Critical review on sputter-deposited Cu2ZnSnS4 (CZTS) based thin film photovoltaic technology focusing on device architecture and absorber quality on the solar cells performance

Solar Energy Materials and Solar Cells 171 (2017) 239–252

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Critical review on sputter-deposited Cu2ZnSnS4 (CZTS) based thin film photovoltaic technology focusing on device architecture and absorber quality on the solar cells performance

MARK

Siarhei Zhuka,b, Ajay Kushwahaa,c, Terence K.S. Wongb, Saeid Masudy-Panaha, ⁎ Aliaksandr Smirnovd, Goutam Kumar Dalapatia, a

Institute of Materials Research and Engineering, A*STAR (Agency for Science, Technology and Research), 2 Fusionopolis Way, #08-03, Innovis,138634 Singapore NOVITAS, School of Electrical and Electronic Engineering, Block S2, Nanyang Technological University, Nanyang Avenue, 639798 Singapore Discipline of Metallurgy Engineering and Materials Science, IIT Indore, Indore, MP 453552, India d Belarusian State University of Informatics and Radioelectronics, P. Brovki 6, Minsk 220013, Belarus b c

A R T I C L E I N F O

A B S T R A C T

Keywords: Sputter-grown Cu2ZnSnS4 Photovoltaic device architecture Series and shunt resistance Interface-layer formation Single step sputter

Thin film photovoltaic Cu2ZnSnS4 (copper zinc tin sulfide or CZTS) is one of the most promising sustainable solar cell absorber material. The CZTS absorber layer containing earth-abundant materials such as copper, zinc, tin and sulfur can be an alternative to existing materials for thin film solar cells. Recently, there has been an increased interest to step-up the efficiency and step-down the manufacturing cost of CZTS-based solar cells. This review critically addresses the advantages and challenges associated with sputter-deposited CZTS solar cells, since sputtering is an industry compatible and relatively low-cost vacuum deposition technique. Various approaches to fabricate CZTS thin films by sputtering are discussed. In addition, the single target quaternary CZTS sputtering technique has been discussed in detail. Current state-of-the art device architectures and methods to improve the quality of interfaces are discussed. This review is intended to highlight current trends and challenges in the field to realize the opportunity of CZTS thin film solar cells for large scale application.

1. 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 ⁎

Corresponding author. E-mail address: [email protected] (G.K. Dalapati).

http://dx.doi.org/10.1016/j.solmat.2017.05.064 Received 14 November 2016; Received in revised form 26 May 2017; Accepted 27 May 2017 0927-0248/ © 2017 Published by Elsevier B.V.

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 [4–7], thermal evaporation [8–10], pulsed laser deposition [11,12], electrodeposition [13–15], spray pyrolysis [16–19], sol-gel [20–22] and hydrazine solution approach [23–25]. 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

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Fig. 1. Reported power conversion efficiency of champion PV devices [26–32].

Fig. 4. Reported power conversion efficiency of sputter-deposited CZTS solar cells versus publication year [5,27,29,73–119].

direct bandgap of about ~1.5 eV, carrier concentration similar to CIGS and high absorption coefficient of order 104 cm−1 for visible wavelengths [42–46]. 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 ShockleyQueisser 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 [43,46,52–72], 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

Fig. 2. Four-quadrant plot of indexed results. The most attractive materials for large-scale future deployment are highlighted red and are in the upper right-hand quadrant. Adapted with permission from (C. Wadia, A.P. Alivisatos, D.M. Kammen, Materials availability expands the opportunity for large-scale photovoltaics deployment, Environ. Sci. Technol. 2009, 43, 2072). Copyright (2009) American Chemical Society.

owing to the following advantages: uniformity of deposited films on large scale, high deposition rate and reproducibility of the process [33–36]. Besides, sputtering enables interface engineering, tuning of crystallinity and composition of the films [37–40]. 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

Fig. 3. Number of journal articles versus publication year (a) kesterite thin films and (b) sputter-grown CZTS solar cells (source: Thomson Reuters Web of Science database).

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Fig. 7. Reported power conversion efficiency of sputter-deposited CZTS solar cells versus thickness of CZTS absorber layer [5,73–78,80,81,83–98,100–105,107–112,114].

Fig. 5. Reported power conversion efficiency of sputter-deposited CZTS solar cells versus FF and comparison of FF with champion CIGS, CdTe, and c-Si solar cells [27,29,73–79,81–119].

2. 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 shortcircuit current (Jsc) and Voc according to Eqs. 1 and 2 [123].

JL ∝ qgo Ldiff

(1)

Voc = Eg / q−(AkT / q)ln (J00 / JL)

(2)

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 prefactor, and JL is the illuminated current density which almost equal to Jsc. Moreover, short Ldiff limits the thickness of the absorber layer in CZTS solar cells. Typical absorber thickness of the best CZTS solar cells made by sputtering is about 1.2 µm (Fig. 7). Although CZTS is a direct bandgap semiconductor with high absorption coefficient, a 1.2 µm thick layer of CZTS cannot harvest sufficient portion of the incident sunlight. Zhang et al. claimed that 2–3 µm is the optimum thickness of CZTS absorber layer according to simulations [124]. One way to enhance light absorption is to increase the thickness of the CZTS absorber layer. Therefore, there is also a need to improve the value of Ldiff, which limits thickness of the absorber layer. Furthermore, enhancement in light absorption of CZTS-based thin film solar cells can be achieved by introduction of Ag or Al nanoparticles into the absorber layer for plasmonic enhancement effect [125,126]. At present there are no reports about CZTS solar cells with metal nanoparticles incorporated absorber layer, which can also be used to improve light management of the PV devices. Dhakal et al. reported CZTS device with η of 6.2% and carried out the characterization of the CZTS absorber [105]. Although the absorber layer was around 1.3 µm thick, only 445 nm of this was contributing to the electron hole pair generation. The calculated Ldiff was 350 nm. Table 1 provides reported information regarding η and Ldiff of kesterite and CIGS solar cells. It can be seen that Ldiff of CZTS is much lower

Fig. 6. Reported power conversion efficiency of sputter-deposited CZTS solar cells versus Voc and indication of Voc deficit (Eg/q – Voc) [27,29,73–119].

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.

Table 1 Reported η and Ldiff of kesterite and CIGS solar cells.

241

Type

CZTS growth technique

Ldiff (μm)

η (%)

Ref.

CZTS CZTSe CZTS CZTSSe CIGS

Thermal evaporation Co-evaporation Sputtering Hydrazine-based process Hydrazine-based process

0.35 2.1 0.35 1 2–3.6

8.4 11.6 6.2 10.2 15

[9] [31] [105] [127] [127]

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contrast with Ldiff for hydrazine process. Defect states, defect complexes and secondary phases within CZTS absorber layer play important role in lowering minority carrier diffusion length (Ldiff). Vacancies, interstitials, antisites, and defect complexes are among possible defects in CZTS [53]. Copper-on-zinc (CuZn) antisites and copper vacancies (VCu) form a shallow acceptor levels with energies of 0.2 eV and 0.02 eV respectively where Vcu are more favorable owing to its lower formation energy [43]. However, other intrinsic defects are adverse since they form mid gap states, deep levels, and donor levels within the bandgap. Donor and acceptor defects can compensate themselves by the formation of defect complexes [53]. Most of such complexes have significant impact on the optoelectronic properties of CZTS while the influence of CuZn+ZnCu defect complex is low [43]. Presence of secondary phases such as Cu2S, SnS, and Cu2SnS3 which have bandgaps lower than that of CZTS are also detrimental for the device performance [58]. The low bandgap secondary phases reduce carrier collection efficiency and enhance carrier recombination [128] owing to a short Ldiff. Doping of kesterite thin films is used to improve electrical conductivity and crystalline properties of the films. Kesterite thin films can be doped by Cd [129,130], Cr [131], Na [132–143], K [143,144], In [145–147], Al [148], Li [149,150], Sb [151,152], Ge [153–155], and Ni [156]. However, it can also lead to formation of additional defect levels within the bandgap. Thus, optimization of the absorber layer composition and post-deposition processing are required to produce CZTS thin films of high quality. More studies should be carried out to investigate the impact of CZTS film composition on formation of various defects and defect complexes. 2.1. Fabrication of CZTS absorber layer via sulfurization of stacked precursors Fig. 8 gives schematic representation of some methods used for CZTS thin films fabrication. Sequential deposition of precursors followed by sulfurization is a widely used method to fabricate CZTS thin films [88,101,108]. Sulfurization is a crucial processing step to convert stacked precursors to the single-phase CZTS film. Sulfurization time, temperature, pressure, source of sulfur, thickness of precursor layers, their content and sequence have a significant impact on properties of the CZTS films [157–159]. Most high efficiency CZTS solar cells have a Cu-poor and Zn-rich composition of the absorber [109]. Composition of the CZTS absorber prepared by sulfurization of stacked precursor films is controlled by the thickness of the precursors. Optimization of the composition is crucial because it reduces defect density and improve the Ldiff [43]. Furthermore, prolonged sulfurization (30 min at 570–580 °C) of stacked precursor films was required to fabricate CZTS-based solar cells, showing η higher than 7% for this technique [92,94,115]. However, loss of volatile Sn occurs during high temperature treatment [86]. Abusnina et al. revealed that Sn deficit caused the formation of Cu2−xS secondary phase at the surface of the film [160]. Wang et al. reported that Sn loss deteriorated stoichiometry of CZTS absorber which in turn led to the formation of cracks and small holes among the grain boundaries [159]. Sugimoto et al. fabricated CZTS thin film with long photoluminescence (PL) lifetime of 36 ns by adjusting the absorber layer composition [27]. It has been shown that a lower Cu/Sn ratio resulted in an increase of the PL lifetime and Eg thus improving Voc. Thus, prevention of Sn loss seems to be crucial to suppress secondary phase formation and fabricate CZTS thin films of high quality. Several approaches to inhibit the loss of volatile species for CZTS films prepared by sulfurization of stacked precursors were reported to date. A method to reduce evaporation by annealing of metal stacked precursors before sulfurization was presented by Wei et al. [159]. Furthermore, this study showed enhancement of crystallinity of CZTS absorber layer associated with the pretreatment. Pawar et al. performed rapid thermal

Fig. 8. Schematic representation of sputter-based methods to fabricate CZTS thin films for PV application.

sulfurization of metallic precursor films for 5 min to suppress evaporation of volatile compounds and formation of resistive MoS2 at the back interface [102]. At an annealing temperature of 580 °C, CZTS absorber was found to be fully sulfurized. Solar cell with η of 4.97% was obtained. Gang et al. proposed to reduce Sn-loss during sulfurization by increasing sulfur partial pressure [86]. Rapid thermal annealing (RTA) of the stacked precursor films was carried out in a graphite box which contained sulfur powder for 10 min at a temperature of 580 °C. The graphite box was located in a RTA chamber with N2 (95%)/H2S (5%) atmosphere. Sulfurization of the sample with Cu / (Zn + Sn) = 0.76 ratio resulted in void-free CZTS film. At the optimum conditions, a η of 5.1% was achieved. In addition, it has been observed that thickness of the CZTS film as well as its grain size were dependent on Cu/ (Zn+Sn) ratio. 2.2. Fabrication of CZTS absorber layer by co-sputtering and sulfurization techniques The co-sputtering technique can also be used to produce CZTS films [29,78,80,161]. This method provides an effective way to uniformly mix elements from targets [111] which in turn enables faster solid state reaction during the thermal treatment process. Reduction of high temperature sulfurization time results in less elemental loss during the sulfurization [114]. 242

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Fig. 9. (a) Temperature profiles for a low-temperature formation process (P2) and a slow-ramping annealing process (P1). (b) J-V measurements and (c) the external quantum efficiency (EQE) curves of the CZTS devices S1 and S2 under standard AM1.5 illumination. Reproduced from Ref. [114] with permission from The Royal Society of Chemistry.

Feng et al. fabricated a CZTS PV device with a η of 8.58% using cosputtering technique and two-step sulfurization [114]. Precursor layer was deposited by co-sputtering of SnS2, ZnS, and Cu targets. Then twostep sulfurization was carried out to minimize Sn loss and form high quality CZTS absorber. At the first step, as-deposited film was annealed at 260 °C in N2 (95%)/ H2S (5%) atmosphere for 75 min to form CZTS crystallites as shown in Fig. 9a. At this temperature, intensive decomposition SnS2 into volatile SnS and S2 was not observed. During the second sulfurization step, the films were annealed in the same atmosphere at a temperature of 510 °C for 15 min to eliminate secondary phases and increase the size of crystallites. CZTS absorber layer thus prepared had atomic ratios Cu / (Zn + Sn) = 0.73 and Zn / Sn = 1.35. J-V and external quantum efficiency (EQE) curves are presented in Figs. 9b and c, respectively. This study revealed that the device obtained using two-step thermal treatment process exhibited higher hole concentration, lower defect density and shallower trapping energy than the device formed by a conventional annealing process. Liu et al. improved quality of CZTS/CdS interface via defect reduction using modified sulfurization approach and post-processing of CZTS thin film in air [118]. Decrease of reverse saturation current by two orders of magnitude by sulfurization of co-sputtered CZTS film in combined S and SnS atmosphere was reported. SnS was introduced to suppress Sn loss and decomposition of the CZTS during high temperature sulfurization. Besides, positive effect of post-annealing in air at 300 °C for 1–2 min on Voc through reduction of surface defect was revealed. The measured η of the PV device was 8.76% showing significant improvement in Voc and FF compared to the reference device. Moreover, pulsed laser annealing (PLA) can potentially be employed to reduce defect density near the surface of CZTS absorber film. The effect of sulfurization conditions on reduction of bulk and surface defects should be investigated in more detail to further enhance η of CZTS-based solar cells. CZTS device prepared using the reactive co-sputtering technique with a η of 4.6% was reported by Scragg et al. [107]. Fully sulfurized film was obtained by co-sputtering of Cu/Sn alloy and Zn targets in H2S atmosphere. The substrate temperature was maintained at 120 °C during the deposition. RTA was carried out in argon atmosphere at a temperature of 550 °C for 3 min to enhance the crystallinity of the asdeposited film. It has been reported that grain size of the annealed film was about 1 µm. Cormier et al. reported one-step fabrication process of CZTS films deposited by co-sputtering of Cu/Sn alloy and Zn targets in a reactive atmosphere of Ar/H2S on the heated substrate [162]. It has been revealed that a substrate temperature higher than 300 °C was crucial to prepare crystallized CZTS thin films.

2.3. Fabrication of CZTS absorber layer by single target sputtering Single target sputtering of quaternary compound CZTS target is suitable for the large scale manufacturing of the CZTS-based PV devices due to many advantages. This single target sputtering approach is relatively simple and cost-effective. Furthermore, it provides good reproducibility and uniform element distribution within deposited films [163]. Along with co-sputtering technique, implementation of single target sputtering enables reduction of sulfurization temperature and time. Nakamura et al. compared properties of CZTS thin films obtained using co-sputtering of Cu/ZnS/SnS targets and single target CZTS sputtering [104]. It has been found that samples sulfurized at 500 °C exhibited similar chemical composition, optical and crystalline properties. However, scanning electron microscopy (SEM) study of cosputtered CZTS thin films revealed the presence of voids while CZTS thin films made by single target sputtering were void free. CZTS PV device fabricated by single target sputtering demonstrated η of 4.4%. This was higher than the device with absorber prepared using the cosputtering method. Lin et al. prepared CZTS sputtering target by mixing and sintering of CuS, ZnS, and SnS2 raw powder with a molar ratio of Cu:Zn:Sn:S = 1.6:1:1:4 [100]. A η of 5.2% was measured for the CZTS photovoltaic device fabricated using this sputtering target. Furthermore, a significant effect of working pressure (WP) on grain structure of the sulfurized CZTS films and the photovoltaic performance of the CZTS devices has been shown in Figs. 10 and 11 respectively. Deposition at low WP of 1 mTorr resulted in formation of CZTS thin film with larger grain size. In addition, a η of the solar cell based on CZTS absorber deposited at low WP was significantly higher than that of CZTS absorber fabricated at high WP of 10 mTorr. Positive effect of low WP was explained by higher kinetic energy of sputtered CZTS nuclei due to larger mean free path. The higher energy leads to better rearrangement during deposition process and, hence, better film quality. Jheng et al. reported one-stage fabrication method of CZTS thin films using single target sputtering approach [89]. The substrate temperature was kept at 50 °C, 100 °C, 150 °C and 200 °C during the deposition processes. It has been revealed that increase of substrate temperature caused significant reduction of sulfur content and increment of copper content in the as-grown films. This study also showed that increase of substrate temperature resulted in a decrease of strain and dislocation density in the deposited films. The substrate temperature of 150 °C was found to be optimal in terms of carrier concentration, carrier mobility and resistivity. Although the absorber deposited at optimum condition had Cu-rich, Sn-rich and S-poor composition with 243

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Fig. 10. (a)–(c) Cross-sectional SEM micrographs of the sulfurized CZTS layers deposited at various WPs without the KCN treatment. Corresponding top-view SEM micrographs of sulfurized CZTS layers deposited at various WPs without ((d)-(f)) and with ((g)-(i)) the KCN treatment. Reprinted from Journal of Alloys and Compounds, 654, Y.-P. Lin, Y-.F. Chi, T.-E. Hsieh, Y-.C. Chen, K-.P. Huang, Preparation of Cu2ZnSnS4 (CZTS) sputtering target and its application to the fabrication of CZTS thin-film solar cells, 498 – 508, Copyright (2016), with permission from Elsevier.

Cu / (Zn + Sn) and Zn / Sn ratio of 1.06 and 0.94 respectively, a η of 2.9% was obtained. A higher η was achieved by Jheng et al. in the following study when the absorber layer was prepared by co-sputtering of CZTS and ZnS target onto heated soda-lime glass (SLG) substrate [164]. Cu-poor / Zn-rich CZTS film was prepared without sulfurization since the substrate temperature was maintained at 500 °C during the deposition. Cu / (Zn + Sn) and Zn / Sn ratios of the best device were 0.87 and 1.26 respectively. This one-stage fabrication process resulted in the solar cell with η of 5.5%. An even higher η of 6% was achieved by implementation of conical ZnO nanorods as an anti-reflection coating (ARC). Fig. 12(a–c) shows difference in the properties of the solar cell with and without ZnO nanorods while Fig. 12d provides cross-section image of the solar cell with the ARC. Kusano et al. proposed a way to minimize composition deviation for CZTS thin films prepared by one-stage approach using CZTS target [103]. The space between cathode and substrate was confined by the temperature-controlled reflector wall to enhance the probability of redeposition of the sputtered or evaporated particles back to the growing film. Feng et al. fabricated CZTS thin films showing high carrier mobility [116]. The target had near stoichiometric composition of Cu:Zn:Sn:S = 24.8:12.7:13:49.5 (atomic ratio %) but the composition of the as-deposited film became Cu-poor, Sn-rich, S-rich: Cu:Zn:Sn:S = 22.3:12.6:14.1:50.9 (atomic ratio %). After the sulfurization in the tube furnace at temperatures from 400 °C to 550 °C for 30 min with sulfur

Fig. 11. J-V characteristics of the CZTS thin film solar cells containing the sulfurized CZTS layers deposited at various WPs. Reprinted from Journal of Alloys and Compounds, 654, Y.-P. Lin, Y-.F. Chi, T.-E. Hsieh, Y-.C. Chen, K-.P. Huang, Preparation of Cu2ZnSnS4 (CZTS) sputtering target and its application to the fabrication of CZTS thin-film solar cells, 498 – 508, Copyright (2016), with permission from Elsevier.

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Fig. 12. (a) External quantum efficiency of CZTS solar cell without (black dotted line) and with (blue line) antireflection coating of conical ZnO nanorods. (b) J-V characteristics of CZTS solar cell with and without antireflection coating of conical ZnO nanorods. (c) Wavelength-dependent reflectance of CZTS solar cell before (black dotted line) and after (blue line) deposition of antireflection coating of conical ZnO nanorods. (d) The cross-sectional field emission SEM image of the fabricated CZTS solar cell. Reprinted from Solar Energy Materials & Solar Cells, 128, B-.T. Jheng, P-.T. Liu, M-.C. Wu, A promising sputtering route for dense Cu2ZnSnS4 absorber films and their photovoltaic performance, 275 – 282, Copyright (2014), with permission from Elsevier.

be adjusted by variation of WP [100] which may be useful in fabrication of CZTS absorber films with enhanced carrier collection efficiency. Moreover, effect of substrate temperature on defect formation needs further investigation.

powder, the S content increased by 1–2 at% while Zn and Sn content reduced slightly. Resistivity and carrier mobility of prepared thin films were found to depend on sulfurization temperature. Carrier mobility and resistivity improved with temperature and reached values of 57.6 cm2/V·s and 33.9 Ω cm respectively at sulfurization temperature of 500 °C. This was explained by the better crystalline quality of the film and reduction of defects. It has been revealed that the above mentioned electrical properties of the CZTS film sulfurized at 550 °C degraded dramatically which was attributed to the formation of voids and grain boundary discontinuities. However, hole concentration increased slightly with temperature. A solar cell with η of 4.4% was fabricated based on the CZTS absorber sulfurized at temperature 500 °C. Sun et al. proposed alternative method to fabricate CZTSSe thin films with controlled S / (S + Se) ratios using CZTS precursor deposited by sputtering of CZTS target [165]. Selenization at a temperature of 550 °C was performed to convert as-deposited CZTS precursor to CZTSSe thin film. Inductively coupled plasma mass spectroscopy (ICPMS) study showed that S / (S + Se) ratio decreased from 1 to 0 after 30 min of selenization thus providing a way to fabricate CZTSe thin films. Table 2 summarizes reported data regarding photovoltaic properties of solar cells based on CZTS absorber fabricated using various sputtering methods. Choice of CZTS target composition can be useful for depositing high quality of CZTS thin films. Besides, Cu content in as-deposited film can

2.4. Chemical etching of secondary phases Chemical treatment is used to selectively remove secondary phases from the surface of CZTS absorber. Fairbrother et al. studied the effect of the KCN and HCl etching on the photovoltaic properties of CZTS solar cells [79]. It has been found that widely used KCN-based etching was not effective in removal of Zn-rich phases while it was useful in elimination of Cu-rich secondary phases. It was established that HCl etching of ZnS secondary phase had a significant influence on photovoltaic properties of the solar cell. According to the study, HCl processing improved Jsc, Rs and η of the solar cell compared to devices based on KCN-treated CZTS absorber. It has been reported that solar cell processed in HCl-based solution achieved η of 5.2%. Katagiri et al. stated that CZTS absorber soaking in deionized water (DIW) improved overall device performance [74]. It has been revealed that DIW-soaking resulted in selective etching of metal oxide particles which did not absorb light as effectively as CZTS. The processing enhanced Jsc and FF via reduction of Rs and resulted in a η of 6.77%. Dhakal et al. used NaCN solution to selectively etch secondary phases at CZTS absorber [105]. The etching was carried out for 2 min in a NaCN solution prepared in 245

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Table 2 Photovoltaic properties of CZTS solar cells based on different fabrication methods of the absorber layer. Deposition method

Sulfurization parameters (temperature and time)

Jsc (mA/cm2)

Voc (V)

FF

η (%)

Substrate

Ref.

Sequential sputtering of precursor

5 min at 580 °C 10 min at 580 °C 1st stage: 20 min at 580 °C, 2nd stage: 60 min at 500 °C 30 min at 570 °C 30 min at 580 °C

18.4 18.38 17.5

0.561 0.573 0.71

0.482 0.49 0.71

4.97 5.13 8.8

Glass Glass Alkali-glass

[102] [86] [94]

19.3 20.14

0.632 0.655

0.616 0.573

7.5 7.6

SLG Alkali-glass

[115] [92]

Co-sputtering of precursors

1st stage: 75 min at 260 °C,2nd stage: 15 min at 510 °C 5 min at 560 °C 3 min at 550 °C NA

21.1 19.47 14.6 20.1

0.625 0.667 0.513 0.56

0.651 0.675 0.608 0.48

8.58 8.76 4.6 5.5

SLG SLG SLG SLG

[114] [118] [107] [164]

Reactiveco-sputtering of precursors

10 min at 560–570 °C 180 min at 580 °C

19.6 15.7

0.667 0.662

0.6 0.55

7.9 5.74

SLG SLG

[93] [73]

Sputtering of quaternary compound CZTS target

NA 5 min at 550 °C 10 min at 550 °C 60 min at 570 °C NA 30 min at 500 °C

14 17.9 9.5 19.17 16.8 19.2

0.563 0.412 0.6 0.513 0.533 0.65

0.558 0.405 0.36 0.527 0.324 0.318

4.4 2.85 3.07 5.2 2.9 4.4

SLG Glass Glass SLG SLG SLG

[104] [96] [91] [100] [89] [116]

wavelength light absorption which limits Jsc of the solar cell [113]. Passivation of the interface is needed to reduce defect density and improve η of the CZTS solar cells. Li et al. studied the influence of ZnS secondary phase segregation near the absorber/buffer interface [112]. It has been suggested that ZnS could effectively passivate the interface owing to small misfit of about 0.5% between CZTS and ZnS. This could reduce interface recombination thus increasing Voc. However, the presence of ZnS increased series resistance (Rs) of the device due to spikelike CBO of 0.86 eV at CZTS/ZnS interface. CZTS PV device with ZnS buffer was fabricated by Kim et al. [97]. ZnS buffer layer was deposited on top of CZTS absorber by RF sputtering technique. It has been reported that optimized sputtering power and thickness of the buffer were 95 W and 30 nm respectively. CZTS / ZnS-based PV device with a η of 2.11% was reported while η of the reference device with CdS buffer prepared by chemical bath deposition (CBD) technique was 4.97%. The low η of the CZTS / ZnS-based solar cell can be attributed to the large spike-like CBO at the interface between CZTS and ZnS which limits electron transport through the heterojunction. Ericson et al. prepared CZTS solar cell with Zn(O,S) alternative buffer having a η of 4.6% [167]. Taking into account large positive CBO between CZTS and ZnS, large negative CBO between CZTS and ZnO, it has been supposed that implementation of Zn(O,S) alternative buffer could result in optimal band alignment. It has also been shown that CBO between buffer and absorber could be varied by adjusting O/S ratio. The buffer layer was grown by atomic layer deposition (ALD) at 120 °C. The best Zn(O,S)-based PV device showing η of 4.6% had 6:1 Zn (O,S) buffer layer composition while the reference solar cell with CdS buffer had a measured η of 7.3%. An alternative Zn0.35Cd0.65S buffer layer was used to fabricate CZTS PV device by Sun et al. [29]. The buffer layer was grown by the successive ionic layer adsorption and reaction (SILAR) method. The composition of the buffer was adjusted to get optimum CBO which was directly measured by X-ray photoelectron spectroscopy (XPS) technique. CZTS solar cell based on this alternative buffer layer had η of 9.2% while a η of 7.8% was obtained by the reference solar cell with CdS buffer. Furthermore, implementation of the alternative buffer layer with a bandgap of 2.7 eV enabled to enhance Voc an FF compared to the reference device. Bras et al. reported CZTS solar cell with sputter-grown In2S3 buffer [106]. Flexible stainless steel substrates were used instead of SLG to withstand fast temperature changes. Sodium precursor layer was deposited prior to Mo back contact deposition to enhance re-crystallization of the CZTS absorber. Single quaternary target of Cu-poor Znrich composition was used to fabricate the absorbers. The device

Fig. 13. Typical schematic diagram of CZTS-based solar cells.

DIW followed by deposition of CdS buffer layer. XPS study revealed absence of secondary phases at the surface of the CZTS absorber. The η of the chemically treated device was doubled and reached a value of 6.2%. 3. 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 CdS which favors formation of defects that deteriorate device performance [121]. Besides, cliff-like conduction band offset (CBO) of about −0.3 eV leads to the carrier recombination at the interface [29,58] while favorable band alignment is associated with spike-like CBO ranging from 0 to 0.4 eV [166]. Furthermore, implementation of CdS buffer with an optical bandgap of 2.42 eV results in parasitic short 246

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Table 3 Reported photovoltaic properties of sputter-deposited CZTS solar cells with and without buffer layer. Type of buffer layer

Buffer layer growth method

Jsc, mA/cm2

Voc, V

FF

η, %

Substrate

Ref.

Zn0.35Cd0.65S CdS ref. device In2S3/CdS In2S3 CdS ref. device ZnS CdS ref. device Zn(O,S) CdS ref. device In2S3/CdS CdS ref. device Zn1−xSnxOy CdS ref. device

SILAR CBD CBD Sputtering CBD Sputtering CBD ALD CBD CBD CBD ALD CBD

19.5 20.4 21.6 13.9 11.2 12.16 18.4 17.2 17.5 17.6 15.9 17.9 17.1

0.7478 0.665 0.708 0.531 0.423 0.311 0.561 0.482 0.652 0.714 0.641 0.682 0.607

0.632 0.574 0.601 0.508 0.38 0.5573 0.482 0.555 0.638 0.527 0.537 0.602 0.559

9.2 7.8 9.19 4.2 2 2.11 4.97 4.6 7.3 6.62 5.47 7.4 5.8

SLG SLG Glass Stainless steela Stainless steela SLG SLG SLG SLG SLG SLG SLG SLG

[29] [29] [113] [106] [106] [97] [97] [167] [167] [119] [119] [168] [168]

a

Fe diffusion barrier and MoNa precursor.

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].

fabrication process was performed without breaking vacuum and took less than 40 min. This approach prevented contamination of the CZTS absorber. The solar cell with In2S3 buffer had a measured η of 4.2% while the η of the solar cell with CdS buffer was only 2%. This improvement was attributed to better heterojunction quality of CZTS/ In2S3. Hiroi et al. used hybrid buffer layer based on a combination of CdS and In2S3 layers to suppress carrier recombination at the interface [113]. As a result, a Voc of 758 mV was achieved, and solar cell showed a η of 9.19%. In addition, reduction of recombination at both absorber/ buffer and buffer/i-ZnO interfaces was obtained. Moreover, it has been suggested that small CBO at absorber/buffer layer interface (0 – 0.1 eV) could cause a decrease of Rsh. Yan et al. also investigated effect of hybrid In2S3/CdS on performance of CZTS solar cells [119]. A Voc of 714 mV and η of 6.62% were achieved for the best device. Improvement in Voc compared to CdS reference device was explained by higher carrier concentration in CZTS due to In doping (In diffuses from buffer layer to CZTS absorber layer) and more favorable band alignment at buffer/absorber interface. Platzer-Bjorkman et al. implemented Zn1−xSnxOy (ZTO) buffer layer instead of CdS to reduce the interface recombination [168]. ZTO buffer was formed using ALD technique. The device with ZTO buffer exhibited activation energy of 1.36 eV while that value of the CZTS device with CdS buffer was only 1 eV. Such increase in activation energy was explained by reduction of recombination at the buffer/absorber interface as a result of improved band alignment at the interface between CZTS and ZTO. It has been shown that implementation of ZTO buffer caused an increase of Voc compared to CdS reference devices. As a result, a η of 7.4% was achieved. Table 3 summarizes reported information regarding impact of the buffer layers on photovoltaic properties of sputter-grown CZTS solar cells. Tajima et al. fabricated solar cell based on two-layered CZTS absorber with world record Voc of 780 mV [94]. CZTS layer near Mo electrode was stoichiometric while CZTS layer near absorber/buffer interface was Cu-poor. It has been suggested that Cu content gradient caused an increase of carrier concentration gradient within the absorber, which in turn could lead to an increase of inner potential between absorber and buffer layers. The best solar cell showed Voc of 710 mV and provided a η of 8.8%. Besides, the extrapolated Voc to 0 K was only about 1.2 V which is significantly less than the bandgap of CZTS. This result was attributed to the recombination at the CZTS/CdS interface.

J = J0 ⎡exp ⎛ ⎢ ⎝ ⎣

q (V − JRs ) ⎞ ⎤ V − JRs −1 + − JL AkT Rsh ⎠ ⎥ ⎦

(3)

where, J is the output current, J0 is the reverse saturation current, V is the voltage across the device terminals. Evaporation of SnS leads to the formation of highly conductive Cu2−xS secondary phase which not only decreases Rsh but also enhances recombination [88,160]. ZnS and MoS2 secondary phases increase Rs [112,115,172]. He et al. reported that Rsh higher than 1000 Ω cm2 and Rs lower than 1 Ωcm2 were needed for the high performance CZTS photovoltaic device [96]. Fig. 14 represents reported data on Rs of the CZTS thin film solar cells made by sputtering, while Fig. 15 shows the data regarding Rsh of the solar cells. It can be seen from these figures that further investigation of absorber/back contact interface is required to improve FF and overall device performance. Several methods to inhibit chemical reaction at the interface between CZTS absorber and Mo back electrode were reported. Yang et al. revealed that thermal pretreatment of Mo layer in N2 atmosphere before deposition of CZTS layer can slow down the formation of the resistive MoS2 layer at the interface between CZTS and Mo [172]. Moreover, it has been stated that high temperature treatment of the Mocoated SLG substrate increased diffusion of Na atoms to the Mo layer. According to the paper, Na could passivate defects at the interfaces and

4. 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

Fig. 14. Reported power conversion efficiency of sputter-deposited CZTS solar cells versus series resistance [5,78,79,81–83,90,93,96,98,99,102,105,109,112,114,122,169,174].

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7.9%. The decrease of η was attributed to enhanced Rs of TiN-based device. Zhou et al. used thin carbon layer to reduce chemical reactions at the interface [173]. It has been revealed that the carbon IL refilled voids at the interface and reconnected absorber to the back contact layer. A solar cell with a η of 5.2% was obtained. Li et al. employed 10 nm ZnO IL [169]. It has been revealed that implementation of this IL caused reduction of the MoS2 thickness from 300 to 80 nm. Furthermore, ZnO layer was converted in ZnS during sulfurization and then was involved to the CZTS formation. The elemental profile study of the sample without ZnO IL showed segregation of elemental Sn within CZTS absorber at the back contact region as can be seen from Fig. 16(a, c). It has been supposed that these highly conductive regions provided shunt passes and significantly reduced Rsh. The application of the ZnO IL limited S diffusion which promoted the reaction between S and elemental Sn causing volatile SnS formation. Sample with ZnO IL exhibited a high density of voids because of Sn loss (Fig. 16(b,d)). The η of CZTS device with ZnO IL was 3.26% while η of CZTS solar cell without ZnO IL was 2.07%. The improvement of η was mainly attributed to the increase of Voc and Rsh. Although the thickness of the resistive MoS2 layer was reduced, Rs increased slightly due to the formation of voids which limited free carrier transport. In another study, Liu et al. fabricated sputter-grown CZTS device with 10 nm ZnO IL providing η of 4.3% [174]. The device exhibited Rsh of 1630 Ωcm2

Fig. 15. Reported power conversion efficiency of sputter-deposited CZTS solar cells versus shunt resistance [78,81–83,90,96,98,99,105,112,114,122,169,174].

grain boundaries within CZTS absorber. Scragg et al. fabricated CZTS solar cell with thin inert TiN intermediate layer (IL) to suppress chemical reactions at the absorber/back interface [93]. A η of 5.5% was obtained for CZTS device with TiN IL, while a η of reference device was

Fig. 16. (a) and (b) are the high –angle annular dark-field (HADDF) images of the CZTS solar cell without and with ZnO IL, respectively. The red arrow, which extends from ZnO to Mo, marks the direction of EDS line scan. (c) and (d) are the elemental profiles determined by EDS scan of the red arrow in (a) and (b), respectively. Reprinted from Materials Letters, 130, W. Li, J. Chen, H. Cui, F. Liu, X. Hao, Inhibiting MoS2 formation by introducing a ZnO intermediate layer for Cu2ZnSnS4 solar cells, 87–90, Copyright(2014), with permission from Elsevier.

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Table 4 Photovoltaic properties of sputter-deposited CZTS solar cells with IL. Intermediate layer

Fabrication method

Thickness (nm)

Jsc (mA/cm2)

Voc (V)

FF

Rs (Ωcm2)

η (%)

Substrate

Ref.

TiN Ref. device Carbon Ref. device ZnO Ref. device

Reactive sputtering

20

Evaporation

25

Sputtering

10

18.7 19.6 20.5 17.5 15.97 10.8

0.621 0.667 0.6 0.59 0.641 0.324

0.471 0.6 0.42 0.4 0.42 0.32

6.9 2 – – 15.1 19.7

5.5 7.9 5.2 4.1 4.3 1.13

SLG SLG SLG SLG SLG SLG

[93] [93] [173] [173] [174] [174]

which is the highest for CZTS solar cells made by sputtering to the best of our knowledge. Table 4 summarizes reported information regarding impact of the IL on CZTS device performance. CZTS solar cells prepared using single target sputtering approach typically exhibit lower FF compared to the solar cells fabricated using other sputtering methods. This may be caused by deviation of CZTS composition in the back contact region due to out-diffusion of elements during high temperature treatment [175]. Thus, investigation of absorber/back contact interface is required to provide new pathways to further improve FF of the solar cells. Low resistivity, suitable work function, low diffusion, low reactivity [72] and good adhesion to CZTS thin films are among requirements for the back contact material. Although Mo is a widely used material for the fabrication of kesterite thin film solar cells, it cannot meet this demand. Thus, further investigations are needed to find alternative pathways owing to described limitations of Mo for CZTS solar cell application. Implementation of metal stacked back electrode may enable to provide an effective way to overcome disadvantages of single layer Mo back contact and enhance FF of CZTS-based photovoltaic devices.

during sputter deposition and potential to further reduce fabrication cost make single target sputtering an attractive method to produce CZTS thin films. Composition optimization of CZTS target may provide a straightforward way to fabricate high quality CZTS thin films. Although some studies reported the fabrication of CZTS absorber without sulfurization, it has been observed that high temperature postdeposition treatment is still needed to produce CZTS films of adequate quality. This fact restricts usage of inexpensive substrates and increases processing cost of the photovoltaic device. Moreover, effect of substrate heating on the film composition and various defects formation needs further investigation. It has been shown post-sulfurization thermal treatment can be useful to reduce surface defects. Thus, implementation of pulsed laser annealing may be useful to effectively modify nearsurface region of the CZTS absorber layer. It has been also found that Rsh of the CZTS solar cells is quite low which deteriorates not only FF but Voc. Therefore, Mo/CZTS interface which is critical for the device performance should be further studied. Further improvement of back contact interface can potentially be achieved by deployment of stacked back contact.

5. Conclusion

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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 the performance of sputter-deposited CZTS devices caused by recombination. It has been shown that recombination at the buffer/absorber interface can be reduced by implementation of alternative buffer materials and precise engineering of conduction band offset at the interface. Furthermore, presence of defects and secondary phases reduces Ldiff which limits the thickness of CZTS absorber layer. Although best sputter-deposited CZTS solar cells have absorber of about 1.2 µm, it is not enough to harvest 90% of photons. The absorber thickness should be increased to absorb more photons thus increasing Jsc. Precise control of CZTS composition and suppression of elemental loss are crucial to fabricate CZTS absorber of high quality. An alternative way to improve performance of CZTS devices while keeping absorber layer thin is to use plasmonic nanostructures to enhance light absorption. Most of the studies employed a two-stage process to prepare CZTS films. However, there is considerable interest in single target sputtering and co-sputtering techniques because they provide effective mixing of CZTS constituent elements. This can shorten thermal treatment time and suppress elemental loss of the CZTS films prepared by this techniques compared to films fabricated by sulfurization of stacked precursors. Moreover, a decrease of sulfurization time may reduce the thickness of highly resistive MoS2 at the back contact and increase FF. It has been shown that reactive co-sputtering and single target sputtering techniques may be employed to prepare CZTS films without sulfurization which is a great advantage for this low cost PV technology. Moreover, relative simplicity, absence of toxic reactive H2S gas 249

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