(735g) Spray Deposition of Cu2ZnSnS4 (CZTS) Semiconductor Photo Electrode for Solar Rechargeable Battery

Purpose:

Copper, Zinc, Tin and Sulfur based semiconductor, Cu₂ZnSnS₄ (CZTS), is an excellent candidate for cost effective solar energy conversion. Major researches on CZTS are focused on devices with solid-solid junction. These photovoltaic devices, in practice, must be associated with energy storage devices due to the intermittent availability of sunlight. Here we show that CZTS photo electrode can be used to recharge a polysulfide-bromide flow battery. The process intensification by the direct conversion of solar energy into chemical energy would lead to reduced fabrication and operating cost.

Methods:

Ultrafast femtosecond laser spectroscopy was used to study the charge transfer dynamics in CZTS semiconductors with varying compositions. The semiconductor materials are prepared via two different methods and then sprayed onto FTO-glass substrate. In one method, the precursor solutions containing metal salts are sprayed at 300^oC onto glass substrate and then the films are annealed at 450^oC in sulfur and tin sulfide atmosphere. In another method, CZTS QD are prepared at as low as 250^oC which is then subjected to Na₂S treatment for ligand exchange in order to remove excess carbon from the surface. Finally, the nanocrystal dispersion is sprayed onto FTO at 250^oC. The photosensitivity of the prepared electrodes were tested using three electrode photo electrochemical test with polysulfide electrolyte.

Result:

The carrier lifetimes, as determined using transient absorption spectroscopy, are reflected in the photoelectrochemical (PEC) performance. The photocurrent generated by the reduction of polysulfide is stable for long period of time. Theoretical calculation shows that the holes generated are oxidizing enough to oxidize the bromide solution into polybromide.

Conclusion:

Solution process of CZTS is an excellent way to prepare low temperature, low cost photoelectrode but the presence of long carbon chain suppress the performance. This drawback can be overcome by treating the nanocrystal with Na₂S solution. The ligand exchanged nanocrystal can be sprayed into thin film which can be used as photocathode in a polysulfide-bromide redox flow battery.