Cu(In,Ga)Se2 Films Research Articles

Fabrication and characterization of Cu (In, Ga) Se2 thin films by electrodeposition: optimization of the thermal treatment with selenium and mechanical disturbance technique

The evaporation technique fabricates solar cells using the Cu(In, Ga)Se2 (CIGS) absorber. This technique has strong limitations in preparing this absorber in a large area, necessitating the electrodeposition technique. However, the morphology and crystallinity of this absorber need to be sufficiently adequate to guarantee proper collection of charge carriers since a cauliflower-type growth is favored. This underscores the need for modifications during the synthesis, thermal treatments, and post-synthesis to improve the morphology and crystallinity, a complex and significant aspect of our research. This work discusses the structural, atomic composition, morphological, and optical results obtained for samples of CIGS films synthesized by the electrodeposition technique. We proudly report that we achieved the best atomic composition, close to the ideal and an adequate morphology, by selenizing the samples with 30 mg and a temperature of 570°C. This success was further enhanced by subjecting these films to constant periodic movement during their synthesis, leading to significant improvements in the crystallinity, a testament to the success of our research.

16.48% Efficient solution-processed CIGS solar cells with crystal growth and defects engineering enabled by Ag doping strategy

Solution-processed Cu(In,Ga)Se2 (CIGS) solar cells suffer from serious carrier recombination and power conversion efficiency (PCE) loss because of the poor film properties and easy formation of defects. Herein, we propose Ag&Se co-selenization strategy to enhance the crystallization and passivate harmful defects of the CIGS films. The formation of Ag-Se phase during the selenization process enables the formation of large grains and suppresses the deep level defects. It is found that Ag doping can enlarge the depletion region width, lower the Urbach energy and prolong the carrier lifetime. As a result, a champion solution-processed CIGS solar cell presents a high efficiency of 16.48% with the highly improved open-circuit voltage (VOC) of 662 mV and fill factor (FF) of 75.8%. This work provides an efficient strategy to prepare high quality solution-processed CIGS films for high-performance CIGS solar cells.

Enhancement of the response speed of CIGS-based photodetector by Te-doping

Cu(In,Ga)Se2 (CIGS) based devices are promising candidates for high performance photodetector applications. The present work investigated the effects of Te-doping on the properties of CIGS layers and studied the performance of photodetectors (PDs) fabricated on these films. The films were vacuum evaporated on glass substrates and their morphological, structural and optical properties were evaluated after an annealing step at 550ºC. Morphological data indicated that undoped CIGS films had non-uniform surface features with large grains separated by nanoparticles. Addition of 6.6 % Te improved the morphology and yielded a smoother layer. Further increase in Te concentration showed appreciable reduction in surface feature size and shape. X-ray diffraction patterns showed that increasing the Te amount in CIGS caused a shift in the XRD peaks towards smaller angles pointing to replacement of Se atoms by Te. Peak splitting at higher Te samples suggested a graded structure with Se and Te amounts changing through the thickness of the film. Raman analysis demonstrated formation of CuInGa(Se,Te)2 (CIGST) compound. According to Tauc’s approximation, CIGS films with and without Te atoms possessed two energy band gaps of Eg1 and Eg2 that were in the ranges of 1.10–1.28 eV and 1.16–1.36 eV, respectively. Electrical data obtained from photodetector devices showed a responsivity of 4.44×10−1 A/W and a detectivity of 8.01×107 Jones for Te-free material, while the rise/fall times were measured as 34/148 ms. A much faster response speed of 19/20 ms was reached for devices fabricated on films with 10.2 % Te. This work demonstrated, for the first time, the fabrication low-cost and high-performance metal-semiconductor-metal (MSM) type CIGST-based PDs.

Evidence of Cationic Antiphase Disorder in Epitaxial Cu(In,Ga)S2 Grown on GaP/Si(001)

A transmission electron microscopy study of epitaxial Cu(In,Ga)S2 (CIGS) films coevaporated on GaP/Si(001), in either Cu‐rich or Cu‐poor conditions, is presented. The spatial distribution and the orientation of the different phases by means of electron diffraction are unveiled. From atomically resolved imaging of the CIGS film's atomic structure, it is concluded that different chalcopyrite domains, sharing cation antiphase symmetries of the cation sublattice, coexist in the films. At least three types of cation antiphase boundaries (CAPBs), which do or do not lead to a violation of the octet rule, depending on the propagation direction, are conceptualized. Even though it is observed that epitaxial CIGS is highly prone to cation antiphase disorder (CAPD), and it is found that the growth of CIGS in Cu‐rich conditions leads to a lower density of CAPBs, as compared to Cu‐poor growth conditions. This opens the question of the influence of CAPBs on CIGS electronic properties.

Aluminum‐Based Chalcopyrite Thin Films and Solar Cells

AbstractAluminum‐based chalcopyrite materials have attracted attention because of the wide controllability range of their material properties and potential for use in energy‐conversion devices. Herein, CuAlSe2‐based thin‐film growth and solar cell device properties are discussed. Ternary CuAlSe2 thin films are relatively unstable and decompose weeks after film growth, even when preserved in a dry box. However, alloying with elemental In led to significant improvements in stability. Cu(In,Al)Se2 (CIAS) solar cells yield better photovoltaic performance than CuInSe2 cells, although the effective range of Al concentration that can improve device performance is narrower than that of elemental Ga in Cu(In,Ga)Se2 (CIGS). A decrease in the alkali metal concentration in CIAS films with increasing Al concentration is observed, indicating that the formation energy of alkali‐metal substitutional defects on the Cu site is high, and/or Al‐related complex defects formed are kinetically stable and difficult to replace with alkali metals once they form. Although the direct observation of alkali metals in the bulk (grain interior) of chalcopyrite CuInSe2 and CIGS films has been difficult to date, this result can serve as indirect evidence of the presence of alkali metals in the bulk of CuInSe2 films.

Effect of the energy band bending on surface by Cu-poor layer of Cu(In, Ga)Se2-related photoelectrode for water splitting

To achieve the field of photoelectrochemical water splitting, the modulating of the valence band maximum (VBM) grading on the surface of Cu(In, Ga)Se2 (CIGS) photoelectrodes via the deposition of additional In and Se (forming Cu-poor-CIGS layer) following CIGS growth was investigated. The resulting downshift of the Fermi level was likely to occur around the surface of the CIGS films for modulating the VBM grading, increasing the onset potential of the CIGS-related photoelectrodes. Moreover, a hole-blocking layer was obtained via modulated VBM grading of the CIGS-related photoelectrode, which increased the photocurrent density of the CIGS-related photoelectrode. Subsequently, a Cu-poor-CIGS/CIGS interface was formed for modulating the VBM grading using Cu-poor-CIGS, which contributed to increasing the photocurrent density owing to the enhancement of charge separation by the depletion layer in the Cu-poor-CIGS/CIGS interface. This study shows that water splitting can be effectively improved by modulating the VBM grading on the surface of CIGS-related photoelectrodes.

Preparation and characterization of Cu-In-Ga-Se thin films by the electrodeposition technique using different metal salts and substrates

Cu(In, Ga)Se2 thin films possess important optoelectronic properties desirable for their application in devices such as solar cells. Solar cells based on this material have reached higher efficiencies than 23%. However, the commercialization of these cells has been restricted due to the use of thin film deposition methods involving costly high vacuum and cost. To reduce costs, it is necessary to use methods that do not use a high vacuum, among which electrodeposition stands out. Unfortunately, solar cells produced with this technique have yet to achieve high conversion efficiencies. Several authors attribute the lower efficiencies in such cells to the use of chemical additives in the preparation, different substrates, different deposition temperatures, etc. Nevertheless, there are very few reports on the influence of other metal salts in electrolytic baths. This work aims to use three different types of metal salts and voltages to produce Cu(In, Ga)Se2 (CIGS) absorber thin films by co-electrodeposition technique. The effect of nucleation type with two different substrates is studied, also report the studies carried out on the atomic composition and structural, morphological, and electrochemical characterization to understand the formation, growth, and morphology of CIGS films and, in this way, to obtain a suitable stoichiometry of thin film solar cells using this absorber.

Mechanism analysis of CuInS2 and Cu(In,Ga)S2 growth via KCN- and H2S-free process and solar-cell application

In this article, CuInS2 (CIS) and Cu(In,Ga)S2 (CIGS) absorbers are prepared via sulfurization by a sulfur powder source for co-evaporated Cu–In(–Ga) metal precursors without toxic KCN and H2S. The CIS and CIGS growth mechanism during sulfurization and their application to solar cells are discussed. X-ray diffraction and Raman spectroscopy analyses indicate that CuS and (In,Ga)2S3 exist at the frontside and the backside, respectively, in the CIGS films at the temperature between 250 °C and 350 °C. Then, these intermediate phases react at 400 °C or higher forming CIGS. Finally, CIS and CIGS solar cells with efficiencies of 3.7% and 7.2% are achieved, utilizing an optimum temperature of 600 °C.

Growth-Promoting Mechanism of Bismuth-Doped Cu(In,Ga)Se2 Solar Cells Fabricated at 400 °C.

The classical high-temperature synthesis process of Cu(In,Ga)Se2 (CIGS) solar cells limits their applications on high-temperature intolerant substrates. In this study, a novel low-temperature (400 °C) fabrication strategy of CIGS solar cells is reported using the bismuth (Bi)-doping method, and its growth-promoting mechanism is systematically studied. Different concentrations of Bi are incorporated into pure chalcopyrite quaternary target sputtered-CIGS films by controlling the thickness of the Bi layer. Bi induces considerable grain growth improvement, and an average of approximately 3% absolute efficiency enhancement is achieved for Bi-doped solar cells in comparison with the Bi-free samples. Solar cells doped with a 50 nm Bi layer yield the highest efficiency of 13.04% (without any antireflective coating) using the low-temperature technology. The copper-bismuth-selenium compounds (Cu-Bi-Se, mainly Cu1.6Bi4.8Se8) are crucial in improving the crystallinity of absorbers during the annealing process. These Bi-containing compounds are conclusively observed at the grain boundaries and top and bottom interfaces of CIGS films. The growth promotion is found to be associated with the superior diffusion capacity of Cu-Bi-Se compounds in CIGS films, and these liquid compounds function as carriers to facilitate crystallization. Bi atoms do not enter the CIGS lattices, and the band gaps (Eg) of absorbers remain unchanged. Bi doping reduces the number of CIGS grain boundaries and increases the copper vacancy content in CIGS films, thereby boosting the carrier concentrations. Cu-Bi-Se compounds in grain boundaries significantly enhance the conductivity of grain boundaries and serve as channels for carrier transport. The valence band, Fermi energy level (EF), and conduction band of Bi-doped CIGS films all move downward. This band shift strengthens the band bending of the CdS/CIGS heterojunction and eventually improves the open circuit voltage (Voc) of solar cells. An effective doping method and a novel mechanism can facilitate the low-temperature preparation of CIGS solar cells.

Effect of K Doping on the Performance of Aqueous Solution‐Processed Cu(In,Ga)Se2 Solar Cell

Copper indium gallium selenide Cu(In,Ga)Se2 (CIGS) solar cells fabricated by vacuum thermal evaporation, magnetron sputtering, or organic solvent solution‐process either lead to high manufacturing cost and film nonuniformity or cause explosion, toxication and pollution. To address these issues, herein, water as a solvent to prepare the CIGS precursor solution is used and 7.04% efficient CIGS solar cells without MgF2 antireflection layer is obtained. Furthermore, for the first time, K doping on CIGS films via adding “K+” into the aqueous precursor solution is conducted. K–In–Se species is formed in the process of selenization and acts as a Se reservoir during CIGS grain growth, resulting in improved quality of the CIGS film with much larger grains. K doping can also inhibit the loss of Ga element. The high‐bandgap K–In–Se phase and higher Ga content combine to result in the increase in the CIGS bandgap. As a result, the K‐doped CIGS solar cell achieves a 33.1% enhancement in efficiency to 9.37%, with the significant increase in the open‐circuit voltage from 520.5 to 597.1 mV and fill factor from 50.77 to 55.36%. These results provide an important preliminary foundation for the development of aqueous precursor solution‐based CIGS solar cells.

Ga]/([Ga]+[In]) profile controlled through Ga flux for performance improvement of Cu(In,Ga)Se2 solar cells on flexible stainless steel substrates

We present flexible Cu(In,Ga)Se2 (CIGSe) solar cells on stainless steel substrates with enhanced photovoltaic performance. CIGSe absorbers with thicknesses of 1.4 and 1.0 µm were deposited by the multilayer precursor method using precursor depositions of Ga–Se/In–Se/Cu–Se and then annealed under In–Se/Ga–Se/In–Se evaporations. The pronounced double-graded [Ga]/([Ga] + [In]) (GGI) profiles of the 1.4-µm-thick and 1.0-µm-thick CIGSe films were obtained using a relatively high Ga flux (up to 5.4 × 10−4 Pa), which shortened the annealing time. The pronounced double-graded GGI profiles of the 1.4-µm-thick and 1.0-µm-thick CIGSe films and CuInSe2 phase separation remarkably enhanced photovoltaic performances because a high back slope in the double-graded GGI profile induces back surface field, and the pronounced double-graded GGI profile with CuInSe2 phase separation yields the minimum GGI values, increasing light absorptionat longer wavelengths and in turn, conversion efficiency (η). Ultimately, η values of the flexible CIGSe solar cells increased to 17.3%, 17.2%, and 14.3% for CIGSe films with thicknesses of 2.5, 1.4, and 1.0 µm, respectively.

Silver-assisted optimization of band gap gradient structure of Cu(In,Ga)Se2 solar cells via SCAPS

Recently, silver (Ag) substitution for copper (Cu) into Cu(In,Ga)Se2 (CIGS) films has attracted widespread attention due to the lower synthesis temperature and the enlarge band gap. In this paper, SCAPS simulation is employed to investigate the influence of Ag doped CIGS absorber layer on the lattice defect and band gap gradient. First, the impact of the [Ga]/([In] + [Ga]) (GGI) and [Ag]/([Ag] + [Cu]) (AAC) on the defect concentrations is calculated systematically. It is demonstrate that the compression (GGI > 0.25) is more likely to cause the tetragonal distortion compared with the tension (GGI < 0.25) on the unit cell. And appropriate substitution of Cu with Ag can reduce the defect concentrations. Further investigations on the variation of the lattice structure indicate the ideal band gap can be enlarged to the range of about 1.20–1.29 eV by silver. With the ideal lattice structure, the efficiency of solar cells has a relative increase of about 48 % compared with that of the reference solar cell. This study offers a new sight for the incorporation of Ag into CIGS films from the perspective of lattice structure, which should be further improve device performance of CIGS solar cells at low temperature deposition process.

Indentation-induced cracking behavior of a Cu(In,Ga)Se2 films on Mo substrate

Abstract Cu(In,Ga)Se2 (CIGS) light absorption films were synthesized by co-sputtering method on Mo/soda-lime glass, and the mechanical behavior of the films was investigated by nanoindentation. The Young's modulus and hardness of the films were approximately 88.5 ± 2.2 and 5.89 ± 0.23 GPa, respectively. The comparison graph of indentation load–displacement curve with the curve based on the theoretical elastic films, while considering the curvature of the indenter tip, revealed that cracks initiated in the CIGS films at approximately 25 mN indentation load. The transmission electron microscopy (TEM) analysis revealed that most of the cracks exhibited intergranular fracture, and the fracture toughness of the films was ~0.22 MPa√m, which is greater than the brittleness of soda-lime glass. Both Palmqvist radial and lateral cracks are observed. Results also reveal that the indentation pressure caused grain subdivision (before cracking initiated), and indentation stress may be absorbed by relatively less dense microstructures of the films.

Photovoltaic characteristics and computational simulation of samarium-ion doped Cu(In, Ga)Se2 thin films prepared via a non-vacuum coating process

The effects of samarium ions on the photovoltaic performance of Cu(In, Ga)Se2 (CIGS) films were investigated through experiments and computational simulation. Compared with pristine CIGS films, the conversion efficiency of the prepared solar cells increased by 26.6% (from 8.62% to 10.91%) when 0.5 mol% samarium ions were doped into CIGS films. The incorporation of 0.5 mol% samarium ions facilitated the formation of the Cu2−xSe phase during the selenization reaction. The Cu2−xSe phase as the flux agent melted at high temperatures and thus promoted the growth of CIGS grains and reduced the number of surface pores on CIGS films. The smooth morphology of 0.5 mol% samarium-ion doped CIGS films improved the coverage of the buffer layers and suppressed the formation of additional shunt paths, thereby decreasing defect density in the CIGS/cadmium sulfide (CdS) interface and the absorber layer. The simulated results revealed the incorporation of 0.5 mol% samarium ions reduced the defect density from 1 × 1011 cm−3 to 1 × 109 cm−3 in the CIGS/CdS interface and from 3 × 1017 cm−3 to 4 × 1016 cm−3 in the absorber layers. The shunt conductance and saturation current of the prepared solar cells also decreased, from 4.38 mS/cm2 to 2.60 mS/cm2 and from 7.25 × 10−3 mA/cm2 to 1.31 × 10−3 mA/cm2, respectively. Excess doping of samarium ions caused a residual Cu2−xSe phase in CIGS films and resulted in the formation of additional shunt paths to deteriorate the performance of CIGS solar cells. The incorporation of appropriate concentrations of samarium ions into CIGS films was an effective approach to improve the photovoltaic properties of CIGS solar cells.

Terahertz Emission and Ultrafast Carrier Dynamics of Ar-Ion Implanted Cu(In,Ga)Se2 Thin Films

We investigated THz emission from Ar-ion-implanted Cu(In,Ga)Se2 (CIGS) films. THz radiation from the CIGS films increases as the density of implanted Ar ions increases. This is because Ar ions contribute to an increase in the surface surge current density. The effect of Ar-ion implantation on the carrier dynamics of CIGS films was also investigated using optical pump THz probe spectroscopy. The fitted results imply that implanted Ar ions increase the charge transition of intra-and carrier–carrier scattering lifetimes and decrease the bandgap transition lifetime.

A new method of generating Ga slope in Cu(In,Ga)Se2 film by controlling Se content in a multi-stacked precursor

An appropriate Ga slope is required in Cu(In,Ga)Se2 (CIGS) film to enhance the cell performance of CIGS thin-film solar cells. In the conventional three-stage-co-evaporation process, the Ga slope was obtained by controlling Ga/In flux during deposition process. However, in two-step process, where a precursor was deposited first and then annealed in a Se environment for mass production, the desirable Ga slope was not achievable with the Ga/In flux control. We observed that the Ga/(Ga + In) ratio was nearly flat in CIGS film for Se-rich precursor and the ratio was nearly zero at surface and very high on bottom side of CISG film for Se-deficient precursor. We were able to generate a CIGS film with a Ga non-zero Ga surface and desired slope in the bulk by devising a precursor with Se-rich layer on top and Se-deficient layer on bottom, resulting in the enhancement of Cell performance.

Effects of annealing atmosphere on the performance of Cu(InGa)Se2 films sputtered from quaternary targets.

Quaternary sputtering without additional selenization is a low-cost alternative method for the preparation of Cu(InGa)Se2 (CIGS) thin film for photovoltaics. However, without selenization, the device efficiency is much lower than that with selenization. To comprehensively examine this problem, we compared the morphologies, depth profiles, compositions, electrical properties and recombination mechanism of the absorbers fabricated with and without additional selenization. The results revealed that the amount of surface Se on CIGS films annealed in a Se-free atmosphere is less than that on CIGS films annealed in a Se-containing atmosphere. Additionally, the lower amount of surface Se reduced the carrier concentration, enhanced the resistivity of the CIGS film and allowed CIGS/CdS interface recombination to be the dominant recombination mechanism of CIGS device. The increase of interface recombination reduced the efficiency of the device annealed in a Se-free atmosphere.

Boosting Cu(In,Ga)Se2 Thin Film Growth in Low-Temperature Rapid-Deposition Processes: An Improved Design for the Single-Heating Knudsen Effusion Cell

The Knudsen effusion cell is often used to grow high-quality Cu(In,Ga)Se2 (CIGS) thin film in co-evaporation processes. However, the traditional single-heating Knudsen effusion cell cannot deliver complete metal selenides during the whole deposition process, particularly for a low-temperature deposition process, which is probably due to the condensation and droplet ejection at the nozzle of the crucible. In this study, thermodynamics analysis is conducted to decipher the reason for this phenomenon. Furthermore, a new single-heating Knudsen effusion is proposed to solve this difficult problem, which leads to an improvement in the quality of CIGS film and a relative increase in conversion efficiency of 29% at a growth rate of about 230 nm·min−1, compared with the traditional efficiency in a low-temperature rapid-deposition process.

Bismuth-doped Cu(In,Ga)Se2 solar cell on flexible stainless steel substrate: Examination of bismuth-doping effectiveness under different substrate temperatures on photovoltaic performances

Bismuth (Bi)-doped flexible Cu(In,Ga)Se2 (CIGS) films on stainless steel (SUS) substrates with different Bi contents are prepared, obtained by covering Bi thin layers (as Bi source) on Mo layers with Bi thickness from 0 (without Bi-doping) to 100 nm. It is disclosed that [Ga]/([Ga] + [In]) profiles are almost identical when Bi content is increased by enhancing the Bi thickness. The CIGS grain size and carrier lifetimes are improved through the proper Bi-doping (Bi thickness of 20–65 nm) under low deposition temperature of 457 °C. The 12.6%-efficient flexible Bi-doped CIGS solar cell under the deposition temperature of 457 °C is consequently obtained. Conversion efficiency of the flexible Bi-doped CIGS solar cell under the low deposition temperature of 486 °C is further enhanced to 14.8%, very close to that (15.1%) of the solar cell under convention deposition temperature (543 °C) without Bi-doping. Furthermore, the Bi-doping is effective for the enhancement of the photovoltaic performances under the deposition temperature range of 457–486 °C, whereas the Bi-doping under high CIGS deposition temperature range of 543–572 °C has the detrimental impact on the cell parameters, resulting from the sever Bi diffusion into the CIGS film.

Silver Surface Treatment of Cu(In,Ga)Se2 (CIGS) Thin Film: A New Passivation Process for the CdS/CIGS Heterojunction Interface

The interface engineering of Cu(In,Ga)Se2 (CIGS)‐based solar cells is challenging for high‐efficiency devices, especially for the CdS/CIGS heterojunction interface. Recently, post‐treatment of the CIGS surface, as an efficient approach to passivate the defects at the CdS/CIGS interface, has attracted widespread attention. Here, a simple Ag surface treatment process is used to realize the passivation of interface defects and the enhancement of the CdS/CIGS heterojunction. This process not only reduces the surface roughness of CIGS films significantly, but also contributes to controlling the Ga composition in the surface layer. Furthermore, characterization techniques reveal that Ag surface treatment can effectively decrease the defect concentrations at the heterojunction interface and enhance the CdS/CIGS heterojunction quality with appropriate Ag deposition duration. Further investigations on the changed defect level caused by the Ag surface treatment indicate that the increased defect level is likely related to the shift of valence band maximum. Eventually, the efficiency of the optimum device has a relative increase of about 18% compared with that of the reference solar cell. This work focuses on revealing the differences of the CIGS surface and CdS/CIGS interface caused by Ag, which provides a new surface processing method for the passivation of the CdS/CIGS heterojunction.