Diffractive Optics for Thin-Film Silicon Solar Cells

129 Pages·2017·5.498 MB·English

by  Christian Stefano Schuster (auth.)

Springer Theses
Recognizing Outstanding Ph.D. Research
Christian Stefano Schuster

Diffractive Optics
for Thin-Film
Silicon Solar Cells
Springer Theses
Recognizing Outstanding Ph.D. Research
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Christian Stefano Schuster
Diffractive Optics
for Thin-Film Silicon
Solar Cells
Doctoral Thesis accepted by
the University of York, UK
123
Author Supervisor
Dr. Christian StefanoSchuster Prof. ThomasF. Krauss
Department ofPhysics Department ofPhysics
University of York University of York
York York
UK UK
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-319-44277-8 ISBN978-3-319-44278-5 (eBook)
DOI 10.1007/978-3-319-44278-5
LibraryofCongressControlNumber:2016947936
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It’s never as good as it seems,
and it’s never better.
George Bernard Shaw
To the photon
that entangled me with the love of my wife
’
Supervisor s Foreword
Humility is a great skill for a supervisor, especially when faced with such a gifted
student as Christian Schuster. As supervisors, we have to accept that our students
will surpass us and that we will eventually learn more from them than they will
from us. By accepting this sometimes uncomfortable truth, however, we will pro-
gress much further than by insisting on our own superiority. Christian Schuster is
certainly one of the students who have taught me a great deal during their Ph.D.
project.
Havingacceptedthisandabandonedtheroleofthealmightyteacher,wecanadd
more value to our students by channelling their enthusiasm and by helping them
filtertheirideas,especiallyiftheyhavesuchabundantenergyasChristianSchuster.
Hewouldbounceintomyofficeallexcitedaboutsomenewidea,andmyanalytical
skillsrequiredtodissecttheproblemwerechallengedmorethanonce.Isometimes
feltlikemybrainmightexplode!Duringthesediscussions,hesometimesfilledthe
entire whiteboard with equations and point out where some assumptions made in
theliteraturemightnotapply,andsuggestedwaysofimprovingthem.Imightpoint
outtheflawsinhisownargument,butthiswouldnotstophimcomingbackwitha
better solution the next day. It was an interesting journey!
Christian’s project started in August 2012 as part of the Marie Curie Training
network “PROPHET” (Postgraduate Research on Photonics as an Enabling
Technology) which was led by the University of Cork (Ireland). We led the
Workpackage 2, “Photonics Enabling Energy Applications”, together with the
Universities of Pavia (Italy), Catania (Italy) and Lancaster (UK). Our goal was to
develop novel photonic nanostructures based on dielectric materials and to
implementthemindifferentsolarcellgeometries.Thisworkwascomplementedby
the work at Catania on plasmonic materials and was supported by the theoretical
and characterisation capabilities at Pavia.
Importantly, Christianspent two months onsecondmentatST Microelectronics
and 3SUN, Catania (Italy), where he gained valuable insights into the manufac-
turingprocessofrealsolarcellpanels.HealsospentonemonthatPaviainorderto
ix
x Supervisor’sForeword
learn more about modelling and simulation and in return, hosted the visit of one
of the Pavia students, Piotr Kowalczewski, in our cleanroom.
Overall, I believe that PROPHET provided a most valuable environment for a
Ph.D. student, which was further enhanced by the many workshops and meetings
we organised throughout the project.
Regarding the content of the thesis, I would like to highlight three areas where
Christian made original contributions, the diversity of topics highlighting his out-
standing ability as a scientist and as an engineer. The thesis provides sufficient
detailtoallowotherstofollowthework,buildonitandtherebyhopefullyenhance
their own projects.
Original Fabrication. He invented a very simple technique for releasing a thin
film of amorphous silicon from its original carrier and transferring it onto another
substrate. Importantly, the technique could be applied to patterned substrates, thus
allowing us to pattern the film from both sides, which increases the degrees of
freedom available for applying nanostructures to the light-trapping problem.
Unsurprisingly, this idea has since been taken up and developed further by a
number of others in the field.
Another technological development that the thesis reports very nicely is the
designandrealisationofasimpletoolfornanoimprintlithography.Nanoimprintis
a topic we considered seriously throughout the project, as it is essential for trans-
ferring any nanostructure onto a solar cell in an industrial process. The tool that
Christian developed is beautifully simple; by using a commercial high-power
UV-LED as the light source that can be bought for a few tens of euros, the tool is
accessibletoalargerangeofresearchgroupswhomaywanttocopythedesignfor
their own purposes.
Critical evaluation. While working on our own light-trapping geometries, it
occurred to Christian that there was no objective figure of merit to describe the
light-trapping efficiency; different groups tended to provide different ways of
quantifying the performance of their respective structures, which led to a myriad
ofclaimsintheliteratureandwhichmadeitverydifficulttocomparedifferenttypes
oflight-trappingstructuresandunderstandwhichwasthebestone.Furthermore,it
became clear that authors made different assumptions when describing the per-
formance of their structures or that they used different models, which made it
difficult to objectively compare results. By defining the “Light-trapping Efficiency
(LTE)”, Christian managed to bring order into this very diverse field and estab-
lished a new benchmark for any future structure, as well as assessing more than
50 different structures reported in the literature. In many cases, he recalculated the
projected performance of a given structure based on the data provided in order to
ensure that all results were treated with the same model assumptions.
Experimental confirmation. During the project, there was a lot of discussion
aboutplasmonic(i.e.metallic)nanostructuresandthemanybenefitstheyofferedfor
light trapping; many high-profile papers had been written about the beneficial
scattering properties of plasmonic structures. Unfortunately, we could not find a
single paper that clearly separated the beneficial scattering properties form their
detrimental absorption losses, and there was a lot of hand-waving in the
Supervisor’sForeword xi
community. We therefore conceived a simple experiment that was designed to put
the question to bed once and for all; by comparing dielectric and plasmonic
nanostructures, both amongst the best that had been reported in the literature, and
applying them to identical amorphous silicon layers, we clearly demonstrated that
there was excessive absorption loss in the long-wavelength range in plasmonics,
morethanpeoplehadexpected.Christianunderpinnedtheexperimentalworkwith
a beautifully simple model that he derived from first principles and that clearly
showed the reason for the high observed losses, namely the multipass nature of
trapped light in a thin film.
Atthebeginningofthejourney,theworldfirmlybelievedinthebenefitoflight
trappingformakingsolarcellscheaper,aslighttrappingwouldreducethethickness
andthereforethematerialrequiredformakingasolarcell.Unfortunately,theworld
changed during the course of the project, and silicon prices dropped dramatically,
such that the material argument no longer applied. Nevertheless, many of the
insights and techniques developed here naturally also apply to other types of solar
cells,suchasthenewlyemergingperovskites;tandemstructuresarealsocomingto
thefore, which will benefitgreatlyfromthelightmanagement strategies discussed
here.
Overall, the reader interested in light trapping applied to thin-film solar cells of
any kind, its conceptual and experimental realisation, will find this thesis a rich
source of information and inspiration.
York, UK Prof. Thomas F. Krauss
July 2016