For the second half of my MSc program, I am pursuing a thesis project at imec (Leuven, Belgium) on CZTSe and CIGS(e) solar cells. The absorber materials are more broadly known as Kesterite and Chalcopyrite compounds. My work deals with the fundamental science of making these quaternary compounds, as well the numerous steps required to make complete cells out of them (absorber layer formation, grain-growth, surface passivation, grid deposition and so on!)
In this blog post, I would like to present the case for thin-film solar cells.
As a solar cell material, Silicon is doing very well. It is the second most abundant element in the earth’s crust (in the form of silica, sand) . As explained in my previous post, William Schokley and Hans-Jaochim Queisser famously calculated that an ideal semiconductor for absorbing our star’s light on the Earth’s surface, should have a band-gap between 1.2-1.34 eV. Silicon sits at a somewhat favourable 1.12eV. Thanks to the efforts of countless scientists and engineers, today we can all easily buy modules with an efficiency of 21% . Silicon is also highly favoured because of the immensely vast technologies developed by the electronic industry, which uses it in every chip, for every transistor, resistor and capacitor!
It is no surprise then, that 93% of the current PV market is served by this wonder material!  It is present in the 1.7 million solar panels in the Solar star installation and the Tesla roof tiles that will go into production soon. Why then, does mankind search for something more? Because this is not the whole story!
1) Absorption: Silicon, with its indirect band-gap is not a good absorber. In simple terms, it means that several semiconductor materials can absorb the same light in about 1/100th material. By using silicon, we are using 100 times more material than we need to, in the form of expensive, heavy and bulky panels.
Due to their crystalline nature, Silicon wafers work best under direct incident light which is difficult to track as the Sun moves in the sky. Amorphous thin-film technologies have a lesser performance drop under indirect light and can produce more energy in a day than crystalline Silicon panels with higher efficiencies! Average energy production is a better performance benchmark than peak efficiency when comparing technologies.
2) Production: Sand is abundant but SiO2 is an extremely stable compound. The extraction of Silicon is now a highly standardized process but it still remains energy intensive and expensive . Alternative materials can be easier and safer to produce with the use of lesser energy and chemicals harmful to the environment. Their production can even be solution based as Perovskites cells have been demonstrated to be. 
3) Band-gap engineering and multi-junction cells: By using other materials, a better band-gap can also be used to approach the theoretical maximum efficiency. Some semiconductors offer ‘tunable’ band-gaps which vary according to the stoichiometric composition.
4) Uniting forces: While new technologies try to compete with Silicon, then can also instead, work with silicon (in ‘tandem’) to absorb the blue, short wavelength light that silicon might leave out. You may have noticed the blue colour of conventional solar panels (particularly the old ones).
Multi-junction solar cells can theoretically go up to 86.6% efficiency under concentrated light . Such panels have already been used in space missions since the 1970s. . Currently, the multi-junction domain is dominated by III-V group materials (In, Ga, As) and not Silicon. 38.9% efficiency modules are commercially available. 
5) Chalcopyrites: CIGS(e) and CdTe solar cells have been the champions of thin-film technologies, with record laboratory efficiencies reaching 22.6 %  which is greater than the multi-crystalline Si record of 21.25 %  and comparable to the mono-crystalline Si record of 25.6 % . CIGS(e) solar cells have also exhibited high volume production and roll-to-roll manufacturing. Several challenges remain for these technologies. Perhaps that is the reason why large corporations such as TSMC solar have tried and failed to sell them. First Solar is the most dominant player in the production as well as research of CdTe solar cells. However, Te is rare and Cd is more toxic than lead or mercury, which has why this technology is getting increasingly unpopular in the European Union. One of the major drawbacks of CIGS(e) technology is the use of Indium, which has an estimated abundance of 0.049 ppm in the earth’s crust, making it rarer than even silver . It’s demand has been rapidly increasing due to its wide spread use in touch screen displays and LCDs. Gallium also carries a risk of increasing costs. The economics of constituent elements could increase the cost of CIGS(e) technology as demand rises. This could stagnate or reverse the downward spiral of the prices of solar modules.
6) Kesterite: Kesterite (CZTS(e)) solar cells seek to solve this problem of CIGS(e) technology by using earth abundant materials such as Copper, Zinc, Tin, Selenium and Sulphur. They are thus far made in the same architecture as CIGS(e) cells and follow a similar line of research. Efforts are also being made to make the production low-temperature, low-cost and with few steps. A record efficiency of 12.6 % has already been demonstrated . Research in this field focuses on earth abundant materials such as copper, zinc, tin and selenium.
Imagine a world where solar panels are light-weight, wafer thin, flexible and cheap/easy to produce and install. They come in all shapes and sizes and can fit onto the phones in your pocket, on the top of camping tents or in the windows of your buildings and cars. The possibilities are endless and we could power our entire civilization with the infinite pool of the sun’s energy which already makes our blue planet burst with life.
Technologies take time to develop. Tracing the historical timeline of silicon solar cells, it is easy to see that the first such cell was demonstrated in 1954 with an efficiency of 6%. In 1958, Vanguard I became the first solar power satellite. It was only decades later that silicon solar cells become widely accepted. Therefore, this project is important to me because with careful scientific analysis, engineering and development, today’s 6% solar cell can change the world tomorrow.
- Nave, R. Abundances of the Elements in the Earth’s Crust, Georgia State University
- Fraunhofer ISE, “Photovoltaics report,” Freiburg, updated: 6 June 2016
- PV-magazine, The trouble with Silicon, [online]. Available: http://www.pv-magazine.com/archive/articles/beitrag/the-trouble-with-silicon-_100001055/86/?#axzz4PDooxOvU %5BAccessed: 6th November 2016]
- imec, Semi-transparent-perovskite-pv-module, [online]. Available: http://www2.imec.be/be_en/press/imec-news/semitransparent-perovskite-pv-module.html [Accessed: 6th November 2016]
- Green, Martin A. (2003). Third Generation Photovoltaics: Advanced Solar Energy Conversion. Springer. p. 65.
- Spectrolab, [online]. Available: http://www.spectrolab.com/history.htm %5BAccessed: 6th November 2016]
- PV magazine, “ZSW sets new thin film world record 29th 22.6 % efficient CIGS PV cell,” ZSW, 15 June 2016. [Online]. Available: http://www.pvmagazine.com/news/details/beitrag/zsw-sets-new-thin-film-solar-world-record-with-226-efficient-cigs-pv-cell_100024995/#axzz4ClVxNMm4. [Accessed 30 August 2016].
- Trinasolar, “Trina Solar Announces New Efficiency Record of 21.25% Efficiency for Multi-crystalline Silicon Solar Cell,” 9 November 2015. [Online]. Available: http://www.trinasolar.com/us/about-us/newinfo_978.html. [Accessed 30 August 2016].
- K. Barbalace, “Periodic Table of Elements,” 1995-2016. [Online]. Available: http://EnvironmentalChemistry.com/yogi/periodic/In.html. [Accessed 30 August 2016].
- W. Wang , M. T. Winkler, O. Gunawan, T. Gokmen, T. K. Todorov, Z. Yu and D. B. Mitzi, “Device Characteristics of CZTSSe Thin-Film Solar Cells with 12.6% Efficiency,” Advanced Energy Materials, vol. 4, 2014.