phenomenon of boron-oxygen LID. In a recent white paper titled “Gallium-doped
monocrystalline silicon fully solves the problem of a PERC module’s LID”, released
by LONGi, the PV technology provider has summarized its findings on the subject,
supported by related studies. Research strongly indicates that application of
gallium-doped silicon wafers can effectively mitigate the initial LID from
which cells using boron-doped p-type silicon wafers have long suffered.
Key features of LONGi’s test
LONGi team conducted a LID Test of Gallium-doped and Boron-doped
PERC cells. The test used LONGi’s bifacial PERC cells (which had the cell
efficiency of about 22.7 per cent). Following is part of the test scheme
including the test item, and type and quantity of cells:
1sun, 75°C – 7 boron-doped cells; 10 gallium-doped cells
×10suns, >100°C – 5 gallium-doped cells
Test results
1sun, 75°C: In order to fully reflect
the LeTID, LONGi’s mass produced cell adopted a test temperature of 75°C. Figure
2 shows the 264h test results at 1sun, 75°C. The boron-doped cell degrades to a
maximum of 2.3 per cent at 8 hours and then recovers to a stable value of 1.3
per cent at 96 hours. The degradation value of gallium-doped cells is basically
stable at 96 hours, which is 1.2 per cent, and then slowly degraded to 1.3 per
cent (216 hours) and then recovered slightly.
Figure 2:
×10suns, >100°C: The LeTID process can be
accelerated by adopting ×10suns, >100°C. The
test results of gallium-doped PERC cells under this method are shown in Figure 3.
Using this test method, the gallium-doped cell also experienced a process of
first degrading and then returning to stability. The degradation reached the
maximum value of 1.05 per cent at 5 minute and began to stabilize at a fairly
low level of 0.3 per cent at 90 minutes.
Figure 3: Accelerated LID Test Results of Gallium-doped Bifacial PERC Cells
Related studies also support LONGi’s test results
Tine U. Naerland from Azizona State University (along with other
researchers) studied the minority carrier lifetime degradation of indium-doped,
gallium-doped and boron-doped silicon wafers without impurities at room
temperature 25°C, as shown in Figure 4.
It can be seen that the minority carrier lifetime of gallium-doped
silicon wafers basically maintains a constant value of about 300μs after 104s
light exposure, while those of boron-doped and indium-doped silicon wafers
degrade continuously and greatly. Therefore, under low-temperature light
conditions, the gallium-doped silicon wafer is relatively stable and basically
has no degradation. However, in the case of actual outdoor exposure, the
working temperature of the cell will exceed 60°C, and the gallium-doped cell
will also have a certain degree of LeTID under the action of temperature. Her
research clearly supplements LONGi’s test results of the LID of gallium-doped
PERC cells and regenerated boron-doped PERC cells at different temperatures.
Another related research has been made by Nicholas Grant and John
Murphy from the University of Warwick who recently studied the viability of
indium doping and found that its relatively deep acceptor level limits its
potential. “Gallium doped silicon has demonstrated very stable and high
lifetimes when subject to extended illumination. There have also not been any
known detrimental recombination active defects,” said Grant in a recent
interaction with a leading solar industry journal. The application of
gallium-doped silicon wafers can effectively mitigate the initial LID from
which cells using boron-doped p-type silicon wafers have long suffered. Hence,
gallium-doped silicon does not require the additional stabilization steps used
to mitigate degradation, unlike the boron-doped status quo. The average
efficiency of gallium-doped cells is 0.09% higher than that of boron-doped cells.
“My team performed stabilization testing and no significant
degradation of the PERC solar cells utilizing gallium-doped silicon substrate
was observed,” he said. “In contrast, we did observe significant degradation
for an equivalent PERC solar cell with a boron-doped silicon substrate under
the same experimental conditions.”
The way forward
LONGi’s testing of gallium-doped PERC cells at different
temperatures clearly reflects that compared with boron-doped cells,
gallium-doped cells show significantly lower degradation and is supported by credible
and detailed literature. This further supports the conclusion in one of its
previous white papers that the use of gallium-doped silicon wafers can solve
the LID problem of PERC technology.
Due to its established potential benefits, LONGi has increased its
efforts towards the production of gallium-doped silicon wafers. Over the past
six months, we have acquired licenses from Shin-Etsu Chemical to manufacture
gallium-based technologies. Shin-Etsu is considered a pioneer of gallium-doped
silicon growth. Technical team at LONGi has been able to solve the problem of
high cost of gallium-doped silicon through in-house technological innovation. Significant improvement on productivity and
performance on Ga doping process has enabled LONGi to deliver Ga doped wafers
at similar price as B doped wafers.
Going forward, LONGi’s R&D team
aims to continue studying the characteristics of gallium-doped silicon in order
to obtain a reasonable resistivity range and higher doping accuracy to improve
the gallium-doping process, from both a cost and quality standpoint. We believe
that with the reduction in degradation, decrease in cost and increase in
reliability, gallium doped silicon solar modules will be more cost-effective in
the future practical application and
will bring better value to our customers.