Se2 Thin Films Research Articles

Cu(In,Ga)Se2 thin film solar cells produced by atmospheric selenization of spray casted nanocrystalline layers

Cu(In,Ga)Se2 (CIGS) is suitable absorber material for thin film photovoltaic owing to its excellent thermo-chemical stability and demonstration of high power conversion efficiency of 23.35% for lab-scale devices. The manufacturing of CIGS thin film solar cells need to be economical for commercially viability. Non-vacuum approach for the synthesis of the CIGS thin film absorber is desired for its cost-effective benefits over vacuum based processes. We report development of solution-processed CIGS absorber layer involving spraying of sonochemically prepared nanocrystal ink to obtain pristine CIGS nanocrystal layer, followed by single-step atmospheric pressure selenization utilizing Se pallets as a Se vapor source. Influence of selenization conditions and NaF doping on grain growth of the CIGS thin film absorber is studied in detail by structural, morphological, compositional and electrical characterization. Solar cells prepared under optimized conditions yielded a maximum efficiency of 6.7% without employing any thermal pre-treatment and toxic Potassium Cyanide etching.

Structural and wavelength dependent optical study of thermally evaporated Cu2Se thin films

Abstract The present work encloses structural and optical characterization of copper (I) selenide (Cu2Se) thin films. The films having thickness 85 nm have been deposited using thermal evaporation technique in initial step of work. The structural and morphological studies of deposited thin films are then done by X-ray diffraction (XRD), scanning electron microscope (SEM), and surface profilometer measurements. Later on, ultraviolet-visible-near-infrared (UV-VIS-NIR) spectrophotometer and Raman spectroscopic measurements are performed for optical characterization of films. The structure and morphology measurements reveal that deposited material of films is crystalline. The optical band gap estimated from the optical transmission spectra of the film has been found 1.90 eV. The mean values of refractive index, extinction coefficient, real and imaginary dielectric constant are received 3.035, 0.594, 9.623, and 3.598, respectively. The obtained results are compared and analyzed for justification and application of Cu2Se thin films.

Improving the performance of low-cost water-based solution-synthesised Cu2ZnSn1-xGex(S,Se)4 absorber thin films by germanium doping

In this work, Cu2ZnSn1-xGexS4 (CZTGS) precursor thin films with varied Ge composition (x = 0, 0.2, 0.4, 0.6, 0.8, and 1) were prepared by using a water-based solution approach. After selenizing at 550 °C for 10 min in a graphite box containing selenium powder, the as-fabricated kesterite Cu2ZnSn1-xGex(S,Se)4 (CZTGSSe) thin films were systematically investigated for phase structure, morphology, valence states of elements, optical properties, and electrical properties. The results showed that the Ge element was successfully incorporated into the host lattice of Cu2ZnSn(S,Se)4 (CZTSSe), and kesterite CZTGSSe films were formed. As the Ge content increased, the crystal grains in the films grew larger and showed uniform morphology on the sample surfaces. The band gap in the CZTGS(Se) thin films could be engineered in the range of 1.21 eV–2.00 eV by tuning the relative compositions of the Ge elements. Furthermore, it was confirmed that the doping of Ge improved the mobility, resistivity, and photoelectric performance of the films. Finally, the Cu2ZnSn0.8Ge0.2(S,Se)4 solar cell device with an efficiency of 5.39% was prepared. The results indicate that non-toxic water solution-based CZTGSSe films showed potential for application in solar cells.

Chlorine termination of selenium dangling bonds decreases tellurium-diffusion-induced acceptor states in hexagonal selenium

In this study, we examined the effects of doping chlorine (Cl) into crystalline selenium (c-Se) on diminishing the tellurium-diffusion-induced shallow acceptor states associated with the dangling bonds that appear at the ends of Se chains by theoretical analysis with the first-principles calculations. In this model, the negatively charged dangling bonds can be neutralized by inserting monovalent Cl atoms. Furthermore, the defect termination by doping Cl into fabricated Se thin films was experimentally demonstrated. Low-temperature cathodoluminescence measurements were implemented to show that acceptor-related emissions from the c-Se films were significantly decreased by Cl termination of the dangling bonds, experimentally confirming the results of the simulations.

Highly improvement in efficiency of Cu(In,Ga)Se2 thin film solar cells

PurposeThe purpose of this study is to design and optimize copper indium gallium selenide (CIGS) thin film solar cells.Design/methodology/approachA novel bi-layer CIGS thin film solar cell based on SnS is designed. To improve the performance of the CIGS based thin film solar cell a tin sulfide (SnS) layer is added to the structure, as back surface field and second absorbing layer. Defect recombination centers have a significant effect on the performance of CIGS solar cells by changing recombination rate and charge density. Therefore, performance of the proposed structure is investigated in two stages successively, considering typical and maximum reported trap density for both CIGS and SnS. To achieve valid results, the authors use previously reported experimental parameters in the simulations.FindingsFirst by considering the typical reported trap density for both SnS and CIGS, high efficiency of 36%, was obtained. Afterward maximum reported trap densities of 1 × 1019 and 5.6 × 1015 cm−3 were considered for SnS and CIGS, respectively. The efficiency of the optimized cell is 27.17% which is achieved in CIGS and SnS thicknesses of cell are 0.3 and 0.1 µm, respectively. Therefore, even in this case, the obtained efficiency is well greater than previous structures while the absorbing layer thickness is low.Originality/valueHaving results similar to practical CIGS solar cells, the impact of the defects of SnS and CIGS layers was investigated. It was found that affixing SnS between CIGS and Mo layers causes a significant improvement in the efficiency of CIGS thin-film solar cell.

Physical properties of CuIn0.7Ga0.3Se2 ingot and thin films prepared by one-step rf-magnetron sputtering from single-target material

Nowadays, large area CuIn1-xGaxSe2 (CIGS) thin film fabrication by one-step radio-frequency magnetron sputtering is still facing difficulties and challenges in uniformity of composition and structure. This study illustrates how we can prepare polycrystalline thin layers of CuIn0.7Ga0.3Se2 on glass and on F-doped SnO2/SLG substrates using single quaternary target. It was found that using direct reaction of stoichiometric proportions of pure elements, we can make high quality CIGS powder to prepare sputtering target. X-ray diffraction study of CIGS powder revealed all possible orientations for the CIGS material. Compositional analysis of the obtained CIGS crystal surface was also investigated. To investigate the defect chemistry, Cathodoluminescence (CL) measurements at different temperatures were carried out. The structural, compositional, morphological, and electrical properties of the as-deposited films were systematically investigated by XRD, SEM, AFM, and four-point probe method. By adjusting the processing parameters at high substrate temperature TS = 300 °C, a CIGS thin films with a preferential orientation along (112) direction which exhibited single chalcopyrite phase were obtained. The optical transmission showed energy gap value up to 1.19 eV and rather sharp absorption fronts which is suitable for high efficient solar cells.

Electrodeposition of CuIn(Al)Se2-based thin films on various substrates

While the electrodeposition of CuInSe2-based compounds have been the topic of many research efforts, effects of the substrate have been rarely addressed in the literature. In the current study, the electrodeposition of CuIn(Al)Se2 thin films on different substrates is performed and effects of the substrate on the morphology and composition of obtained layer are studied. CuIn(Al)Se2 thin films are electrodeposited on different substrates, including copper, stainless steel (SS), and titanium foils as well as fluorine-doped tin oxide (FTO) transparent electrode. Electrodepositions are performed in sulfate solution of Cu2+, Al3+, In3+, and Se4+ ions. The results from cyclic voltammetry (CV), composition and phase analyses indicate that the incorporation of In and Al is a function of the co-deposition potential in cathodic scanning process and the lowest co-deposition potential of − 0.9 V is observed on FTO substrate. Characterizations of deposited films by scanning electron microscopy, energy-dispersive X-ray spectroscopy and X-ray diffraction analysis have also revealed that the deposited films on FTO and Ti substrates contained a layer with high amount of In, Al (In + Al > 7 wt%) and CuIn(Al)Se2 phase with chalcopyrite structure while the deposited film on Cu substrate mainly consists from binary Cu-Se phases. The deposited films on FTO and SS substrates have large particles (> 400 nm) with partly laminar morphology while fine microstructures are obtained for films deposited on Cu (300 nm) and Ti (80 nm) substrates. In addition, by combining the crystallographic information and thermodynamic relations, a method is proposed to predict the favored phase on each support which are in a good agreement with experimental observations.

Dielectric behavior of amorphous thin films of Se–Te–Sn-Ge system

Studies of the dielectric behavior of chalcogenide glasses (ChGs) are very important for a better understanding of the transport mechanisms. The characterization of different electric properties of the ChGs in thin-film form is an essential demand of the scientific and industrial research fields. This paper reports the dielectric behavior of amorphous thin films of Ge containing Se–Te–Sn (STS) alloy. Our analysis shows that sufficient dielectric dispersion is present in amorphous thin films of each sample. Relatively few works have been done in the direction of comparing the results of dielectric constant and loss in bulk glassy samples and amorphous thin film samples of some glassy alloys of SeTeSnGe system to decide the optimized condition for technological applications and industrial production. We have also made an attempt to compare and discuss the behavior of the compositional dependence of dielectric constant and loss of thin-film samples with the results of other Ge containing glassy systems reported in published works that might shed light for using these glasses in several interesting applications.

Lithium-Doping Effects in Cu(In,Ga)Se2 Thin-Film and Photovoltaic Properties.

The beneficial effects of heavy alkali metals such as K, Rb, and Cs in enhancing Cu(In,Ga)Se2 (CIGS) photovoltaic efficiencies are widely known, though the detailed mechanism is still open for discussion. In the present work, the effects of the lightest alkali metal, Li, on CIGS thin-film and device properties are focused upon and compared to the effects of heavy alkali metals. Till date, the beneficial effects of elemental Li on Cu2ZnSnS4 photovoltaic devices in enhancing efficiencies have been reported. On the other hand, it is shown in the present work that the beneficial effects of Li on CIGS are not so significant. In contrast to the effects of Na or Rb in enhancing CIGS(112) growth orientation, Li was revealed not to affect CIGS growth orientation. The most distinctive feature observed between Li and other alkali metals was the elemental depth profile in CIGS films. Namely, Na and heavier alkali metals show a concentration peak near the surface (relatively Cu-poor) region of CIGS films, whereas elemental Li showed no such trend, suggesting that Li has no significant effect on CIGS surface modification. Nonetheless, Li was found to have some effect in enhancing the PL peak intensity and photovoltaic performance of CIGS, though the effect is relatively small in comparison to that obtained with other alkali metals.

Manipulation of [Ga]/([Ga] + [In]) profile in 1.4-μm-thick Cu(In,Ga)Se2 thin film on flexible stainless steel substrate for enhancing short-circuit current density and conversion efficiency of its solar cell

Flexible 1.4-μm-thick Cu(In,Ga)Se2 (CIGS) films on stainless steel (SUS) substrates are fabricated by the process named “multi-layer precursor method” as the absorbers of the thin-film solar cells. It is shown that the [Ga]/([Ga] + [In]) (GGI) or bandgap (Eg) profiles were easily controlled to enhance photovoltaic performances, especially short-circuit current density (JSC) for the increased conversion efficiency (η). It is determined that the double-grading GGI profile is most pronounced with the presence of the CuInSe2 (CIS) phase in the CIGS film, when annealing time and temperature during the fabrication are reduced to 1 min and 610 °C, respectively. Consequently, the minimum (mini) Eg value in the Eg profile is reduced to approximately 1.05 eV close to the Eg of the CIS material, resulting in the enhancement of external quantum efficiency in the long wavelength region. The JSC of the flexible CIGS solar cell is ultimately increased up to 40 mA/cm2, thus enhancing the η to 17.5% despite the reduction of CIGS thickness to 1.4 μm. It is disclosed that the reduced CIGS thickness to 1.4 µm with very short annealing time (1 min) leads to the possible reduction of the production cost of the flexible thin-film solar cell with the high η.

Investigations of potassium-induced surface treatment of Cu(In,Ga)Se2 (CIGSe) thin film solar cells prepared by two-stage process using elemental selenium

The influences of alkaline doping of CIGS absorbers by a potassium fluoride post-deposition treatment (KF-PDT) were investigated systematically. Three factors that influence the CIGS solar cell performances were investigated: (i) deposition temperature, (ii) post-deposition annealing, and (iii) the thickness of the potassium fluoride (KF) layer. It is deduced that Cu vacancies were more effectively occupied by K instead of In, which prevents the formation of InCu donor defects and facilitates the subsequent Cd doping into the CIGS region. We confirmed large densities of deep defects induced by KF-PDT, leading to significant FF losses of CIGS solar cells. The potassium diffusion and redistribution during post-annealing is critical to micro-structure and crystallinity of CIGS absorber, as well as the defect properties. We pointed out the effects of surface treatment extend to the bulk of CIGS, dependent on the chemical compositions. With modified post-annealing process towards homogeneous potassium distribution and high-quality CGIS crystals, room temperature KF-PDT remains an effective strategy to enhance the performances of CIGS solar cells prepared by two-stage process.

Ellipsometric and Spectrometric Studies of (Ga0.2In0.8)2Se3 Thin Film

Thermal evaporation technique is used to deposite (Ga0,2In0,8)2Se3 thin films. The refractive index and extinction coefficient dispersions are obtained from the spectral ellipsometry measurements. The dispersion of the refractive index is described in the framework of the Wemple–Di Domenico model. The optical transmission spectra of a (Ga0,2In0,8)2Se3 thin film are studied in the temperature range 77–300 K. The temperature behavior of the Urbach absorption edge, as well as the temperature dependences of the energy pseudogap and Urbach energy, are investigated. The influence of different types of disordering on the optical absorption edge of a (Ga0,2In0,8)2Se3 thin film is discussed. Optical parameters of a (Ga0,2In0,8)2Se3 thin film and a single crystal are compared.

Solution-Processed Anti Reflective Transparent Conducting Electrode for Cu(In,Ga)Se2 Thin Film Solar Cells

Silver nanowire (AgNW) networks have been adopted as a front electrode in Cu(In,Ga)Se2 (CIGS) thin film solar cells due to their low cost and compatibility with the solution process. When an AgNW network is applied to a CIGS thin film solar cell, reflection loss can increase because the CdS layer, with a relatively high refractive index (n ~ 2.5 at 550 nm), is exposed to air. To resolve the issue, we apply solution-processed ZnO nanorods to the AgNW network as an anti-reflective coating. To obtain high performance of the optical and electrical properties of the ZnO nanorod and AgNW network composite, we optimize the process parameters – the spin coating of AgNWs and the concentration of zinc nitrate and hexamethylene tetramine (HMT – to fabricate ZnO nanorods. We verify that 10 mM of zinc nitrate and HMT show the lowest reflectance and 10% cell efficiency increase when applied to CIGS thin film solar cells.

Microstructural investigation of inkjet printed Cu(In,Ga)Se2 thin film solar cell with improved efficiency

Inkjet printed copper indium gallium diselenide (CIGS) thin film solar cell has attracted tremendous attention because of its various technological benefits as a non-vacuum process. Focused efforts in selenization of inkjet printed films to make the process feasible, are desired. In this work, microstructural investigation of inkjet printed precursor film selenized by rapid thermal processing (RTP) is presented. The optimization of selenization time for transforming metal nitrates precursor ink to CIGS thin film is investigated. Based on the results, the growth mechanism to form CIGS from inkjet printed CIG precursor films is proposed. Systematic study on the molybdenum diselenide (MoSe2) phase evolution during the two-step atmospheric pressure selenization process at the CIGS-Mo interface and its effect on device performance are carried out. Non-uniform inter-diffusion of indium (In) and gallium (Ga) during selenization, resulting in double-layered CIGS, one of the major reason limiting the performance of the devices is investigated through XRD, Raman, FESEM, EDS and Mott-Schottky analysis. The significant improvement in device efficiency from 0.4% to 4.2% is achieved due to microstructural improvement in CIGS films. Investigation on the mechanism of microstructural growth with selenization time affecting final device performance is presenting in this work.

Towards an optimized gallium gradient for Cu(In,Ga)Se2 thin film via an improved constant low-temperature deposition process

The gallium gradient of the CIGS film plays a key role for device performance of solar cells. However, a three-stage co-evaporation deposition process at low substrate temperature often produces a pronounced steeper gallium (Ga) gradient, which is detrimental to the transport of carriers and leads to lower conversion efficiency. In this work, we propose a modified low-temperature deposition process by introducing an overlapping process between the 1st stage and the 2nd stage. This modified low-temperature process mitigates the diffusion of In and Ga and facilitates the formation of ideal back gradient. Meanwhile, we have demonstrated the phase transition process of the back gradient during the overlapping process by characterization techniques. With the design of new reaction models during the overlapping process, growth mechanisms of the CIGS film in the modified low-temperature process could be visually displayed. This modified low-temperature process has improved the device efficiency of CIGS solar cells from 14.1% to 16.7%.

SYSTEMATIZED AND SIMPLIFIED PROCESSING OF CuInGaSe2 THIN FILMS TO BE APPLIED ON SOLAR CELLS

Results of processing and characterization of Cu(In,Ga)Se2 (CIGS) thin films growth by thermal co-evaporation with adequate physical properties to be used in solar cells are presented in this work. Cadmium Sulfide (CdS) as window material with 40 nm of thickness were deposited by chemical bath deposition (CBD) technique. A Molybdenum (Mo) layer was deposited as back contact on CIGS solar cells. Different CIGS thin films were processed by a simplified co-evaporation technique and revealed a good polycrystalline quality with an alfa-chalcopyrite phase. The stoichiometry of the CIGS thin films can be accurately controlled using a fully automated single vacuum photovoltaics manufacturing system. Automated system operations (ASO) is the set of software and hardware that allows computer systems, network devices or machines to function without any manual intervention. ASOs allow computer systems to work without a human operator physically located at the site where the system is installed. CIGS solar cells fabricated in this work, showed photovoltaic efficiencies close to 11 %.

Performance of Graphene-CdS Hybrid Nanocomposite Thin Film for Applications in Cu(In,Ga)Se2 Solar Cell and H2 Production.

A graphene–cadmium sulfide (Gr–CdS) nanocomposite was prepared by a chemical solution method, and its material properties were characterized by several analysis techniques. The synthesized pure CdS nanoparticles (NPs) and Gr–CdS nanocomposites were confirmed to have a stoichiometric atomic ratio (Cd/S = 1:1). The Cd 3d and S 2p peaks of the Gr–CdS nanocomposite appeared at lower binding energies compared to those of the pure CdS NPs according to X-ray photoelectron spectroscopy analyses. The formation of the Gr–CdS nanocomposite was also evidenced by the structural analysis using Raman spectroscopy and X-ray diffraction. Transmission electron microscopy confirmed that CdS NPs were uniformly distributed on the graphene sheets. The absorption spectra of both the Gr–CdS nanocomposite and pure CdS NPs thin films showed an absorption edge at 550 nm related to the energy band gap of CdS (~2.42 eV). The Cu(In,Ga)Se2 thin film photovoltaic device with Gr–CdS nanocomposite buffer layer showed a higher electrical conversion efficiency than that with pure CdS NPs thin film buffer layer. In addition, the water splitting efficiency of the Gr–CdS nanocomposite was almost three times higher than that of pure CdS NPs.

Picosecond laser ablation and depth profile of Cu(In, Ga)Se2 thin film layer

Laser ablation based on picosecond laser was used to achieve the micro-analysis of Cu(In, Ga)Se2 (CIGS) thin film with the ablation crater diameter of 50μm and the ablation crater central depth of 93 ± 13 nm. We achieved the depth profile of CIGS thin film with different laser shot number. The evolutions of spectral lines intensities of Ca from glass substrate and Ga and In from CIGS thin film layer, and intensity ratios of Ca/Ga and Ca/In could exhibit the change of the ablation volume, which could estimate the thin film thickness of single CIGS thin film layer. The average plasma temperature was calculated to be about 5243 ± 100 K, and the average electron density was calculated to be about 4.5×1016 cm−3. It is shown that our experimental setup is suitable to achieve a precise control and monitor the element compositions in each CIGS thin film layer in the research and in the production of CIGS solar cells.

Interface Formation between CdS and Alkali Postdeposition-Treated Cu(In,Ga)Se2 Thin-Film Solar Cell Absorbers-Key To Understanding the Efficiency Gain.

A combination of X-ray photoelectron/Auger electron spectroscopy and soft X-ray emission spectroscopy has been employed to investigate the impact of different alkali postdeposition treatments (PDTs) on the chemical structure of the (buried) CdS/Cu(In,Ga)Se2 heterojunction: the key interface in chalcopyrite-based thin-film solar cells. Chemical bath deposited (CBD) CdS layers of different thicknesses on NaF PDT (CIGSeNaF) and NaF + KF PDT (CIGSeNaF+KF) Cu(In,Ga)Se2 absorbers prepared at low temperature (to facilitate the use of flexible, e.g., polyimide, substrates) were studied. While we find the CdS/CIGSeNaF interface to be mainly free of significant chemical interaction, in the proximity of the CdS/CIGSeNaF+KF interface, an elemental redistribution involving Cd, In, K, S, and Se is revealed. For the early stages of the CBD-CdS process, our findings are in agreement with the conversion of the K-In-Se-type layer present on the CIGSeNaF+KF surface into a mixed Cd-In-(O,OH,S,Se)-type layer, probably having some Cd-In and (S,O)-Se composition gradients. For long CBD times-independent of employed PDT-we find the buffer material to be best described by a Cd(O,OH,S)-like species rather than by a pure CdS buffer. These findings shed light on the observed performance leap of corresponding CdS/CIGSeNaF+KF-based solar cells.

Cu(In,Al)Se2 Photovoltaic Thin Film Solar Cell from Electrodeposited Stacked Metallic Layers

An improved technique for synthesis of Cu(In,Al)Se2 is demonstrated, associated with sequential electrodeposition of a stacked layer in the order of Cu/Al/In followed by annealing in selenium vapor. Many advantages such as adjustable constitutes composition, good polycrystalline structure, pure chalcopyrite phase, uniform and compact surface morphology are obtained compared to the film fabricated by conventional electrodeposition process. The film thickness and the concentration of each metal deposited were controlled by the flexibility parameters of deposition time. The influence of Al content on the crystal structure, surface morphology, photoelectrochemical performance, optical and electronic properties of the films were investigated. The crystal size decrease and the energy bandgap of Cu(In,Al)Se2 thin film increase gradually with the increasing Al were revealed. Impedance potential test reveals the manufactured Cu(In,Al)Se2 thin films are all p-type semiconductor and the carrier concentration increases with the Cu/(Al + In) and Al/(In + Al) ratio. Photoelectrochemical investigation of Cu(In,Al)Se2 films verified that a higher photocurrent was obtained with a relative lower Al content due to a narrower bandgap leading to lower-energy photon absorption and a lower carrier density and a larger grain size both benefitting the transfer of photogenerated carriers and decrease the recombination of charge carriers. The obtained optimum Cu(In,Al)Se2 thin film based solar cell has been theoretical modeling and simulated. A high PCE of 17.08% was gained implying its potential application in photovoltaic devices.