Optical and grain boundary potential characteristics of sulfurized BiFeO₃ thin films for photovoltaic applications

Sulfurized BiFeO₃ (BFO) thin films have been investigated with the purpose of reducing their band gap for photovoltaic applications. A strong dependence of the degree of sulfurization on the structure and optical properties of the BFO thin films was observed. The sulfurization process substantially reduced the optical band gap from 2.83 eV for the reference sample to ∼1.90 eV in a sample sulfurized at 200 °C, a temperature at which the BFO phase was still dominant. The existence of the secondary Bi₂S₃ phase was found to be initiated from the film surface and became dominant at higher temperatures. XPS analysis suggests potential Bi-Fe(iii)-Fe(ii)-S-O compounds as a result of the change of the oxidation state of Fe with the progress of sulfurization. The sulfurized BFO film exhibited relatively higher positively charged grain boundaries than the reference film, suggesting its improved applicability in photovoltaic devices.