CZTSSe

 

Authors: Scott McClary, Caleb Miskin, Rakesh Agrawal, Purdue University, Davidson School of Chemical Engineering.

As seen elsewhere on this site, CdTe and Cu(In,Ga)Se2 (CIGSe) solar cells have achieved remarkable efficiencies exceeding 20%. However, elements such as indium and tellurium are rare, and cadmium is quite toxic, which may limit the implementation of these devices at the terawatt scale. In recent years, Cu2ZnSnS4 (CZTS), Cu2ZnSnSe4 (CZTSe), and Cu2ZnSn(S,Se)4 (CZTSSe) have emerged as replacement materials for thin-film PV due to promising optoelectronic properties and the use of nontoxic, earth-abundant elements.

History of CZTSSe

CZTS was first investigated as a solar cell material in 1988 at Shinshu University in Japan. The researchers deposited thin films of CZTS using sputtering and measured p-type conductivity, a direct band gap of 1.45 eV, and absorption coefficients exceeding 104 cm-1 in the visible range.[1] They reported a solar cell the next year with a Voc of 0.165 V, though the Jsc was very low. In 1996, the first solar cells based on CZTS and CZTSe absorbers using cadmium sulfide/zinc oxide window layers were reported with efficiencies exceeding 0.6%.[2][3] Further research continued over the next decade, with improvements to processing conditions and window layers, resulting in efficiencies nearing 7%.[4] Around this time, research into solution processing of CZTSSe cells began.[5][6] In 2013, IBM’s Watson Research Center was able to push the efficiency of a lab-scale solar cell to 12.6% using a hydrazine-based solution process; this stands as the world record for a CZTSSe-based solar cell today.[7]

Structures of CZTSSe

Crystal structures of semiconductor materials.
Crystal structures of semiconductor materials. (Courtesy of Dr. Bryce Walker)

CZTS may crystallize in a zincblende-derived structure similar to other semiconductor materials such as silicon and CIGSe. Conceptually, this kesterite CZTS cell is formed by substituting half of the In/Ga sites in CIGSe with Zn and the other half with Sn. However, CZTS can have two other forms. The stannite structure has the same tetragonal coordination, but different symmetry due to alternate placement of cations in the crystal lattice. The wurtzite-derived structure is a hexagonal close-packed array. All three have the nominal Cu2ZnSnS4 stoichiometry, though the kesterite structure is used most often in solar cells.[8] Note that the CZTSSe and CZTSe compounds adopt analogous structures to those of CZTS.

Fabrication of CZTSSe Solar Cells

Materials/CZTS_architecture.png
Typical architecture of a CZTSSe-based solar cell.

Thin-film solar cells with CZTSSe absorber layers are typically fabricated using a substrate configuration, as shown above. The layers are all deposited on a conductive substrate (typically molybdenum-coated soda lime glass), which serves as the back contact. Some cells are also fabricated using a superstrate configuration in which all of the layers are sequentially deposited on a transparent conducting surface, often tin-doped indium oxide (ITO) or fluorine-doped tin oxide (FTO) coated on glass. The cell is then flipped over and illuminated through the superstrate for operation.

Materials/CZTS_architecture.png
Sample schematic of CZTS solar cell production: Nanocrystal-based processing

Fabrication methods for CZTSSe solar cell absorber layers can be broadly classified into two different categories: those that are vacuum-based and those that are solution-based (i.e. non-vacuum based). Vacuum-based processes include (but are not limited to) sputtering, evaporation, and atomic layer deposition, while solution-based processes include nanocrystal “inks”, molecular precursors, electrodeposition, sol-gel, and spray pyrolysis.

To fabricate a typical substrate CZTS solar cell, one starts with a substrate with a metallic back contact, such as Mo. Then, the CZTS material is deposited on a substrate using one of the processes above. It is often annealed at high temperatures (~500-600 °C) in gaseous atmospheres such as sulfur, selenium, hydrogen sulfide, and argon. Then, a thin n-type window layer (typically CdS) is deposited by chemical bath deposition or sputtering, followed by sputtering of a hole-blocking layer of intrinsic ZnO and a transparent conducting layer of ITO. Patterned metal grids (e.g. nickel/aluminum) are evaporated to complete the device. Individual cells are often defined by mechanical scribing down to (but not through) the Mo-back contact. The solar cell can then be tested under simulated sunlight to evaluate its performance.

CZTSSe Limitations and Future Research Directions

CZTSSe is an inherently challenging material to work with due to its complexity. Forming a phase-pure material is difficult, as its thermodynamic stability window is quite narrow; additionally, Sn-compounds are volatile, and CZTS breaks down at high temperatures. Many secondary phases (particularly Cu2SnS3 and ZnS) may coexist in the absence of carefully controlled reaction conditions.[9] Additionally, the stannite and kesterite phases have similar formation energies, so it is likely that a mixture of both can form during synthesis. To complicate matters further, it is difficult to distinguish between these unwanted phases through traditional characterization techniques such as X-ray diffraction.

Crystal structure defects can form quite easily in CZTS. One of the most common defects is an antisite defect – Cu cations can occupy Zn sites and vice versa due to their similar ionic radii. These defects are partially compensated by targeting a Cu-poor, Zn-rich composition during materials synthesis. However, these defects are not eliminated completely and can give rise to band tail states – essentially, these are trap states that arise due to electrostatic potential fluctuations in the material. These states lower the Voc considerably and severely limit the cell’s efficiency.[10][11]

Current research is focused primarily on understanding and mitigating the defects within the CZTSSe crystal structure. One potential method is to replace either the Cu or Zn with another atom of a significantly different size (e.g. Cu with Ag or Zn with Ba) to prevent formation of antisite defects, though this could change other material properties such as the conductivity type or crystal structure.[12][13] Another critical area of research focuses on optimizing the solar cell structure, particularly through use of alternative n-type layers that may have energy bands that align more favorably with CZTS.[14]

Further Reading

Copper Zinc Tin Sulfide-Based Thin Film Solar Cells, 1st ed.; Ito, K., Ed.; John Wiley & Sons, Ltd., 2015.

Mitzi, D. B.; Gunawan, O.; Todorov, T. K.; Wang, K.; Guha, S. The Path towards a High-Performance Solution-Processed Kesterite Solar Cell. Sol. Energy Mater. Sol. Cells 2011, 95 (6), 1421–1436.

Zhou, H.; Hsu, W.-C.; Duan, H.-S.; Bob, B.; Yang, W.; Song, T.-B.; Hsu, C.-J.; Yang, Y. CZTS Nanocrystals: A Promising Approach for next Generation Thin Film Photovoltaics. Energy Environ. Sci. 2013, 6 (10), 2822–2838

Acknowledgement

The material on this page was developed as part of the 2016 Hands-On PV Experience (HOPE) Workshop at NREL.

http://www.nrel.gov/pv/hands-on-photovoltaic-experience.html