Scientists at Swansea University in Wales (UK) have shown that Cu2ZnSn(S,Se)4 (CZTS), which is based on Earth-abundant elements, could potentially be an alternative to conventional Cu(In,Ga)Se2 (CIGS) in solar cells.
CZTS-based solar cells are currently much less efficient than CIGS devises—12.7% compared with 22.3% maximum conversion. The Swansea team is now using Raman mapping analysis to explore how to improve the performance of CZTS-based solar cells. “Recently, we successfully demonstrated that large-area Raman mapping could be used to show the effect of removal of secondary phases by chemical etching of CZTS samples,” reports Dr Zhengfei Wei, whose team in the SPECIFIC Innovation and Knowledge Centre in Swansea’s College of Engineering has investigated CZTS. “The Raman measurements provide chemical and crystallographic information about the material — in this case the ratio of SnS to CZTS, which showed a clear reduction after etching.”
In their experiments, the researchers etched CZTS samples in different durations using an aqueous Na2S solution. “We used a Renishaw inVia confocal Raman microscope to characterize the samples by both Raman spectroscopy and photoluminescence,” says Wei, noting that Raman spectroscopy is a powerful technique to identify a number of CZTS and secondary phases at the surface, which could provide essential feedback for the optimization of process parameters and film growth.
A pioneering approach: Largest Raman mapping for CZTS-based materials
Raman mappings on a 1 mm x 10 mm have been found to be an effective and sensitive method to illustrate the successful removal of surface secondary phases on CZTS. “To our knowledge,” says Wei, “1 mm x 10 mm is the largest Raman mapping for CZTS-based materials so far.”
He elaborates that large-area mapping is useful to gain a macroscopic picture of the distribution of the defects. Additionally, the team conducted room-temperature photoluminescence measurements to confirm the effect of those secondary phases on CZTS within the Raman mapping area.
CZTS as alternative to CIGS?
Scientists have suggested physical mechanisms such as interface losses, electrostatic or bandgap fluctuations and defect-driven recombination to be culprits in causing the roughly 10-point efficiency gap between CZTS solar cells and their CIGS-based counterparts. Wei says secondary phase materials at the pn-junction interface could lead to one or more of the above energy loss mechanisms, and that secondary phases such as Cu2SnS3 or SnS — having a low-bandgap at the interface — can cause a decrease in open circuit voltage, while ZnS — with high bandgap — can act as a photocurrent barrier and is usually responsible for a high series resistance. “We use large area Raman mapping to reveal the effect of chemical etching (1M Na2S aqueous solution) on the removal of secondary phases (predominantly SnS) at the surface of CZTS films,” he points out.
Improving CZTS solar cells
The research advance accomplished at Swansea could potentially influence the design of future generations of solar cells.
“The combination of both large-area Raman mapping and photoluminescence represents a powerful new tool for locating surface impurities following etching and can be used as a critical guide in improving CZTS device performance,” Wei confirms. He says what is most exciting to him about the outcome of this study is, “Raman mapping is used to rapidly gather comprehensive 2D distribution of crystalline phases, which could play a significant role in monitoring absorber film quality during industrial production.”
What lays ahead for the Swansea team
“These results enable us to quickly determine the best etching parameter and further help us to apply the most suitable etching in device fabrication of CZTS solar cells,” Wei says, projecting that future studies will focus on exploring the links between Raman mapping and solar cell performance within CZTS solar cells.
The full study, titled “Raman mapping analysis for removal of surface secondary phases of CZTS films using chemical etching," is published in Applied Physics Letters.
Written by Sandra Henderson, Research Editor, Solar Novus Today