Progress on research in high-efficiency silicon solar cells is presented. Topics addressed include: recombination measurement techniques, doped crystals, minority carrier transport, oxygen process in solar cells, solar cell models, loss mechanics in solar cells, high-efficiency metal insulators, dendritic web cells, and surface and bulk loss reduction by low-energy hydrogen doping

Burger, D. R.

English

NASA Technical Reports Server

tHIGH-EFFICIENCY DEVICE RESEARC 8 6 - 2 9 8 6 9
|. Dale R. Burger, Chairman
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Progress reports on research in high-efficiency silicon solar cells were
• p_esented by II contractors. The presentations covered the issues of: heavy
doping, bulk and surface recombination, and cell modelling.
The University of Florida's theoretical work on heavily doped silicon
was described. Heavily doped polysilicon was used as a back-surface passivant
replacing the usual back-surface field (BSF). Very good first results were
achieved and there is the promise of a simple, low temperature deposition
process. Short-circuit current-decay measurement methods were also covered.
A survey of bulk recombination measurement techniques was presented by
the University of Pennsylvania. Classical methods were reviewed along with
their limiting assumptions and simplifications. A moduated light measurement
system was built and showed the large effects of junction capacitance.
Techniques for extension of classical methods for measurement of multiparameter i
: multlregression measurements were identified and analyzed.
Measurement of minor_ty carrier transport parameters in heavily doped
silicon was covered by Stanford University. The basic transport equations
were used to define two independent parame* rs. Use of special vertical and
lateral transistor test devices permitted the measurement of both parameters.
Prior studies were normalized to show excellent agreement over the heavy
doping region. I
The State University of New York at Albany presentation featured oxygen I
and carbon related defects, both native and process-induced. A s,_Inary of
oxygen processes in silicon versus process temperature was shown along with i
experimental results. The anamolous diffusion of oxygen was explained by the
dissociation of the (V.O) center allowing 0i to move through the lattice. |
Modelling and correlation studies of solar cells was discussed by the
Research Triangle Institute. Recursive relationships were used to generate
solutions at a number of mesh points within the emitter region. Photoexcited
hole concentration and built-in electric field were calculated as a function
of position. Simulated and experimentally determined I-V curves were shown to
have good fit.
C. T. Sah Associates discussed loss mechanisms in high-efficiency solar
cells. Fundamental limitations and practical solutions were stressed. Present
cell efficiency is limited by many recombination sites: emitter, base,
contacts, and oxide/silicon interface. Use of polysilicon passivatiou was
suggested. After reduction of these losses, a 25% efficient cell could be
built. A floating emitter cell design was shown that had the potential of low
-'_i recombination losses, i
Fabrication and characterization of high-efficiency metal insulator, i
i-- _ n-p (MINP) cells was described by the University of Washington Joint Center
! for Graduate Studies. Particular attention was paid to development of
1986019875-302
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HIGH-EFFICIENCY DEVICE RESEARCH
measurement methods for surface recombination and density of surface states.
A modified Rosier test structure has been used successfully for density of
surface states. Silicon oxide and silicon nitride passivants were studied.
Heat treatment after plasma enhanced chemical vapor deposition (CVD) of
silicon nitride was shown to be beneficial. A more optimum emitter •
concentration profile was modelled.
Westinghouse Electric Corporation's Research and Development Center
showed their work on high-efficiency dendritic web cells. The influence of
twin planes and heat treatment on the location and effect of trace impurities
was of particular interest. Proper heat treatment often increases efficiency
• by causing impurities to pile up at twin planes. Oxide passivation had a
beneficial effect on efficiency. A very efficienct antireflective (AR)
coating of zinc selenide and magnesium fluoride was designed and fabricated.
An aluminu_ back-surface reflector was also effective.
Pennsylvania State University examined surface and bulk loss reduction
by low-energy hydrogen doping. Hydrogen ions provided a suppression of space
charge recombination currents. Implantation of hydrogen followed by the Spire
anneal cycle caused more redistribution of boron than the Spire anneal which i
could complicate processing,
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1986019875-303


High-efficiency device research

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