2, University of Oslo, Oslo, , Norway
Cu2ZnSnS4 (CZTS) is interesting for tandem solar cell application due to its bandgap of 1.5 eV and possibilities for further bandgap increase through alloying. In recent years significant efforts has been devoted to investigate the properties of the CZTS solar cell largely on the molybdenum (Mo) back contact resulting in a record efficiency of 10%1. However, there have been very sparse studies made on the transparent back contacts (TBC), which are essential for both tandem and bifacial solar cell structures2,3. Recent investigations have reported an efficiency of 2.9 % and 2.1 % for CZTS deposited on transparent ITO and FTO back contact respectively2,4. The high temperatures involved during the sulphurization of the CZTS absorber results in the diffusion of atoms from the TBC into the CZTS layers thereby degrading the device performance5. The use of a barrier interlayer between the TBC/CZTS was shown to have a beneficial effect on the device performance3.
This work investigates the role of the molybdenum oxide (MoOx) interfacial layer between the transparent FTO electrode and the CZTS absorber. Commercially available FTO glass, hereafter referred as FT1, with a sheet resistance of 13 Ω/o was used as the substrate. Additionally two FTO substrates, namely FM1 and FM2, were coated with an interfacial MoOx layer with a thickness of 5 and 40 nm respectively. The CZTS precursors were sputtered using binary metal targets of CuS, ZnS, SnS in argon environment followed by annealing in sulphur (S) atmosphere at 580 °C to form the absorbers. Analysis using secondary ion mass spectroscopy (SIMS) shows an out diffusion of fluorine (F) atoms from the FTO into the CZTS absorber. Furthermore, the concentration of F atoms in the CZTS increased as a function of MoOx thickness. From the current-voltage (IV) characteristics, it was observed that the samples FT1 and FM2 were shunted with no light-to-electricity conversion. However, the presence of thin MoOx interlayer, sample FM1, had a beneficial effect on the device performance resulting in an efficiency of 3.5 %. Experiments are underway in order to optimize and understand how the annealing time and temperature of the sulphurization process effects the back contact/CZTS interface and its implications on the overall performance of the devices.
1 M.A. Green, Y. Hishikawa, W. Warta, E.D. Dunlop, D.H. Levi, J. Hohl-Ebinger, and A.W.H. Ho-Baillie, Prog. Photovolt. Res. Appl. 25, 668 (2017).
2 J. Ge, J. Chu, J. Jiang, Y. Yan, and P. Yang, ACS Appl. Mater. Interfaces 6, 21118 (2014).
3 M. Espindola-Rodriguez, D. Sylla, Y. Sánchez, F. Oliva, S. Grini, M. Neuschitzer, L. Vines, V. Izquierdo-Roca, E. Saucedo, and M. Placidi, ACS Sustain. Chem. Eng. (2017).
4 S. Mahajan, E. Stathatos, N. Huse, R. Birajdar, A. Kalarakis, and R. Sharma, Mater. Lett. 210, 92 (2018).
5 J. Ge, J. Chu, J. Jiang, Y. Yan, and P. Yang, ACS Sustain. Chem. Eng. 3, 3043 (2015).