Mo-coated Glass Research Articles

Enhanced grain orientation in Sb2Se3 thin films deposited on Mo/BSG substrates via RF-sputtering and selenization

The alignment of the Sb2Se3 absorber grains along the c-axis, resulting from their quasi-1D ribbon-like growth, has been found to enhance current density, thereby enabling high power conversion efficiencies in Sb2Se3-based thin film solar cells. This study presents a controllable method for depositing Sb2Se3 thin films with highly oriented grains and a compact morphology, making them suitable for large-scale solar cell applications. A combination of magnetron sputtering and post-deposition isothermal selenization techniques was used. This process involved sputtering antimony onto Mo-coated Borosilicate glass (BSG) substrates, followed by selenizing the films at different temperatures in argon-gas filled quartz ampoules. The influence of several partial Ar-gas pressures and selenization temperatures on the grain growth was studied to achieve enhanced preferred grain orientations and compact morphology of the Sb2Se3 thin films. Selenization at 380 °C resulted in dominant vertical grain orientations with respect to the substrate surface of the Sb2Se3, as confirmed by scanning electron microscopy and X-ray diffraction analysis, without compromising surface compactness. Although occasional micro-voids remain in the films, further optimization could eliminate these imperfections. The developed sequential method, where Sb layer was deposited by RF-sputtering process, followed by an annealing at elevated temperature in the presence of Se and argon gas pressure, holds potential for large-scale production and the fabrication of other quasi-1D chalcogenide thin films.

Influence of Deposition Parameters on Performance of Zn(O,S) Buffer Layer Deposited by Solution-Based Continuous Flow Reactor for CIGS Solar Cells

CdS thin films are commonly used as buffer layers in hetero-junction structured CIGS thin film solar cells. However, the toxicity of Cd has raised concerns regarding environmental safety and alternative materials have been explored. ZnS is a leading candidate to replace Cd, with the presence of Zn(O,S) impurities known to be required for efficient CIGS solar cell fabrication. In this study, we synthesized Zn(O,S) films using a solution-based deposition method combining a continuous flow reactor (CFR) with another solutionbased deposition method. X-ray diffraction (XRD) and scanning electron microscopy (SEM) were used to characterize the structural and physicochemical properties of the films, confirming the efficacy of the CFR process for depositing Zn(O,S) thin films in a short period of time. We successfully deposited Zn(O,S) thin films using the combined CFR process. Cd-free B:ZnO/Zn(O,S)/CIGS solar cells were fabricated on Mo-coated glass substrates to investigate the effects of experimental parameters, such as deposition time and method, on the performance of the Zn(O,S) buffer layer in the devices.

A Novel combined thermal evaporation and solvothermal approach to produce highly crystalline and compact CIG(S1-xSex)2 thin films

This research introduces a novel approach that combines thermal evaporation with cost-effective solvothermal methods to produce highly crystalline CIGSe thin films. By avoiding the use of expensive and pure target materials and reducing the selenization step, this method offers an alternative to co-evaporation or sputtering techniques. The study investigates the growth mechanism and sulfurization of the films, resulting in improved crystallinity, absence of secondary impurities, and compact morphology. The sulfurization process is achieved at a low temperature using a safe and convenient method, enhancing the film properties for high-performance solar cells. In this study, the CIGSe powder was synthesized using the solvothermal technique and thermally evaporated onto Mo-coated glass substrates. The resulting films exhibited improved crystallinity and a single-phase structure upon RTP annealing at 600 °C. Characterization techniques confirmed the presence of chalcopyrite crystal structure and the coexistence of CISe, CIGSe, and CGSe phases within the films. The sulfurization process, conducted at a relatively low temperature of 500 °C using an RTP furnace, led to compact morphology, increased grain size, and the absence of secondary phases. These findings demonstrate the feasibility of the proposed approach for fabricating efficient and environmentally friendly CIGSe-based solar cells.

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.

CZTS solar cells on graphite without Mo-coated glass substrate

The use of graphite substrates has been demonstrated in thin-film Cu2ZnSnS4 (CZTS) solar cells and can serve as alternative electrodes for next-generation, thin-film solar cells. From the early stages of CZTS development, Mo-coated substrates composed of copper (Cu), zinc (Zn), tin (Sn), and sulfur (S) have been employed for stability at high temperature. However, Mo has become a rare metal in recent years; correspondingly, there are potential risk of supply shortages and depletion problems. We focus herein on graphite as an electrode and substrate owing to its versatility and low cost. The CZTS precursor was prepared by radiofrequency sputtering. Subsequently, NaF was deposited using the E-B vapor deposition method to control the Na composition ratio. CZTS films were obtained by gas-phase sulfurization at 898 K for 1 h. CZTS solar cells with MgF2/Al/AZO/CdS/CZTS/graphite structure were prepared, and their characteristics were evaluated. J–V measurement of the precursor with a 20 nm thick NaF yielded η = 2.37%, V oc = 543 mV, J sc = 10.7 mA cm−2, and FF = 40.8%. These results suggest that CZTS on graphite with NaF control has the potential for realizing the low cost CZTS solar cells.

Effect of sodium incorporation on CuIn(S,Se)2 thin films fabricated by aqueous spray deposition and their photovoltaic properties

Selenium-rich CuIn(S,Se)2 (CISSe) thin films on a Mo-coated glass substrate were fabricated by spray pyrolysis of an aqueous precursor solution containing Cu(NO3)2, In(NO3)3, and methylthiourea followed by selenization at 560 °C. Addition of low-concentration NaNO3 to the precursor solution (Na/(Cu + In) = 0.2) effectively improved the crystallinity of the Cu-poor CuInS2 precursor film as confirmed by XRD measurement. Furthermore, final CISSe film obtained from this precursor film had an improved grain growth with bigger grain sizes than that of the CISSe film from the precursor film without the addition of Na. As a result, solar cell based on that CISSe film had a conversion efficiency of 2.66 %, significantly improved from only 0.28 %. An increase of the CISSe film thickness from 900 nm to 2.2 μm led to further improvement of the conversion efficiency to 4.35 %.

Insight into the Effect of Selenization Temperature for Highly Efficient Ni-Doped Cu2ZnSn(S,Se)4 Solar Cells.

Cu2Ni0·05Zn0·95Sn(S,Se)4 (CNZTSSe) films were synthesized on Mo-coated glass substrates by the simple sol–gel means combined with the selenization process, and CNZTSSe-based solar cells were successfully prepared. The effects of selenization temperature on the performance and the photoelectric conversion efficiency (PCE) of the solar cells were systematically studied. The results show that the crystallinity of films increases as the selenization temperature raises based on nickel (Ni) doping. When the selenization temperature reached 540 °C, CNZTSSe films with a large grain size and a smooth surface can be obtained. The Se doping level gradually increased, and Se occupied the S position in the lattice as the selenization temperature was increased so that the optical band gap (Eg) of the CNZTSSe film could be adjusted in the range of 1.14 to 1.06 eV. In addition, the Ni doping can inhibit the deep level defect of SnZn and the defect cluster [2CuZn + SnZn]. It reduces the carrier recombination path. Finally, at the optimal selenization temperature of 540 °C, the open circuit voltage (Voc) of the prepared device reached 344 mV and the PCE reached 5.16%.

Deposition of (Ag,Cu)2Zn(Sn,Ge)S4 thin films on Mo-coated glass substrate by vacuum magnetron sputtering and post-sulfurization techniques

Cation substitution is a useful way to improve the properties of semiconducting Cu2ZnSnS4 thin film. In this work, partial Cu and Sn in Cu2ZnSnS4 are substituted by Ag and Ge, respectively. The (Ag,Cu)2Zn(Sn,Ge)S4 thin films were successfully fabricated using vacuum magnetron sputtering and post-sulfurization techniques. The formation of Ag & Ge co-doped Cu2ZnSnS4 structure with secondary phase is proved by XRD and Raman results. The Ag and Ge ratios depend on the composition of Cu-Ag target and the sputtering time of Ge, respectively. The direct optical band gap values of thin films increase with the increase of Ge content. When the sputtering time of Ge is 90 s, the Urbach energy of (Ag,Cu)2Zn(Sn,Ge)S4 thin films reaches the minimum value of 339 meV, revealing the reduced band tail state by Ge incorporation.

Effects of incorporation of Ag into a kesterite Cu2ZnSnS4 thin film on its photoelectrochemical properties for water reduction.

Kesterite Cu2ZnSnS4 (CZTS) thin films in which the Cu site was partially replaced with Ag were prepared by spray deposition on an Mo-coated glass substrate. Successful replacement of Cu components in the CZTS lattice with Ag up to an Ag/(Cu + Ag) ratio of 0.20 was achieved. Samples with relatively low contents of Ag (Ag/(Cu + Ag) ratios of 0.05 and 0.10) showed obvious grain growth compared to that of bare CZTS, whereas samples with higher Ag contents showed an appreciable decrease in grain sizes. Photoelectrochemical properties for water reduction (H2 production), which was examined after surface modifications with an In2S3/CdS double layer and Pt catalyst for H2 evolution, depended strongly on such morphological differences; a maximum conversion efficiency, i.e., half-cell solar to hydrogen efficiency, of 2.4% was achieved by the photocathode based on the film with an Ag/(Cu + Ag) ratio of 0.10. Minority carrier dynamics examined by photoluminescence measurements indicated that such an active sample of PEC H2 production had a relatively long carrier lifetime, suggesting that the suppression of carrier recombination at grain boundaries in the bulk of these kesterite films is one of the important factors for enhancing PEC functions.

Vapor Phase Synthesis of SnS Facilitated by Ligand-Driven "Launch Vehicle" Effect in Tin Precursors.

Extraordinary low-temperature vapor-phase synthesis of SnS thin films from single molecular precursors is attractive over conventional high-temperature solid-state methods. Molecular-level processing of functional materials is accompanied by several intrinsic advantages such as precise control over stoichiometry, phase selective synthesis, and uniform substrate coverage. We report here on the synthesis of a new heteroleptic molecular precursor containing (i) a thiolate ligand forming a direct Sn-S bond, and (ii) a chelating O^N^N-donor ligand introducing a “launch vehicle”-effect into the synthesized compound, thus remarkably increasing its volatility. The newly synthesized tin compound [Sn(SBut)(tfb-dmeda)] 1 was characterized by single-crystal X-ray diffraction analysis that verified the desired Sn:S ratio in the molecule, which was demonstrated in the direct conversion of the molecular complex into SnS thin films. The multi-nuclei (1H, 13C, 19F, and 119Sn) and variable-temperature 1D and 2D NMR studies indicate retention of the overall solid-state structure of 1 in the solution and suggest the presence of a dynamic conformational equilibrium. The fragmentation behavior of 1 was analyzed by mass spectrometry and compared with those of homoleptic tin tertiary butyl thiolates [Sn(SBut)2] and [Sn(SBut)4]. The precursor 1 was then used to deposit SnS thin films on different substrates (FTO, Mo-coated soda-lime glass) by CVD and film growth rates at different temperatures (300–450 °C) and times (15–60 min), film thickness, crystalline quality, and surface morphology were investigated.

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.

Photoelectrochemical water splitting on the Pt-In2S3/CuInS2 photoelectrode under solar light irradiation: Effects of electrolytes on the solar energy to hydrogen conversion

An effective CuInS 2 for PEC water splitting was developed. Phosphate ions not only suppress the chemical bias for the HER process, but also reduce recombination center at the electrode/electrolyte interface. The HC-STH efficiency achieved 1.5% with faraday efficiency of 96 %. Solar energy to hydrogen (H 2 ) conversion system has been developed by the photoelectrochemical (PEC) water splitting. The CuInS 2 (CIS) photoelectrodes were successfully fabricated by electrodeposition (ED) of copper and indium metallic precursors on Mo-coated glass, followed by sulfurization under sulfur evaporation. The CIS was modified with In 2 S 3 by the chemical bath deposition (CBD), followed by photo-deposition of Pt particles. The Pt-In 2 S 3 /CIS photoelectrodes provided cathodic photocurrent by water splitting in the electrolyte solution under irradiation from the simulated sunlight (AM 1.5G). The half-cell solar-to-hydrogen (HC-STH) efficiencies by the Pt-In 2 S 3 /CIS achieved 1.5% at + 0.26 V vs. RHE with faraday efficiency of 96% in 0.5 M potassium phosphate (pH 7), while 0.16% at + 0.16 V vs. RHE in 0.5 M sodium sulfate (pH 7). It was clearly demonstrated that phosphate buffer solution play an important role in control of chemical bias by a removal of OH − as well as passivating surface defect sites, resulting in improvement of PEC water splitting.

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%.

The preparation of Cu2ZnSnS4 thin film solar cell based on oxygen containing precursor

In this paper, we reported a new method to fabricate Cu2ZnSnS4 (CZTS) thin film solar cells. Oxygen containing precursor thin films were deposited on the Mo-coated soda lime glass (SLG) by sputtering ZnO, SnO2 and Cu targets. For getting higher quality CZTS thin films, the effects of different annealing temperatures of 380 °C, 480 °C, 580 °C and 650 °C on CZTS films were systematically investigated. The results showed that the optimum annealing temperature was 580 °C. Through optimizing the sulfurization process, phase-pure CZTS thin films with compact and large grains were obtained. Composition analysis showed that O was not detected after sulfurizing process which indicated all of O was replaced by S. And due to the stability of SnO2, the loss of tin can be avoided. In addition, ZnO not only can provide zinc sources, but also can decrease the thickness of MoS2 as a barrier layer. Finally, CZTS solar cells based on sputtering oxides targets were obtained and the preliminary fabricated CZTS thin film solar cells had an efficiency of 5.08%, with an open-circuit voltage of 645 mV, a short-circuit current density of 21.74 mA/cm2 and a fill factor of 36.23%.

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.

Effect of metal coating material on field emission of vertically grown layered MoS2 nanosheets

A series of two-dimensional MoS2 nano-sheet (MSNS) films were deposited on different metal coated substrate (Al or Mo-coated alumina ceramic substrate and Al or Mo-coated glass substrate) by using the vacuum electron beam vapor deposition (EBVD) technique. The film thickness of MSNS and metal coating was well-controlled by appropriately modifying the growth conditions and deposition time. The microstructure and surface morphology of MoS2 films were characterized by scanning electron microscope (SEM) and x-ray diffraction (XRD). The field emission (FE) test results indicated that the FE properties of the glass-Al/MSNS sample are superior to the glass-Mo/MSNS sample. And in the case that the device is working properly, the FE current density of ceramic-Al/MSNS sample is always higher than that of ceramic-Mo/MSNS sample when the external electric field is the same. All the experimental results are explained reasonably.

Influence of WSe2 buffer layer at back electrode on performance of Cu2ZnSn(S,Se)4 solar cells

CZTSSe-based solar cells with a structure of Al/ITO/ZnO/CdS/CZTSSe/p-WSe2/Mo (denoted as WSe2-CZTSSe solar cell) were prepared on the Mo-coated soda-lime glass (SLG) with a pre-sputtering metal tungsten (W) layer. Comparing with the conventional CZTSSe solar cell with a structure of Al/ITO/ZnO/CdS/CZTSSe/n-MoSe2/Mo (denoted as CZTSSe solar cell), the WSe2-CZTSSe solar cell shows a significant improvement in open-circuit voltage (VOC), short-circuit current density (JSC) and fill factor (FF), and so its power conversion efficiency (PCE) enhances. The largest enhancement of the PCE is from 4.13% of the CZTSSe solar cell to 5.45% of the WSe2-CZTSSe solar cell. The increased VOC and JSC are mainly due to the enhancement of photogenerated current density (JL) and decrease of reversion saturation current density (J0), while the increased FF is ascribed to the increase of shunt resistance (RSh). The influence mechanism of the WSe2 buffer layer on JL, J0 and RSh is also investigated in detail. Our results present a route of increment of PCE by reducing and even removing back contact barrier between CZTSSe and MoSe2 layers.

Fabrication of Pt/In2S3/CuInS2 thin film as stable photoelectrode for water splitting under solar light irradiation

Hydrogen has been expected as a new energy carrier for realizing low carbon society. This paper demonstrates H2 production from water splitting for development of artificial photosynthesis on the copper indium sulfide (CuInS2) electrodes. The photoelectrode was fabricated by electrodeposition (ED) of Cu and In metallic precursors on Mo-coated glass, followed by sulfurization under sulfur (S) vapor environment instead of H2S gas. In order to improve surface properties, the CuInS2 surface was modified with In2S3 by the chemical bath deposition (CBD). The Pt particles as a co-catalyst for proton reduction were, then, photo-electrochemically deposited on the CuInS2 or In2S3/CuInS2 electrodes. They exhibited photo-electrochemical (PEC) water splitting in 0.1 M Na2SO4 aqueous solution to produce H2, of which the efficiency is in the order of: Pt-In2S3/CuInS2 > Pt-CuInS2 >> CuInS2. The half-cell solar-to-hydrogen (HC-STH) of 0.7 % with almost 100 % Faraday efficiency was obtained on the Pt-In2S3/CuInS2 photoelectrode. High photoelectrochemical property is attributed to the band-bending of p-n homo-junction between CuInS2 and In2S3.

Sodium-assisted passivation of grain boundaries and defects in Cu2ZnSnSe4 thin films

The long stagnation of the photo-conversion efficiency of kesterites below 13% is a source of frustration in the scientific community. In this study, we investigated the effects of sodium on the passivation of grain boundaries and defects in Cu2ZnSnSe4 (CZTSe) grown on a soda-lime glass (SLG) and borosilicate (BS) glass. Because BS glass does not inherently contain sodium, we placed a thin layer of NaF between CZTSe and Mo. The composition of the samples is Cu-poor and Zn-rich. The distribution of sodium and its contributions to phase formation and defects were examined by cross-sectional energy-dispersive X-ray profiling, Raman scattering spectroscopy and imaging, surface potential and photoluminescence. From the experimental results, it can be strongly claimed that sodium ions segregate predominantly near the grain boundaries and reduce CuZn-related defects. These local surface imaging analyses provided the exact locations of the secondary phases. In particular, the photo-assisted scanning probe method enabled us to observe the changes in the optoelectrical properties of the thin films and the carrier behavior within the materials. Further studies with distinct alkali ions and optimal processing conditions will pave a way to improve the performance of kesterite solar cells.

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.