Life was comfortable back in 2007. While starting to work on quaternary semiconductors for thin-film solar cells, there was very little literature. It was possible to read all of the papers in the field. Since the report of a 10% efficient solar cell made from Cu2ZnSn(S,Se)4 in 2010, interest in the field exploded and there now stands over 1000 publications. I feel sorry for any new graduate student beginning a project…
From reading quite a few of these papers and attending conferences and workshops over the years, here is a decent reading list to get started:
1. Development of CZTS-based thin film solar cells (Thin Solid Films, 2009) – An important historical overview of the development of the field. Like many technologies, it all started in Japan.
2. New routes to sustainable photovoltaics: evaluation of Cu2ZnSnS4 as an alternative absorber material (Physica Status Solidi B, 2008) – An important paper from Jonathan Scragg (whose PhD thesis turned into the first book on kesterite solar cells) with layers made by electrodeposition.
3. The crystal structure of kesterite type compounds: A neutron and X-ray diffraction study (Solar Energy Materials and Solar Cells, 2011) – X-ray diffraction has trouble distinguishing between Cu and Zn. Neutron diffraction confirms the ground-state crystal structure (not stannite) and the tendency for cation disorder.
4. Kesterite Thin-Film Solar Cells: Advances in Materials Modelling of Cu2ZnSnS4 (Advanced Energy Materials, 2012) – The complexity of these materials has provided a fertile ground for theory and simulation, with early efforts reviewed here on structure, defects and band energies.
5. 8.6% Efficient CZTSSe Solar Cells Sprayed from Water–Ethanol CZTS Colloidal Solutions (Journal of Physical Chemistry Letters, 2014) – simple, clean and easy to scale up, with more recent reports of reproducible 10% efficiency from this approach.
6. Device Characteristics of CZTSSe Thin-Film Solar Cells with 12.6% Efficiency (Advanced Energy Materials, 2014) – The current record device with 12.6% efficiency. The one to beat!
7. Influence of compositionally induced defects on the vibrational properties of device grade Cu2ZnSnSe4 absorbers for kesterite based solar cells (Applied Physics Letters, 2015) – Precision Raman spectroscopy is becoming increasingly useful for identifying secondary phases and quantifying structural disorder in kesterites. The team at IREC are leading the way.
8. Suns-VOC characteristics of high performance kesterite solar cells (Journal of Applied Physics, 2014) – What is limiting performance to less than 20%? This is one of several important detailed charactertisation papers that point to issues with the back contact.
For research, 2014 has been an extremely fun year. There have been many new projects going in unexpected directions, which keeps things fresh and interesting. My research group composition has been changing too (Out: Lee to Kytoto, Davide to Oxford and Rachel to Queen Mary; In: Suzy from York, Ruoxi from Fudan, Katrine from Aarhus), which alters the dynamic. There is no such thing as a quiet or normal week.
Publications from 2014:
- Automated procedure to determine the thermodynamic stability of a material and the range of chemical potentials necessary for its formation relative to competing phases and compounds
- Electronic structure and band alignment of zinc nitride, Zn3N2 (OA)
- From kesterite to stannite photovoltaics: Stability and band gaps of the Cu2(Zn,Fe)SnS4 alloy (OA)
- N incorporation and associated localized vibrational modes in GaSb
- Prediction of Electron Energies in Metal Oxides (OA)
- Three-electron two-centred bonds and the stabilisation of cationic sulfur radicals (OA)
- Crystal electron binding energy and surface work function control of tin dioxide
- Defect chemistry of Ti and Fe impurities and aggregates in Al2O3 (OA)
- Computational screening of structural and compositional factors for electrically conductive coordination polymers (OA)
- Relativistic quasiparticle self-consistent electronic structure of hybrid halide perovskite photovoltaic absorbers
- Ultra-thin oxide films for band engineering: design principles and numerical experiments
- Ab initio thermodynamic model of Cu2ZnSnS4 (OA)
- Atomistic Origins of High-Performance in Hybrid Halide Perovskite Solar Cells (OA)
- Thermal physics of the lead chalcogenides PbS, PbSe, and PbTe from first principles
- Origin of deep subgap states in amorphous indium gallium zinc oxide: Chemically disordered coordination of oxygen
- Design of I2−II−IV−VI4 Semiconductors through Element Substitution: The Thermodynamic Stability Limit and Chemical Trend
- Electronic Chemical Potentials of Porous Metal−Organic Frameworks (OA)
- Ellipsometric characterization and density-functional theory analysis of anisotropic optical properties of single-crystal SnS
- Molecular ferroelectric contributions to anomalous hysteresis in hybrid perovskite solar cells (OA)
- Identification of critical stacking faults in thin-film CdTe solar cells
- Stacking-dependent energetics and electronic structure of ultrathin polymorphic V2VI3 topological insulator nanofilms
- Ligand design for long-range magnetic order in metal–organic frameworks (OA)
- Transferable Force Field for Metal−Organic Frameworks from First-Principles: BTW-FF (OA)
- Tunable Trimers: Using Temperature and Pressure to Control Luminescent Emission in Gold(I) Pyrazolate-Based Trimers (OA)
- Electronic Structure Modulation of Metal−Organic Frameworks for Hybrid Devices (OA)
We have done better for fully open access (OA) publications this year, but still room for improvement. Generally, we don’t pay for gold open access with the American Physical Society (e.g. Physical Review B or Physical Review Letters) because they have the most generous policy for hosting on personal websites and institutional databases.
2013 was the fastest year on record (plenty done, with even more to do). Christmas is a nice time to step back and refocus. In terms of research it has been a year where my group matured to produce a series of excellent papers that wouldn’t have been possible without their combined skills (and backgrounds from chemistry, physics, chemical engineering and materials science). I am indebted to them and our many talented collaborators around the world.
Publications from 2013:
- “PbO2: from semi-metal to transparent conducting oxide by defect chemistry control“
- “Thermodynamic and electronic properties of tunable II–VI and IV–VI semiconductor based metal–organic frameworks“
- “Prediction of (TiO2)x(Cu2O)y alloys for efficient photoelectrochemical water splitting“
- “Classification of Lattice Defects in the Kesterite Cu2ZnSnS4and Cu2ZnSnSe4 Earth-Abundant Solar Cell Absorbers“
- “Electronic origin of the conductivity imbalance between covalent and ionic amorphous semiconductors“
- “Band alignment in SnS thin-film solar cells: Possible origin of the low conversion efficiency“
- “Microscopic origin of the optical processes in blue sapphire” (OA)
- “Electron excess in alkaline earth sub-nitrides: 2D electron gas or 3D electride?” (OA)
- “Dynamical response and instability in ceria under lattice expansion“
- “Band alignment of rutile and anatase TiO2“
- “Engineering the Optical Response of the Titanium-MIL-125 Metal−Organic Framework through Ligand Functionalization” (OA)
- “Electronic Structures of Antimony Oxides“
- “Limits to Doping of Wide Band Gap Semiconductors” (OA)
- “Helical frontier orbitals of conjugated linear molecules” (OA)
- “One-dimensional embedded cluster approach to modeling CdS nanowires“
- “Dielectric response of Fe2O3 crystals and thin films“
- “Oxidation of GaN: An ab initio thermodynamic approach“
- “Structural and electronic properties of hybrid perovskites for high-efficiency thin-film photovoltaics from first-principles” (OA)
- “Polymorphism of indium oxide: Materials physics of orthorhombic In2O3“
- “Synthesis, Characterization, and Electronic Structure of Single-Crystal SnS, Sn2S3, and SnS2” (OA)
One goal next year is to increase the proportion of fully open access (OA) publications. Institutional repositories are nice, but they don’t compare to direct access on a journal website.
I am not sure what over research fields are like, but it is quite common in computational chemistry / materials science to keep small analysis codes, scripts and tricks hidden within a research group. With an increasing number of open access projects and packages, this situation is changing.
I regularly get mails from students requesting pieces of information relating to papers I have published. My new policy is to put everything useful and requested online. The best route is not clear, but for the moment, I am adding it to my group’s GitHub. The latest additions include the crystal structure of MIL-125, an inverse spinel geometry, indium oxide nanoclusters and kesterite polytypes (see here).
In terms of version control for codes, both GitHub and Bitbucket do a fine job, especially when paired with Sourcetree. On paper, Bitbucket should be the winner as they offer both public and private repositories for free, but they can’t compete with the Github mascot (octocat):
Now it’s time to learn some python with Code Academy.
Spring is the time for the best research ideas; summer is the time for writing up the work; autumn is the time for peer-review and winter is time to reap the rewards. The bounty of the last few months:
- “Phase Stability of the Earth-Abundant Tin Sulfides SnS, SnS2, and Sn2S3” L. Burton and A. Walsh, Journal of Physical Chemistry C 116, 24262 (2012).
The first publication written by my first PhD student (an iconic moment). Lee has spent a year making sense of both experiment and theory of tin sulfides towards applications in solar cells. One of his early findings was that the recently reported zinc-blende phase cannot exist (based on thermodynamic, crystallographic, computational and chemical grounds).
- “Abundance of CuZn+SnZn and 2CuZn+SnZn defect clusters in kesterite solar cells” S. Chen, L. Wang, A. Walsh, X. G. Gong and S.-H. Wei, Applied Physics Letters 101, 223901 (2012).
Attending the recent European Kesterite Workshop was inspiring: many groups across the continent working towards the common goal of low cost and sustainable solar energy. The defect chemistry of the quaternary semiconductor Cu2ZnSnS4 (CZTS) is epic; in the latest chapter we have identified a number of defect complexes that are likely to impact the performance in CZTS solar cells.
- “Electronic structure of CuCrO2 thin films grown on Al2O3(001) by oxygen plasma assisted molecular beam epitaxy” D. Shin, J. S. Foord, R. G. Egdell and A. Walsh, Journal of Applied Physics 112, 113718 (2012).
When my long-time collaborator Russ Egdell visited Bath for a seminar last summer, we sat down to discuss our on-going projects. One issue he had was understanding why the material CuCrO2 adopted a peculiar orientation (015) when grown on Al2O3 substrates. With the aid of crystal structure visualisation and a few back-of-the-envelope calculations we were able to explain it in a few hours. This nice paper is the result!
- “Thermodynamic and electronic properties of tunable II-VI and IV-VI semiconductor based metal-organic frameworks from computational chemistry” C. H. Hendon, D. Tiana, T. P. Vaid and A. Walsh, Journal of Materials Chemistry C 1, 95 (2013).
The first publication written by my second PhD student (another iconic moment). A first step towards systematically tuning the electronic properties of metal organic frameworks. From screening 24 compounds, five potential hybrid semiconductors were identified.
It has been a busy year in terms of university and network activities, but chemistry comes first. The first third of the year has been a mix of code developments, research reviews and new science.
- “Introducing k-Point Parallelism into VASP” A. Maniopoulou, E. R. M. Davidson, R. Grau-Crespo, A. Walsh, I. J. Bush, C. R. A. Catlow and S. M. Woodley, Computer Physics Communications 183, 1696 (2012).
VASP is the most used electronic structure code on the national supercomputer, but it cannot take full advantage of the thousands of available processors. This paper is the result of a collaboration between the HPC Materials Chemistry Consortium and NAG to add a new mode of parallelism into the code. It works, and will hopefully be adopted into the mainstream version of VASP soon. The approach makes hybrid density functional theory calculations less painful to run!
- “Structural and electronic properties of CuSbS2 and CuBiS2: potential absorber materials for thin-film solar cells” J. T. R. Dufton, A. Walsh, P. M. Panchmatia, L. M. Peter, D. Colombara and M. S. Islam, Physical Chemistry Chemical Physics 14, 7729 (2012).
Solar cells work, but to enable widespread adoption of thin-film technologies, new absorber materials need to be developed that are made from earth abundant elements. Copper based compounds are the current hot topic, and this work presents a theoretical understanding of the structure and bonding in two candidate materials that have recently been synthesised in the Department of Chemistry at Bath.
- “Kesterite Thin-Film Solar Cells: Advances in Materials Modelling of Cu2ZnSnS4” A. Walsh, S. Chen, S.-H. Wei and X. G. Gong, Advanced Energy Materials 4, 400 (2012).
People are excited about CZTS solar cells, and with good reason. While the elements are abundant and low cost, the challenge is in the complex solid-state chemistry associated with a four-component system. This paper reviews our work performed over the past five years exploring the materials chemistry and physics of the kesterite system.
- “Synthesis, Characterization, and Calculated Electronic Structure of the Crystalline Metal–Organic Polymers [Hg(SC6H4S)(en)]n and [Pb(SC6H4S)(dien)]n” D. L. Turner, K. H. Stone, P. W. Stephens, A. Walsh, M. P. Singh, and T. P. Vaid, Inorganic Chemistry 51, 370 (2012).
The development of semiconducting metal-organic frameworks is the primary subject of my recently funded European grant. To complement our predominately theoretical research, we are collaborating with the group of Tom Vaid at the University of Alabama. Novel lead and mercury sulfide networks that show some real promise were synthesized and characterised.
- “Surface structure of In2O3(111) (1×1) determined by density functional theory calculations and low energy electron diffraction” K. Pussi, A. Matilainen, V. R. Dhanak, A. Walsh, R. G. Egdell, K. H. L. Zhang, Surface Science 606, 1 (2012).
It may be increasingly rare and expensive, but indium makes my favourite oxide material. So simple yet so complex. Last year we predicted the surface structure of In2O3 based on my calculations, and this year we managed to get experimental validation by one of the largest LEED studies performed to date.