I09-Surface and Interface Structural Analysis
|
Huw
Shiel
,
Oliver S.
Hutter
,
Laurie J.
Phillips
,
Jack E. N.
Swallow
,
Leanne A. H.
Jones
,
Thomas J.
Featherstone
,
Matthew J.
Smiles
,
Pardeep K.
Thakur
,
Tien-Lin
Lee
,
Vinod R.
Dhanak
,
Jonathan D.
Major
,
Tim D.
Veal
Diamond Proposal Number(s):
[23160]
Abstract: Sb2Se3 is a promising material for use in photovoltaics, but the optimum device structure has not yet been identified. This study provides band alignment measurements between Sb2Se3, identical to that used in high-efficiency photovoltaic devices, and its two most commonly used window layers, namely, CdS and TiO2. Band alignments are measured via two different approaches: Anderson’s rule was used to predict an interface band alignment from measured natural band alignments, and the Kraut method was used in conjunction with hard X-ray photoemission spectroscopy to directly measure the band offsets at the interface. This allows examination of the effect of interface formation on the band alignments. The conduction band minimum (CBM) of TiO2 is found by the Kraut method to lie 0.82 eV below that of Sb2Se3, whereas the CdS CBM is only 0.01 eV below that of Sb2Se3. Furthermore, a significant difference is observed between the natural alignment- and Kraut method-determined offsets for TiO2/Sb2Se3, whereas there is little difference for CdS/Sb2Se3. Finally, these results are related to device performance, taking into consideration how these results may guide the future development of Sb2Se3 solar cells and providing a methodology that can be used to assess band alignments in device-relevant systems.
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Dec 2020
|
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I09-Surface and Interface Structural Analysis
|
Jack E. N.
Swallow
,
Christian
Vorwerk
,
Piero
Mazzolini
,
Patrick
Vogt
,
Oliver
Bierwagen
,
Alexander
Karg
,
Martin
Eickhoff
,
Jörg
Schörmann
,
Markus R.
Wagner
,
Joseph William
Roberts
,
Paul R.
Chalker
,
Matthew J.
Smiles
,
Philip
Murgatroyd
,
Sara
Mohamed
,
Zachary W.
Lebens-Higgins
,
Louis F. J.
Piper
,
Leanne A. H.
Jones
,
Pardeep K.
Thakur
,
Tien-Lin
Lee
,
Joel B.
Varley
,
Juergen
Furthmüller
,
Claudia
Draxl
,
Tim D.
Veal
,
Anna
Regoutz
Diamond Proposal Number(s):
[21430, 24670]
Abstract: The search for new wide band gap materials is intensifying to satisfy the need for more advanced and energy effcient power electronic devices. Ga2O3 has emerged as an alternative to SiC and GaN, sparking a renewed interest in its fundamental properties beyond the main β-phase. Here, three polymorphs of Ga2O3, α, β, and ε, are investigated using X-ray diffraction, X-ray photoelectron and absorption spectroscopy, and ab initio theoretical approaches to gain insights into their structure - electronic structure relationships. Valence and conduction electronic structure as well as semi-core and core states are probed, providing a complete picture of the influence of local coordination environments on the electronic structure. State-of-the-art electronic structure theory, including all-electron density functional theory and many-body perturbation theory, provide detailed understanding of the spectroscopic results. The calculated spectra provide very accurate descriptions of all experimental spectra and additionally illuminate the origin of observed spectral features. This work provides a strong basis for the exploration of the Ga2O3 polymorphs as materials at the heart of future electronic device generations.
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Sep 2020
|
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B18-Core EXAFS
I11-High Resolution Powder Diffraction
|
Jacinthe
Gamon
,
Arnaud J.
Perez
,
Leanne A. H.
Jones
,
Marco
Zanella
,
Luke M.
Daniels
,
Rhun E.
Morris
,
Chiu C.
Tang
,
Tim
Veal
,
Laurence J.
Hardwick
,
Matthew S
Dyer
,
John B.
Claridge
,
Matthew J.
Rosseinsky
Open Access
Abstract: Multiple anion materials are of particular interest for the discovery of new crystal structures and offer an original way to modulate physical properties, including energy storage materials with enhanced performances. Through careful synthesis optimization, a new Na2Fe2OS2 phase was prepared by two different routes: high temperature solid-state synthesis and simple mechanochemical synthesis. The long-range and local structure of Na2Fe2OS2 was studied by Rietveld refinement of neutron and X-ray diffraction data combined with EXAFS data refinement. The phase comprises an amorphous and a crystalline part which has an anti-K2NiF4 structure, corresponding to the n = 1 member of the homologous anti-Ruddlesden-Popper [AX][ABX3]n series. Its electrochemical properties as a cathode material were studied in Na half cells and Na-ion full cells, revealing that the material becomes fully amorphous upon initial desodiation to Na0.5Fe2OS2, but maintains a reversible capacity of 135 mAh·g-1 in full cells where up to 1.2 Na+ can be reversibly extracted and reinserted when compensating for the Na lost in SEI formation. The stability of the pristine material and its structural evolution upon charging are discussed, paving the way for further optimization of this material. Being composed exclusively of earth-abundant elements and stable under dry air, Na2Fe2OS2 perfectly illustrates the great opportunity of multiple anion chemistry to explore new structure types and develop better energy storage systems.
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Sep 2020
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I09-Surface and Interface Structural Analysis
|
Christopher H.
Don
,
Huw
Shiel
,
Theodore D. C.
Hobson
,
Christopher N.
Savory
,
Jack E. N.
Swallow
,
Matthew J.
Smiles
,
Leanne A. H.
Jones
,
Thomas J.
Featherstone
,
Pardeep K.
Thakur
,
Tien-Lin
Lee
,
Ken
Durose
,
Jonathan D.
Major
,
Vinod R.
Dhanak
,
David O.
Scanlon
,
Tim D.
Veal
Diamond Proposal Number(s):
[21431]
Open Access
Abstract: The presence of a lone pair of 5s electrons at the valence band maximum (VBM) of Sb2Se3 and the resulting band alignments are investigated using soft and hard X-ray photoemission spectroscopy in parallel with density functional theory (DFT) calculations. Vacuum-cleaved and exfoliated bulk crystals of Sb2Se3 are analysed using laboratory and synchrotron X-ray sources to acquire high resolution valence band spectra with both soft and hard X-rays. Utilising the photon-energy dependence of different orbital cross-sections and corresponding DFT calculations, the various orbital contributions to the valence band could be identified, including the 5s orbital's presence at the VBM. The ionization potential is also determined and places the VBM at 5.13 eV below the vacuum level, similar to other materials with 5s2 lone pairs, but far above those of related materials without lone pairs of electrons.
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Aug 2020
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I09-Surface and Interface Structural Analysis
|
Leanne A. H.
Jones
,
Wojciech M.
Linhart
,
Nicole
Fleck
,
Jack E. N.
Swallow
,
Philip A. E.
Murgatroyd
,
Huw
Shiel
,
Thomas J.
Featherstone
,
Matthew J.
Smiles
,
Pardeep K.
Thakur
,
Tien-lin
Lee
,
Laurence J.
Hardwick
,
Jonathan
Alaria
,
Frank
Jaeckel
,
Robert
Kudrawiec
,
Lee A.
Burton
,
Aron
Walsh
,
Jonathan M.
Skelton
,
Tim D.
Veal
,
Vin R.
Dhanak
Diamond Proposal Number(s):
[21431]
Open Access
Abstract: The effects of Sn
5
s
lone pairs in the different phases of Sn sulphides are investigated with photoreflectance, hard x-ray photoemission spectroscopy (HAXPES), and density functional theory. Due to the photon energy-dependence of the photoionization cross sections, at high photon energy, the Sn
5
s
orbital photoemission has increased intensity relative to that from other orbitals. This enables the Sn
5
s
state contribution at the top of the valence band in the different Sn-sulphides, SnS,
Sn
2
S
3
, and
SnS
2
, to be clearly identified. SnS and
Sn
2
S
3
contain Sn(II) cations and the corresponding Sn
5
s
lone pairs are at the valence band maximum (VBM), leading to
∼
1.0
–1.3 eV band gaps and relatively high VBM on an absolute energy scale. In contrast,
SnS
2
only contains Sn(IV) cations, no filled lone pairs, and therefore has a
∼
2.3
eV room-temperature band gap and much lower VBM compared with SnS and
Sn
2
S
3
. The direct band gaps of these materials at 20 K are found using photoreflectance to be 1.36, 1.08, and 2.47 eV for SnS,
Sn
2
S
3
, and
SnS
2
, respectively, which further highlights the effect of having the lone-pair states at the VBM. As well as elucidating the role of the Sn
5
s
lone pairs in determining the band gaps and band alignments of the family of Sn-sulphide compounds, this also highlights how HAXPES is an ideal method for probing the lone-pair contribution to the density of states of the emerging class of materials with
n
s
2
configuration.
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Jul 2020
|
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I09-Surface and Interface Structural Analysis
|
Theodore D. C.
Hobson
,
Laurie J
Phillips
,
Oliver S
Hutter
,
Huw
Shiel
,
Jack E. N.
Swallow
,
Christopher N.
Savory
,
Pabitra K
Nayak
,
Silvia
Mariotti
,
Bhaskar
Das
,
Leon
Bowen
,
Leanne A. H.
Jones
,
Thomas J.
Featherstone
,
Matthew J.
Smiles
,
Mark A
Farnworth
,
Guillaume
Zoppi
,
Pardeep K.
Thakur
,
Tien-lin
Lee
,
Henry J.
Snaith
,
Chris
Leighton
,
David O.
Scanlon
,
Vinod R.
Dhanak
,
Ken
Durose
,
Tim D.
Veal
,
Jonathan D
Major
Diamond Proposal Number(s):
[21431]
Open Access
Abstract: The carrier type of Sb2Se3 was evaluated for both thin films and bulk crystals via a range of complementary techniques. X-ray photoelectron spectroscopy (XPS), hot-probe, hall effect and surface photo-voltage spectroscopy showed material synthesized from Sb2Se3 granulate mate-rial to be n-type with chlorine identified as an unintentional n-type dopant via secondary ion mass spectrometry analysis. The validity of chlorine as a dopant was con-firmed by synthesis of intrinsic crystals from metallic precursors and subsequent n-type doping by the addition of MgCl2. Chlorine was also shown to be a substitutional n-type shallow dopant by density functional theory calculations. TiO2/Sb2Se3 n-n isotype heterojunction solar cells of 7.3% efficiency based are demonstrated with band alignment analyzed via XPS.
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Mar 2020
|
|
|
|
Philip A. E.
Murgatroyd
,
Matthew J.
Smiles
,
Christopher N.
Savory
,
Thomas P.
Shalvey
,
Jack E. N.
Swallow
,
Nicole
Fleck
,
Craig M.
Robertson
,
Frank
Jaeckel
,
Jonathan
Alaria
,
Jonathan D.
Major
,
David O.
Scanlon
,
Tim D.
Veal
Abstract: The van der Waals material GeSe is a potential solar absorber, but its optoelectronic properties are not yet fully understood. Here, through a combined theoretical and experimental approach, the optoelectronic and structural properties of GeSe are determined. A fundamental absorption onset of 1.30 eV is found at room temperature, close to the optimum value according to the Shockley-Queisser detailed balance limit, in contrast to previous reports of an indirect fundamental transition of 1.10 eV. The measured absorption spectra and first-principles joint density of states are mutually consistent, both exhibiting an additional distinct onset $\sim$0.3~eV above the fundamental absorption edge. The band gap values obtained from first-principles calculations converge, as the level of theory and corresponding computational cost increases, to 1.33 eV from the quasiparticle self-consistent GW method, including the solution to the Bethe-Salpeter equation. This agrees with the 0~K value determined from temperature-dependent optical absorption measurements. Relaxed structures based on hybrid functionals reveal a direct fundamental transition in contrast to previous reports. The optoelectronic properties of GeSe are resolved with the system described as a direct semiconductor with a 1.30 eV room temperature band gap. The high level of agreement between experiment and theory encourages the application of this computational methodology to other van der Waals materials.
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Mar 2020
|
|
I09-Surface and Interface Structural Analysis
|
Benjamin A. D.
Williamson
,
Thomas J.
Featherstone
,
Sanjayan S.
Sathasivam
,
Jack E. N.
Swallow
,
Huw
Shiel
,
Leanne A. H.
Jones
,
Matthew J
Smiles
,
Anna
Regoutz
,
Tien-lin
Lee
,
Xueming
Xia
,
Christopher
Blackman
,
Pardeep K.
Thakur
,
Claire J.
Carmalt
,
Ivan P.
Parkin
,
Tim D.
Veal
,
David O.
Scanlon
Diamond Proposal Number(s):
[18195, 21431]
Abstract: Transparent conducting oxides (TCOs) are ubiquitous in modern consumer electronics. SnO2 is an earth abundant, cheaper alternative to In2O3 as a TCO however, its performance in terms of electrical properties lags behind that of In2O3. Based on the recent discovery of mobility and conductivity enhancements in In2O3 from resonant dopants, we use a combination of state-of-the-art hybrid density functional theory calculations, high resolution photoelectron spectroscopy and semiconductor statistics modelling to understand what the optimal dopant is to maximise performance of SnO2-based TCOs. We demonstrate that Ta is the optimal dopant for high performance SnO2, as it is a resonant dopant which is readily incorporated into SnO2 with the Ta 5d states sitting ca. 1.4 eV above the conduction band minimum. Experimentally, the electron effective mass of Ta doped SnO2 was shown to be 0.23m0, compared to 0.29m0 seen with conventional Sb doping, explaining its ability to yield higher mobilities and conductivities.
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Feb 2020
|
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I09-Surface and Interface Structural Analysis
|
Jack E. N.
Swallow
,
Benjamin A. D.
Williamson
,
Sanjayan
Sathasivam
,
Max
Birkett
,
Thomas J.
Featherstone
,
Philip A. E.
Murgatroyd
,
Holly J.
Edwards
,
Zachary W.
Lebens-higgins
,
David A.
Duncan
,
Mark
Farnworth
,
Paul
Warren
,
Nianhua
Peng
,
Tien-lin
Lee
,
Louis F. J.
Piper
,
Anna
Regoutz
,
Claire J.
Carmalt
,
Ivan P.
Parkin
,
Vin R.
Dhanak
,
David O.
Scanlon
,
Tim D.
Veal
Diamond Proposal Number(s):
[18428]
Open Access
Abstract: Transparent conductors are a vital component of smartphones, touch-enabled displays, low emissivity windows and thin film photovoltaics. Tin-doped In2O3 (ITO) dominates the transparent conductive films market, accounting for the majority of the current multi-billion dollar annual global sales. Due to the high cost of indium, however, alternatives to ITO have been sought but have inferior properties. Here we demonstrate that molybdenum-doped In2O3 (IMO) has higher mobility and therefore higher conductivity than ITO with the same carrier density. This also results in IMO having increased infrared transparency compared to ITO of the same conductivity. These properties enable current performance to be achieved using thinner films, reducing the amount of indium required and raw material costs by half. The enhanced doping behavior arises from Mo 4d donor states being resonant high in the conduction band and negligibly perturbing the host conduction band minimum, in contrast to the adverse perturbation caused by Sn 5s dopant states. This new understanding will enable better and cheaper TCOs based on both In2O3 and other metal oxides.
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Sep 2019
|
|
|
|
Thomas J.
Whittles
,
Tim D.
Veal
,
Christopher N.
Savory
,
Peter J.
Yates
,
Philip A. E.
Murgatroyd
,
James T.
Gibbon
,
Max
Birkett
,
Richard J.
Potter
,
Jonathan D.
Major
,
Ken
Durose
,
David O.
Scanlon
,
Vinod R.
Dhanak
Abstract: The earth-abundant semiconductor Cu3BiS3 (CBS) exhibits promising photovoltaic properties and is often considered analogous to the solar absorbers copper indium gallium diselenide (CIGS) and copper zinc tin sulfide (CZTS) despite few device reports. The extent to which this is justifiable is explored via a thorough X-ray photoemission spectroscopy (XPS) analysis: spanning core levels, ionization potential, work function, surface contamination, cleaning, band alignment, and valence-band density of states. The XPS analysis overcomes and addresses the shortcomings of prior XPS studies of this material. Temperature-dependent absorption spectra determine a 1.2 eV direct band gap at room temperature; the widely reported 1.4–1.5 eV band gap is attributed to weak transitions from the low density of states of the topmost valence band previously being undetected. Density functional theory HSE06 + SOC calculations determine the band structure, optical transitions, and well-fitted absorption and Raman spectra. Valence band XPS spectra and model calculations find the CBS bonding to be superficially similar to CIGS and CZTS, but the Bi3+ cations (and formally occupied Bi 6s orbital) have fundamental impacts: giving a low ionization potential (4.98 eV), suggesting that the CdS window layer favored for CIGS and CZTS gives detrimental band alignment and should be rejected in favor of a better aligned material in order for CBS devices to progress.
|
Jul 2019
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