Effect of Cl ion implantation on electrical properties of CuInSe2 thin films

The effects of Cl ion implantation on the properties of CuInSe₂ epitaxial thin films have been investigated. Using five kinds of accelerating energies, the doped layer with a constant profile of Cl concentration along the depth direction was fabricated. From the results of reflection of high-energy electron diffraction, the damages due to implantation were removed by annealing at 400°C in N₂. The conductivity type in all implanted films was n-type, and the carrier concentration was increased with increasing Cl concentration in the thin films. Consequently, it is considered that Cl acts as a donor in CuInSe₂.     

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.     

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.     

Blue-photon modification of nonstandard diode barrier in CuInSe2 solar cells

The current–voltage curves of many ZnO/CdS/CuInSe₂ solar cells display significant distortion when only red light illumination is employed. This distortion generally disappears or partially disappears for a period of time following illumination with blue light. Similarly, the dark diode curve shortly after illumination containing blue light is shifted significantly from the equilibrium dark curve. This effect is more common than generally realized and indicates a mechanism that is potentially detrimental to photovoltaic efficiency. A model is presented that is based on a low free-electron concentration and a high concentration of deep levels in the CdS window layer. This model is consistent with observed variations in current–voltage, capacitance, and laser scan data with illumination wavelength and history.     

Single crystal growth and properties of γ-phase in the CuInSe2+2CdS⇔CuInS2+2CdSe reciprocal system

The intermediate solid solution, γ-phase, exists in the CuInSe₂+2CdS⇔CuInS₂+2CdSe reciprocal system. It crystallizes in the cubic structure and has a wide homogeneity range. Single crystals of the γ-phase are grown by a modified Bridgman method and their composition, crystal structure, optical and electrical properties are studied. The band gap varies from 1.43 to 1.05 eV along the ‘Cu₃Cd₂In₃S₈’-‘CuCd₂InSe₄’ compositional section. The crystals are photosensitive, mostly p-type, with hole concentrations in the 10¹⁵-10¹⁶ cm⁻³ range and mobilities up to 18 cm²/V s. The results indicate that the γ-phase can be considered as a new absorbing material for thin-film solar cells.     

Na incorporation in Mo and CuInSe2 from production processes

Results of characterization of thin films of Mo deposited by DC magnetron sputtering on soda-lime glass (Mo/SLG) and CuInSe₂ (CIS) on Mo/SLG are presented. The primary objective of the work was to clarify the factors determining the concentration of Na in commercial-grade CIS. Mo films were deposited by three laboratories manufacturing CIS thin film solar cells. Analysis was by secondary ion mass spectrometry, scanning electron microscopy and X-ray diffraction. Changes in Mo deposition parameters in general affected the Na level but there was no obvious link to any single Mo deposition parameter. Oxygen content directly affected the Na level. The Na behavior was not obviously connected to film preferred orientation. Selenization of the Mo layers was also examined. Elemental Se vapor was found to produce significantly less selenization than H₂Se. The amount of selenization was also strongly dependent upon Mo deposition conditions, although a specific source of the change in reaction rate was not found. Na distributions in the CIS deposited on the Mo were not limited by the diffusivity of the Na. The Na concentration in the CIS was increased by annealing the Mo films both with and without intentionally added Na. The Na level in the CIS appears to be set more by the CIS deposition process than by the Na concentration in the Mo so long as the Mo contains sufficient Na to saturate the available sites in the CIS.     

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.     

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.     

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.     

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.     

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.     

Photoluminescence properties of stoichiometric CuInSe2 crystals

We studied photoluminescence (PL) properties of stoichiometric CuInSe₂ (CIS) single microcrystals. Temperature and laser power dependencies of the PL spectra were measured. Two bands at 0.973 (A-band) and 0.991 eV (B-band) governed the obtained PL spectra. Measured dependencies showed very similar properties for both bands: the j-shift that was generated by altering the laser power was 2 meV per decade for both bands and thermal activation energies were 46 and 32 meV for A- and B-band, respectively. Based on these results, we assume that both bands come from the same shallow donor–shallow acceptor recombination process with different bandgap energies for CIS. The solid solution phase of CIS with K or Na is proposed as the source of distinctive bandgap.     

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.     

Electrodeposition of p–i–n type CuInSe2 multilayers for photovoltaic applications

Copper indium diselenide polycrystalline thin films of p-, i- and n-type electrical conductivity were grown using a one-step electrodeposition process in a single bath. The bulk structure and the stoichiometry of the layers were determined using X-ray diffraction and X-ray fluorescence. The material composition was correlated with the electrical conductivity type variation, detected by the photoelectrochemical cell. Atomic force microscopy analysis showed copper-rich films deposited at low cathodic potentials (0.6 V vs Ag/AgCl) are of spherical and granular morphology and the grain sizes were 0.3–0.5 μm, while stoichiometric CIS films deposited at 1.0 V vs Ag/AgCl have grain sizes of 0.1–0.4 μm. The initial studies of optoelectronic properties (V_oc, J_sc and FF) of the four-layer solar cell devices (glass/FTO/n-CdS/n-CIS/i-CIS/p-CIS/Au) are presented.     

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.     

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.     

Study of the sub-bandgap absorption and the optical transitions in CuInSe2 polycrystalline thin films

Optical transitions near the fundamental band edge are studied for CuInSe₂ films having various Cu/In ratios by analysing the variations of the absorption coefficient with incident photon energy. The results indicate different transitions depending upon the Cu/In ratio. There are sub-bandgap absorption for near stoichiometric and Cu-rich films. The results are compared to some literature data.     

CuInSe2 thin film preparation through a new selenisation process using chemical bath deposited selenium

Copper indium selenide thin films were prepared through a novel and an eco-friendly selenisation process. In this method, selenium film required for selenisation was prepared using chemical bath deposition technique, at room temperature. Thus, totally avoided usage of highly toxic H₂Se or selenium vapour. Here, the process involved annealing the Stacked layer, Se/In/Cu in which Cu and In were deposited using vacuum evaporation technique. Investigations on the solid-state reaction between the layers were done by analysing structural and optical properties of films formed at different annealing temperatures. Optimum annealing condition for the formation of copper indium selenide thin film was found to be 673 K for 1 h in high vacuum. Compositional dependence of the growth process was also studied using various Cu/In ratios. Optical band gap was decreased with increase in Cu/In ratio. Carrier concentration and hence conductivity were found to be increased with increase in Cu/In ratio. The films obtained were p-type and highly Cu-rich films were degenerate.     

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.     

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%).