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Review of Back Contact Silicon Solar Cells for low-cost Application
David D. Smith
Sandia National Laboratories, Albuquerque, NM 90 \g
Motivation AVG 1 1999
Back contact solar cells hold significant promise for increased performance in
photovoltaics for the near future. Two major advantages which these cells possess are a
lack of grid shading loss and coplanar interconnection. Front contacted cells can have up
to 10 % shading loss when using screen printed metal grids. A front contact cell must
also use solder connections which run from the front of one cell to the back of the next
for series interconnection. This procedure is more difficult to automate than the case of
co-planar contacts.
The back contact cell design is not a recent concept. The earliest silicon solar cell
developed by Bell Labs was a back contact device[l]. There have been many design
modifications to the basic concept over the years. To name a few, there is the
Interdigitated Back Contact (IBC) cell [2], the Stanford Point contact solar cell [3], the
Emitter Wrap Through (EWT)[4,5], and its many variations [6,7,8,9,10]. A number of
these design concepts have demonstrated high efficiency. The SunPower back contact
solar cell holds the efficiency record for silicon concentrator cells[11]. The challenge is
to produce a high efficiency cell at low cost using high throughput techniques. This has
yet to be achieved with a back contact cell design.
The focus of this paper will be to review the relevant features of back contact
cells and progress made toward the goal of a low cost version of this device.
Design classifications
All of the back contact cells developed to date fall into two categories, which can
be referred to as the IBC cell and the EWT cell. IBC includes all of those cell designs
which rely upon carrier collection at a rear junction alone. The EWT class of cells can
accomplish carrier collection at both sides and relies upon current conduction from the
front to the back through some sort of perforation in the cell.
The IBC cell must be fabricated on material with a long minority carrier diffusion
length. The distance from any point in the cell to the junction must be much less than the
diffusion length. This results from two- or three-dimensional carrier collection in the
IBC cell. If the junction is present along a finger pattern at the back, then the current
collection is two dimensional. If the junction is formed at points on the back surface, as
in the case of the Stanford Point Contact cell, then the collection is three dimensional.
This imposes a restriction on gridline geometry requiring fine lines and tight tolerances.
As material quality decreases, the restrictions on grid geometry become more severe.
Another requirement of the IBC type cell is excellent front surface passivation,
since the junction is on the back and most photogeneration occurs at the front. This
surface passivation must remain stable as well.
The IBC cell has the advantage of allowing the rear junction to be optimized for
electrical performance, namely a low Jo junction. The front surface can be optimized for
optical performance[12]. Reduced junction area has the potential to give very low totalI
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Smith, David D. Review of Back Contact Silicon Solar Cells for Low-Cost Application, article, August 4, 1999; Albuquerque, New Mexico. (https://digital.library.unt.edu/ark:/67531/metadc792114/m1/1/: accessed April 18, 2024), University of North Texas Libraries, UNT Digital Library, https://digital.library.unt.edu; crediting UNT Libraries Government Documents Department.