The influence of the vacuum annealing process on electrodeposited CuInSe2 films

CuInSe₂ films were electrodeposited on mechanical polished Mo substrates. The applied potential was adjusted to get a stoichiometric composition. The as-deposited films were annealed in a high vacuum system for a short time. The films have been characterized by X-ray diffraction, scanning electron microscopy, energy dispersive X-ray analysis, Auger electron spectroscopy. The results indicate that the crystallization of the films was greatly improved by the short time vacuum annealing process without significant change in composition. The capacitance–voltage measurement shows characteristic p-type behaviors. This annealing process after electrodeposition was proved to be a useful method to prepare the polycrystalline CuInSe₂ films for solar cell application.     

The formation of CuInSe2 thin-film solar cell absorbers by laser annealing of electrodeposited precursors

The formation of the compound semiconductor CuInSe₂ by laser annealing of electroplated precursor films in inert gas atmosphere represents an entire non-vacuum production process of thin-film solar cell absorbers. Besides this technological aspect, the impact of extreme annealing rates on structural properties of the resulting semiconductor is interesting from a fundamental research point of view. For this reason, we compared absorbers processed by laser annealing with absorbers annealed with moderate heating rates in the range of 1 K/s by means of X-ray powder diffraction (XRD), scanning electron microscopy (SEM) and energy-dispersive X-ray analysis (EDX). All absorbers processed with laser or furnace annealing consist of crystalline CuInSe₂ in the chalcopyrite crystal structure with a high degree of cation disorder. We show that laser annealing does not lead to unintentional selenium loss during the semiconductor formation process.     

Optical and electrical properties of flash-evaporated amorphous CuInSe2 films

Amorphous films of CuInSe₂ were deposited on glass substrate by flash evaporation of source materials. The films were found to be p-type semiconductors. The direct optical band-gap energy was obtained to be 1.21–1.41 eV. The film DC conductivity ranged from 1.2–5.7 S cm⁻¹ at 285 K for different film thickness with corresponding activation energy of 55.5–301 meV. From temperature dependence of conductivity, the carrier transport was interpreted to be due to band conduction above 270 K.     

Preparation and study of structural and optical properties of CSVT deposited CuInSe2 thin films

A simple close-spaced vapour transport (CSVT) system has been designed and fabricated. Copper indium diselenide (CuInSe₂) thin films of wide range of thickness (4000–60000 Å) have been prepared using the fabricated CSVT system at source temperatures 713, 758 and 843 K. A detailed study on the deposition temperature has been made and the temperature profile along with the reaction kinetics is reported. The composition of the chemical constituents of the films has been determined by energy dispersive X-ray analysis. The structural characterization of the as-deposited CuInSe₂ films of various thicknesses has been carried out by X-ray diffraction method. The diffractogram revealed that the CuInSe₂ films are polycrystalline in nature with chalcopyrite structure. The structural parameters such as lattice constants, axial ratio, tetragonal distortion, crystallite size, dislocation density and strain have been evaluated and the results are discussed. The surface morphology of the as-deposited CuInSe₂ thin films has been studied using scanning electron microscope. The transmittance characteristics of the CuInSe₂ films have been studied using double beam spectrophotometer in the wavelength range 4000–15000 Å and the optical constants n and k are evaluated. The absorption coefficient has been found to be very high and is of the order of 10⁵–10⁶ m⁻¹. CuInSe₂ films are found to have a direct allowed transition and the optical band gap is found to be in the range 0.85–1.05 eV.     

Effect of bath temperature and annealing on the formation of CuInSe2

CuInSe₂ films of 2 μm thickness were electrodeposited potentiostatically, from aqueous solution containing thiocyanate as a complexing agent, on Mo substrates. For all the experiments, the potential of the potentiostatic deposition of the materials was chosen to be −1 V, whereas the bath temperature of electrolyte was varied from 20 to 80 °C. It was found that the electrodeposited CuInSe₂ was characterized by an amorphous layer and densely-packed nanometric grains with a good homogeneity. After vacuum annealing at 200 °C, glancing angle X-ray diffraction revealed the presence of the CuInSe₂ phase whereas annealing under selenium atmosphere lead to the growth of molybdenum selenide compound MoSe₂, in addition to a better crystallization of the copper indium diselenide compound. Scanning electron microscopic revealed that despite an increase in the grains dimensions, there was no significant change in the films surface morphology when the bath temperature was varied from 20 to 80 °C. At the same time, the composition of the electrodeposited Cu-In-Se layers becomes richer in copper. This increase in copper concentration is mainly compensated by a deficit in selenium atoms.     

Scanning tunneling microscopy of electrodeposited CuInSe2 nanoscale multilayers

We have investigated the electrochemical deposition of modulated thin films based on the Cu_xIn_2−xSe₂ system. CuInSe₂ is a leading alternative to silicon for use in thin film photovoltaic solar cells due to its optical absorption and electrical characteristics. Alternating layers of two different compositions based on the Cu_xIn_2−xSe₂ system were potentiostatically deposited. These nanometer-scale layers are used to form reduced-dimensionality structures such as superlattices that can be used in concentrator solar cells. We have used X-ray diffraction, energy-dispersive spectroscopy, and scanning tunneling microscopy to characterize our asdeposited thin films. The ability of the scanning tunneling microscope to resolve the individual nanoscale layers of our multilayered thin films is shown and is used to determine modulation wavelengths.     

Depth profile analysis of CuInSe2 (CIS) thin films grown by the electrodeposition technique

In this study, we report the results obtained from the auger electron spectroscopy (AES) depth profiling of CIS thin films grown by the electrodeposition technique. This result enables one to do a comparison between the bulk and superficial elemental compositions. The AES result is also compared with that obtained by the inductively coupled plasma (ICP) spectroscopy. These results support our proposition that the electrodeposited CIS film has a Cu-rich bulk region and an In rich surface, which leads to the formation of an n-layer (CuIn₂Se_3.5) on the top of the p-type CIS (CuInSe₂) phase     

Temperature dependence of photoacoustic spectra in CuInSe2 thin films grown by molecular beam epitaxy

CuInSe₂ (CIS) thin films, the thickness of about 1.0 μm and composition of Cu/In ratio (γ = 1.79), were grown on (0 0 1)-oriented GaAs substrate by molecular beam epitaxy (MBE) at substrate temperature of T_s, = 450°C. The samples have been characterized by means of piezoelectric photoacoustic (PPA) measurements between liquid helium (4.2 K) and room temperature (300 K). Two distinct PPA signals due to band gap of CIS and GaAs are observed in the whole temperature range from 4.2 to 300 K and the PPA signals of CIS decrease curvilinearly at the temperature range. Since the PPA signals of CIS thin films can be obtained up to room temperature, the PPA measurements are quite effective to obtain the optical characterizations, especially for the nonradiative recombination processes.     

Characterization of co-sputtered Cu-In alloy precursors for CuInSe2 thin films fabrication by close-spaced selenization

Sputtering technique for Cu–In precursor films fabrication using different Cu and In layer sequences have been widely investigated for CuInSe₂ production. But the CuInSe₂ films fabricated from these precursors using H₂Se or Se vapour selenization mostly exhibited poor microstructural properties. The co-sputtering technique for producing Cu–In alloy films and selenization within a close-spaced graphite box resulting in quality CuInSe₂ films was developed. All films were analysed using SEM, EDX, XRD and four-point probe measurements. Alloy films with a broad range of compositions were fabricated and XRD showed mainly In, CuIn₂ and Cu₁₁In₉ phases which were found to vary in intensities as the composition changes. Different morphological properties were displayed as the alloy composition changes. The selenized CuInSe₂ films exhibited different microstructural properties. Very In-rich films yielded the ODC compound with small crystal sizes whilst slightly In-rich or Cu-rich alloys yielded single phase CuInSe₂ films with dense crystals and sizes of about 5 μm. Film resistivities varied from 10⁻²–10⁸ Ω cm. The films had compositions with Cu/In of 0.40–2.3 and Se/(Cu+In) of 0.74–1.35. All CuInSe₂ films with the exception of very Cu-rich ones contained high amount of Se (>50%).     

Photoluminescence properties of sodium incorporation in CuInSe2 and CuIn3Se5 thin films

CuInSe₂ and CuIn₃Se₅ thin films have been deposited using sodium compounds such as Na₂Se and Na₂S onto Corning 7059 glass substrates by the two-stage co-evaporation method. Enhanced grain growth and preferred (1 1 2) grain orientation as well as a decrease in resistivity with respect to undoped films were observed with sodium incorporation. A clear correlation between the photoluminescence spectra and the resistivity of the films was found by comparing the properties of films with and without Na incorporation. These observations suggest that compensation is reduced due to the suppression of donor-type defects by the presence of Na.     

Off-stoichiometry effect on the CuInSe2 dielectric function

The dielectric function of CuInSe₂ has been studied for various composition ratios of Cu/In. The crystals were cut from ingots of 10 mm diameter and 40 mm length, grown by the classical Bridgman method. Spectroscopic ellipsometry measurements have been performed at room temperature in the range of 1.5–5.5 eV. From the measured spectra of the imaginary part of the dielectric function ₂, the broadening effect of the Cu/In ratio has been examined in connection with photoluminescence measurements. All transition edges were found to broaden as the In content increases. The effect of copper d levels has been observed to dominate in the 2.5–3.5 eV range.     

Sub-micrometer thick CuInSe2 films for solar cells using sequential elemental evaporation

CuInSe₂ thin films were prepared using sequential vacuum evaporation of In, Se and Cu at moderately low substrate temperatures, avoiding any treatment using toxic H₂Se gas. The samples were annealed at 400 °C at a pressure of 10⁻⁵ mbar to form CuInSe₂. Structural, optical, electrical, compositional and morphological characterizations were carried out on these films. We could obtain highly stoichiometric film, using this simple method, without opting for co-evaporation or high substrate temperature for deposition.     

Comparison of point defects in CuInSe2 and CuGaSe2 single crystals

Hall-effect and photoluminescence measurements have been carried out on as-grown and In/Ga-annealed CuInSe₂ and CuGaSe₂ single crystals grown by chemical vapor transport. Various defect levels in these related compounds have been identified and compared. V_Cu and V_Se show similar properties and activation energies in both materials. A tremendous difference is observed in the behavior of III_Cu antisite defects. Ga_Cu levels in CuGaSe₂ are much deeper than In_Cu in CuInSe₂, and furthermore, the formation of In_Cu is much easier compared to Ga_Cu. This is related to the higher formation energy of Ga_Cu in CuGaSe₂. Due to this difference in the defect chemistry of both compounds, it has not been possible until now, to prepare n-type CuGaSe₂ crystals, whereas CuInSe₂ is easily transformed from p- to n-type by annealing in vacuum or In-atmosphere.     

A comparative study of CuInSe2 and CuIn3Se5 films using transmission electron microscopy, optical absorption and Rutherford backscattering spectrometry

CuInSe₂ and CuIn₃Se₅ films were grown by stepwise flash evaporation onto glass and Si substrates held at different temperatures. Transmission electron microscopy (TEM) studies revealed that the films grown above 370 K were polycrystalline, with CuInSe₂ films exhibiting larger average grain size than CuIn₃Se₅. Optical absorption studies yielded band gaps of 0.97±0.02 and 1.26±0.02 eV for CuInSe₂ and CuIn₃Se₅, respectively. Rutherford backscattering spectrometry (RBS) study of the films on Si showed that CuInSe₂/Si structures included an inhomogeneous interface region consisting of Cu and Si, whereas CuIn₃Se₅/Si structures presented sharp interface.     

Formation of CuInSe2 thin films on flexible substrates by electrodeposition (ED) technique

This work reveals the formation of electrodeposition (ED) of CuInSe₂ (CIS) film on flexible substrate. Ternary compounds were co-deposited on Au coated plastic substrate from an aqueous acidic solution containing 1 mM CuCl₂, 5 mM InCl₃ and 1 mM SeO₂ adjusted to pH=1.65. It was found that the film stoichiometry improves when the growth solution consisted of 1 M triethanolamine (TEA) and 0.1 M Na-citrate. The optimal ED-CIS film was obtained after annealing at 150°C for 1 h in a nitrogen (N₂) atmosphere. Optical absorption study showed that the energy gap of the annealed material is 1.18 eV. Good and reliable quality ED-CIS film was grown in this research with the potential use in fabricating flexible solar cells. This was supported by various analytical techniques, including energy dispersive spectroscopy (EDS), X-ray diffraction spectroscopy (XRD), scanning electron microscopy (SEM), ultraviolet/visible spectroscopy, Fourier transform infrared spectrometer (FTIR) and photoelectrochemical (PEC) characterization.     

Fabrication of CuInSe2 films and solar cells by the sequential evaporation of In2Se3 and Cu2Se binary compounds

Cu₂Se/In_xSe(x≈1) double layers were prepared by sequentially evaporating In₂Se₃ and Cu₂Se binary compounds at room temperature on glass or Mo-coated glass substrates and CuInSe₂ films were formed by annealing them in a Se atmosphere at 550°C in the same vacuum chamber. The In_xSe thickness was fixed at 1 μm and the Cu₂Se thickness was varied from 0.2 to 0.5 μm. The CuInSe₂ films were single phase and the compositions were Cu-rich when the Cu₂Se thickness was above 0.35 μm. And then, a thin CuIn₃Se₅ layer was formed on the top of the CuInSe₂ film by co-evaporating In₂Se₃ and Se at 550°C. When the thickness of CuIn₃Se₅ layer was about 150 nm, the CuInSe₂ cell showed the active area efficiency of 5.4% with V_oc=286 mV, J_sc=36 mA/cm² and FF=0.52. As the CuIn₃Se₅ thickness increased further, the efficiency decreased.     

Measurement of CuInSe2 solar cell AC parameters

The AC parameters (cell capacitance and cell resistance) of Copper Indium Diselenide (CuInSe₂) solar cell are measured using time-domain technique. The cell capacitance is calculated from the open circuit voltage decay (OCVD) and cell resistance with solar cell I–V characteristics measured in dark. The solar cell exhibits high parallel resistance and low parallel capacitance. The doping concentration and built in voltage are derived from the 1/C_P² versus bias voltage graph. The built-in voltage of the solar cell shows good agreement with measurements published in the literature.     

Complexing agent effect on the stoichiometric ratio of the electrochemically prepared CuInSe2 thin films

The electrodeposition of CuInSe₂ is investigated to improve the stoichiometric properties of CuInSe₂ layers on indium tin oxide (ITO)-coated glass substrates and to develop one-step electrodeposition method for solar cell applications. XPS was utilized for the characterization of the surface properties of CuInSe₂ layers. The influence of the complexing agent, e.g. benzotriazole, bulk concentration of Cu and Se and deposition potentials on the stoichiometric properties, are discussed.     

Electrodeposition of CuInSe2 from ethylene glycol at 150 °C

Polycrystalline chalcopyrite thin films were potentiostatically electrodeposited from ethylene glycol solution onto SnO₂-coated glass substrates at 150 °C. The thickness of the layers was estimated using talysurf at 1.0 μm after deposition for 60 min. X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses were used to identify and characterise compounds formed at different potentials. It was found that Cu_1.75Se formation was dominant at −0.80 V vs Se and indium assimilation increased at more negative voltages forming a mixture of compounds including numerous Cu-Se binary phases and copper indium diselenide (CuInSe₂) at the cathode. As-deposited materials showed poor crystallinity and therefore films were annealed in Ar/5%H₂ in the presence of Se to improve the material quality for all investigations. Although the films were deposited at 150 °C, no noticeable improvement of the CuInSe₂ was observed, suggesting growth from aqueous media at room temperature to be preferable.     

n-GaAlAs on p-GaAs heterostructure solar cells grown by molecular beam epitaxy

p-GaAs substrate was used as the starting material in molecular beam epitaxial growth. n-type GaAlAs for heterostructure and n-GaAs capping layer were then grown after a buffer layer deposition on the substrate. The n-GaAlAs on p-GaAs heterostructure solar cells, with active area of 13.25 mm² under 100 mW/cm² AM1 illumination light source, provide a typical output as follows: V_oc=0.73 V, I_sc=6 mA, FF=0.7 and η=23% (active area). Spectral response measurements from 500 to 850 nm reflects the window effect of GaAlAs and band edge of GaAs materials.