Current progress in Cu 2 BaSn(S, Se) 4 thin film

Abstract

The inexhaustible solar energy is regarded as a candidate to displace the diminishing fossil fuel resources and reduce greenhouse emissions. Photovoltaic and photoelectrochemical process provide a pathway for directly converting solar light into electricity power and clean hydrogen energy. It is significant to explore a promising and abundant material for utilization the solar energy. Cu2ZnSn(S, Se)4 has a direct bandgap, suitable bandgap (1.0–1.5 eV), high absorption efficient, which has been used as a absorbed layer in thin film solar cell achieving a power conversion efficiency of 12.6% and a photocathode gaining a photocurrent density of 13 mA/cm2. However, the close ionic sizes between Cu+ (0.91 A) and Zn2+ (0.88 A) in Cu2ZnSn(S, Se)4 leads to the antisite disorder and associated band tailing, and limits the performance improvement. According to the theoretical calculation, substituted the Cu or Zn with the large radius atom can efficiently reduce the antisite disorder. The large Ba substitutes Zn in Cu2ZnSn(S, Se)4 forming Cu2BaSn(S, Se)4, which can suppress the antisite defects. According to the Schockley-Queisser theory, the photocurrent density of the photocathode based on bare Cu2BaSn(S, Se)4 thin film is 14.5 mA/cm2 under AM 1.5G illumination and the maximum power conversion efficiency of the thin film solar cell based on a pure Cu2BaSn(S, Se)4 ( E g =2.0 eV) is 22%. Therefore, it has attracted a great deal of attentions in Cu2BaSn(S, Se)4 thin films. From a special perspective, this review summarizes the progress of Cu2BaSn(S, Se)4 thin films in recent years. First, we outline the physical properties of Cu2BaSn(S, Se)4. According to the experimental and theoretical results, it reveals that the Cu2BaSn(S, Se)4 has less cationic disordering and associated band tailing than that in Cu2ZnSn(S, Se)4, which indicates the Cu2BaSn(S, Se)4 will be a promising candidate to Cu2ZnSn(S, Se)4. Moreover, the Cu2BaSn(S, Se)4 has two crystal structures and the phase transition from trigonal ( P3 1 ) to orthorhombic ( Ama 2 ) with a increasing Se/(S+Se) ratio. Due to the phase transition, the bandgap decreases with an increasing Se/(S+Se) ratio, achieved a lowest bandgap, then the bandgap increases with an increasing Se/(S+Se) ratio. In other word, the bandgap of Cu2BaSn(S, Se)4 can be controlled by adjusted the Se/(S+Se) ratio. Meanwhile, the Cu2BaSn(S, Se)4 shows a direct bandgap which is beneficial to the performance of photoelectrical devices. Therefore, Cu2ZnSn(S, Se)4 will be an excellent semiconductor material in photoelectrical field. Second, the methods for preparing Cu2BaSn(S, Se)4 thin films is a hot topic. Here, we classified the methods for growing Cu2BaSn(S, Se)4 thin films to physical deposition approach and chemical solution approach. The advantages and disadvantages of the method are summarized in this review. It is worth to noting, the morphology of the thin films is related with the composition of Cu2BaSn(S, Se)4. Third, we summarized the application field of Cu2BaSn(S, Se)4 thin films. For example, the solar cells based on Cu2BaSn(S, Se)4 get a highest power efficiency conversion of 5.2% (total area 0.425 cm2) and the Cu2BaSn(S, Se)4 applied in photoelectrochemical electrode achieved a current-density of 12.08 mA/cm2 at 0 V vs. reversible hydrogen electrode. Finally, we conclude the prospective and challenge in the future.