CdTe-Cadmium telluride
CdTe polycrystalline thin film solar cells have shown an immense potential in scalability, up to now this is the sole thin film technology that has reached its maturity for industrial production, by the amazing results in terms of market and efficiency of First Solar that is ranked among the top ten solar modules manufacturers [1].
These modules have demonstrated long-term stable performance and high efficiency up to 22% under AM1.5 illumination [2]. Amongst several attractive features, high chemical stability of CdTe and a simple compound formation are the most important ones for large area production of solar modules.
For this reason high efficiency devices can be obtained with a large variety of fabrication processes such as vacuum evaporation (15,6% from EMPA, CH) [3], sputtering (14% by University of Toledo, USA) [4], screen printing (10% from Matsushita) [5], close space sublimation (16.7% from NREL) [6] and vapour transport deposition (22% from First Solar) [1].
Further simplifications have been introduced recently like substituting the CdCl2 (carcinogenic) annealing step with a recrystallization treatment obtained by MgCl2 deposition [7] and annealing or treatment with Chlorine containing gases [8].
A lot has been discussed about the presence of cadmium in the finished product; this has been universally accepted as a perception issue more than a real problem since the thin film architecture allows an extremely low amount of cadmium (in a 60x120cm module there is less Cd than in a AA battery) and stored in an inert, non toxic compound [9]. Moreover CdTe modules have performed the lowest environmental impact and the lowest pay back time [10].
Recent works on stable back contact have demonstrated that it is possible to produce devices with a remarkable stability versus time and with an additional higher performance in low light irradiation or high temperature conditions [11].
Moreover, there are new promising device configurations like bifacial solar cells [12], ultra-thin solar cells [13] [14] and flexible devices [15]. The highest efficiencies in CdTe solar cells have been obtained using CSS deposition methods, requiring a high substrate temperature (500÷550 °C). Instead, conventional physical vapor deposition (PVD) process where CdTe is evaporated in a vacuum evaporation (VE) system at lower substrate temperatures (typically 300°C) has provided solar cells with efficiencies of more than 15%. For these reasons VE process is attractive for a very simple in-line deposition of large area CdTe solar modules on soda-lime glass substrates, as well as on polymer foils thereby facilitating the roll-to-roll manufacturing of flexible solar modules [16].
Flexible CdTe/CdS solar cells of 14% efficiency in superstrate and 12% efficiency in substrate configurations have been developed [17]. Flexible superstrate solar cells have been directly grown on commercially available polyimide foils or ultra thin glass.
References:
- http://www.pv-tech.org/editors-blog/top-10-solar-module-manufacturers-in-2015
- http://investor.firstsolar.com/releasedetail.cfm?ReleaseID=956479
- http://www.swissphotonics.net/libraries.files/EMPA_ThinFilmsAndPhotovoltaics_Flyer_March2015.pdf
- Akhlesh Gupta1,a) and Alvin D. Compaan, All sputtered 14% CdS/CdTe thin-film Appl. Phys. Lett. 85, 684 (2004);
- Nakayama, N.; Matsumoto, H.; Nakano, A.; Ikegami, S.; Uda, H.; Yamashita, T. Screen printed thin film CdS/CdTe solar cell Japanese Journal of Applied Physics, vol. 19, Apr. 1980, p. 703-712
- http://www.nrel.gov/docs/fy02osti/31025.pdf
- J. D. Major, R. E. Treharne, L. J. Phillips, K. Durose, C. R. Corwine, A. O. Pudov, M. Gloeckler, S. H. Demtsu, J. R. Sites, D. Albin, J. R. Sites, S. S. Hegedus, B. E. Mccandless, R. W. Birkmire, W. a Buchanan, R. W. Birkmire, V. V. Plotnikov, X. Liu, and A. D. Compaan, “A low-cost non-toxic post-growth activation step for CdTe solar cells,” Nature, no. 4, pp. 523–526, Jun. 2014.
- A. Salavei, I. Rimmaudo, F. Piccinelli, P. Zabierowski, and A. Romeo, “Study of difluorochloromethane activation treatment on low substrate temperature deposited CdTe solar cells,” Sol. Energy Mater. Sol. Cells, vol. 112, pp. 190–195, 2013.
- Vasilis M Fthenakis, Life cycle impact analysis of cadmium in CdTe PV production, Renewable and Sustainable Energy Reviews, Volume 8, Issue 4, August 2004, Pages 303-334,
- http://iea-pvps.org/index.php?id=315&eID=dam_frontend_push&docID=2395
- http://www.firstsolar.com/~/media/Images/Our%20Advantage/TechnologyAdvantage_BR_19AUG15.ashx?la=en
- A. Romeo, G. Khrypunov, S. Galassini, H. Zogg, A.N. Tiwari, Bifacial configurations for CdTe solar cells, Solar Energy Materials and Solar Cells, Volume 91, Issues 15–16, 22 September 2007, Pages 1388-1391
- N.R. Paudel, K.A. Wieland, A.D. Compaan, Ultrathin CdS/CdTe solar cells by sputtering, Solar Energy Materials and Solar Cells, Volume 105, October 2012, Pages 109-112
- Ivan Rimmaudo, Andrei Salavei, Bing Lei Xu, Simone Di Mare, Alessandro Romeo, Superior stability of ultra thin CdTe solar cells with simple Cu/Au back contact, Thin Solid Films, Volume 582, 1 May 2015, Pages 105-109
- T. M. Barnes et al., "High-efficiency flexible CdTe superstrate devices," Photovoltaic Specialist Conference (PVSC), 2014 IEEE 40th, Denver, CO, 2014, pp. 2289-2292.
- A. Romeo, G. Khrypunov, F. Kurdesau, M. Arnold, D.L. Bätzner, H. Zogg, A.N. Tiwari, High-efficiency flexible CdTe solar cells on polymer substrates, Solar Energy Materials and Solar Cells, Volume 90, Issues 18–19, 23 November 2006, Pages 3407-3415
- L. Kranz, C. Gretener, J. Perrenoud, R. Schmitt, F. Pianezzi, F. La Mattina, P. Blösch, E. Cheah, A. Chirila, C. M. Fella, H. Hagendorfer, T. Jäger, S. Nishiwaki, A. R. Uhl, S. Buecheler, and A. N. Tiwari, “Doping of polycrystalline CdTe for high-efficiency solar cells on flexible metal foil,” Nat Commun, vol. 4, Aug. 2013.
