CuInSe2 Thin Films Research Articles

On the preparation of CuInSe2 thin films via two‒stage selenization

A two-stage selenization method for preparing CuInSe2 thin films in a carrier-gas (nitrogen) flow is developed. The dependences between the morphology and structure of CuInSe2 thin films and the selenization temperature are studied via electron microscopy and X-ray diffraction analysis. It is demonstrated that the film incorporates copper and indium selenides in the temperature range 300°C ≤ T < 400°C and a stoichiometric film with ordered chalcopyrite is formed at Т = 400°С. The possible mechanism whereby a CuInSe2 thin film is generated with the participation of selenization centers, namely, Cu2Se and In2Se3 grains, is discussed.

Transient and modulated charge separation at CuInSe2/C60 and CuInSe2/ZnPc hybrid interfaces

Spectral dependent charge transfer and exciton dissociation have been investigated at hybrid interfaces between inorganic polycrystalline CuInSe2 (untreated and Na-conditioned) thin films and organic C60 as well as zinc phthalocyanine (ZnPc) layers by transient and modulated surface photovoltage measurements. The stoichiometry and electronic properties of the bare CuInSe2 surface were characterized by photoelectron spectroscopy which revealed a Cu-poor phase with n-type features. After the deposition of the C60 layer, a strong band bending at the CuInSe2 surface was observed. Evidence for dissociation of excitons followed by charge separation was found at the CuInSe2/ZnPc interface. The Cu-poor layer at the CuInSe2 surface was found to be crucial for transient and modulated charge separation at CuInSe2/organic hybrid interfaces.

Determination and interpretation of the optical constants for solar cell materials

Solar cell materials in thin film form often exhibit quite rough surface, which makes the accurate determination of the optical constants using spectroscopic ellipsometry (SE) quite difficult. In this study, we investigate the effect of the rough surface on the SE analysis and establish an analysis procedure, which is quite helpful for the correction of the underestimated roughness contribution. As examples, the roughness analyses for CuInSe2 and CH3NH3PbI3 hybrid-perovskite thin films are presented. Moreover, to interpret the dielectric functions of emerging solar cell materials, such as CH3NH3PbI3 and Cu2ZnSnSe4, the optical transition analyses are performed based on density functional theory (DFT). The excellent agreement observed between the experimental and DFT results allows the detailed assignment of the transition peaks, confirming the importance of DFT for revealing fundamental optical characteristics.

Grain-boundary character distribution and correlations with electrical and optoelectronic properties of CuInSe2 thin films

Thin-film solar cells based on polycrystalline Cu(In,Ga)Se2 absorbers exhibit record conversion efficiencies of up to 22.6%. There is still a lack of a quantitative connection between the grain-boundary character distribution (GBCD) and the corresponding electrical and optoelectronic properties. The present work uses microstructural data from a CuInSe2 thin film acquired by electron backscatter diffraction (EBSD) to evaluate the GBCD. The most prominent features of the GBCD of CuInSe2 are Σ3 twin boundaries and the Σ9 and Σ27a symmetric tilt grain boundaries. Moreover, combining EBSD with electron-beam-induced current and cathodoluminescence (measurements on the same identical area) on a CuInSe2/Mo/glass stack provide the means to relate the grain-boundary character with the corresponding electrical and optoelectronic signals across the grain boundary. In part, determining this relationship is accomplished by means of correlation analysis using measurement data from more than 100 grain boundaries. However, the crystallographic, electrical and optoelectronic data showed no strong correlations, which is attributed to atomic reconstruction found in atomic planes adjacent to planar defects in polycrystalline CuInSe2 thin films and corresponding reductions of excess charge densities at these defects.

Agitation dependent properties of copper indium diselenide thin films prepared by electrochemical route

CuInSe2 (CIS) thin films have been prepared by potentiostatic electrochemical synthesis route in an aqueous medium. A conventional three-electrode electrochemical geometry was used to study the influence of agitation on electrodeposition of CIS. The co-deposition potentials for Cu-In-Se, −0.4V and −0.6V were optimized by using cyclic voltammetry measurements. The diffusive controlled growth with instantaneous nucleation is observed by Chronoamperometric measurements. X-ray diffraction (XRD) and Raman spectroscopy, UV–Vis spectroscopy, energy dispersive X-ray analysis (EDAX) and scanning electron microscopy (SEM) were employed to study the structural, optical, compositional and morphological properties, respectively. Polycrystalline CIS thin films with tetragonal structure were obtained without agitation, however the secondary phases of CuxSey were attributed with agitation. The Raman results are found to be in good agreement with XRD and EDAX analyses. Without agitation the energy band gap (Eg) was estimated ~1.1eV. The higher values of Eg~1.5 to 1.8eV measured for the samples grown with agitation are proposed due to the inhomogeneous growth of precursors and the change in surface morphology. Uniform, compact and well adherent CIS layers were deposited with and without agitation. The deposition potentials and agitation conditions were found to be greatly affected on the surface morphology. The stoichiometric CIS layer was deposited at −0.6V without agitation.

Microstrain distribution mapping on CuInSe2 thin films by means of electron backscatter diffraction, X-ray diffraction, and Raman microspectroscopy

The investigation of the microstructure in functional, polycrystalline thin films is an important contribution to the enhanced understanding of structure–property relationships in corresponding devices. Linear and planar defects within individual grains may affect substantially the performance of the device. These defects are closely related to strain distributions. The present work compares electron and X-ray diffraction as well as Raman microspectroscopy, which provide access to microstrain distributions within individual grains. CuInSe2 thin films for solar cells are used as a model system. High-resolution electron backscatter diffraction and X-ray microdiffraction as well as Raman microspectroscopy were applied for this comparison. Consistently, microstrain values were determined of the order of 10−4 by these three techniques. However, only electron backscatter diffraction, X-ray microdiffraction exhibit sensitivities appropriate for mapping local strain changes at the submicrometer level within individual grains in polycrystalline materials.

Formation and characterization of CuInSe2 thin films from binary CuSe and In2Se3 nanocrystal-ink spray

CuInSe2 (CISe) thin films were grown on Mo-coated glass substrates by spray coating technique using nanocrystalline ink. The ink was prepared by dissolving binary CuSe and In2Se3 nanoparticles in iso-butanol. The grown films were subsequently annealed in vacuum and selenium atmosphere, and examined by various characterization techniques. The vacuum-annealed films showed the CuSe binary phase along with the CuInSe2 phase, whereas the selenized films showed a single phase CuInSe2. The elemental analysis of the selenized film exhibited near stoichiometry, and the photoluminescence peak was observed in the region of band-to-band transitions (0.97 eV), which indicated the good structural quality of the selenized films. The developed method opens a new prospect for the finetuning of CISe composition by pre-adjusting the composition of each element in the binary to best fit of CISe in the photovoltaic devices.

Doping mechanism in pure CuInSe2

We investigate the dopant concentration and majority carrier mobility in epitaxial CuInSe2 thin films for different copper-to-indium ratios and selenium excess during growth. We find that all copper-poor samples are n-type, and that hopping conduction in a shallow donor state plays a significant role for carrier transport. Annealing in sodium ambient enhances gallium in-diffusion from the substrate wafer and changes the net doping of the previously n-type samples to p-type. We suggest that sodium incorporation from the glass might be responsible for the observed p-type doping in polycrystalline Cu-poor CuInSe2 solar cell absorbers.

Colloidal CuInSe2 nanocrystals and thin films for low-cost photovoltaics

We report on the synthesis of colloidal CuInSe2 nanocrystals (NCs) using simple solution-based method and its vacuum thermally evaporated films. The tetragonal phase CuInSe2 NCs grow in the shape of nanoparticles with diameter varies between 30 and 40nm according to high resolution transmission electron microscopy and electron diffraction measurements. The deposited films have been annealed for different times (up to 45min) in vacuum and at different temperatures (150–346°C). Optical characterization of the CuInSe2 NCs films using UV–Vis-NIR absorption, transmittance, excitation and photoluminescence measurements were performed. Optical properties of the deposited CuInSe2 films were quantified using developed theoretical model based on transmission and reflectance data. We found a strong dependence of the structural, morphologies and optical absorption coefficient of the prepared films on both annealing temperatures and annealing times. A remarkable enhancement of the optical absorption coefficient (α∼3×105cm−1) of the formed CuInSe2 NCs films with annealing was demonstrated and suggested the potential use of colloidal CuInSe2 for low cost photovoltaics.

Microstrain distributions in polycrystalline thin films measured by X-ray microdiffraction

Microstrain distributions were acquired in functional thin films by high-resolution X-ray microdiffraction measurements, using polycrystalline CuInSe2 thin films as a model system. This technique not only provides spatial resolutions at the submicrometre scale but also allows for analysis of thin films buried within a complete solar-cell stack. The microstrain values within individual CuInSe2 grains were determined to be of the order of 10−4. These values confirmed corresponding microstrain distribution maps obtained on the same CuInSe2 layer by electron backscatter diffraction and Raman microspectroscopy.

Suppression of surface recombination in CuInSe2 (CIS) thin films via Trioctylphosphine Sulfide (TOP:S) surface passivation

CuInSe2 (CIS) solar cells are promising candidates for thin film photovoltaic applications, one key limitation in their performance is surface recombination in these thin films. We demonstrate that passivating CIS films with Trioctylphosphine Sulfide (TOP:S) solution increases photoluminescence (PL) intensity by a factor of ∼30, which suggests that this passivation significantly reduces surface recombination. X-ray photoelectron spectroscopy (XPS) reveals that TOP:S forms both –S and –P bonds on the CIS film surface, which leads to a ∼4-fold increase in the surface Na peak intensity. This value is significantly higher than what would be expected from high temperature annealing alone, which has been linked to improvements in surface morphology and device efficiency in CIGS solar cells. We use Energy-Dispersive X-ray Spectroscopy (EDS) to measure the solid-state transport of Na within CIS films with and without passivation. EDS spectra on CIS film cross-sections reveals a saddle-shaped Na profile in the as-fabricated films and a concentration gradient towards the film surface in the passivated films, with 20% higher surface Na content compared with the unpassivated films. We employ Hybrid (B3PW91) Density Functional Theory (DFT) to gain insight into energetics of Na defects, which demonstrate a driving force for Na diffusion from bulk towards the surface. DFT Calculations with TOP:S-like molecules on the same surfaces reveal a ∼ 1eV lower formation energy for the NaCu defect. The experiments and computations in this work suggest that TOP:S passivation promotes Na diffusion towards CIS film surfaces and stabilizes surface Na defects, which leads to the observed substantial decrease in surface recombination.

Polyol process synthesis of metal selenide nanomaterials and their photovoltaic application

A mixed solvent of monoethanolamine and thioglycolic acid is successfully used to dissolve Se powder to prepare a highly reactive and soluble Se solution at room temperature in air. The as-prepared Se solution is very stable for more than half a year without any observed precipitation even in air. We used the Se precursor solution to synthesize various metal selenide nanomaterials such as Sb2Se3 nanorods, CuSe nanoplates and CuInSe2 nanocrystals by using a facile and environmentally-friendly triethylene glycol process. The structure, morphology, chemical composition and optical properties of the synthesized metal selenide nanomaterials are characterized. Moreover, the CuInSe2 nanocrystals are used to prepare ethanol-based colloidal ink and then deposited thin films by a dip-coating process. The synthesized CuInSe2 nanocrystal thin films are annealed at 550 °C in a selenizing atmosphere. A power conversion efficiency of 2.65% has been achieved by using the CuInSe2 thin films after selenization as a light-absorbing layer under AM 1.5 illumination.

CuInSe&lt;sub&gt;2&lt;/sub&gt; (CIS) as A light Absorption Layer of Photovoltaic Solar-Cells

Arrested precipitation methode used to synthesize CuInSe2 (CIS) nanocrystals were added to a hot solvent with organic capping ligands to control nanocrystal formation and growth. CIS thin films deposited onto Soda-Lima Glass (SLG) substrate by spray-coat, then selenized in Ar-atmosphere to form CIS thin films. PVs were made with power conversion efficiencies of 0.631% as-deposited and 0.846% after selenization, for Mo coated, under AM 1.5 illuminations. (XRD) and (EDX) it is evident that CIS have chalcopyrite structure as the major phase with a preferred orientation along (112) direction and Cu:In:Se nanocrystals is nearly 1:1:2 atomic ratio.

Polarimetric sensors on the basis of CdS/CuInSe2 heterocontact: Influence of the heat treatment on photosensitivity spectrum

CuInSe2 thin films were grown by vacuum evaporation of presynthesized material from a single source. The room-temperature photoconductivity spectra of the film show a strongly dependence on the evaporation conditions and, especially, on the evaporation temperature. A shift of the long-wavelenght photoactive absorption edge and the evolution of the photoconductivity spectra of the films were observed. The influence can be explained by allowing for deviations of the compositions of the condensed phase from CuInSe2 stoichiometry.

Orientation-distribution mapping of polycrystalline materials by Raman microspectroscopy.

Raman microspectroscopy provides the means to obtain local orientations on polycrystalline materials at the submicrometer level. The present work demonstrates how orientation-distribution maps composed of Raman intensity distributions can be acquired on large areas of several hundreds of square micrometers. A polycrystalline CuInSe2 thin film was used as a model system. The orientation distributions are evidenced by corresponding measurements using electron backscatter diffraction (EBSD) on the same identical specimen positions. The quantitative, local orientation information obtained by means of EBSD was used to calculate the theoretical Raman intensities for specific grain orientations, which agree well with the experimental values. The presented approach establishes new horizons for Raman microspectroscopy as a tool for quantitative, microstructural analysis at submicrometer resolution.

Fabrication and characterization of copper–indium–diselenide (CuInSe2, CIS) thin film using one-step electro-deposition process

We had successfully fabricated CuInSe2 (CIS) thin film by electro-deposition technique. Electro-deposited ternary selenide precursor was thermally annealed under nitrogen conditions to form CuInSe2 films. The structure, composition, chemical state and optical characterization of CuInSe2 films were measured using several techniques, including Scanning electron microscopy (SEM), Atomic force microscopy (AFM), Atomic emission spectroscopy (AES), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), Ultraviolet Spectrophotometry (UV) and electrochemical workstation. The results showed that the CuInSe2 thin films formed using electro-deposition had dense grain structure whose size was various with the increase of the deposited voltage and annealing temperature. The band gap of CuInSe2 was 1.43 eV when deposition voltage was 1.7 V, and CIS film exhibited the most excellent property.

Ordered Vacancy Compound Formation by Controlling Element Redistribution in Molecular-Level Precursor Solution Processed CuInSe2 Thin Films

Ordered Vacancy Compound Formation by Controlling Element Redistribution in Molecular-Level Precursor Solution Processed CuInSe₂ Thin Films

Composition Control of CuInSe2 Thin Films Using Cu/In Stacked Structure in Coulometric Controlled Electrodeposition Process.

Cu/In bi-metal stacked structures were prepared on Mo coated soda lime glass substrates using electrodeposition method. These metallic precursors were selenized at 550 °C for 60 min to synthesize the CuInSe2 (CIS) thin films in a thermal evaporator chamber with an Se overpressure atmosphere. The composition ratios of CIS thin films were systematically controlled using the coulometric method of the electrodeposition, where the accumulated coulomb of In layers was varied from 1062 to 6375 mC/cm2. As a result, the stoichiometric CIS film was obtained in the Cu/In coulomb ratio of 0.6. Highly crystallized CIS films were produced from the liquid Cu-Se phase in the Cu/In coulomb ratio of ≥0.6. In contrast, the crystallinity and grain size were degraded in the In-rich region. We found that the Cu/In composition ratio of CIS films was linearly proportional to the precursor thickness determined by the coulomb ratio.

Effect of iodine utilisation conditions on CSVT deposited CuInSe2 films

In this work, we investigate the effect of the carrier gas (iodine) utilisation conditions on the growth of CuInSe2 (CIS) thin films deposited by the close spaced vapour transport technique. This study shows that the CIS films deposited at 430°C, with heated iodine and open reactor, are of high quality in structural, optical and electrical properties. Analysis by X-ray diffraction showed that these films are polycrystalline and have a chalcopyrite structure. The preferential orientation according to the plane (112) was obtained. The morphological study of these films showed that they present a compact aspect with crystallites of uniform size. The energy dispersive spectrometer analysis showed that these films are slightly copper rich. The optical and electrical studies at room temperature gave the following results: an energy gap of 1 eV, an absorption coefficient higher than 105 cm− 1, p type conductivity and a low resistivity of 0·12 Ω cm.

Probing diffusion of In and Ga in CuInSe2/CuGaSe2 bilayer thin films by x-ray diffraction

The bilayer thin films of CuInSe2 (CIS) and CuGaSe2 (CGS) are fabricated by a molecular beam deposition (MBD) technique by sequential depositions of CGS followed by CIS and vise versa on Mo-coated soda-lime glass (SLG) and Mo/Al2O3-coated (Na blocking) SLG substrates. The thicknesses of CIS/CGS layers are adjusted to have the overall x=[Ga]/([In]+[Ga]) at approximately 0.4 similar to what is generally used in CIGS thin film solar cells. The growth temperature and the Cu-ratio y=[Cu]/([In]+[Ga]) of the CIS and CGS layers are systematically varied for Cu-rich (y>1) and Cu-poor (y<1) conditions. X-ray diffraction (XRD) technique is used to analyze the shift of diffraction peaks of the preferred orientations of the bilayers compared to those of the single-layer CIS, CGS and CIGS thin films. The XRD results show that higher Ga diffusion is observed in the Cu-rich CIS/CGS bilayer rather than others. The results suggest that the diffusion of Ga is enhanced by the excess Cu–Se phase that is dependent upon the substrate temperature. The Na from the SLG substrate is one of the important factors affecting Ga diffusion. The results show alloying CIGS patterns rather than separated CIS and CGS patterns as observed in the bilayers with Na.