Effects of annealing temperature and cooling rate on photo-electrochemical performance of pristine polycrystalline metal-chalcogenide film electrodes

摘要

Photoelectrochemical (PEC) conversion processes are recently emerging in solar energy technology. To avoid high cost and special preparation conditions, thin films of polycrystalline nano-size materials are being considered PEC-based solar energy conversions. Nano-scale metal-chalcogenide (MX where M = Cd, Cu, Zn or others; X = S, Se, or Te) based films are being widely studied in such processes. This is because metal-chalcogenide film electrodes have band gap values suitable for visible light and can be prepared by simple methods. Being in the nano-scale thickness, they demand lower starting materials and are therefore less hazardous to environment. However, with their narrow-to-medium band gap values, pristine metal-chalcogenide film electrodes are unstable to photo-corrosion under PEC conditions. They also exhibit low PEC conversion efficiency, which encourages researchers to consider multi-junction thin film systems. Such a strategy adds to the cost and undermines the virtue of simplicity of metal chalcogenide films in their pristine form. Attempts have been widely made to enhance stability and conversion efficiency of such film electrodes in their pristine forms, using simple methods. Among those attempts are controlled annealing temperatures and, to a lesser extent, controlled cooling rates. This review is devoted to study effects of annealing temperature and cooling rate on stability and performance of pristine metal chalcogenide film electrodes under PEC conditions. Basic science models, known for atomic migrations inside metal crystals, are used here to rationalize effects of annealing and cooling rate on film electrode physical properties. By modulating physical properties of semiconductor film electrodes their PEC characteristics (most notably conversion efficiency and stability) can be optimized. In this respect, generalizations based on reported literature are highlighted. Recommendations on best practice in pristine metal-chalcogenide electrode annealing temperature and cooling rate are also presented.