Tailoring the surface morphology of Cu ₂ ZnSnS ₄ thin films for photovoltaic application

Thin films of Cu ₂ ZnSnS ₄ (CZTS) with controlled surface morphology have been obtained using sol-gel technique. Large cracks present in the precursor films were replaced with small cracks/nano-pinholes when the pH of the corresponding synthesis solution was lower than a value laying in the range of 4.5–3. The tendency to form large cracks in a precursor film has been attributed to the presence of large polymeric molecules that form in the corresponding solution during condensation reaction. By limiting the extent of condensation reaction via controlling the pH of the synthesis solution, the formation of large cracks in the corresponding precursor films was successfully prevented. Further, the pH was controlled by combining the chloride and acetate salts of copper. The small cracks and nano-pinholes present in the precursor films were also undesirable due to the formation of large crystals at these sites during sulfurization. This issue was successfully addressed by applying a capping layer of zinc sulfide on top of the precursor films prior to their sulfurization. A CZTS film with morphology controlled in this fashion as well as having silver alloying was incorporated into a photovoltaic device. This device had an open circuit voltage and a short circuit current density of 0.6 V and 9.47 mA/cm ² , respectively.