Cu₂ZnSnS₄ (CZTS) is a kesterite-type material with promising properties among the materials suitable to be used as absorbers in solar cells. It is highly absorbent (α ≥ 10⁵ cm^-1), with a direct bandgap energy value (E_GAP) suitable for solar light absorption (E_GAP close to 1.5 eV) [1]. The constituting elements are not toxic, expensive or scarce, as is the case in competing family compounds such as chalcopyrites (CuInGaSe₂). CZTS films are thus excellent candidates for low-cost, highly efficient and environmentally friendly solar cells [2].

There are different electroplating routes to obtain CZTS. In a previous work [3] we explored electrodeposition from a single bath containing the four elements and found it difficult to stabilize the sulfur source in the precursor mixture. Moreover, sulfur was found to incorporate into the material mainly during the sulfurization stage. In this work, the synthesis started with the deposition of a Cu-Zn-Sn precursor (CZT), to be continued by the incorporation of S during a heat treatment in the presence of vaporized sulfur that transforms the precursor into CZTS. The purpose of this investigation was to adjust the pH of the precursor bath to 5.9, so that CZTS could be deposited on top of ZnO, in order to prepare solar cells in superstrate configuration. The effect of electrodeposition time and annealing temperature were also explored.

Thin films of CZTS were prepared by sulfurization of metallic precursors (CZT) previously electrodeposited during 5 to 10 min at -1.15 V_SCE using conducting glass (FTO) as primary substrate. The precursor solution consisted of 100 mmol/L ZnSO₄, 30 mmol/L CuSO₄, 10 mmol/L SnSO₄ and 100 mmol/L of sodium citrate as complexing agent. Citrates and hydroxyls concentrations were adjusted so that pH = 5.9.

Electrodeposited CZT precursors underwent a reactive sulfurization annealing stage in a three-temperature zone thermal reactor. First, the samples were placed in the cold zone of the tube (room temperature). Then the second zone was heated to 500 °C and in the third zone the sulfur powder (0.3 g) was allowed to reach 350 °C (melting starts at around 150 °C). The vaporized sulfur reached the samples assisted by argon flux. After 30 to 60 minutes, the samples were left to cool down naturally.

CZTS films were characterized by X-ray diffraction (XRD), Raman spectroscopy, scanning electronic microscopy (SEM), UV-Vis spectroscopy and photoelectrochemical techniques. SEM results showed good coverage and homogenous morphology. Also, cross-section images showed thickness values close to 1 micron and higher than expected, even for samples electrodeposited for just 10 min.

Raman spectroscopy and XRD results confirmed the formation of crystalline CZTS after sulfurization, with no secondary phases detected in the film. Photocurrent analysis confirmed the p-type nature of the semiconductor. Direct energy gap values close to 1.5 eV were estimated for CZTS films using transmittance spectra. CZTS films obtained with short electrodeposition times are good-quality absorbers suitable to be used in kesterite thin films solar cells. Preliminary results from solar cells in superstrate configuration show a promising photovoltaic response.

References

[1] S. Siebentritt and S. Schorr, Progress in Photovoltaics: Research and Applications, 20 (2012) 512-519.

 [2] T. K.Todorov, J. Tang, S. Bag, O. Gunawan, T. Gokmen, Y. Zhu, D. B. Mitzi Advanced Energy Materials, 3 (2013) 34–38.

 [3] M. Valdes, M. Modibedi, M. Mathe, T. Hillie, M. Vazquez . Electrochimica Acta 128 (2014) 393-399.