NCPV preprints for the 2. world conference on photovoltaic solar energy conversion Page: 10 of 144
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IMPROVED PERFORMANCE OF SELF-ALIGNED, SELECTIVE-EMnTER SILICON SOLAR CELLS
D. S. Ruby, P. Yang', S. Zaidi, S. Brueck, M. Roy' and S. Narayanan'
'SandaNational Laboratories, Albuquerque, NM 87185-0752 USA
2University of New Mexico, Albuquerque, NM 87106 USA
3Solarex (a business unit ofAmoco/Enron Solar), Frederick, MD 21701 USA
Tel.: 505-844-0317, Fax 505-844-6541, email: email@example.com
ABSTRACT: We improved a self-aligned emitter etchback technique that requires only a single emitter diffusion and
no alignments to form self-aligned, patterned-emitter profiles. Standard commercial screen-printed gridlines mask a
plasma-etchback of the emitter. A subsequent PECVD-nitride deposition provides good surface and bulk passivation
and an antireflection coating. We used full-size multicrystalline silicon (mc-Si) cells processed in a commercial
production line and performed a statistically designed multiparameter experiment to optimize the use of a
hydrogenation treatment to increase performance. We obtained an improvement of almost a full percentage point in
cell efficiency when the self-aligned emitter etchback was combined with an optimized 3-step PECVD-nitride surface
passivation and hydrogenation treatment We also investigated the inclusion of a plasma-etching process that results in
a low-reflectance, textured surface on multicrystalline silicon cells. Preliminary results indicate reflectance can be
significantly reduced without etching away the emitter diffusion.
Keywords: Passivation-1: Silicon-Nitride-2: Texturisation- 3
The purpose of our work is to improve the performance
of standard commercial screen-printed solar cells by
incorporating high-efficiency design features without
incurring a disproportionate increase in process complexity
or cost. Our approach uses plasma processing to replace the
heavily doped homogenous emitter and non-passivating
antireflection coating (ARC) with a high-performance
selectively patterned diffusion covered with a passivating
ARC. A slight variation of the plasma step can effectively
texture even multicrystalline silicon (mc-Si) surfaces to
significantly reduce front surface reflectance.
1.1 Passivated, Patterned Emitter
Plasma-enhanced chemical vapor deposition (PECVD)
is now recognized as a perfonnance-enhancing technique
that can provide both surface passivation and an effective
ARC layer . For some solar-grade silicon materials, it
has been observed that the PECVD process results in the
improvement of bulk minority-carrier diffusion lengths as
well, presumably due to bulk defect passivation .
In order to gain the full benefit from improved emitter
surface passivation on cell performance, it is necessary to
tailor the emitter doping profile.so that the emitter is lightly
doped between the gridlines, but heavily doped under them
. This is especially true for screen-printed gridlines,
which require very heavy doping beneath them for
acceptably low contact resistance. This selectively
patterned emitter doping profile has historically been
obtained by using expensive photolithographic or screen-
printed alignment techniques and multiple high.
temperature diffusion steps [3,4].
We have attempted to build on a self-aligned emitter
etchback technique described by Spectrolab that requires
only a single emitter diffusion and no alignments .
Sandia is a multiprogram laboratory operated by Sandia
Corporation, a Lockheed Martin Company, for the U.S.
Department of Energy under Contract DE-AC04-
Reactive ion etching (RIE) using SF6 etches back the
emitter but leaves the gridlines and emitter regions beneath
them unetched. This removes the heavily diffused region
and any gettered impurities between gridlines while leaving
the heavily doped regions under the metal for reduced
contact resistance and recombination. This leaves a low-
recombination emitter between gridlines that requires good
surface passivation for improved cell performance.
Therefore, we follow the etchback with a surface-
passivating PECVD-nitride layer. The nitride also provides
a good ARC and can be combined with plasma-
hydrogenation treatments for bulk defect passivation.
1.2 Textured, Low-Reflectance Emitter
Several groups have reported interest in plasma-etching
techniques to texture Mc-Si cells, because mc-Si cannot
benefit sufficiently from the anisotropic etches typically
used for single-crystal Si In contrast to laser or
mechanical texturing, plasma-etching textures the entire
cell at once, which is necessary for high-throughput.
Inomata et al. used C1rbased RIE on mc-Si to fabricate a
17.1% efficient cell, showing that plasma-texturing does
not resultinperformance-imiting surface damage .
We developed a variation of the SF6 emitter etchback
process, which results in good surface texturing. Use of
SF6 keeps the process compatible with the metal gridlines.
This allows the texturing to be done after the metallization
step as part of the emitter-etchback process.
2. EXPERIMENTAL PROCEDURE
The textured, self-aligned selective-emitter (SASE)
plasma-etchback and passivation process is shown in
Figure 1. The SASE concept uses cells that have received
standard production-line processing through the printing
and firing of the gridlines. Then the cells undergo reactive
ion etching (RE) to first texture and etch away the most
heavily-doped part of the emitters in the regions between
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NCPV preprints for the 2. world conference on photovoltaic solar energy conversion, article, September 1, 1998; Golden, Colorado. (digital.library.unt.edu/ark:/67531/metadc707815/m1/10/: accessed December 16, 2018), University of North Texas Libraries, Digital Library, digital.library.unt.edu; crediting UNT Libraries Government Documents Department.