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CdTe solar cells


By
Carlos G Torres

June 26, 2017 - Posted in Discussion
My previous research back in Mexico focused on the development of low-cost Cadmium Telluride (CdTe) solar cells. They is considered as thin film technology because the active layers are just a few microns thick. I want to show you a quick overlook of this II-VI device technology.

Historically, the thin film (a-Si, CdTe, and CIGS) materials have been considered to be the low-cost option for PV. However, Si has made great strides in recent years. The massive expansion in capacity that is taken place in China has increased the gap considerably between Si and thin film PV[1]. The lowest price per watt reported for CdTe, as far as I know, is 0.49USD/W[2] while for Si is around 30-40USD/W[3]. The absence of many thin film PV companies seems to suggest that many of them are still struggling to achieve low costs and competence with Si[4].

Nowadays, CdTe represents the second most important technology in the manufacture of solar cells/panels, way behind Si. Fig. 1 a) shows the vast difference between thin film and Si PV production, and the enormous increase in the total annual PV production after 2005. According to Fraunhofer Institute for Solar Energy Systems[5], the market share of all thin film technologies amounted to about 8%. The thin film market share is illustrated in Fig. 1 b). CdTe thin film technology holds the first pace in thin film PV, representing ~5% of the world market.

Carlos 1.JPG

Fig.1 a) Annual PV Production by Technology Worldwide (in GWp) and b) PV Production by Technology Percentage of Global Annual Production 5

I would say that today there are two important players in CdTe PV. The National Renewable Energy Laboratory (NREL) supported by the USA government, has made a strong bet (investment) for this technology and has been at the forefront of research and development (R&D). For example, Busrst et al.[6] in 2016 fabricated CdTe solar cells with open-circuit voltage breaking the 1 V barrier. The second player is an American company named First Solar. In 2016, they exceeded the US$ 1 billion in cumulative R&D spending[7]. They have the record in efficiency for a CdTe solar cell (21.0±0.4)[8].

Finally, I want to show and explain to you the different films of this device. Figure 2 illustrates the schematic of a typical CdTe solar cell. The sunlight incidence is from top to bottom. The layers are:

1)    Transparent conductive oxide (TCO) (Cd2SnO4 and SnO2): Must be highly conductive to transport current efficiently and almost transparent to allow photons to get into the active part of the cell.
2)    Intermediate layer (CdS): help in both growth and electrical properties between the CdTe and TCO. 
3)    Active layer (CdTe): Absorbs most of the light and act as the primary photoconversion layer.
4)    Metallic contact: The primary contact (Cu, Cux/Te, ZnTe:Cu) is usually a heavily doped that allows a low-loss electrical contact to the CdTe, and the secondary contact is a metal that carries the current (Au, C, Ag paste).

Carlos 2.JPG
Fig.2 Schematic illustration of a typical CdTe superstrate thin-film PV device[9].

Reference

1 Bradsher, K. (April 7, 2017). When Solar Panels Became Job Killers. The New York Times.
2 Osborne, M. (November 4, 2013). First Solar hits cost reduction milestone. PVTECH. Retrieved from:https://www.pv-tech.org/news/has_first_solar_retaken_the_lowest_cost_pv_manufacturer_mantle
3 Wesoff, E. (June 15, 2017) The Top Solar Manufacturers Are Not Necessarily Making the Highest-Quality Modules. Green Tech Media. Retrieved from:https://www.greentechmedia.com/articles/read/top-solar-manufacturers-are-not-making-the-highest-quality-modules
4 Ferekides, C. S., personal communication, November 6, 2013.
5 Phillips, S. (2016). Photovoltaics Report. Fraunhofer Institute for Solar Energy Systems.
6 Burst, J.M., et al. CdTe solar cells with open-circuit voltage breaking the 1 V   barrier. (2016). Nature Energy, 1, 1-7.
7 Osborne, M. (June 3, 2017). First Solar to surpass US$1 billion in cumulative R&D spending in 2016. PVTECH. Retrieved from:https://www.pv-tech.org/news/first-solar-to-surpass-us1-billion-in-cumulative-rd-spending-in-2016
8 Green, M.A., et al. Solar cell efficiency tables (version 49) (2017). Prog. Photovolt: Res. Appl., 25, 3–13.
9 National Renewable Energy Laboratory. Cadmium Telluride Solar Cells. Retrieved from:https://www.nrel.gov/pv/cadmium-telluride-solar-cells.html



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