The main results of the project can be summarized as follows:

a) We demonstrated CZTS(Se) absorber layer sprayed in a sequence of Cu-, followed by Zn-, Sn- and sulfur-containing alcoholic solutions. Sequential spraying provided a greater flexibility in terms of molarity and reduced the possibility of instable solutions. The effect of different sulfur source on the Zn/Sn metal ratio was investigated and a reduction of Zn loss was found when using thioacetamide. Reducing the Sn concentration led to the desired Zn-rich film formation.

b) CZTSe absorber with state-of-the-art properties was developed by spin-coating metal salts dissolved in DMSO solution and subsequent selenization.  The effect of sodium content in improving the CZTSe properties was demonstrated.

c) We demonstrated that high quality metallic precursor layers for CZTS(Se) can be deposited electrochemically, using mild aqueous solutions and low temperatures.  The stoichiometry of the metals in the final kesterite absorber can be tuned by adjusting their respective layer thickness.  Kesterite films could be obtained after the chalcogenization process.

d) Buffer layers of ZnS and InS_x with appropriate properties were developed by chemical spray pyrolysis and chemical bath deposition, respectively, as alternatives to the standard CdS buffer used in kesterite cells.

e) Transparent and conductive ZnO and aluminum-doped ZnO (AZO) layers were developed using electrochemical deposition, from aqueous solutions and temperatures below 90^oC.

f) ZnO, and aluminum-doped ZnO layers were developed by the spray pyrolysis technique from exclusively water-based solutions, at temperatures below 400 ^oC and with high quality structural, optical and electrical properties.

g) CZTSe solar cells formed by spray pyrolysis showed an energy conversion efficiency of 0.7%. CZTSe cells formed by spin coating showed efficiency larger than 8%.  ECD-grown kesterite cells suffered from shunts.  The work for addressing the shunt issue is currently on-going.  

Following conclusions could be drawn:

a) The chemical spray pyrolysis method for CZTS(Se) is feasible but not suitable for producing efficient absorber films.  On the contrary, the spin-coating technique is the solution process of choice for efficient CZTS(Se) solar cells.

b) The sequential potentiostatic electrodeposition of metallic thin film precursors is a very promising technique for kesterite CZTS(Se) absorber, since it is upscalable, uncomplicated in terms of solution preparation, we can easily adjust the composition through the thickness of the metallic constituents and the deposition takes place close to room temperature.

c) Alternative buffer layers of ZnS and In2S3 are not yet capable to replace CdS in producing efficient CZTS(Se) cells.

d) It is possible to produce intrinsic and doped-ZnO layers by CSP using exclusively water-based solutions, with layer properties similar to solutions with alcohol-content.  Water-based solutions greatly facilitate the processing, as the high temperatures used for spray pyrolysis demand strict safety measures when an alcohol-containing solvent is employed.

e) Compact and highly transparent ZnO and doped-ZnO layers can be fabricated by ECD.

f) It is possible to prepare a complete solar cell using exclusively low-cost, solution-based processing for all components, i.e. transparent contact, absorber, buffer.      

Peer-reviewed publications

[1] S. Abermann, Non-vacuum processed next generation thin film photovoltaics: Towards marketable efficiency and production of CZTS based solar cells, Sol. Energy. 94 (2013) 37–70. doi:10.1016/j.solener.2013.04.017.

[2] S. Edinger, J. Bekacz, M. Richter, R. Hamid, R.A. Wibowo, A. Peić, T. Dimopoulos, Influence of the acetic acid concentration on the growth of zinc oxide thin films prepared by spray pyrolysis of aqueous solutions, Thin Solid Films. (2015). doi:10.1016/j.tsf.2015.04.027.

[3] R.A. Wibowo, R. Hamid, T. Maier, T. Dimopoulos, Galvanostatically-electrodeposited Cu–Zn–Sn multilayers as precursors for crystallising kesterite Cu2ZnSnS4 thin films, Thin Solid Films. 582 (2015) 239–244. doi:10.1016/j.tsf.2014.10.060.

[4] M. Werner, C.M. Sutter-Fella, H. Hagendorfer, Y.E. Romanyuk, A.N. Tiwari, Cu₂ZnSn(S,Se)₄ solar cell absorbers processed from Na-containing solutions in DMSO: Cu₂ZnSn(S,Se)₄ solar cell absorbers, Phys. Status Solidi A. 212 (2015) 116–120. doi:10.1002/pssa.201431146.