| AVS 62nd International Symposium & Exhibition | |
| Energy Frontiers Focus Topic | Tuesday Sessions |
| Session EN-TuP |
| Session: | Energy Frontiers Poster Session |
| Presenter: | Rony Snyders, University of Mons, Belgium |
| Authors: | P.-A. Cormier, University of Mons, Belgium R. Snyders, University of Mons, Belgium |
| Correspondent: | Click to Email |
Cu2ZnSnS4(CZTS) has attracted significant attention for thin film solar cells because it is composed on earth abundant and non-toxic elements and, has an optimal band gap of 1.5 eV. The control of the film stoichiometry is critical during the growth of CZTS thin films. We previously demonstrated the possibility to grow close to stoichiometric and phase pure crystallized CZTS thin films by reactive magnetron co-sputtering [1]. The films were close to stoichiometric (Zn/Sn=1.1-1.4, Cu/[Zn+Sn]=0.9-1.1). Nevertheless, it has been suggested that Cu-poor Zn-rich CZTS have would present the best solar cell performances [2].
Therefore, in the present work, aiming to better control the film stoichiometry and to deeper understand the relationship between the chemical composition of the deposited film and its phase constitution and ultimately to reach the best performances of the cell, two Cu poor CuSn targets presenting Cu/Sn ratio of 1.5 (CuSn1.5) and 1 (CuSn1) have been utilized. In both case, the effect of the sputtering power (PCuSn) on the material properties has been studied. The films were characterized by multiwavelength Raman spectroscopy (785 and 325 nm), by X-Ray Diffraction (XRD), and by Energy-dispersive X-ray spectroscopy (EDX).
As expected, the phase constitution is affected by increasing PCuSn. Using the CuSn1.5 target, its evolution is similar than the one observed using the CuSn2 target [1]: at low PCuSn, the films mainly contain CZTS, with a low content of ZnS. Increasing the power, CZTS disappears to the profit of SnS, Cu4Zn clusters, S6 and ZnS. On the contrary, using the CuSn1 target does not allow the formation of CZTS dominated films. Indeed, even for the low PCuSn conditions, ZnS dominates the Raman spectra. For medium power (PCuSn = 45W), the film is composed by SnS, S6, ZnS and CZTS while for higher powers, the films are similar than the one obtained with the CuSn1.5 and CuSn2 targets and the CZTS is replaced by Cu4Zn clusters. For all sets of data, the concentration in Zn is stable. Therefore, the decomposition of the CZTS material seems to be related to the increase of the Cu and Sn concentrations and to the decrease of the S one, which deviates from the theoretical values in CZTS. Based on these data, a phase diagram can be built.
Finally, one film of each region was synthesized and used in solar cell devices in order to investigate the influence of the phase constitution on the cell performance with efficiency up to 1.8%.
[1] Cormier et al., Acta Materiallia, accepted for publication subjected to minor revisions (2015)
[2] Mitzi et al., Solar Energy Materials & Solar Cells 95 (2011)