CZTS
Encyclopedia
Copper zinc tin sulfide (CZTS) is a quaternary semiconducting compound which has received increasing interest since the late 2000’s for applications in 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. Recent material improvements for CZTS have increased efficiency to just above 10% in laboratory cells, but more work will be needed for successful commercialization [1].
and CdTe are two of the most promising thin film solar cells and have recently seen growing commercial success. Despite continued rapid cost reductions, concerns about material price and availability as well as toxicity have been raised. Although current material costs are a small portion of total solar cell cost, continued rapid growth of thin film solar cells could lead to increased material cost and limited supply.
For CIGS, indium is the most scarce element and has been subject to growing demand due to the rapid expansion of indium tin oxide (ITO) used in flat screen displays and mobile devices (FIGURE ELEMENT SCARCITY). 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 raw material cost 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 [2]. Tellurium is even more scarce than indium although demand has also been historically lower. Tellurium abundance in the earth’s crust is similar to 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 material costs are ~5x less than 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 (Figure 1)[3]. In addition, CZTS doesn’t have any concerns with toxicity 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).
Some literature reports have identified CZTS in the related stannite structure, but conditions under which a stannite structure may occur are not yet clear. Theoretical 1st principle calculations show that while stannite is only 2.86 meV higher than the kesterite structure which suggests that it may be possible to form under non-equilibrium conditions[4]. Structural determination (via techniques like XRD) is made more difficult 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 difficulty identifying the true structure and false reports of stannite structure.
Many different secondary phases are possible in quaternary compounds like CZTS and their presence can have large effects on solar cell performance. Secondary phases can provide shunting current paths through the solar cell or act as centers of recombination both degrading solar cell performance. From the literature it appears that all secondary phases have a detrimental effect on CZTS performance (although some are worse than others). Unfortunately many secondary phases in CZTS 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 XRD due to peak overlap of ZnS and Cu2SnS3 with CZTS. Other methods like Raman scattering are being explored to help characterize CZTS.
A particular challenges 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 once formed, CZTS will decompose into binary and ternary compounds in vacuum at temperatures above 500C (SOURCE). The element volatility and difficulty achieving single phase material has stymied the success of many traditional vacuum methods. Currently the record CZTS devices have been achieved through certain chemical methods which allow CZTS formation at low temperatures avoiding volatility problems.
. The British Geological Survey
Risk List 2011 gave indium a "relative supply risk index" of 6.5, where the maximum was 8.5.
In 2010, a solar energy conversion efficiency of about 10% was achieved in a CZTS device. CZTS technology is now being developed by several private companies.
Copper indium gallium selenide
Copper indium gallium selenide is a I-III-VI2 semiconductor material composed of copper, indium, gallium, and selenium. The material is a solid solution of copper indium selenide and copper gallium selenide...
) making it well suited for use as a thin film solar cell absorber layer, but unlike CIGS
CIGS
CIGS may refer to:* Chief of the Imperial General Staff, a pre-1964 military position in the British Army* Copper indium gallium selenide , a semiconductor absorber material for solar cells...
(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
Indium
Indium is a chemical element with the symbol In and atomic number 49. This rare, very soft, malleable and easily fusible post-transition metal is chemically similar to gallium and thallium, and shows the intermediate properties between these two...
in CIGS and tellurium in CdTe, as well as toxicity of cadmium
Cadmium
Cadmium is a chemical element with the symbol Cd and atomic number 48. This soft, bluish-white metal is chemically similar to the two other stable metals in group 12, zinc and mercury. Similar to zinc, it prefers oxidation state +2 in most of its compounds and similar to mercury it shows a low...
have been a large motivator to search for alternative thin film solar cell
Thin film solar cell
A thin-film solar cell , also called a thin-film photovoltaic cell , is a solar cell that is made by depositing one or more thin layers of photovoltaic material on a substrate...
materials. Recent material improvements for CZTS have increased efficiency to just above 10% in laboratory cells, but more work will be needed for successful commercialization [1].
Motivation
CIGSCIGS
CIGS may refer to:* Chief of the Imperial General Staff, a pre-1964 military position in the British Army* Copper indium gallium selenide , a semiconductor absorber material for solar cells...
and CdTe are two of the most promising thin film solar cells and have recently seen growing commercial success. Despite continued rapid cost reductions, concerns about material price and availability as well as toxicity have been raised. Although current material costs are a small portion of total solar cell cost, continued rapid growth of thin film solar cells could lead to increased material cost and limited supply.
For CIGS, indium is the most scarce element and has been subject to growing demand due to the rapid expansion of indium tin oxide (ITO) used in flat screen displays and mobile devices (FIGURE ELEMENT SCARCITY). 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 raw material cost 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 [2]. Tellurium is even more scarce than indium although demand has also been historically lower. Tellurium abundance in the earth’s crust is similar to 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 material costs are ~5x less than 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 (Figure 1)[3]. In addition, CZTS doesn’t have any concerns with toxicity 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).
History
CZTS was first created in 1966 [5]and was later shown to exhibit the photovoltaic effect by Ito in 1988. Some of the first research was performed by Friedlemier who created devices of up to 2.3% efficiency for CZTS and demonstrated CZTSe devices for the first time[6]. Much of the ground work for optimizing the deposition process was performed Katigiri’s group which led from .6% efficiency in 1998 and increasing to 5.7% in 2005 [7]. These improvements, alongside the beginnings of CIGS production on a commercial scale in the mid-2000’s catalyzed research interest in CZTS and related compounds.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 bi-valent Zn and IV-valent Sn which forms in the kesterite structure (See Figure).Some literature reports have identified CZTS in the related stannite structure, but conditions under which a stannite structure may occur are not yet clear. Theoretical 1st principle calculations show that while stannite is only 2.86 meV higher than the kesterite structure which suggests that it may be possible to form under non-equilibrium conditions[4]. Structural determination (via techniques like XRD) is made more difficult 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 difficulty identifying the true structure and false reports of stannite structure.
Material Properties
Carrier concentrations and absorption coefficient are similar to CIGS (#’s and SOURCE). Other properties such as carrier lifetime (and related diffusion length) has been found to be particularly low for CZTS with best cells achieving between 3-9 ns. This low carrier lifetime may be due to high numbers of active defects, or possibly recombination at grain boundaries.Many different secondary phases are possible in quaternary compounds like CZTS and their presence can have large effects on solar cell performance. Secondary phases can provide shunting current paths through the solar cell or act as centers of recombination both degrading solar cell performance. From the literature it appears that all secondary phases have a detrimental effect on CZTS performance (although some are worse than others). Unfortunately many secondary phases in CZTS 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 XRD due to peak overlap of ZnS and Cu2SnS3 with CZTS. Other methods like Raman scattering are being explored to help characterize CZTS.
Fabrication
CZTS has been successfully made by a variety of vacuum and non-vacuum techniques. Current mostly methods mirror what has what has been successful with CIGS although it has been found that the optimal fabrication conditions often differ. Methods can be broadly categorized across two variables vacuum deposition vs. non vacuum and single step vs. sulfization/selenization 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 due to their potential lower capital costs and flexibility to coat large areas.A particular challenges 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 once formed, CZTS will decompose into binary and ternary compounds in vacuum at temperatures above 500C (SOURCE). The element volatility and difficulty achieving single phase material has stymied the success of many traditional vacuum methods. Currently the record CZTS devices have been achieved through certain chemical methods which allow CZTS formation at low temperatures avoiding volatility problems.
CZTS in solar cells
After the photovoltaic effect was discovered in CZTS in 1988 this material 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 indiumIndium
Indium is a chemical element with the symbol In and atomic number 49. This rare, very soft, malleable and easily fusible post-transition metal is chemically similar to gallium and thallium, and shows the intermediate properties between these two...
. The British Geological Survey
British Geological Survey
The British Geological Survey is a partly publicly funded body which aims to advance geoscientific knowledge of the United Kingdom landmass and its continental shelf by means of systematic surveying, monitoring and research. The BGS headquarters are in Keyworth, Nottinghamshire, but other centres...
Risk List 2011 gave indium a "relative supply risk index" of 6.5, where the maximum was 8.5.
In 2010, a solar energy conversion efficiency of about 10% was achieved in a CZTS device. CZTS technology is now being developed by several private companies.