Talking about strategies for high efficiency kesterite solar cells

Dr. Edgardo A. Saucedo visits BUAP after the MRS congress in Cancun, Mexico and gives us a motivational talk about kesterites PV technology. He is the leader of the Solar Energy Materials and Systems Group at IREC and leads the European STARCELL project too.

 Kesterites are a promising PV material, it's metal elements, Copper, Zinc, and Tin are abundant on earth crust. This material owns a tetragonal lattice similar to commercial PV technology. As Dr. Saucedo said, they come from the royal family of photovoltaics. 

Kesterites comes from the royal family of photovoltaics

Source: PV-Education Crystal structure of the royal family of photovoltaics

The pace of research on kesterites has been slow due to the emergence of perovskites who has reached a conversion efficiency of 24%(early 2019) while kesterite holds a record of 12.6% reported by IBM in 2013. As you can see, there is a difference of 6 years between both reports [1].

STARCELL born to push forward kesterites PV technology and it is formed by universities and industrial partners. As you can see in the next image, the project is conceived by institutions which have the infrastructure for developing solar cells. You can read a complete description in the following link (About Starcell-Project)

STARCELL organization structure for CZTS development of solar cells and scale-up to PV modules

Challenges of kesterite technology: Doping and alloying 

The main technological challenge of kesterites is their Low Voc.  For example, the sulfo-selenide kesterite or CZTSSe has a Voc of 513 mV [2] compared to commercial CIGSe solar cells which reach 734 mV [3].  Dr. Saucedo said that some bulk properties of the kesterite related to recombination process should be enhanced controlling the following techniques:

• Doping (intrinsic and extrinsic) with less than 1% of element incorporation 
• Extrinsic: Incorporation of germanium (Ge) or sodium (Na)
• Intrinsic:  Defect points like vacancies, substitution, or interstitials. 
• Alloying with enough element incorporation where there is a change in the structure of the material. 

How to obtain the best kesterite deposition conditions for a solar cell?

Solar cell community who developed kesterites with selenium already know that the kesterite CZTSSe layer has to be nearly stochiometric: Cu2ZnSnSe4. This means that the content of copper should be almost 10% less than the real (Cu-Poor) and excess of Zn (Zn-rich) with a content of 105%.

Tips for crystallization and grain growth (Kesterites):

• After working with the sulfurization and selenization process, you would realize that selenium is noble and more controllable during heat treatment. You will love it.
• For the selenization process, he recommends a big annealing chamber,  in this configuration, a homogeneous temperature at the center can be maintained. Therefore the process will keep reproducible. 
• The graphite box for selenization reaction its a self-design developed at IREC, here we have proven several designs. 
• The graphite box works well just ten consecutive times, but more than these tries selenium content runs out. 

References

[1] M.A. Green, E.D. Dunlop, D.H. Levi, J. Hohl-Ebinger, M. Yoshita, A.W.Y. Ho-Baillie, Solar cell efficiency tables (version 54), Prog. Photovolt: Res. Appl. 27 (2019) 565–575. doi:10.1002/pip.3171.

[2] S. Giraldo, Z. Jehl, M. Placidi, V. Izquierdo-Roca, A. Pérez-Rodríguez, E. Saucedo, Progress and Perspectives of Thin Film Kesterite Photovoltaic Technology: A Critical Review, Adv. Mater. 31, (2019). doi:10.1002/adma.201806692.

[3] M.A. Green, E.D. Dunlop, D.H. Levi, J. Hohl-Ebinger, M. Yoshita, A.W.Y. Ho-Baillie, Solar cell efficiency tables (version 54), Prog. Photovolt: Res. Appl. 27 (2019) 565–575. doi:10.1002/pip.3171.