The world must meet the requirement for clean electricity (without carbon emissions) generation of 30 terawatts by 2050, associated with the expected increase in global energy demand [1
]. Solar photovoltaic systems have great potential to address the challenge of future clean electricity supply on a large scale [2
]. The quaternary semiconductor Cu2
(CZTS) has gained wide attention and has been intensively investigated as a new generation photovoltaic (PV) absorber material. CZTS is theoretically derived from the CuInS2
(CIS) structure, where in the formula unit of Cu2
, two trivalent (In) atoms are substituted with one divalent (Zn) atom and one tetravalent (Sn) atom. This isoelectronic substitution produces a material with many properties similar to the initial compound, however, with the added advantage of containing abundant and cheap elements. The Zn content (79 ppm) and the Sn content (2.2 ppm) in the Earth crust are about 1500 and 45 times greater than that of In, respectively. An evaluation of the minimum cost of raw materials for commercialized PV technologies and emerging PV technologies was done by Wadia et al. [3
]. The cost for CZTS is much lower than that of other existing PV technologies.
The desirable properties of CZTS include p-type conductivity, a high absorption coefficient (104
, equivalent to 90% of the incident light) and a bandgap of around 1.5 eV (the theoretical optimum value for solar energy conversion [4
]). Another advantage of the similarity between CZTS and CIS is that CZTS may be substituted directly into the standard device structure. The potential of CZTS was recognized by Ito and Nakazawa, who prepared synthetic CZTS films using a powder source and atom beam sputtering and demonstrated a photovoltaic effect at the junction between CZTS and cadmium–tin–oxide [5
The crystalline Cu2
has a tetragonal lattice, in fact, a face-centered pseudo-cubic lattice with F-43m (216) space group (“Zinc blende” type structure) where all the atoms (Cu, Zn, Sn and S) are tetrahedrally coordinated. Since the Cu+
cations must be arranged regularly, the unit cell becomes tetragonal and its crystallographic space group can be either I-42m (121) (similar to the natural mineral “stannite”—a Zn-poor form of Cu2
), where the metal atom planes on the c
axis alternate as: Zn–Sn (in a 2D checkerboard array)/Cu (in a cubic array); or I-4 (82) (similar to the natural mineral “kesterite”—a Zn-rich form of Cu2
), where the metal atom planes on the c
axis alternate as: Cu–Sn (in a 2D checkerboard array)/Cu–Zn (in a 2D checkerboard array) [6
]. Because the Zn-rich mineral has the I-4 space group, it seems reasonable to consider that the most stable phase of Cu2
is a “kesterite” type structure. Another reason could be that the “stannite” type structure is more ordered: higher symmetry elements and a “segregation” of the metal atoms in the planes perpendicular on the c
axis (Cu vs. Zn/Sn).
Therefore, the two crystallographic structures (“stannite” vs. “kesterite”) differ in the ordering of the Cu+
cations, but these cations have the same number of electrons (28), meaning that they have equal “atomic X-rays scattering factors”, so the positions or their ordered/disordered distribution [7
] and the presence of these cations in the unit cell cannot be easily determined by X-ray diffraction. Other structural techniques such as neutron diffraction [7
] or Raman scattering are necessary. Another difficulty in the interpretation of XRD data, comes from the fact that the polycrystalline tetragonal (Cu2
) and cubic (ZnS, Cu0.67
S) phases have unit cell parameters with very close values.
The formation of polycrystalline phases such as Cu2
, and/or secondary phases in Cu–Zn–Sn–S thin films is influenced by their elemental composition. Thus, the deviation from stoichiometric CZTS ratios in the thin films can lead to the formation of secondary phases such as binary Cu2-x
], ZnS [11
] or ternary Cu4
], irrespective of growth techniques [13
]. The best performances of such cells are obtained for thin film compositions quite different from the precise Cu2
one (stoichiometric CZTS), especially those with copper deficiencies [14
]. The effects of the chemical composition variation in the Cu–Zn–Sn–S thin films have started to be studied by combinatorial deposition [15
]. Chemical composition tuning and defect engineering are needed in order to achieve better solar cell performances in Cu–Zn–Sn–S thin films [16
This study explores the formation of secondary crystalline phases and their effect on the optical properties in off-stoichiometric Cu2S–ZnS–SnS2 thin films, obtained by magnetron co-sputtering from three binary chalcogenide targets. Moreover, it also provides useful information for the future development of thin-film CZTS-like solar cells: the influence of chemical composition on the structural and optical properties.
2. Materials and Methods
The library was synthesized on nine SiO2
substrates that were cleaned by successive sonication in different liquids (acetone (Chemical Company, Iasi, Romania), ethanol (Chemical Company, Iasi, Romania), deionized water (prepared using a Thermo Scientific Smart2Putre UV water treatment system, Hatvan Hungary)), dried in a nitrogen flow (Linde, Bucharest Romania) and placed next to each other in the deposition equipment as in Figure 1
a. Cu–Zn–Sn–S thin films were deposited by RF magnetron co-sputtering (Gencoa Ltd., Liverpool, UK) from Cu2
S, ZnS and SnS2
binary chalcogenide targets (99.99% purity, Mateck GmbH, Jülich,, Germany, 2 inch in diameter). The magnetron sputtering system is a custom-built setup that consists of a cylindrical deposition chamber (Excel Instruments, Maharashtra, India) with hemispherical up and down caps (Excel Instruments, Maharashtra, India). Three magnetrons are equidistantly placed on the bottom hemisphere. The substrates are placed on a holder in the upper part of the chamber. The distance between the center of the P5 sample and each target was of 11 cm, whereas the rest of the samples were closer to at least one of the targets, resulting in a continuous variation of composition. The angle between the targets and the substrates was 45 degrees. After initially evacuating the chamber at 10−6
Torr, Ar gas (Linde, Bucharest Romania) was introduced at a rate of 30 sccm and the pressure inside the chamber, during deposition, was maintained constant at 5 × 10−3
Torr. A few minutes of pre-sputtering was performed in order to remove any unwanted contaminants from the target surfaces prior to the deposition process. The sputtering power was set at 80, 50 and 20 W for Cu2
S, ZnS and SnS2,
respectively, leading to a sputtering rate of 0.5 Å/s for each material. The sputtering rates were optimized using an Inficon Q-bridge monitoring software (Bad Ragaz, Switzerland) connected to a quartz microcrystal (Inficon, Bad Ragaz, Switzerland). The deposition time was approximately 35 min, in order to obtain thin films with a thickness of 300 nm. The substrates were not heated during deposition.
Sulfurization annealing was performed by placing the samples in a quartz tube, inserted in a tubular furnace, at 450 °C for 1 h. A continuous flow of 83 sccm Argon was used to transport the sulfur vapors obtained from the evaporation of an upstream sulfur powder. After annealing, the furnace was turned off and the samples were cooled to room temperature in Ar flow in order to avoid oxidation.
The determination of the elemental concentration in the films was carried out by means of energy dispersive X-ray (EDX) spectroscopy using a Zeiss EVO 50 XVP scanning electron microscope (Carl Zeiss, Oberkochen, Germany) equipped with a Bruker Quantax 200 detector (Bruker AXS Microanalysis GmbH, Berlin, Germany).
The investigation of the Cu2S–ZnS–SnS2 thin films structure was performed by grazing incidence X-ray diffraction (GIXRD) at an incidence angle of 0.3° with a Rigaku SmartLab diffractometer (Rigaku, Tokyo, Japan) provided with Cu Kα radiation (λ = 1.54178 Å) and HyPix-3000 2D Hybrid Pixel Array Detector (Rigaku, Tokyo, Japan) (in 0 D mode). The identification of the crystalline phases was performed using the DIFFRAC.SUITE Software package (Bruker, Billerica, USA).
The optical transmission and absorbance spectra were measured using a Variable Angle Spectroscopic Ellipsometer (J.A. Woollam Co., Lincoln, NE, USA), equipped with a high-pressure Xenon discharge lamp (Hamamatsu Photonics K.K., Japan), incorporated in an HS-190 monochromator (J.A. Woollam Co., Lincoln, NE, USA).
Raman spectra were recorded at room temperature, in the 200–450 cm−1 range, in backscattering configuration, with a LabRAM HR Evolution spectrometer (Horiba Jobin-Yvon, Palaiseau, France) equipped with a confocal microscope. A He–Ne laser (Horiba Jobin-Yvon, Palaiseau, France) operating at 633 nm was focused using an Olympus 100× objective (Olympus, Tokyo, Japan) on the surface of the samples. Accurate and automated calibration was performed on a standard Si wafer (provided by Horiba Jobin-Yvon, Palaiseau, France) by checking the Rayleigh and Raman signals. The laser excitation power was adjusted to avoid laser-induced heating in the thin films.
Combinatorial Cu2S–SnS2–ZnS2 thin film samples, with a gradient of chemical composition, were synthesized by magnetron co-sputtering on silicate glass substrates using Cu2S, SnS2 and ZnS binary targets. A ratio of Cu/Zn > 1.5 indicates the formation of secondary Cu–S crystalline phases in P3, P6, P8 and P9, whereas a ratio of Zn/Sn > 1 is a sign for the formation of Zn–S crystalline phases in P9. The XRD diffractograms indicate that annealing in a sulfurized environment increases the polycrystalline fraction of the thin films, but amorphous phases are still present. The sample richest in Sn (P1) is the most amorphous in the as-deposited state, however, it has the highest crystallization rate after annealing, whereas the samples richest in Cu and Zn (P3, P8 and P9) are the most crystallized in the as-deposited state, and they have the lowest crystallization rate after annealing. In all the samples, the majority crystalline phase, obtained by XRD, is a Cu–Sn–S phase that has a stannite type structure, which might be confused with a Cu–Zn–Sn–S phase if a moderate Zn ‘doping’ is present or a macrostrained CZTS can be considered taking into account the presence of Zn inferred from EDX spectroscopy. The CZTS phase could not be detected by XRD also because the peaks are broad and a deconvolution was not possible, but the Raman results show that the CZTS phase was present in each sample. The presence of binary secondary phases (SnS2, ZnS and Cu–S) has been observed in most of the samples, except for P4 and P7 (in P5 only 2%). The Raman peak from ~336 cm−1 is very narrow for P3, P5, P6, P8, P9, while for the others it is broad, which means that its width is inversely proportional with the average crystallite size of the CZTS phase. The bandgap of all samples lies in the interval of 1.41–1.68 eV. The results show that the probability of having secondary crystalline phases increases as we travel farther away from CZTS and also the bandgap changes outside the optimum range for photovoltaic applications. Finally, this study offers useful insight into how chemical composition influences the structural and optical properties of Cu–Zn–Sn–S films, knowledge which can be used in the materials design of future solar cells.