Selenium Flux Effect on Cu(In,Ga)Se2 Thin Films Grown by a 3-stage Co-evaporation Process

An investigation into the effects of Se flux on absorber thin film growth at each step of a 3-stage co-evaporation process was conducted to further optimize the performance of CIGS solar cells. In ‘step I’ forming an In-Ga-Se precursor thin film during the 3-stage process, Se flux affected the preferred orientation of the CIGS crystal structure, but not the film morphology. In ‘step II’, no correlation was found between Se flux and the crystal structure, although excessively high Se flux employed throughout the 3-stage process degraded the solar cell performance. A CIGS thin film, with a (220/204) crystal orientation, minor physical surface defects and ∼20 nm thick MoSe₂ at CIGS/Mo interface, was obtained by fine control of Se flux conditions (high Se flux at ‘step I’ and low Se flux at ‘step II’) at optimum substrate temperatures. The solar cell fabricated using the aforementioned CIGS thin film showed the highest conversion efficiency of 20.02 %.     

Flexible CZTSSe thin film solar cells fabricated at low temperature with relieved residual stress by Sb incorporation

The troublesome residual stress is always a stumbling block that drags the progress pace of flexible CZTSSe thin film solar cells, which urgently needs to be noticed and solved. In this paper, low-temperature prepared CZTSSe absorber with relieved residual stress (0.558 GPa) is realized by Sb incorporation. Owing to the evaporated 20 nm Sb layer under CZTS precursor, the crystalline quality and band mismatching of CZTSSe/CdS interface are simultaneously improved. Additionally, the spatial potential fluctuation extracted from the PL results is found to decrease from 63.26 meV to 41.57 meV, indicating a reduction in band tailing and disorder of CZTSSe absorber. Compared with the general solar cells fabricated at 580 °C, flexible devices with Sb incorporation can maintain a slightly higher performance at a lower temperature about 60 °C. The best power conversion efficiency (PCE) of 4.41% is obtained in the solar cell with 550 °C-selenized CZTSSe absorber after incorporating 20 nm Sb layer, featuring 351.20 mV Voc, 25.73 mA/cm² Jsc and 48.79% FF. Finally, low-temperature prepared flexible CZTSSe thin film solar cell can retain over 83% of the original PCE after bending at 180° for 40 cycles. The mechanical durability paves a promising way for flexible CZTSSe thin film solar cell in roll-to-roll production.     

Smooth and highly-crystalline Ag-doped CIGS films sputtered from quaternary ceramic targets

Sputtering with copper indium gallium selenide (CIGS) ceramic targets could produce smooth CIGS thin films that are preferred for preparing two-terminal tandem devices. However, grain sizes prepared in this way are small and device efficiency was low. To increase the grain size, in this report, an Ag layer was pre-sputtered beneath CIGS. The Ag doping layer increased the grain size and improved the crystalline alignment. Consequently, the Ag-doped films exhibited improved charge mobility. From X-ray diffraction, scanning electron microscopy and X-ray photoelectron spectroscopy characterizations, we obtained an optimized Ag thickness of 15 nm. Short-circuit current density (J_SC), open-circuit voltage (V_OC), and fill factor (FF) were all improved after doping with 15-nm Ag. Increasing the annealing temperature from 550 °C to 575 °C, the grains was enlarged further, with the power conversion efficiency (PCE) increasing to 14.33% and V_OC to 545 mV. Upon the smooth CIGS film, a thin conformal perovskite layer was fabricated without polishing. This work demonstrates a simple way to fabricate smooth and highly-crystalline CIGS films that can be used for tandem solar cells.     

Comprehensive characterization of CIGS absorber layers grown by one-step sputtering process

We have demonstrated that the use of a one-step sputtering process allowed for the fabrication of copper indium gallium diselenide (CIGS) thin films by RF magnetron sputtering without an additional selenization process. The CIGS thin films deposited at different substrate temperatures were synthesized on soda-lime glass (SLG) substrates using a single quaternary CIGS target. The film composition ratios of ([Cu]/[In]+[Ga]), ([Ga]/[In]+[Ga]), and ([Se]/[Cu]+[In]+[Ga]) were almost consistent with those of the sputtering target. X-ray diffraction (XRD) and Raman results showed that the crystallinity of the CIGS thin films was gradually improved as substrate temperatures increased. Transmission electron microscopy (TEM) showed that the films grown at 600?°C have a columnar structure with the grain size of ~100?nm. In addition, for the CIGS films grown at 600?°C, TEM-EDX analysis revealed that the synchronized fluctuation of the Cu and Se signals was observed in the direction of the film depth, while the In and Ga signals were constant. As a result, the CIGS solar cell made using the film showed a degraded cell efficiency of 2.5%, which might be have been caused by not only Cu-rich and Se-poor compositions but the locally unstable composition in the CIGS films fabricated by one-step sputtering.     

Selenization process in simple spray-coated CIGS film

The synthesis process to produce a decent thin-film CIGS layer with simple, easy, and low-cost are essential factors in CIGS solar cell technology. This study synthesized a thin layer of CIGS by a simple spray coating method and selenization process. The temperature substrate and distance between the nozzle and target are controlled to provide an even thickness coating. A one-stage selenization was carried out with temperature variations of 350, 400, and 500 °C and a three-stage selenization process as the comparison was made at 400 and 520 °C. The surface morphology, thickness, and optical properties of CIGS films were investigated as a function of the temperature of selenization. The complete CIGS solar cell prototype consists of a window, buffer, and CIGS absorber layer. The buffer layer used is ZnS deposited by the chemical bath deposition method, while the window layer of ZnO is deposited by the spin coating method. The SEM characterization showed a thinner layer of CIGS as temperature, and the heating rate of the selenization increased. The increasing selenization temperature also affects the Selenium deficiency that causes low Ga/(In + Ga) content and increases the bandgap energy. By the grain growth model and the kinetic study yielded estimated activation energy, which one step selenization of the process is higher than the three steps of the selenization process. PV82QJC8Y.     

Surface modifications of CIGS absorbers and their effects on performances of CIGS solar cells

The surface roughness of CIGS absorber and residual Cu–Se phase on the surface are two major factors greatly affecting the performance of CuInGaSe₂ (CIGS) solar cells with the absorbers fabricated by the selenization annealing of Cu–In-Ga precursors. In this work, Br₂–CH₃OH solution is used to etch the surfaces of CIGS films to address these two challenges. The effects of the etching treatment on CIGS films and devices are investigated. It is found that etching significantly reduces the film roughness and removes the Cu–Se phase. Thus, the performance of the device is improved within a certain range. In addition, it is found that CIGS films with suitable surface roughness effectively absorb incident light and possess a higher J_sc. However, excessive etching causes the film to be too thin, and the carrier recombination at the back electrode increases. Based on these characteristics, this work optimizes the Br₂ etching process.     

Study of the Electrical Properties and the Internal Quantum Efficiency of SnO2-p/n- Si Thin Film Solar Cells

SnO₂-p/n-Si thin film solar cells were prepared by a vacuum evaporation technique, and SnO₂ studied as a transparent antireflection coating with the n–type silicon wafer to fabricate a thin film. The oxidation of the Si surface takes place simultaneously with the evaporation process. Dark forward and reverse I-V characteristics for the SnO₂–p/n-Si structure in the temperature range (25–300 °C), for a thickness of 5000 Å are measured. The illuminated current-voltage characteristics of a PN junction solar cell have been investigated. The capacitance-voltage (C–V) measurements at room temperature are carried out to show the variation of capacitance with frequency. The relative IQE for the fabricated SnO₂–p/n-Si structure with 5000 Å thickness at 300 °C are calculated. The fabricated structure has a maximum response at a wavelength in the range, 0.8–1.05 μm.     

Preparation and Characterization of CIGSe Solar Cells by Ink Printing on Alumina Substrates with Self‐Synthesized Selenide Powders via an Environment‐Friendly and Cost‐Effective Dry Synthesis Route

CIGSe solar cells with an ink‐printing absorber layer were prepared on Mo‐coated alumina substrates. The use of alumina substrates can extend the process window to higher temperatures. The inks contained single‐phase CIGSe powder, which was formed by firing different selenide powders of Cu₂Se, In₂Se₃, and Ga₂Se₃ at 800°C. All these powders were synthesized with an environment‐friendly and cost‐effective powder process. The printed inks were sintered at 600–800°C. The solar cells had power conversion efficiency of 0.50%, an open‐circuit voltage of 27 mV, a short‐circuit current density of 37 mA/cm², and a fill factor of 0.50.     

Investigation of CIGS Solar Cells on Polyethylene Terephthalate Substrates

CIGS nanoparticles ink with composition of Cu (Ga_0.3 In_0.7) Se₂ was prepared by using dissolved copper, indium, gallium acetylacetonate, and Se powder in oleylamine using hot injection methods. The structural properties of the CIGS films deposited on the polyethylene terephthalate (PET) substrate were studied using an X-ray diffraction technique. The as-deposited CIGS films were found to be a chalcopyrite-type structure with crystallite grain size of about 43.8 nm. An optical study shows that the band-gap energy of the CIGS film is 1.25 eV. The flexible CIGS solar cell on a PET substrate with the best conversion efficiency of 4.21% is demonstrated.     

Ecologic ceramic substrates for CIGS solar cells

In this work the use of porcelain stoneware tiles as alternative substrates for CIGS thin film solar cells for the development of specific applications as those related to building integration (BIPV, BAPV) are reported. Two types of porcelain stoneware have been compared: the conventional tile (STD) and the ecological tile (ECO). The ECO ceramic paste formulation has been designed adding industrial wastes (recycled glass, sludges and chamote). Chemical, technological and functional properties of the paste have been performed. The CIGS absorber has been synthesized by an easy and low-cost way of preparation using co-precipitation method. The solar cell device has been completed and fully characterized.The achieved results indicate better performances for ECO paste compared to the STD, stated in improved mechanical resistance, thickness and morphology. It is demonstrated that both tiles are suitable for solar cell applications, but ECO substrate developed higher energy conversion efficiency of 1.3%.     

Gas-pressure assisted sintering of copper indium gallium selenide thin films

Defects, such as cracks, porous structure, small grains that easily occur in the fabrication of copper indium gallium selenide (CIGS) thin film absorbers using non-vacuum process have been the major obstacle to practical application of this technology so far. A gas-pressure assisted sintering process has been developed to achieve dense, crack-free, large-grained CIGS films. The gas-pressure assisted sintering effects on the microstructure, crystalline, and electric properties were investigated by scanning electron microscopy, X-ray diffraction, and Hall-effect analyzer. A uniform microstructure with a large grain size and small amount of isolated residual pores and good electric properties can be obtained by pre-sintering at 500°C under 6 bar N₂ overpressure and then annealing at 500°C for 20 minutes under a selenium atmosphere.     

Non-vacuum deposition of CIGS absorber films for low-cost thin film solar cells

Thin film solar cells composed of chalcopyrite Cu(In_1?x Ga _x )(Se_1?y S _y )₂ (CIGSSe) absorbers have gained considerable attention in recent years in an effort to develop sustainable technologies for harnessing clean energy. Nonvacuum solution methods can reduce production costs by replacing vacuum-based deposition methods with large-scale, high-throughput processes. The efficient use of materials can reduce production costs. Non-vacuum processes generally rely on two sequential steps: solution-coating, followed by a post-annealing process. Depending on the point at which the CIGS phase evolves, non-vacuum processes can be categorized as nanoparticle (NP) approaches or molecular precursor approaches. These two types of liquid processes are believed to be compatible with a variety of applications, such as roll-to-roll coating for the production of flexible, portable devices. Additional thermal treatments using a gaseous chalcogen or oxygen can improve the absorber quality. This review describes the current status of chalcopyrite thin film solar cells fabrication methods via low-cost solution routes. An analysis of recently published reports describing liquid-based deposition methods is introduced, and the features of the development steps are compared. Finally, a discussion and future outlook are offered.     

Polyvinylbutyral‐assisted synthesis and characterization of SnS mesoporous fibers via electrospinning process

Tin sulfide (SnS) has the potential to be used as a low‐cost absorber material for applications in thin film photovoltaic solar cells. In this study, polyvinylbutyral/SnS (PVB/SnS) composite fibers were synthesized through a relatively simple electrospinning process. SnS mesoporous fibers were obtained from PVB/SnS composite fibers after sintering treatment at 500°C for 1 h in N₂ atmosphere. The SnS mesoporous fibers were then characterized using X‐ray powder diffraction, scanning electron microscopy, thermogravimetric analysis, and transmission electron microscopy. The optical properties of SnS mesoporous fibers were also recorded by UV–vis absorption spectroscopy. The results showed that the synthesized SnS mesoporous fibers exhibited a single phase, stoichiometric composition, with good crystallinity, a size ranging from 100 to 200 nm, and a band gap of 1.49 eV. The as‐prepared SnS mesoporous fibers are thus a suitable material to achieve visible light absorption in a thin film solar cell. © 2015 Wiley Periodicals, Inc. J. Appl. Polym. Sci. 2015 , 132, 42388.     

Investigation of bulk hybrid heterojunction solar cells based on Cu(In,Ga)Se2 nanocrystals

This work presents the systematic studies of bulk hybrid heterojunction solar cells based on Cu(In, Ga)Se₂ (CIGS) nanocrystals (NCs) embedded in poly(3-hexylthiophene) matrix. The CIGS NCs of approximately 17 nm in diameter were homogeneously blended with P3HT layer to form an active layer of a photovoltaic device. The blend ratios of CIGS NCs to P3HT, solvent effects on thin film morphologies, interface between P3HT/CIGS NCs and post-production annealing of devices were investigated, and the best performance of photovoltaic devices was measured under AM 1.5 simulated solar illumination (100 mW/cm²).     

A new method for measuring wetness of flowing steam based on surface plasmon resonance

Stacked precursors of Cu-Zn-Sn-S were grown by radio frequency sputtering and annealed in a furnace with Se metals to form thin-film solar cell materials of Cu₂ZnSn(S,Se)₄ (CZTSSe). The samples have different absorber layer thickness of 1 to 2 μm and show conversion efficiencies up to 8.06%. Conductive atomic force microscopy and Kelvin probe force microscopy were used to explore the local electrical properties of the surface of CZTSSe thin films. The high-efficiency CZTSSe thin film exhibits significantly positive bending of surface potential around the grain boundaries. Dominant current paths along the grain boundaries are also observed. The surface electrical parameters of potential and current lead to potential solar cell applications using CZTSSe thin films, which may be an alternative choice of Cu(In,Ga)Se₂. PACS number: 08.37.-d; 61.72.Mm; 71.35.-y     

Comparative study of the electrical characteristics of ALD-ZnO thin films using H2O and H2O2 as the oxidants

ZnO thin films were deposited via atomic layer deposition (ALD) using H₂O and H₂O₂ as oxidants with substrate temperatures from 100°C to 200°C. The ZnO films deposited using H₂O₂ (H₂O₂-ZnO) showed lower growth rates than those deposited with H₂O (H₂O-ZnO) at these temperature range due to the lower vapor pressure of H₂O₂, which produces fewer OH⁻ functional groups; the H₂O₂-ZnO films exhibited higher electrical resistivities than the H₂O-ZnO films. The selection of H₂O₂ or H₂O as oxidants was revealed to be very important for controlling the electrical properties of ALD-ZnO thin films, as it affected the film crystallinity and number of defects. Compared to H₂O-ZnO, H₂O₂-ZnO exhibited poor crystallinity within a growth temperature range of 100-200°C, while H₂O₂-ZnO showed a strong (002) peak intensity. Photoluminescence showed that H₂O₂-ZnO had more interstitial oxygen and fewer oxygen vacancies than H₂O-ZnO. Finally, both kinds of ZnO thin films were prepared as transparent resistive oxide layers for CIGS solar cells and were evaluated.     

Fabrication of Sb2S3 thin films by sputtering and post-annealing for solar cells

The semiconductor antimony sulfide (Sb₂S₃) is a potential absorber materials for the top sub-cell of Si-based tandem solar cells because of its appropriate band-gap, simple binary composition, nontoxic elements, and long-term stability. In this study, polycrystalline Sb₂S₃ films were fabricated by post-annealing of radio frequency (RF) magnetron sputtered precursors using an Sb2S3 target. The effects of the post-annealing temperature and atmosphere on Sb₂S₃ film properties and device performances were investigated. A high-performance device having a 2.41% power conversion efficiency was obtained by making use of a uniform Sb2S3 absorber layer. This preliminary experimental study shows that Sb2S3 thin films could be used as top sub-cell absorber materials for third-generation high efficiency, stable, and environmentally friendly Sb₂S₃/Si tandem solar cells.     

Electrochemical study of one-step electrodeposition of copper–indium–gallium alloys in acidic conditions as precursor layers for Cu(In,Ga)Se2 thin film solar cells

This paper investigates one step electrodeposition of copper indium gallium metallic precursor layers for preparing CuIn_1−xGa_xSe₂ (CIGS) absorber layers in thin film solar cells (0 ≤ x ≤ 1). Electrodeposition was carried out in acidic aqueous solutions at about pH 2. At first partial single or binary electrodeposition systems Cu, In, Ga, Cu–Ga, Cu–In were investigated by cyclic voltammetry. Then ternary Cu–In–Ga electrodeposition system was studied. The nature of the supporting electrolyte (sodium sulfate vs. sodium chloride) and the influence of sodium citrate were more specifically investigated. The applied potential, the pH and the nature of the electrolyte were optimized to obtain x values around 0.3 needed for high efficiency devices. Depositions were carried out under potentiostatic conditions in a paddle cell configuration. The electrodeposited Cu–In–Ga alloys were annealed under Se atmosphere at temperatures between 400 and 600 °C to produce CIGS absorbers. Films were characterized by XRF, SEM and XRD analysis. Device efficiencies up to 9.3% are achieved for optimal gallium content.     

Annealing Effect on the Properties of Cu(In0.7Ga0.3)Se2 Thin Films Grown by Femtosecond Pulsed Laser Deposition

This work reports the crystallization, microstructure, and surface composition of CuIn_0.7Ga_0.3Se₂ (CIGS) thin films grown by femtosecond pulsed laser deposition at different annealing temperatures. The structural and optical properties of the CIGS films were characterized by X‐ray diffraction, Raman scattering, UV‐visible spectroscopy, and Hall effect measurement. The results indicate that binary crystals of CuSe initially formed on the as‐deposited film, but then completely turned into a quaternary chalcopyrite structure after annealing at 400°C. Phase transformation significantly affects the surface morphology, Hall properties, and band gap. Transmission electron microscopy further revealed that an interface between the Mo substrate and CIGS crystallites contains an amorphous layer even at the high temperature of 500°C. For the application of photovoltaic devices, we also report on the photoresponse of both as‐deposited and annealed films as demonstrated by preliminary tests.     

Ceramic Enamels as New Back Contacts for Cu (In,Ga) Se2‐Based Photovoltaic Tile

In this work, we investigated the properties of silver and gold enamels as potential back contacts for Cu (In, Ga) Se₂ (CIGS) solar cells. The enamels were deposited on ceramic tiles by nonvacuum printing techniques. Thus, we are proposing a development of integrated photovoltaic tile for the first time. We also explained the CIGS synthesis procedure using coprecipitation of selenite precursors. To deposit the precursor powders on the substrate, a doctor blade method is applied. The interface morphology between ceramic tile, back contact, and CIGS absorber was studied as a critical factor for the final solar cell performance. The thermal treatment effect on the back contact properties was also reported. Excellent compatibility between CIGS and gold layer was observed, keeping thickness and chemical composition adequate for photovoltaic applications. The band gap energy confirms assembly effectiveness. Unsatisfied results of silver diffusion toward CIGS absorber were obtained when silver enamels were used.