Zinc sulfide (ZnS) is a wide direct band gap, high optical absorption coefficient, reasonable work function. It has attracted considerable attention due to its excellent electrical and optical properties with its distinct properties has become the potential candidate for many applications^[1-2].

ZnS layers were electrodeposited from an aqueous electrolyte containing 0.3 M ZnCl₂ and 0.03 M (NH₄)₂S₂O₃ in 800 mL of de-ionized water. Electropurification of the ZnCl₂ was carried out for 48 h prior to the addition of (NH₄)₂S₂O₃ in order to remove any possible impurity ions present in the solution. Finally, the pH of the electrolyte containing both precursors was adjusted to 3.00 ± 0.02. The temperature of the electrolyte was 30 °C. Uniform and transparent ZnS layers were cathodically deposited on cleaned glass/İTO substrates using a simple two-electrode deposition system at a cathodic potential of 1550 mV established using a cyclic voltammogram. The deposited layers using an average deposition current density of ~65 µA·cm^-2 and deposition time of 60 min have thickness of ~150 nm. These were then annealed in air at 350 °C for 10 min.

Prior to the deposition of Cu₂ZnSnS₄, the glass/İTO/ZnS substrates were cleaned with methanol and deionised water. The deposition of Cu₂ZnSnS₄ layers was also done using a two-electrode system at a cathodic deposition potential of 1450 mV also established using a cyclic voltammogram. The Cu₂ZnSnS₄ deposited on glass/İTO had a thickness ~300 nm while that deposited on glass/İTO/ZnS had a thickness ~150 nm. This therefore brings the total thickness of the ZnS/Cu₂ZnSnS₄ bi-layer to ~250 nm comparable to the ~300 nm of Cu₂ZnSnS₄ grown on glass/İTO. The CdTe deposition electrolyte contained 1 M CdSO₄ (99.0%) and 1 mM TeO₂ (99.999%) in 800 mL of de-ionized water. To do this, a cyclic voltammogram was recorded using the two-electrode system, to determine the reduction potential of Cd²⁺. The TeO₂ was first dissolved in H₂SO₄ and then added into the bath after the electro-purification of CdSO₄, and the pH of the electrolyte adjusted to 2.00 ± 0.02. After depositing and characterizing few CdTe samples on glass/İTO substrates, the final cathodic deposition potential for CdTe was taken as 2038 mV. CdTe thin layers with thickness of ~1.70 µm were then deposited on annealed glass/İTO/CdS and glass/İTO/ZnS/CdS substrates previously cleaned with methanol and de-ionised water. Typical deposition time for the CdTe used in this work was 4 h, with an average deposition current density of ~176 µA·cm^-2. To complete the solar cell fabrication, the annealed glass/İTO/Cu₂ZnSnS₄/p-CdTe 2-layer structure and glass/İTO/n-ZnS/n-Cu₂ZnSnS₄/p-CdTe 3-layer structure were etched for 5 s in aqueous solution of 1.0 g of K₂Cr₂O₇ acidified with 10 mL of dilute H₂SO₄ in 10 mL of deionised water, rinsed in deionized water and then etched in a warm solution containing 0.5 g each of NaOH and Na₂S₂O₃ in 50 mL of deionised water for 2 min. The thickness of the gold contacts was ~100 nm each with a diameter of 2 mm. This makes ZnS a suitable candidate for use as effective buffer/window layer in CdTe-based multilayer graded bandgap solar cells. It is important to note what happens to the ZnS/Cu₂ZnSnS₄/CdTe structure in the annealing process. The glass/ITO/ZnS/Cu₂ZnSnS₄/CdTe/In solar cell is also similar to the glass/ITO/ZnS/Cu₂ZnSnS₄/CdTe/In counterpart in structure and is used as a control experiment in this work to compare the advantages of the architecture with ZnS as wide bandgap buffer/window layer. The result of using ZnS as the buffer/window layer is directly reflected in the improved high short-circuit current density (Jsc) as well as improved open-circuit voltage (Voc), fill factor (FF) and ultimately, the conversion efficiency (η) of the 3-layer device, are compared to the device.

However, to ensure that the observed high Jsc values are genuine, the diodes producing them were isolated by carefully removing the CdTe material around them and repeating the I-V measurements. It is therefore possible in these solar cells for photons with energy lower than the energy bandgap of CdTe to create useful electron-hole pairs that contribute to photo-generated current.

Keywords: Solar cells, 3-layer device

References
[1] Razykov, T.M.; Ferekides, C.S.; Morel, D.; Stefanakos, E.; Ullal, H.S.; Upadhyaya, H.M. Solar photovoltaic electricity: Current status and future prospects. Sol. Energy 2011, 85, 1580–1608.
[2] Contreras-Puente, G.; Vigil, O.; Ortega-Lopez, M.; Morales-Acevedo, A.; Vidal, J.;Albor-Aguilera, M.L. New window materials used as heterojunction partners on CdTe solar cells.Energies 2015, 8 4434