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.
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.