Critical review on sputter-deposited Cu2ZnSnS4 (CZTS) based thin film photovoltaic technology focusing on device architecture and absorber quality on the solar cells performance
Introduction
Humanity needs to meet the power generation requirement of 30 TW (1 TW = 1012 W) by 2050 without carbon emissions associated with the expected increase of global energy demand [1]. Solar photovoltaics (PV) has great potential to meet future large-scale electricity supply with low-carbon emission [2]. Fig. 1 illustrates the power conversion efficiency (η) of the different champion solar cells developed worldwide. Crystalline-silicon (c-Si) is the most dominant PV technology. Silicon-based photovoltaic devices are preferable due to their durability, compatibility with the technology of microelectronics and high efficiency. However, this technology is material intensive owing to the low absorption of Si, which acts as a bottle-neck for further reduction in cost. Besides, the processing cost of c-Si solar cells is high, it also requires complex processes including high temperature treatment and ion implantation.
Chalcogenide-based thin film PV technologies have the potential to reduce the cost of the PV technology. CdTe and Cu(In,Ga)(S,Se)2 (CIGS) technologies have shown tremendous progress and finally reached commercial production. However, scarcity of In, Ga, and Te as well the toxicity of Cd are issues of concern. In addition, according to the study of Wadia et al., both these PV technologies are unable to meet annual world electricity demand [3]. Besides, raw material cost of CIGS and CdTe is more expensive and their annual electricity potential is lower compared to other promising alternative PV materials indicated in Fig. 2.
Kesterite semiconductors consisting of copper, zinc, tin and sulfur and/or selenium are considered as promising alternatives to CdTe and CIGS owing to the abundance and non-toxicity of its constituents. Fig. 3a shows the growth of the number of publications on kesterite semiconductors since 2006. Kesterite thin films have been prepared using various vacuum and non-vacuum based techniques including sputtering [4], [5], [6], [7], thermal evaporation [8], [9], [10], pulsed laser deposition [11], [12], electrodeposition [13], [14], [15], spray pyrolysis [16], [17], [18], [19], sol-gel [20], [21], [22] and hydrazine solution approach [23], [24], [25].
The champion kesterite solar cell based on Cu2ZnSn(S,Se)4 absorber achieved record η of 12.6% in 2014. This PV device was fabricated by the hydrazine pure solution approach. However, hydrazine is both highly toxic and reactive [28] which limits its application in the industry. Sputtering is a suitable method for high volume manufacturing owing to the following advantages: uniformity of deposited films on large scale, high deposition rate and reproducibility of the process [33], [34], [35], [36]. Besides, sputtering enables interface engineering, tuning of crystallinity and composition of the films [37], [38], [39], [40]. Fig. 3b depicts the annual number of publications on selenium free sputter-deposited CZTS thin films which clearly indicate growing interest in CZTS thin film solar cells made by sputtering.
High abundance and non-toxicity make pure sulfide CZTS preferable to Cu2ZnSn(S,Se)4. CZTS exists in two crystalline forms: stannite and kesterite, of which the latter is more useful owing to its enhanced stability [41]. CZTS is a quaternary p-type semiconductor showing direct bandgap of about ~1.5 eV, carrier concentration similar to CIGS and high absorption coefficient of order 104 for visible wavelengths [42], [43], [44], [45], [46]. P-type conductivity is achieved owing to intrinsic defects such as copper vacancies [46]. The bandgap of CZTSSe can be tuned from 1.0 eV to 1.5 eV by increase of S/(Se+S) ratio from 0 to 1 [47], [48] providing a degree of flexibility in device fabrication with the material. Similar to CIGS, the grain boundaries of CZTS provide an enhanced minority carrier collection [49]. Carrier mobility and resistivity of CZTS thin films are in the range of 0.1 – 35 cm2/V s and 3.4 × 10−3 – 600 Ω cm, respectively [50]. According to the Shockley-Queisser limit, CZTS PV devices have the potential to reach a η of 28% [51].
Although a number of reviews on kesterite solar cells have been published to date [43], [46], [52], [53], [54], [55], [56], [57], [58], [59], [60], [61], [62], [63], [64], [65], [66], [67], [68], [69], [70], [71], [72], there is no review focused on the sputter-deposited pure sulfide (selenium free) CZTS thin film solar cells, which have shown significant progress recently. In addition, the η of these cells is still too low compared to the best CIGS and CdTe thin film solar cells. Fig. 4 presents η of the sputter-deposited CZTS thin film solar cells versus publication year. The wide dispersion of η can be attributed to differences in the quality of CZTS absorber layer, interface quality and device architecture.
The low power conversion performance of CZTS thin film solar cell is mainly due to poor fill factor (FF) and low open-circuit voltage (Voc). As can be seen from Fig. 5, typical FF of the best CZTS solar cells is about 0.6 while FF of the best CIGS and CdTe solar cells are close to 0.8 [120]. Moreover, the performance of sputter-grown CZTS solar cells suffers from huge Voc deficit (Eg/q – Voc), where Eg is the bandgap and q is the electron charge (Fig. 6). Obviously, there is plenty of room for improvement to reach the high η of CIGS and CdTe PV technologies. Bulk recombination and recombination at the absorber/buffer interface significantly affect FF and Voc of CZTS PV devices [58]. Bulk recombination is attributed to the presence of various defect states and defect complexes within the CZTS absorber layer [43]. Presence of low bandgap secondary phases also negatively affects Voc of CZTS solar cells. Careful control of film composition as well as adjustment of sulfurization parameters is crucial during fabrication of high quality CZTS thin films. Various sputtering techniques used to prepare CZTS thin films and their influence on PV properties of CZTS PV devices are presented in subsections 2.1–2.3. Recombination at the absorber/buffer interface is caused by cliff-like nature of CZTS/CdS heterojunction and defects at the interface, which favor intensive recombination at the interface [58], [121]. Although CdS is a widely used material in fabrication of CZTSSe and CIGS solar cell it is not ideal for CZTS PV devices. Alternative device architectures and their effect on device performance are discussed in Section 3. In addition, poor quality back interface reduces FF and device performance. A summary of various approaches to improve the quality of absorber/back contact interface through intermediate layers is given in Section 4.
The aim of the paper is to show current trends and present status in the field to not only enlighten challenges related to the CZTS thin film solar cells but also provide the insights for the further development of CZTS thin film solar cells.
Section snippets
CZTS as light absorber in photovoltaics
Minority carrier diffusion length (Ldiff) is an important parameter which is used to characterize quality of absorber material. It is well known that short Ldiff of the absorber negatively affects both short-circuit current (Jsc) and Voc according to Eqs. (1), (2) [123].
Here, go is the optical generation rate; Eg is the bandgap; A is the diode ideality factor; k is the Boltzmann constant; T is the temperature; J00 is the weakly temperature-dependent
Effect of buffer layer in device architecture
The typical structure of CZTS thin film solar cell is similar to that of a CIGS device and is shown in Fig. 13. A heterojunction is formed at the interface between p-type CZTS absorber and n-type CdS buffer layer. CdS is widely used as a buffer layer in CZTS-based solar cells [89], [106]. However, the carrier recombination at the interface between CZTS absorber and CdS buffer results in a decrease of Voc. The recombination at the interface can be caused by the 7% lattice misfit between CZTS and
Effect of back electrode interface quality on solar cell performance
Decomposition of CZTS in the presence of Mo during sulfurization at a temperature higher than 500 °C is an important problem for the fabrication of high quality interface between absorber layer and Mo back electrode [72]. Evaporation of volatile compounds results in void formation which increases series resistance (Rs) [169] and reduces shunt resistance (Rsh) [113], [170] thus reducing FF. Eq. (3) shows effect of Rs and Rsh on the output current of a solar cell [171].
Conclusion
Earth abundant low cost pure-sulfide CZTS is a promising material for photovoltaic applications. Sputtering is a relatively low cost method to deposit thin films in a vacuum therefore sputter-grown CZTS thin film solar cells are considered in the present paper. η of the champion CZTS PV device prepared by sputtering is 9.2% [29]. However, it is too low compared to leading chalcogenide-based thin film PV technologies such as CIGS and CdTe.
Low Voc and FF are considered as main reasons limiting
References (175)
Organic photovoltaics: technology and market
Sol. Energy Mater. Sol. Cells
(2004)- et al.
Growth of Cu2ZnSnS4 thin films using sulfurization of stacked metallic films
Thin Solid Films
(2010) - et al.
Obtaining phase-pure CZTS thin films by annealing vacuum evaporated CuS/SnS/ZnS stack
J. Cryst. Growth
(2016) - et al.
Performance limiting factors of Cu2ZnSn(SxSe1−x)4 solar cells prepared by thermal evaporation
Sol. Energy Mater. Sol. Cells
(2016) - et al.
Studies of compositional dependent CZTS thin film solar cells by pulsed laser deposition technique: an attempt to improve the efficiency
J. Alloy. Compd.
(2012) - et al.
Pulsed laser deposition of Cu2ZnSnS4 thin films from single quaternary sulfide target prepared by combustion method
Mater. Lett.
(2016) - et al.
One-step electrodeposition for targeted off-stoichiometry Cu2ZnSnS4 thin films
Appl. Surf. Sci.
(2016) - et al.
CZTS absorber layer for thin film solar cells from electrodeposited metallic stacked precursors (Zn/Cu-Sn)
Appl. Surf. Sci.
(2016) - et al.
Composition controlled preparation of Cu–Zn–Sn precursor films for Cu2ZnSnS4 solar cells using pulsed electrodeposition
J. Alloy. Compd.
(2015) - et al.
Effect of copper content and sulfurization process on optical, structural and electrical properties of ultrasonic spray pyrolysed Cu2ZnSnS4 thin films
Mater. Chem. Phys.
(2016)
Effects of chlorine and carbon on Cu2ZnSnS4 thin film solar cells prepared by spray pyrolysis deposition
J. Alloy. Compd.
Impact of sol-gel precursor treatment with preheating temperature on properties of Cu2ZnSnS4 thin film and its photovoltaic solar cell
J. Alloy. Compd.
Effects of sulfurization temperature on phases and opto-electrical properties of Cu2ZnSnS4 films prepared by sol-gel deposition
Thin Solid Films
Physical and electrical characterization of high-performance Cu2ZnSnSe4 based thin film solar cells
Thin Solid Films
Review of physical vapor deposited (PVD) spectrally selective coatings for mid- and high-temperature solar thermal applications
Sol. Energy Mater. Sol. Cells
Nanometer thick tunable AlHfN coating for solar thermal applications: transition from absorber to antireflection coating
Sol. Energy Mater. Sol. Cells
Titanium doped cupric oxide for photovoltaic application
Sol. Energy Mater. Sol. Cells
Preparation and evaluation of Cu2ZnSnS4 thin films by sulfurization of E-B evaporated precursors
Sol. Energy Mater. Sol. Cells
Optical properties of reactively sputtered Cu2ZnSnS4 solar absorbers determined by spectroscopic ellipsometry and spectrophotometry
Sol. Energy Mater. Sol. Cells
M.L. Free, A study of energy band gap versus temperature for Cu2ZnSnS4
thin films, Phys. B Condens. Matter
A review on pulsed laser deposited CZTS thin films for solar cell applications
J. Alloy. Compd.
Composition dependence of structure and optical properties of Cu2ZnSn(S,Se)4 solid solutions: an experimental study
J. Alloy. Compd.
Technological status of Cu2ZnSn(S,Se)4 thin film solar cells
Sol. Energy Mater. Sol. Cells
Recent trends in direct solution coating of kesterite absorber layers in solar cells
Sol. Energy Mater. Sol. Cells
Why are kesterite solar cells not 20% efficient?
Thin Solid Films
Non-vacuum processed next generation thin film photovoltaics: towards marketable efficiency and production of CZTS based solar cells
Sol. Energy
Cu2ZnSnS4-type thin film solar cells using abundant materials
Thin Solid Films
Effects of sulfurization temperature on CZTS thin film solar cell performances
Sol. Energy
On the formation mechanisms of Zn-rich Cu2ZnSnS4 films prepared by sulfurization of metallic stacks
Sol. Energy Mater. Sol. Cells
Fabrication of Cu2ZnSnS4 solar cell on a flexible glass substrate
Thin Solid Films
Comparison of Cu2ZnSnS4 thin films and solar cell performance using Zn target with ZnS target
J. Alloy. Compd.
Influence of hydrogen sulfide annealing on copper-zinc-tin-sulfide solar cells sputtered from a quaternary compound target
Thin Solid Films
Annealing behavior of reactively sputtered precursor films for Cu2ZnSnS4 solar cells
Thin Solid Films
Cu2ZnSnS4 solar cells grown by sulphurisation of sputtered metal precursors
Thin Solid Films
Effects of substrate temperature on the Cu2ZnSnS4 films deposited by radio-frequency sputtering with single target
Thin Solid Films
Kesterite Cu2ZnSnS4 solar cell from sputtered Zn/(Cu & Sn) metal stack precursors
J. Alloy. Compd.
Sputter grown sub-micrometer thick Cu2ZnSnS4 thin film for photovoltaic device application
Mater. Lett.
Optimization of sputtered ZnS buffer for Cu2ZnSnS4 thin film solar cells
Thin Solid Films
Cu2ZnSnS4 solar cells fabricated by short-term sulfurization of sputtered Sn/Zn/Cu precursors under an H2S atmosphere
Thin Solid Films
Influence of sulfurization pressure on Cu2ZnSnS4 thin films and solar cells prepared by sulfurization of metallic precursors
J. Power Sources
Preparation of Cu2ZnSnS4 (CZTS) sputtering target and its application to the fabrication of CZTS thin-film solar cells
J. Alloy. Compd.
Solar energy. Is it time to shoot for the sun?
Science
Materials availability expands the opportunity for large-scale photovoltaics deployment
Environ. Sci. Technol.
Characterization of a Cu2ZnSnS4 solar cell fabricated by sulfurization of metallic precursor Mo/Zn/Cu/Sn
Phys. Status Solidi Appl. Mater. Sci.
Investigation of blister formation in sputtered Cu2ZnSnS4 absorbers for thin film solar cells
J. Vac. Sci. Technol. A Vac., Surf., Film.
Studies on the disorder in DC magnetron sputtered Cu2ZnSnS4 (CZTS) thin films grown in sulfide plasma
Surf. Coat. Technol.
Thin film solar cell with 8.4% power conversion efficiency using an earth-abundant Cu2ZnSnS4 absorber
Prog. Photovolt. Res. Appl.
Ge-alloyed CZTSe thin film solar cell using molecular precursor adopting spray pyrolysis approach
RSC Adv.
Cu2ZnSnS4 thin film solar cells with 5.8% conversion efficiency obtained by a facile spray pyrolysis technique
RSC Adv.
Cu2ZnSnS4 solar cells with a single spin-coated absorber layer prepared via a simple sol-gel route
Int. J. Energy Res.
Cited by (113)
Photovoltaic efficiencies of microwave and Cu<inf>2</inf>ZnSnS<inf>4</inf> (CZTS) superstrate solar cells
2023, Materials Today SustainabilityFacile single step synthesis of Cu<inf>2</inf>ZnSnS<inf>4</inf> thin films by sputtering from a single target and their electrical characterization
2022, Journal of Alloys and Compounds