CZTS

CZTS
	; CZTS crystal structure. Orange: Cu, grey: Zn/Fe, blue: Sn, yellow: S.
Names
	Other names copper zinc tin sulfide
Identifiers
CAS Number	12158-89-3 ;
Properties
Chemical formula	Cu2ZnSnS4
Molar mass	439.471 g/mol
Appearance	Greenish black crystals
Density	4.56 g/cm3
Melting point	990 °C (1,810 °F; 1,260 K)
Band gap	1.4–1.5 eV
Structure
Crystal structure	Tetragonal
Lattice constant	a = 0.5435 nm, c = 1.0843 nm, Z = 2
	Except where otherwise noted, data are given for materials in their standard state (at 25 °C [77 °F], 100 kPa). Infobox references

Last updated May 25, 2024

Copper zinc tin sulfide (CZTS) is a quaternary semiconducting compound which has received increasing interest since the late 2000s for applications in thin film solar cells. The class of related materials includes other I₂-II-IV-VI₄ such as copper zinc tin selenide (CZTSe) and the sulfur-selenium alloy CZTSSe. CZTS offers favorable optical and electronic properties similar to CIGS (copper indium gallium selenide), making it well suited for use as a thin-film solar cell absorber layer, but unlike CIGS (or other thin films such as CdTe), CZTS is composed of only abundant and non-toxic elements. Concerns with the price and availability of indium in CIGS and tellurium in CdTe, as well as toxicity of cadmium have been a large motivator to search for alternative thin film solar cell materials. The power conversion efficiency of CZTS is still considerably lower than CIGS and CdTe, with laboratory cell records of 11.0 % for CZTS and 12.6 % for CZTSSe as of 2019^[update].^[5]

Crystal structure

CZTS is a I₂-II-IV-VI₄ quaternary compound. From the chalcopyrite CIGS structure, one can obtain CZTS by substituting the trivalent In/Ga with a bivalent Zn and IV-valent Sn which forms in the kesterite structure.

Some literature reports have identified CZTS in the related stannite structure, but conditions under which a stannite structure may occur are not yet clear. First-principle calculations show that the crystal energy is only 2.86 meV/atom higher for the stannite than kesterite structure suggesting that both forms can coexist.^[6] Structural determination (via techniques like X-ray diffraction) is hindered by disorder of the Cu-Zn cations, which are the most common defect as predicted by theoretical calculations and confirmed by neutron scattering. The near random ordering of Cu and Zn may lead to misidentification of the structure. Theoretical calculations predict the disorder of the Cu-Zn cations to lead to potential fluctuations in the CZTS and could therefore the cause for the large open circuit voltage deficit, the main bottle neck of state-of-the-art CZTS devices. The disorder can be reduced by temperature treatments. However, other temperature treatments alone do not seem to be able to yield highly ordered CZTS.^[7] Other strategies need to be developed to reduce this defect, such as tuning of the CZTS composition.

Material properties

Carrier concentrations and absorption coefficient of CZTS are similar to CIGS. Other properties such as carrier lifetime (and related diffusion length) are low (below 9 ns) for CZTS. This low carrier lifetime may be due to high density of active defects or recombination at grain boundaries. Defect formation in CZTS is prevalent due to low defect formation energies of zinc-copper antisite defects and copper vacancies.^[8] These defects create 'effective' charge in the crystal structure, which is stabilized by the aggregation of different defects that compensate for the charge disparity to become effectively neutral. As a result, electron-trapping states are formed, which enables recombination. Having deep-level defect states lowers the open-circuit voltage and the conversion efficiency of a CZTS solar cell.

Many secondary phases are possible in quaternary compounds like CZTS and their presence can affect the solar cell performance. Secondary phases can provide shunting current paths through the solar cell or act as recombination centers, both degrading solar cell performance. From the literature it appears that all secondary phases have a detrimental effect on CZTS performance, and many of them are both hard to detect and commonly present. Common phases include ZnS, SnS, CuS, and Cu₂SnS₃. Identification of these phases is challenging by traditional methods like X-ray diffraction (XRD) due to the peak overlap of ZnS and Cu₂SnS₃ with CZTS. Some impurities such as ZnS and Cu₈GeS₆ can be selectively removed from the CZTS or CZGS by treatment with hot diluted HCl.^[9] Polymorphism could be another source of defects in CZTS. Band gap of different polymorphs have been found to range from 1.12 to 1.45 eV. Other methods like Raman scattering are being explored to help characterize CZTS polymorphs. ^[10]

Fabrication

CZTS has been prepared by a variety of vacuum and non-vacuum techniques. They mostly mirror what has been successful with CIGS, although the optimal fabrication conditions may differ. Methods can be broadly categorized as vacuum deposition vs. non-vacuum and single-step vs. sulfidation and selenidation reaction methods. Vacuum-based methods are dominant in the current CIGS industry, but in the past decade there has been increasing interest and progress in non-vacuum processes owing to their potential lower capital costs and flexibility to coat large areas.

The record-holding CZTS solar cells is made by spin coating a hydrazine-based slurry.^[11] Due to its reducing character, hydrazine can stabilize sulfide and selenide anions in solution without adding impurities into the mix.^[12] To prevent defect formation, copper-poor and zinc-rich solutions were used.

A particular challenge for fabrication of CZTS and related alloys is the volatility of certain elements (Zn and SnS) which can evaporate under reaction conditions. Once CZTS is formed, element volatility is less of a problem but even then CZTS will decompose into binary and ternary compounds in vacuum at temperatures above 500 °C. This volatility and difficulty of preparing a single-phase material has resulted in the success of many traditional vacuum methods. Currently the best CZTS devices have been achieved through certain chemical methods which allow CZTS formation at low temperatures avoiding volatility problems.

A continuous flow process using ethylene glycol as a solvent has been developed at Oregon State University which may be suitable for industrial scale mass production.^[13]

Motivation for development

CIGS and CdTe are two of the most promising thin-film solar cells and have recently seen growing commercial success. Despite continued rapid cost reduction, concerns about material price and availability as well as toxicity have been raised. Although current material costs are a small portion of the total solar cell cost, continued rapid growth of thin-film solar cells could lead to increased material price and limited supply.

For CIGS, indium has been subject to growing demand because of the rapid expansion of indium tin oxide (ITO) used in flat screen displays and mobile devices. The demand coupled with limited supply helped prices quickly climb to over $1000/kg before the global recession. While processing and capital equipment make up the majority of the costs for producing a CIGS solar cells, the price of the raw material is the lower bound for future costs and could be a limiting factor in decades ahead if demand continues to increase with limited supply. Indium exists mostly in low concentration ore deposits and is therefore obtained mainly as a byproduct of zinc mining. Growth projections based on many assumptions suggest that indium supply could limit CIGS production to the range of 17–106 GW/yr in 2050.^[14] Tellurium is even scarcer than indium, although demand has also been historically lower. Tellurium abundance in the Earth's crust is similar to that of gold, and projections of future availability range from 19 to 149 GW/yr in 2050.

CZTS (Cu₂ZnSnS₄) offers to alleviate the material bottlenecks present in CIGS (and CdTe). CZTS is similar to the chalcopyrite structure of CIGS but uses only earth-abundant elements. Raw materials are about five times cheaper than those for CIGS, and estimates of global material reserves (for Cu, Sn, Zn and S) suggest we could produce enough energy to power the world with only 0.1% of the available raw material resources.^[15] In addition, CZTS is non-toxic, unlike CdTe and to a lesser extent CIGS (although selenium is sometimes alloyed with CZTS and CdS is sometimes used as the n-type junction partner). In addition to these economical and environmental benefits, CZTS exhibits much greater radiation hardness than other photovoltaic materials, making it an excellent candidate for use in space.^[16]

Development of solar cells

CZTS was first created in 1966^[17] and was later shown to exhibit the photovoltaic effect in 1988.^[18] CZTS solar cells with efficiency up to 2.3% were reported in 1997, as well as CZTSe devices.^[19] The solar cell efficiency in CZTS was increased to 5.7% in 2005 by optimizing the deposition process.^[20] Recently, a 3.4% bifacial device, using In substituted CZTS (CZTIS) absorber material and transparent conducting back contact was reported in 2014,^[21] which can produce photocurrent on either side of illumination; later, the device efficiency based on this bifacial configuration has been boosted to 5.8% in 2016.^[22] Additionally, it has been demonstrated that sodium has an enhancing effect on the structural and electrical properties of CZTS absorber layers.^[23] These improvements, alongside the beginnings of CIGS production on a commercial scale in the mid-2000s catalyzed research interest in CZTS and related compounds.

Since 1988 CZTS was considered as an alternative to CIGS for commercial solar cell systems. The advantage of CZTS is the lack of the relatively rare and expensive element indium. The British Geological Survey Risk List 2011 gave indium a "relative supply risk index" of 6.5, where the maximum was 8.5.^[24]

In 2010, a solar energy conversion efficiency of about 10% was achieved in a CZTS device.^[25] CZTS technology is now being developed by several private companies.^[26] In August 2012, IBM announced they had developed CZTS solar cell capable of converting 11.1% of solar energy to electricity.^[27]

In 2013 Rajeshmon et al. reported an efficiency of 1.85% on spray pyrolysed CZTS/In₂S₃ solar cell. ^[28]

In November 2013, the Japanese thin-film solar company Solar Frontier announced that in joint research with IBM and Tokyo Ohka Kogyo (TOK), they had developed a world-record setting CZTSSe solar cell with a 12.6% energy conversion efficiency.^[29]

In 2018, CZTS nanoparticles were used as a hole transport layer for perovskite solar cells as a method to increase device stability and affordability, yielding a reported conversion efficiency of 9.66%.^[30]

Related Research Articles

Indium tin oxide (ITO) is a ternary composition of indium, tin and oxygen in varying proportions. Depending on the oxygen content, it can be described as either a ceramic or an alloy. Indium tin oxide is typically encountered as an oxygen-saturated composition with a formulation of 74% In, 8% Sn, and 18% O by weight. Oxygen-saturated compositions are so typical that unsaturated compositions are termed oxygen-deficient ITO. It is transparent and colorless in thin layers, while in bulk form it is yellowish to gray. In the infrared region of the spectrum it acts as a metal-like mirror.

Photovoltaics (PV) is the conversion of light into electricity using semiconducting materials that exhibit the photovoltaic effect, a phenomenon studied in physics, photochemistry, and electrochemistry. The photovoltaic effect is commercially used for electricity generation and as photosensors.

<span class="mw-page-title-main">Copper indium gallium selenide</span> Chemical compound

Copper indium gallium (di)selenide (CIGS) is a I-III-VI₂ semiconductor material composed of copper, indium, gallium, and selenium. The material is a solid solution of copper indium selenide (often abbreviated "CIS") and copper gallium selenide. It has a chemical formula of CuIn_1−xGa_xSe₂, where the value of x can vary from 0 (pure copper indium selenide) to 1 (pure copper gallium selenide). CIGS is a tetrahedrally bonded semiconductor, with the chalcopyrite crystal structure, and a bandgap varying continuously with x from about 1.0 eV (for copper indium selenide) to about 1.7 eV (for copper gallium selenide).

<span class="mw-page-title-main">Zinc phosphide</span> Chemical compound

Zinc phosphide (Zn₃P₂) is an inorganic chemical compound. It is a grey solid, although commercial samples are often dark or even black. It is used as a rodenticide. Zn₃P₂ is a II-V semiconductor with a direct band gap of 1.5 eV and may have applications in photovoltaic cells. A second compound exists in the zinc-phosphorus system, zinc diphosphide (ZnP₂).

Building-integrated photovoltaics (BIPV) are photovoltaic materials that are used to replace conventional building materials in parts of the building envelope such as the roof, skylights, or façades. They are increasingly being incorporated into the construction of new buildings as a principal or ancillary source of electrical power, although existing buildings may be retrofitted with similar technology. The advantage of integrated photovoltaics over more common non-integrated systems is that the initial cost can be offset by reducing the amount spent on building materials and labor that would normally be used to construct the part of the building that the BIPV modules replace. In addition, BIPV allows for more widespread solar adoption when the building's aesthetics matter and traditional rack-mounted solar panels would disrupt the intended look of the building.

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HelioVolt Corporation was a privately held solar energy company based in Austin, Texas that suspended operations in 2014. The company manufactured photovoltaic (PV) solar modules using a thin film semiconductor process based on copper indium gallium selenide (CIGS) to produce CIGS solar cells. HelioVolt manufactured these thin film modules for commercial rooftop, utility-scale ground mount, residential, building-integrated photovoltaics (BIPV) and custom installations. The company raised over $230 million in investments, including over $80 million by SK Group.

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Tin(II) sulfide is a chemical compound of tin and sulfur. The chemical formula is SnS. Its natural occurrence concerns herzenbergite (α-SnS), a rare mineral. At elevated temperatures above 905 K, SnS undergoes a second order phase transition to β-SnS (space group: Cmcm, No. 63). In recent years, it has become evident that a new polymorph of SnS exists based upon the cubic crystal system, known as π-SnS (space group: P2₁3, No. 198).

Global Solar Energy is a US-based manufacturer of CIGS solar cells, a thin-film based photovoltaic technology, with manufacturing operations in Tucson, Arizona, United States, and Berlin, Germany. In 2013, it was bought by Chinese renewable energy company Hanergy.

Cadmium telluride (CdTe) photovoltaics is a photovoltaic (PV) technology based on the use of cadmium telluride in a thin semiconductor layer designed to absorb and convert sunlight into electricity. Cadmium telluride PV is the only thin film technology with lower costs than conventional solar cells made of crystalline silicon in multi-kilowatt systems.

Thin-film solar cells are made by depositing one or more thin layers of photovoltaic material onto a substrate, such as glass, plastic or metal. Thin-film solar cells are typically a few nanometers (nm) to a few microns (μm) thick–much thinner than the wafers used in conventional crystalline silicon (c-Si) based solar cells, which can be up to 200 μm thick. Thin-film solar cells are commercially used in several technologies, including cadmium telluride (CdTe), copper indium gallium diselenide (CIGS), and amorphous thin-film silicon.

<span class="mw-page-title-main">Copper indium gallium selenide solar cell</span>

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<span class="mw-page-title-main">Kesterite</span>

Kësterite is a sulfide mineral with a chemical formula of Cu₂(Zn,Fe)SnS₄. In its lattice structure, zinc and iron atoms share the same lattice sites. Kesterite is the Zn-rich variety whereas the Zn-poor form is called ferrokesterite or stannite. Owing to their similarity, kesterite is sometimes called isostannite. The synthetic form of kesterite is abbreviated as CZTS. The name kesterite is sometimes extended to include this synthetic material and also CZTSe, which contains selenium instead of sulfur.

Solar Frontier Kabushiki Kaisha is a Japanese photovoltaic company that develops and manufactures thin film solar cells using CIGS technology. It is a fully owned subsidiary of Showa Shell Sekiyu and located in Minato, Tokyo, Japan. The company was founded in 2006 as Showa Shell Solar, and renamed Solar Frontier in April 2010.

Light soaking refers to the change in power output of solar cells which can be measured after illumination. This can either be an increase or decrease, depending on the type of solar cell. The cause of this effect and the consequences on efficiency varies per type of solar cell. Light soaking can generally cause either metastable electrical or structural effects. Electrical effects can vary the efficiency depending on illumination, electrical bias and temperature, where structural effects actually changes the structure of the material and performance is often permanently altered.

References

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