TwinPeak Passivated Emitter Rear Cell Technology

As part of the roadmap to improve the efficiency of its cells, REC has
developed a new cell structure that combines the cost-effective basis
of a polysilicon platform with the latest technical advances to compete
strongly with mono p-type and n-type products on the market.
Offering an increase in power output from a polycristalline cell, one
of the key technologies in solar panels achieving this high level of
performance is the passivation of the rear side of the cell.
What is PERC technology?
Based on a change in the design of the rear of the cell which improves
the capture of light falling on its surface, REC has introduced PERC
technology (also known as backside passivation) into its cell production
process and been able to bring it to full production level.
In a conventional solar cell, there is an aluminum metallization layer
which makes contact across the full area of the back of the cell. REC’s
PERC technology first coats the backside of the cell with a special
dielectric layer that has tiny holes made by a laser. The aluminum
metallization is then applied on top of the dielectric layer and contacts
the silicon wafer only through the microscopic holes (fig. ).
How does PERC technology improve performance?
PERC technology increases the overall panel performance by increasing
a cell’s ability to capture light. A regular solar cell consists of two layers
of silicon with different electrical properties – known as the base and
the emier. A strong electrical field is generated where the two layers
meet, which pulls negatively charged particles (electrons) into the
emier when they reach this interface. The electrons are generated by
light entering the cell and releasing electrons from the silicon atoms.
Electrons travel freely through the cell and contribute to the electrical
current only if they are able to reach the interface (fig. .
Different wavelengths of light generate electrons at different levels of
the cell structure, shorter wavelengths (blue light) will generate more
electrons near the front of the cell, compared to longer wavelengths
(red light) which will generate electrons at the back of the cell or even
pass through the wafer without generating current.
The introduction of PERC technology increases the cell efficiency
through the dielectric layer that reflects back into the cell any light that
has passed through to the rear without generating electrons. Through
this reflection, the photons are essentially given a second chance to
generate current (fig. ).
The extra energy yield of cells with PERC technology is added to by
the improved ability to capture light at longer wavelengths, e.g., when
the sun is at an angle (early mornings and evenings) or under cloudy
conditions. At such times a higher quantity of blue light (wavelengths
between  to  nm) is absorbed by the atmosphere as it has
a longer path to travel to the Earth’s surface than when the Sun is
directly overhead. Blue light is generally converted to energy near the
top of the cell, whereas red light (wavelengths between  to  nm)
penetrates further through the cell and is converted to energy near the
boom. Red light is less easily absorbed by the Earth’s atmosphere
and as a result, cells which capture more red light are generally more
powerful (fig. ). The ‘reflective’ properties of the PERC technology
ensure increased absorption of red light, even in weak or diffuse light
conditions, delivering beer energy yields.
Wavelengths above  nm are not absorbed by the silicon wafer.
Instead, in standard cells, such wavelengths are merely absorbed
into the backside metallization, generating heat which increases the
temperature of the cell and reduces its conversion efficiency. As the
PERC layer reflects this light back through the cell and out of the panel,
it reduces the amount of absorption by the aluminum metallization
layer and therefore heat build up internal to the cell. This reduction
in absorption helps the cell to work at a cooler temperature and has a
positive effect on energy yield.
Maximizing cell performance
How REC’s use of Passivated Emier Rear Cell technology improves the
capture of light and optimizes cell performance
Light is absorbed by the
aluminum metallization.
Fig. : A cell with PERC technology will generate more current due to the reflection of light
at the backside of the cell.
Reflected light will generate
additional current.
Dielectric
layer
Small metal
contacts
CONVENTIONAL CELL
PERC CELL
Light Light
Figure : Working principle of a silicon solar cell.
Front side
metallization
emier
Light
Silicon wafer
(base)
Aluminum
metallization
. If an electron reaches the interface
between the emier and the base, it
is pulled into the emier, creating a
voltage difference over the cell
. The electrons can move
freely through the silicon wafer
. Light peels off electrons
from silicon atoms
REC has introduced an innovative cell design into production that includes Passivated Emier Rear Cell
technology (PERC). This technology has been developed for use on a polycrystalline platform by REC and is one
of the crucial steps in allowing the production of polysilicon cells with average efficiencies of above  percent.
Figure : PERC technology improves the internal reflection of light at long wavelengths.
Wavelength (nm)
    





Reflectance

Conventional cell
Cell with PERC technology
Fig. : The structure of a conventional cell (l) compared to a cell with PERC technology (r)
CONVENTIONAL CELL PERC CELL
Dielectric
PERC layer
Small metal
contacts
Base layer (silicon wafer) Base layer (silicon wafer)
Emier layerEmier layer
Aluminum metallization Aluminum metallization