Mo-coated Soda-lime Glass Substrates Research Articles

Photoelectrochemical and Photovoltaic Performance of As-deposited Ink-based CuInS2 Heterojunction Thin Film

In this report, we demonstrate low-temperature ink-based fabrication of CuInS2-based photoelectrodes for hydrogen production capable of producing photocurrent densities as high as 8.8 mA.cm−2. In this process, molecular ink made of metal chlorides and thiourea dissolved in methanol was spin coated onto Mo-coated soda lime glass substrates. Samples were then placed on a hot plate (sample surface temperature: 250 °C) to induce CuInS2 formation and remove the solvent and by-products. No further processing, such as high temperature sulfurization, was performed. The CuInS2 films were then integrated as photocathode for hydrogen evolution by photoelectrodeposition of Pt nanoparticles. Photocurrent density of 0.4 mA.cm−2 at 0 V vs. RHE was detected from such electrodes. However, samples coated with CdS/ZnO/ITO overlayers forming heterojunction prior to Pt deposition exhibited significant improvement of the photocurrent density, reaching 8.8 mA.cm−2 at 0 V vs. RHE, highlighting the improved charge transfer by the p-n junction formed at the CuInS2/CdS interface. The charge transfer resistance of the CuInS2/CdS/ZnO/ITO-Pt photoelectrode was almost 5 times lower than that of the CuInS2-Pt photoelectrode as revealed by electrochemical impedance spectroscopy analysis. In addition, the onset potential was shifted by ca. 0.45 V toward the anodic direction, reaching 0.75 V vs. RHE. Solar cell made of identical structure (Mo/CuInS2/CdS/ZnO/ITO) showed power conversion efficiency of 2.1% with short-circuit photocurrent density and open circuit voltage of 7.4 mA.cm−2 and 820 mV, respectively.

Effect of Cu-In-Ga Target Composition on Hybrid-Sputtered Cu(In,Ga)Se2Solar Cells

High-efficiency Cu(In,Ga)Se <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (CIGSe) thin-film solar cells are typically fabricated by a multistage coevaporation process. The Cu-rich composition achieved during the second stage is known to favor the grain growth of the polycrystalline CIGSe film and has a beneficial effect on the solar cell performance. In this article, we analyze the effect of copper stoichiometry on the grain size of sputtered CIGSe absorbers and the efficiency of respective solar cells. CIGSe absorber layers were deposited on Mo-coated soda-lime glass substrates by pulsed hybrid reactive magnetron sputtering from Cu-poor and Cu-rich Cu-In-Ga targets and simultaneous Se evaporation. CIGSe films sputtered with the Cu-rich target show larger grain size and lead to better solar cell performance. Furthermore, we also introduce a two-stage process, which further increases the solar cell performance, consisting of an initial CIGSe layer deposited from the Cu-rich target followed by an indium-selenide postdeposition step. This two-stage process effectively eliminates the unwanted Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2-</sub> <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">x</sub> Se impurity phase and renders the otherwise required and toxic KCN etching unnecessary.

Improving the Cu2ZnSn(S,Se)4-Based Photovoltaic Conversion Efficiency by Back-Contact Modification

Back-contact modification using a 10-nm ZnS layer in CZTSSe-based solar cell can play a crucial role in improving photovoltaic conversion efficiency. An ultrathin layer of ZnS is deposited over Mo-coated soda lime glass substrate before depositing CZTSSe using sputtering. The crystal structure of deposited CZTSSe thin films over ZnS is recognized as (112)-oriented, polycrystalline in nature, and free from the presence of any secondary phases such as Cu <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> (S,Se) or Zn(S,Se). The bandgap of CZTSSe thin films deposited over ultrathin ZnS is observed to increase from 1.49 (deposited over Mo directly) to 1.58 eV at room temperature, as determined by spectroscopic ellipsometry. In addition, numerical simulation has been performed using SCAPS software. The impact of ZnS layer has been simulated by using the defects in the absorber and at the interface of ZnS/CZTSSe. The simulated results have been validated with experimentally fabricated CZTSSe device. Simulated device with ZnS intermediate layer is observed to give rise to a photovoltaic conversion efficiency of 15.2%.

Investigation on the properties of Cu 2 ZnSnSe 4 and Cu 2 ZnSn(S,Se) 4 absorber films prepared by magnetron sputtering technique using Zn and ZnS targets in precursor stacks

Cu2ZnSnSe4 and Cu2ZnSn(S,Se)4 absorber films were grown on soda lime glass and Mo-coated soda lime glass substrates by deposition of the precursor films via RF magnetron sputtering method in the stacking orders of Cu/Sn/Zn/Mo/SLG and Cu/Sn/ZnS/Mo/SLG using metallic Zn or binary sulfide ZnS targets and subsequently carrying out of selenization process. It was aimed to find out the effect of Zn or ZnS target types and the small amount of S originated from ZnS target material used in the deposition of precursor films on the structural, morphological, optical, and electrical characteristics of the films to be selenized. XRD and Raman spectroscopy analysis showed that the kesterite CZTSe structure was predominantly formed in both cases where Zn and ZnS targets were used. According to the EDX analysis, S content in the prepared film using ZnS target was only around 1.79 at%. This indicated that a considerable amount of S in the film was driven out during the selenization process. Scanning electron microscopy analysis revealed that ZnS target material contributed to the achievement of the absorber films with larger grain size. It was also determined that the thickness of the interfacial MoSe2 film between the absorber and Mo films decreased by using ZnS target in the precursor film. This was attributed to ZnS layer with a high melting point acting as a barrier layer over Mo film and retarding the diffusion of Se into the Mo film during the selenization process. In addition, the band gap energy values of the Cu2ZnSnSe4 and Cu2ZnSn(S,Se)4 films were found to be 1.18 and 1.28 eV, respectively.

Influence of Sulfurization Time on the Properties of Cu2ZnSnS4 Thin Films Deposited on Mo-coated Soda Lime Glass Substrates by Co-sputtering Technique

In this work, Cu2ZnSnS4 (CZTS) absorber films were prepared by sulfurization of co-sputtered precursor films at different time periods. The influence of sulfurization time on physical characteristics of the grown films was investigated in detail. The sulfurized CZTS films exhibited the characteristic diffraction peak of the kesterite phase corresponding to the (112) crystal plane, the main Raman mode of kesterite phase at around 337 cm−1 and Cu-deficient and Zn-rich composition. From the atomic force microscopy surface and cross-sectional scanning electron microscopy images, it was revealed out that the films were uniform, compact without any cracks and consisted of micron-sized and closely packed grains. In addition, the films showed tail-to-tail, band-to-tail and band-to-band transitions at around 1.41 eV (E1), 1.45 eV (E2) and 1.51 eV (E3), respectively. Secondary ion mass spectroscopy analysis showed that the sodium ions deeply diffused through the molybdenum layer into the CZTS film from the soda lime glass substrate for all samples. All of the films exhibited an optical band gap of around 1.4 eV, an absorption coefficient over 104 cm−1, p-type conductivity with a high concentration of free holes in the order of 1017-1018 cm−3 and low mobility in the range from 0.63 cm²/V.s to 8.11 cm²/V.s.

Single-step growth of high quality CIGS/CdS heterojunctions using Pulsed Laser Deposition

This work reports on the utilization of pulsed laser deposition (PLD) for the preparation of CdS thin films and CdS/Cu(In,Ga)Se2 heterojunction structures on soda-lime glass (SLG) and Mo-coated SLG substrates, respectively, under various process conditions. Single phase, stoichiometric and high optical quality CdS films are obtained at a fluence of 1.1 J/cm2 and at deposition temperatures of 200–400 °C. The results of this investigation were used to grow CdS on Cu(In,Ga)Se2/Mo/SLG. Both Cu(In,Ga)Se2 and CdS layers have been deposited sequentially using PLD without interrupting the deposition process. The influence of CdS deposition temperature on the properties of CdS/Cu(In,Ga)Se2 heterojunction has been extensively studied and is reported herein for the first time. Low series and high shunt resistances are obtained for the samples where CdS was grown at 200 and 300 °C. The CdS/Cu(In,Ga)Se2 diode grown at CdS deposition temperature of 300 °C exhibits the lowest ideality factor and leakage current, indicating the better quality of the diode. The results of this work demonstrate that high-quality CdS/Cu(In,Ga)Se2 diodes are obtained using pulsed laser deposition in a single-step growth process, eliminating the need for selenization of Cu(In,Ga)Se2 and the use of other growth techniques such as chemical bath deposition for CdS.

Influence of Ge/(Ge+Sn) composition ratio in Cu2Sn1-xGexS3 thin-film solar cells on their physical properties and photovoltaic performances

Effect of Ge/(Ge + Sn) composition ratio of Cu2Sn1-xGexS3 (CTGS) films is examined. The CTGS films are fabricated by sulfurization of the 700-nm-thick Cu–SnS2/Ge stacked precursors on Mo-coated soda-lime glass (SLG) substrates. It is revealed that the thickness of Ge precursor (Ge thickness) from 0 to 420 nm enhances the Ge/(Ge + Sn) composition ratio in the CTGS thin films from 0 to about 0.58, thereby increasing bandgap energy (Eg) from approximately 0.97 (Cu2SnS3: CTS) to 1.23 eV (CTGS), respectively. Not only does Ge precursor act as Ge source to form the CTGS films but also it prevents the formation of undesirable MoS2 secondary phase. In addition, the CTGS solar cells with a structure of SLG/Mo/CTGS/CdS/ZnO/ZnO:Al/Ni/Al are fabricated with the Ge thickness from 0 to 420 nm. It is found that the optimized Ge thickness of 200 nm resulting in the Ge/(Ge + Sn) composition ratio of about 0.32 eliminates the excessive MoS2 secondary phase and increases the Eg, thereby increasing conversation efficiency (η) to 5.6% of the CTGS solar cell, higher than that (3.6%) of the CTS solar cell. However, the η is reduced when the Ge thickness over 200 nm because CTGS quality observed by Urbach energy is deteriorated and the conduction band offset at the CdS/CTGS interface demonstrates the large cliff type heterostructure, thus increasing the open-circuit voltage deficit.

Investigation of carrier recombination of Na-doped Cu2SnS3 solar cell for its improved conversion efficiency of 5.1%

Na-doped Cu2SnS3 (CTS) solar cells are fabricated and their carrier recombination is examined. The CTS absorbers on Mo-coated soda-lime glass substrates are grown by the sulfurization of NaF/Cu–SnS2 (900 nm) precursors. The NaF thickness is varied from 0 to 100 nm to vary the Na content in the resulting CTS absorbers. It is disclosed that the large grain with the uniformity of the material distribution (Cu, Sn, and S) in the Na-doped CTS absorber is obtained. With optimal Na doping (NaF thickness of 60 nm), the open-circuit voltage deficit (VOC,def) is obviously reduced, whereas short-circuit current density deficit (JSC,def) is not varied much. The reduction of VOC,def is attributable to the decrease in the carrier recombination across the device. The conversion efficiency (η) is consequently increased to approximately 4.7%. However, with severe Na doping (NaF thickness of over 75 nm), the VOC,def is clearly increased owing to the increase in the carrier recombination, thereby reducing the η. This is occurs because the Sn2S3 and Na2S secondary phases near the surface of the CTS film are formed with the severe Na doping. To further increase the η, the JSC,def is reduced through the decrease in CdS buffer thickness. Ultimately, the 5.1%-efficient CTS solar cell is obtained with the Na-doped CTS absorber prepared with the optimal NaF thickness of 60 nm.

The impact of different Ag/(Ag + Cu) ratios on the properties of (Cu1−xAgx)2ZnSnS4 thin films

In this work, the effects of 0%, 3%, 6% and 10% Ag doped on the properties of Cu2ZnSnS4 (CZTS) thin films and solar cells are investigated by sputtering different Ag intermediate layer on Mo-coated soda lime glass substrate. The result shows that the partial substitution of Ag with Cu for (Cu1−xAgx)2ZTS4 (CAZTS) thin films not only can obtain the uniform and compact grains with larger size, but also can reduce the amount of CuZn anti-site and Cu vacancies (VCu) and decrease the thickness of MoS2 and voids at the back contact. Therefore, the CAZTS solar cells can attain a higher open circuit voltage (Voc), short circuit density (Jsc), fill factor (FF) and efficiency compared with pure CZTS solar cells, which may be attributed to reducing the recombination at boundary grains and the interface between Mo and absorbers as well as the series resistance (Rs) due to the high-quality grains. However, the different Ag contents of CAZTS absorbers create the different propriety of CAZTS devices. In short, the performance of CAZTS devices are enhanced markedly under the optimal Ag-incorporation content, and the power conversion efficiency is successfully increased from 3.45 (x = 0) to 5.44% (x = 6%).

Structural and microchemical characterization of Cu(In,Ga)Se2 solar cells with solution-grown CdS, Zn(O,S), and Inx(O,S)y buffers

Thin films of Cu(In,Ga)Se2 (CIGS) with a [Ga]/([Ga] + [In]) ratio of about 0.3 were deposited on Mo-coated soda-lime glass substrates by co-evaporation of the elements. In combination with three different solution-grown buffer layers, namely CdS, Zn(O,S), and Inx(O,S)y, and sputtered ZnO/ZnO:Al or Zn0.75Mg0.25O/ZnO:Al double layers as the front contact they were completed to three different types of CIGS-based solar cells. The as-fabricated cell efficiencies are 16.9%, 14.7%, and 14.6%, respectively. The microstructure of all the three devices was analyzed by transmission electron microscopy (TEM). Pores occur at the interface between CIGS absorber and Mo back electrode, and stacking faults and microtwins are observed in the CIGS absorber. Zn(O,S) and Inx(O,S)y buffers are nanocrystalline in contrast to larger grain sizes in the CdS buffer. The structure of the CIGS/CdS interface is frequently coherent, while that of the CIGS/Zn(O,S) and CIGS/Inx(O,S)y interfaces is incoherent. High-resolution TEM imaging demonstrates that CdS buffer occurs in the cubic zincblende and hexagonal wurtzite structure with a high stacking fault density in zincblende CdS. The difference in crystalline quality of the buffers and CIGS/buffer interfaces might contribute to the different efficiencies of the investigated CIGS-based thin-film solar cells. Chemical analyses by energy-dispersive X-ray spectroscopy show interdiffusion and chemical inhomogeneities in Zn(O,S) and Inx(O,S)y buffers.

CIGS thin film solar cells with graded-band gap fabricated by CIS/CGS bilayer and CGS/CIS/CGS trilayer systems

High efficiency CuIn1-xGaxSe2 (CIGS) thin film solar cells are usually deposited on Mo-coated soda-lime glass (SLG) substrates by the three-stage co-evaporation process which is intended to create double-graded band gap. It requires short total deposition time and rapid changing of substrate temperature in order to obtain the double-graded band gap which is quite difficult for high source-temperatures. When the total deposition time cannot be made short enough, uniform band gap is rather achieved. In this work, the depositions of CIS/CGS bilayer are employed in order to create back grading while CGS/CIS/CGS trilayers are for the double-grading, owing to non-uniformity and different diffusivity of In and Ga constituents in the films. The bilayers with back grading show the increasing short-circuit current density (Jsc) due to the assisting back surface field, but the open-circuit voltage (Voc) is relatively low due to the reduction of Ga content at the front surface. For the CGS/CIS/CGS trilayers, double grading and the increasing of Voc are observed when compared with the bilayers due to the increasing Ga content near the junction. The efficiencies of the devices fabricated from the CIS/CGS bilayer and CGS/CIS/CGS trilayer absorbers show the maximum value of 12.5% and 15.5%, respectively. The external quantum efficiency (EQE) of the bilayer and trilayer absorbers exhibit the enhancement in the long wavelengths compared to that of the three-stage process absorbers.

Characterization of a co-evaporated Cu2SnS3 thin-film solar cell

Cu2SnS3 (CTS) absorber layer was deposited onto a Mo-coated soda-lime glass substrate by sulfurizing a co-evaporated Cu–SnS precursor thin film. Phase composition and quality of the layer were examined by X-ray diffraction, Raman spectroscopy and scanning electron microscopy. A solar cell with such CTS absorber layer was characterized by current-voltage, external quantum efficiency, capacitance-voltage, and admittance spectroscopy. The CTS solar cell exhibited a maximum conversion efficiency of 2.01%. Capacitance-voltage measurement showed that CTS absorber had hole carrier concentration of 6.37 × 1016 cm−3 and depletion width of 82 nm at room temperature. Admittance spectroscopy showed capacitance steps that might have originated from a deep-level with activation energy and pre-exponential factor of about 0.11 eV and 8.31 × 104 s−1∙K−2, respectively. The concentration of the deep-level defects was estimated to be 5.23 × 1018 cm−3.

Effect of substrate temperature on the structural and optical properties of radio frequency sputtered tin sulfide thin films for solar cell application

Tin monosulfide (SnS) thin films were deposited on both soda lime glass (SLG) and Mo-coated SLG substrates by radio-frequency (RF) sputtering at a substrate temperature (Ts) from room temperature (RT) to 420 °C. The structural and optical properties were investigated by performing the scanning electron microscopy, electron dispersive spectroscopy, atomic force microscopy, X-ray diffraction, micro-Raman, and ultraviolet-visible-near infrared spectroscopy measurements. For Ts greater than RT showed the well-defined grain features with increased grain sizes. At 350 °C, the films showed the [S/Sn] ratio close to the stoichiometric SnS. The X-ray diffraction studies unveiled the orthorhombic crystal structure of SnS with the (111) preferred orientation for all the Ts. The Raman analysis confirmed the single phase of SnS without any secondary phases. The optical properties revealed a direct energy bandgap of SnS was around 1.2–1.4 eV and the bandgap decreased with increasing Ts. We fabricated the SnS/CdS heterojunction solar cell with the SnS grown at 350 °C. The device with a structure of SLG/Mo/SnS/CdS/i-ZnO/ITO/Ni/Ag exhibited the power conversion efficiency of 0.35% with an open-circuit voltage of 0.143 V, short-circuit current density of 6.7 mA·cm−2, and fill factor of 40.3%.

CIGS thin film and device performance produced through a variation Ga concentration during three-stage growth process

In this study, Cu(In,Ga)Se2 (CIGS) absorber is grown on Mo-coated soda-lime glass substrates through co-evaporation deposition. Several types of Ga distribution in CIGS was examined by adjusting the [Ga]/([In]+[Ga]) ratio at the first and third stages. The electrical properties of the film were also determined. The carrier concentration increased with increasing Ga content. The highest carrier concentration of 8.46 × 1015 cm−3 were obtained when the [Ga]/([In]+[Ga]) ratio (0.36) was 0.4: 0.3 at the first and third stage. The best device conversion efficiency of 12.5% was achieved because the suitable [Ga]/([In]+[Ga] ratio, band-gap gradient distribution and low recombination rate.

Importance of CdS buffer layer thickness on Cu2ZnSnS4-based solar cell efficiency

Cu2ZnSnS4 (CZTS) thin films were grown on Mo-coated soda lime glass (SLG) substrates by the sulfurization of DC magnetron-sputtered Zn, Sn and Cu metallic precursors under a sulfur atmosphere at 550 °C for 45 min. Understanding the composition and structure of the CZTS absorber layer is necessary to obtain efficient solar cells. With this aim, x-ray diffractometry, Raman spectroscopy, scanning electron microscopy, energy dispersive spectroscopy and x-ray photoelectron spectroscopy were used to investigate the CZTS absorber layers. CZTS absorber films were obtained and found to be Cu-poor and Zn-rich in composition, which are both qualities desired for efficient solar cells. CdS was used as a buffer layer and was grown by the chemical bath deposition technique. The optical properties of CdS films on SLG were searched for using a spectroscopic ellipsometer and the results revealed that the bandgap increases with film thickness increment. CZTS-based solar cells with different CdS buffer layer thicknesses were prepared using a SLG/Mo/CZTS/CdS/ZnO/AZO solar cell configuration. The influence of the CdS buffer layer thickness on the performance of the CZTS solar cells was investigated. Device analysis showed that electrical characteristics of solar cells strongly depend on the buffer layer’s thickness. Highly pronounced changes in VOC, fill factor and JSC parameters, which are the main efficiency limiting factors, with changing buffer layer thicknesses were observed. Our experiments confirmed that decreasing the CdS thickness improved the efficiency of CZTS solar cells down to the lowest thickness limit.

Photoelectrochemical Properties of Surface-Modified ZnSe:Cu(In,Ga)Se2 Photocathodes for Efficient and Durable Overall Water Splitting

Overall water splitting using a photoelectrochemical (PEC) cell composed of a photocathode and photoanode connected in a series is an attractive method to produce hydrogen from water under sunlight. Because driving forces of two photoelectrodes are combined for the water splitting reaction, narrow-gap materials can be used to achieve high solar-to-hydrogen conversion efficiency (STH). However, the STHs obtained from a PEC cell without any external bias voltage reported so far have been less than 1%. This is because of insufficient onset potentials and photocurrent values of photoelectrodes. For the sake of overcoming the drawbacks, (ZnSe)0.85(CuIn0.7Ga0.3Se2)0.15 ((ZnSe)0.85(CIGS)0.15) thin film photocathodes have recently been developed and reported to show a high onset potential of 0.89 VRHE and long absorption edge of 850-900 nm.2 These properties are suitable for use in the PEC cell. (ZnSe)0.85(CIGS)0.15 thin films are prepared by co-evaporation onto Mo-coated soda-lime glass substrates. Subsequently, CdS, a binary of Mo/Ti and Pt are deposited onto the photocathode surface as a buffer layer, surface conductor and hydrogen evolution reaction (HER) catalyst, respectively.3 The surface-modified (ZnSe)0.85(CIGS)0.15 photocathodes show a relatively high photocurrent value of 12 mA cm-2 at 0 VRHE and sufficient stability in a span of hours at potentials of more negative than 0.5 VRHE under simulated sunlight in a neutral potassium phosphate buffer solution.4 However, at potentials more positive than 0.5 VRHE, the photocurrent value attributed to HER dramatically decreases by half in just one hour mainly due to self-oxidation of the surface sulfide layer, accompanying detachment of the Mo/Ti and Pt at the surface. The poor stability at the positive potentials has made it difficult to construct durable PEC cell using the photocathodes. In this work, effects of surface modifications onto PEC properties of (ZnSe)0.85(CIGS)0.15 photocathodes are investigated. For the sake of suppressing the surface corrosion, the CdS layer was passivated with In2S3, which was formed by using chemical bath deposition (CBD).5 Figure 1 shows the current-time curves of the surface-modified (ZnSe)0.85(CIGS)0.15 photocathodes at 0.6 VRHE under simulated sunlight. Without the CBD treatment, the photocurrent value decreased by 50% in one hour under light irradiation. On the other hand, the In2S3-modified photocathode showed relatively stable PEC HER and the decline of the photocurrent value in one hour was decreased to 25%. It is highly possible that the improvement of stability originates from low solubility of indium oxide or hydroxide generated by the self-oxidation process, while the cadmium oxide and hydroxide are relatively soluble, which can cause corrosion of the surface. Furthermore, effects of oxide-coating onto the stability of the (ZnSe)0.85(CIGS)0.15 photocathode during PEC hydrogen evolution have also been investigated. Among various kinds of coating processes, direct formation of the oxide layer by photoelectrodeposition without annealing the photocathode is an effective method without exerting a bad influence on the underlying sulfide and selenide materials. In the presentation, the details of preparation conditions and PEC properties will be discussed. References J. W. Ager, M. R. Shaner, K. A. Walczak, I. D. Sharp and S. Ardo, Energy Environ. Sci. 2015, 8, 2811–2824.H. Kaneko, T. Minegishi, M. Nakabayashi, N. Shibata, Y. Kuang, T. Yamada and K. Domen, Adv. Funct. Mater. 2016, 26, 4570–4577.H. Kumagai, T. Minegishi, N. Sato, T. Yamada, J. Kubota and K. Domen, J. Mater. Chem. A 2015, 3, 8300–8307.H. Kaneko, T. Minegishi, M. Nakabayashi, N. Shibata and K. Domen, Angew. Chemie Int. Ed. 2016, 55, 15329–15333.F. Jiang, Gunawan, T. Harada, Y. Kuang, T. Minegishi, K. Domen and S. Ikeda, J. Am. Chem. Soc. 2015, 137, 13691–13697. Figure 1

Structural properties of CZTS thin films on glass and Mo coated glass substrates: a Rietveld refinement study

Due to the difficulty in analyzing a few properties of CZTS (Cu2ZnSnS4) films coated on opaque substrates, the process conditions are often optimized on normal soda lime glass substrates and then transfer the process to Mo coated soda lime glass (SLG) substrates for device fabrication. This study concentrates on the analysis of structural properties of CZTS films on normal glass and Mo coated SLG substrates. CZTS films were coated on these two substrates by spin coating followed by sulphurization in N2 + H2S atmosphere. Phase purity of the films was analyzed by GIXRD and Raman spectroscopy. W–H analysis was used for estimating crystallite size and lattice strain of the films on the two substrates. Detailed structural studies were carried out with Rietveld refinement technique using GSAS package. Kesterite model with space group I4 (SG No: 82) was chosen for the refinement. Formation of kesterite phase on both substrates was confirmed from the refined lattice constant values. Deviation in bond angle with respect to kesterite structure was also analyzed. Different structural parameters were observed for the films on these substrates. The study indicates that it is important to optimize the properties of CZTS film on Mo coated SLG substrate separately, without following the same preparation conditions used for coating film on glass substrates.

Complexing agent-assisted highly dense CuInSe2 thin films prepared by one-step electrochemical deposition

Highly dense CuInSe2 (CISe) thin films were electrochemically deposited on Mo-coated soda-lime glass substrates in aqueous solution. Three compounds, CuCl2, InCl3, and SeO2, were used as precursors with glycine or oxalic acid as complexing agents. The deposition process was carefully selected through preliminary study of the individual electrochemical properties of the three precursors. To achieve ideal stoichiometry and uniform deposition of CISe thin films, the reaction temperature and acidity of precursor solution were investigated. As the reaction temperature increased, Cu deficiency was observed in the CISe thin film. The density and surface morphology of CISe films prepared by one-step electrodeposition were affected strongly by the acidity of deposition solution. The addition of HCl to solution improved the density and surface roughness of as-deposited CISe films. Denser and larger grain size CISe polycrystalline films were prepared in oxalic acid solution. Two-step heat treatments for the prepared CISe samples were applied to fabrication of CISe photovoltaic devices without an additional Se source.

Scalable Low-Band-Gap Sb2Se3 Thin-Film Photocathodes for Efficient Visible-Near-Infrared Solar Hydrogen Evolution.

A highly efficient low-band-gap (1.2-0.8 eV) photoelectrode is critical for accomplishing efficient conversion of visible-near-infrared sunlight into storable hydrogen. Herein, we report an Sb2Se3 polycrystalline thin-film photocathode having a low band gap (1.2-1.1 eV) for efficient hydrogen evolution for wide solar-spectrum utilization. The photocathode was fabricated by a facile thermal evaporation of a single Sb2Se3 powder source onto the Mo-coated soda-lime glass substrate, followed by annealing under Se vapor and surface modification with an antiphotocorrosive CdS/TiO2 bilayer and Pt catalyst. The fabricated Sb2Se3(Se-annealed)/CdS/TiO2/Pt photocathode achieves a photocurrent density of ca. -8.6 mA cm-2 at 0 VRHE, an onset potential of ca. 0.43 VRHE, a stable photocurrent for over 10 h, and a significant photoresponse up to the near-infrared region (ca. 1040 nm) in near-neutral pH buffered solution (pH 6.5) under AM 1.5G simulated sunlight. The obtained photoelectrochemical performance is attributed to the reliable synthesis of a micrometer-sized Sb2Se3 (Se-annealed) thin film as photoabsorber and the successful construction of an appropriate p-n heterojunction at the electrode-liquid interface for effective charge separation. The demonstration of a low-band-gap and high-performance Sb2Se3 photocathode with facile fabrication might facilitate the development of cost-effective PEC devices for wide solar-spectrum utilization.

High pressure Raman scattering of a co-evaporated Cu2SnSe3 thin film

We fabricated Cu2SnSe3 thin film on a Mo-coated soda-lime glass substrate by a co-evaporation method. The X-ray diffraction pattern revealed the formation of polycrystalline Cu2SnSe3 with a monoclinic structure (space group Cc). Raman scattering measurements were also performed on the thin film at a pressure ranging from 1atm to 7.01GPa. The Raman spectrum was resolved into 4 Lorentzian peaks observed at 184cm−1, 206cm−1, 236cm−1, and 252cm−1 at 1atm, which correspond to A′, A″, A″, and A′ symmetry, respectively. The effects of pressure on these Raman-active phonon modes were discussed. The measured elastic properties of Cu2SnSe3 under high pressure were also compared with those of Cu2ZnSnSe4.