Electronic Characterisation of Earth‐Abundant Sulphides for Solar Photovoltaics

385 Pages·2018·20.93 MB·English

by  Thomas James Whittles

Springer Theses
Recognizing Outstanding Ph.D. Research
Thomas James Whittles
Electronic
Characterisation
of Earth-Abundant
Sulphides for Solar
Photovoltaics
Springer Theses
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Thomas James Whittles
Electronic Characterisation
‐
of Earth Abundant Sulphides
for Solar Photovoltaics
Doctoral Thesis accepted by
the University of Liverpool, Liverpool, UK
123
Author Supervisor
Dr. ThomasJames Whittles Dr. V.R.Dhanak
Department ofPhysics andStephenson University of Liverpool
Institute for RenewableEnergy Liverpool
University of Liverpool UK
Liverpool
UK
ISSN 2190-5053 ISSN 2190-5061 (electronic)
SpringerTheses
ISBN978-3-319-91664-4 ISBN978-3-319-91665-1 (eBook)
https://doi.org/10.1007/978-3-319-91665-1
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’
Supervisor s Foreword
Thegenerationanduseofenergycontinuetoincreasebecauseofhumanpopulation
increaseandtechnological advancement,andbytheyear2050,theworld’senergy
need is projected to be about 30 TW per year. More than 80% of this currently
comes from burning fossil fuels, which has led to adverse environmental and
climate changes. The case for alternative renewable energy sources is therefore
clear and while all renewable resources (solar, wind, hydroelectric, biomass, and
geothermal)playanimportantrole,onlysolarcanmeetthislevelofdemand,since
thesuncontinuouslyprovidesmore energy totheearth inonehourthan theworld
that consumes in an entire year. Currently, commercial solar cell technologies are
based on silicon, cadmium telluride (CdTe), or copper indium gallium diselenide
(CIGS) as the photovoltaic (PV) solar absorbers. While the latter two are thin-film
technologies that are cheaper than silicon, there are serious concerns about the
long-term sustainability and toxicity of some of the elements used in these thin
films. There is thus justifiable search for new solar cell materials using
earth-abundant, cheap, and environmentally friendly elements. The search for a
suitable solar absorber combines sophisticated computational screening together
with experimental characterisation of the electronic structure of the material.
Efficiency losses in solar cells can occur because of an inappropriate bandgap or
because of band misalignments with the other layers within the cell; therefore,
studying band alignments in heterojunctions is of vital importance. The bandgap,
position of the band levels, and optical absorption characteristics are important
properties for solar absorbers. Determining the band structures of materials can
allow relationships between the energy levels to be drawn, leading to an under-
standingofhowelectronicorbitalsofthedifferentatomshybridiseandgiveriseto
the electronic band positions. This also allows the bandgap and band alignment to
be determined. Also, from a solar cell device perspective it is important to deter-
mine the effect that imperfections have on the absorber material. These imperfec-
tions can manifest as unwanted material made of the constituent elements of the
absorber, but in a different phase, or with some form of external contaminant such
asoxygenorcarbon;thus,electroniccharacterisationisextremelyimportantinthis
search for newPVmaterials.TheworkofThomas Whittles presentedinthisbook
v
vi Supervisor’sForeword
demonstrates the use of electronic characterisation, combined with theoretical
insights into the origin of electronic levels in some of these earth-abundant PV
materials, and crucially identifies the important parameters required in the search
for new materials. The methods and procedures that are shown are not confined to
PV materials, but are general tothe electronic characterisation of any materials for
devices.
Liverpool, UK Dr. V. R. Dhanak
May 2018
Abstract
This thesis explores the electronic characterisation of materials for use as absorber
layers within photovoltaic solar cells: an attractive solution to the energy crisis.
Primarily, XPS was used to characterise CuSbS , Cu BiS , SnS, and Cu ZnSnS .
2 3 3 2 4
Allofthesematerialscanbeclassifiedasearth-abundant:animportantfactorwhen
considering materials that are both readily available and environmentally friendly.
To varying degrees, all of these materials are established as potential absorber
layers, with reports of successful, albeit low-efficiency devices. Also, the literature
isoftenscantwithregardstomorefundamentalcharacterisationofthesematerials,
specifically in terms of how the underlying electronic structure affects properties
that are pertinent to solar cells. Where XPS is utilised, it often lacks rigour and is
obfuscated by the complexities of the spectra.
Workisthenpresentedwithtwoaims.First,touseacombinationofhigh-quality
XPS measurements and density of states calculations, in order to give insight into
theelectronicstructureofthematerials.Second,topresentthefullpotentialofXPS,
as applied to solar absorbers, and how this technique complements others that are
used for characterisation.
Withoutexception,eachmaterialdemonstratednaturalbandpositionsthatmake
them unsuitable for use with established solar cell technologies. That is, a low IP
(4.71–5.28 eV), resulting in a large CBO (0.5–0.85 eV) with CdS, for example.
This offers some explanation to the poor efficiencies. These properties were found
to be a consequence of the bonding nature of the valence and conduction bands,
differing to the conventional absorber materials because of the influence of
lone-pair electrons or other second-cation states.
The power of XPS was demonstrated for application to absorber materials. The
presence, effects, and formation pathways of contamination of the materials were
elucidated using XPS, showing how the presence of such can act adversely to the
performance of cells, especially with regards to oxidation, and also how these
factorshavebeenoverlookedinthepast.Whencoupledwithothercomplementary
techniques, it has the ability to aid in the phase identification of a grown material
andalsotohelpdeterminethepresenceofunwantedphases,whichcausedetriment
vii
viii Abstract
to the device. For these applications, the methods, fitting procedures, and analysis
considerations detailed in this thesis should be followed.
Researchinterestinthesematerialsshouldbemaintainedbasedonthefindings,
with new approaches to cell design being aided by the characterisation methods
developed here.
Publications and Presentations
Publications Directly Associated with this Thesis
(cid:129) Peccerillo, E.; Major, J.; Phillips, L.; Treharne, R.; Whittles, T. J.; Dhanak, V.;
Halliday, D.; Durose, K. Characterization of Sulfurized CuSbS Thin Films for
2
PV Applications. In 2014 IEEE 40th Photovoltaic Specialist Conference
(PVSC); IEEE, 2014; pp 0266–0269.
(cid:129) Burton,L.A.;Whittles,T.J.;Hesp,D.;Linhart,W.M.;Skelton,J.M.;Hou,B.;
Webster,R.F.;O’Dowd,G.;Reece,C.;Cherns,D.;Fermin,D.J.;Veal,T.D.;
Dhanak, V. R.; Walsh, A. Electronic and Optical Properties of Single Crystal
SnS : An Earth-Abundant Disulfide Photocatalyst. J. Mater. Chem. A 2016, 4
2
(4), 1312–1318.
(cid:129) Whittles,T.J.;Burton,L.A.;Skelton,J.M.;Walsh,A.;Veal,T.D.;Dhanak,V.
R.BandAlignments,ValenceBands,andCoreLevelsintheTinSulfidesSnS,SnS ,
2
andSn S :ExperimentandTheory.Chem.Mater.2016,28(11),3718–3726.
2 3
(cid:129) Gupta, S.; Whittles, T. J.; Batra, Y.; Satsangi, V.; Krishnamurthy, S.; Dhanak,
V. R.; Mehta, B. R. A Low-Cost, Sulfurization Free Approach to Control
Optical and Electronic Properties of Cu ZnSnS via Precursor Variation. Sol.
2 4
Energy Mater. Sol. Cells 2016, 157, 820–830.
(cid:129) Whittles, T. J.; Veal, T. D.; Savory, C. N.; Welch, A. W.; de Souza Lucas, F.
W.; Gibbon, J. T.; Birkett, M.; Potter, R. J.; Scanlon, D. O.; Zakutayev, A.;
Dhanak, V. R. Core-Levels, Band Alignments and Valence Band States in
CuSbS . Manuscript in Preparation 2017.
2
(cid:129) Whittles,T.J.;Veal,T.D.;Yates,P.;Savory,C.N.;Scanlon,D.O.;Major,J.D.;
Durose, K.; Dhanak, V. R. Core-levels, Band Alignments and Valence Band
StatesinCu BiS foritsuseasaPVAbsorber.ManuscriptinPreparation2017.
3 3
Presentations Directly Associated with this Thesis
(cid:129) EUPVSEC 2014, Amsterdam NL
Poster: Tin Sulfide for Low-Cost Solar Cells
(cid:129) Postgraduate Poster Day 2015, University of Liverpool UK
Poster: Earth Abundant Materials for Solar Cells
ix