Yan, Di
Description
While heavily doped regions are an integral part of conventional
silicon solar cells for carrier separation and selective
transport, they also introduce deleterious effects that constrain
their performance. This thesis addresses the fundamental
limitations imposed by heavy doping in silicon solar cells and
explores the potential of utilising doping in a manner that
circumvents them. In particular, selectively doped regions and
novel carrier-selective...[Show more] passivating contacts are developed.
In heavily doped silicon, energy band gap narrowing (BGN) occurs.
Subsequently, the pn product in equilibrium increases, which
tends to limit the performance of silicon solar cells. In
principle, heavily doped silicon should be analyzed using
Fermi-Dirac statistics coupled with a BGN model. Nevertheless,
applying Schenk’s theoretical BGN model to experimental samples
underestimates their corresponding recombination current
parameters. Based on a large number of samples and using
well-proven models for Auger recombination and carrier mobility,
updated empirical expressions for the BGN are derived here for
both n-type and p-type silicon. The study confirms that the BGN
values in p-type silicon are slightly larger than in n-type
silicon. Both updated BGN models contribute to a more complete
understanding of the losses and ramifications caused by heavily
doped regions in silicon solar cells.
A possible approach to reduce the impact of heavily doped regions
is to reduce the area they occupy, restricting it to underneath
the metal contacts. In this thesis a process to implement such
selectively doped (SD) silicon solar cells is developed. The
process, based on a controlled etch-back of the diffused region
offers the advantage of being self-aligned, which avoids the
critical mask alignment step. The TMAH based etch-back solution
used here provides a well-controlled etching rate, uniformly
etched surface and selectivity to metallic layers (Al or Ag). The
poof-of-concept SD silicon solar cells have reached a conversion
efficiency of 17.5%.
Inspired by silicon heterojunction solar cells and polysilicon
emitter BJT, carrier-selective passivating contacts based on
heavily doped silicon films are developed. Two approaches of
depositing intrinsic polysilicon or amorphous silicon are
demonstrated. In both cases, a thermal diffusion process is used
to dope and recrystallise the films. The diffusion processes, the
intrinsic silicon film thickness, and the interfacial layer
conditions are optimized in terms of the trade-off between the
recombination current parameter and the contact resistivity. The
interfacial layer is found to be critical for blocking the
penetration of dopants, for stopping the epitaxial regrowth
between the top silicon film and the underlying crystalline
silicon, as well as for enhancing the response to a subsequent
hydrogenation treatment. Examination of these carrier-selective
passivating contact structures before and after the thermal
diffusion processes by using XRD, micro-PL and XPS indicates that
allowing a moderate level of phosphorus into the silicon oxide
and into the silicon substrate is necessary to achieve a low
recombination and a low contact resistivity. The
electron-selective passivating contacts have been successfully
implemented at the rear of n-type silicon solar cells, showing
open circuit voltage of > 672 mV and efficiency values up to
20.8%.
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