Se2 Thin Films Research Articles

Short-range versus long-range structure in Cu(In,Ga)Se2, Cu(In,Ga)3Se5, and Cu(In,Ga)5Se8

The Cu-poor phases Cu(In,Ga)3Se5 and Cu(In,Ga)5Se8 play an important role both for understanding the Cu-(In,Ga)-Se material system and for growing high-efficiency Cu(In,Ga)Se2 thin film solar cells. Using extended X-ray absorption fine structure spectroscopy, we have studied the element-specific short-range structure of Cu(In,Ga)Se2, Cu(In,Ga)3Se5, and Cu(In,Ga)5Se8 alloys spanning the entire compositional range. The materials feature different local atomic arrangements and the element-specific average bond lengths remain nearly constant despite significant changes of the lattice constants with increasing In to Ga ratio and decreasing Cu content. In particular, the average bond lengths of Cu-Se and Ga-Se are almost identical while the average In-Se bond length is significantly longer in all three phases. The distance between lattice sites with mixed site occupation therefore corresponds to the weighted average of different element-specific bond lengths rather than to the individual bond lengths themselves. Furthermore, the increasing number of vacancies with decreasing Cu content lead to both a significant unit cell contraction and a slight bond length expansion. The crystallographic long-range structure and the element-specific short-range structure thus describe different structural aspects that are certainly interrelated but obviously not identical.

Effect of e-beam evaporated elemental metal stack precursors on the property of Cu(InGa)Se2 thin films through two-step process

Two-step process is considered to be more simple than co-evaporation method in Cu(InGa)Se2 (CIGS) thin films preparation process. However, research on CIGS thin films prepared by two-step process based on evaporation of elemental metals Cu, In and Ga is hardly reported. In this work, four types of metal stacks engineered as In/Ga/Cu/Ga/In (type A), Cu/Ga/In (type B), Cu/In/Ga (type C) and Cu/Ga/In/Cu (type D) were prepared and effects of metal stack precursors on properties of CIGS thin films formed by two-step process were studied. All types of precursors consisted of Cu–In, Cu–Ga and In phases. CIGS thin film from type A precursor showed poor compactness and relative high surface roughness due to the ordered defect compounds on the surface. Type B precursor exhibited a better compactness and crystal quality which led to an optimal structural property of films among all types of CIGS thin films. The reduction of Ga/(Ga + In) and the gaps appeared at the Mo/CIGS interface of CIGS thin film from type C precursor were explained by the re-evaporation of Ga element and compensation of In element during selenization. Grain size in CIGS thin film from type D is a little smaller than that from type B. Models of selenization process in different types of precursors were given. CIGS solar cell from type B exhibited the highest conversion efficiency of 9.2%.

Influence of process parameters on the properties of pulsed laser deposited CuIn0.7Ga0.3Se2 thin films

This work reports on the influence of pulsed laser deposition growth parameters on the properties of CuIn0.7Ga0.3Se2 thin films. Polycrystalline CuIn0.7Ga0.3Se2 thin films were grown on soda-lime glass substrates under various growth conditions. Morphological, compositional, structural, electrical and optical properties of CuIn0.7Ga0.3Se2 thin films were investigated as a function of laser fluence, background gas and substrate temperature. A threshold of laser fluence, 0.8 J/cm2, and background pressure, 0.01 mbar, determined through a systematic parametric investigation, for obtaining stoichiometric films. A minor secondary phase of Cu2−xSe was observed by X-ray diffraction, and found to gradually diminish with increasing deposition temperature. The film grown at 500 °C shows the purest CuIn0.7Ga0.3Se2 chalcopyrite phase. The electrical properties of the films, i.e., dark resistivity, carrier concentration and mobility, are shown to be mostly affected by the Cu2−xSe phase and the crystal quality of the films. All films exhibit high absorption coefficients of ∼2–3 ∗ 104 cm−1 in the vicinity of the band-edge; a blue shift of the energy gap with deposition temperature is attributed to the relaxation of the lattice strain, as corroborated by the respective shift of (1 1 2) peak of the XRD patterns. The monotonic increase of the photoluminescence intensity accompanied by the concomitant decrease of the emission linewidth and Stokes shift indicate an improvement in the material quality and uniformity as deposition temperature increases in accordance with the structural and electrical measurement findings. Long PL lifetime of ∼50 ns is measured in the band-edge region for the film grown at 500 °C, suggestive of a single phase material with low defect density. Our results overall indicate that high-quality, stoichiometric CuIn0.7Ga0.3Se2 thin films can be obtained using pulsed laser deposition, a rapid single-step growth method that eliminates the need for post-selenization.

DC and RF sputtered molybdenum electrodes for Cu(In,Ga)Se2 thin film solar cells

In this study, molybdenum (Mo) thin films are deposited at different working pressures and different target-substrate distances (Dt-s) (8 cm and 10 cm) under the excitation of the radio frequency (RF) or direct current (DC) power supply for applications in Cu(In,Ga)Se2 (CIGS) thin film solar cells. The various material properties including dynamic deposition rate, electrical and optical properties, crystal structure and morphology are systematically investigated and compared for these RF and DC sputtered Mo thin films. Mo bilayer, i.e. bottom Mo layer prepared at a high working pressure and top Mo thin film sputtered at a low working pressure are employed as back electrodes in CIGS thin film solar cells. CIGS thin film solar cells with DC Mo/DC Mo bilayer have relatively higher efficiency as compared to the others with RF Mo layers, which might be related to a relatively easy migration of sodium (Na) passing through the DC Mo layers and entering into the CIGS absorbers. A high efficiency CIGS thin film solar cell of 16.2% has been achieved in this work.

Direct insights into RbInSe2 formation at Cu(In,Ga)Se2 thin film surface with RbF postdeposition treatment

Scanning transmission electron microscopy-energy dispersive X-ray spectroscopy measurements performed on Cu(In,Ga)Se2 (CIGS) thin-film devices grown with RbF postdeposition treatment (RbF-PDT) revealed the formation of a RbInSe2 compound at the p-CIGS/n-CdS heterointerface. However, this type of Rb-compound formation was not observed with elemental In-free ternary CuGaSe2 (CGS) devices. The film surface of CIGS grown with RbF-PDT was found to be Ga-depleted; thus, the practical interface structure turned out to be CIGS/(CuInSe2, RbInSe2)/CdS. In contrast to the significant improvements observed in In-containing CIGS photovoltaic device performance with RbF-PDT, no significant improvements have been observed in In-free CGS devices thus far. These results suggest that the presence of elemental In plays a key role in obtaining beneficial alkali Rb effects for enhancing device performance as well as surface modification with RbF-PDT.

Luminescence and Stimulated Emission of Polycrystalline Cu(In,Ga)Se2 Films Deposited by Magnetron-Assisted Sputtering

The stimulated emission of Cu(In,Ga)Se2 alloy thin films formed by magnetron-assisted sputtering onto a sodium-fluoride layer deposited onto a molybdenum layer on a glass substrate is observed. The structural and optical parameters of the films are determined by scanning electron microscopy, local X-ray spectral microanalysis, X-ray structural analysis, and low-temperature luminescence (T = 10 K) measurements in the range of excitation levels of 1.6–75 kW cm–2 provided by nanosecond nitrogen-laser pulses. The stimulated emission threshold corresponds to ~25 kW cm–2. Comparative analysis of the emission of Cu(In,Ga)Se2 thin films suggests that the introduction of sodium results in significant improvement of the structural quality, specifically, in a decrease in the density of energy states in the band tails and in a decrease in the concentration of nonradiative-recombination centers.

Nanoscale Growth Kinetics of Cu(In,Ga)Se2 Absorbers

We investigate the kinetics of elemental segregation during the synthesis of Cu(In1−x,Gax)Se2 (CIGS) thin films via a rapid thermal growth process. Changes in elemental concentration are mapped rapidly via in situ X-ray fluorescence (XRF) microscopy. In situ XRF enables the quantification of Cu, In, Ga, and Se concentrations with 200 nm spatial resolution over a region of 7 μm × 7 μm, large enough to map multiple CIGS grains. Reacting elemental precursor stacks at 500, 550, and 600 °C, we find that copper is the driving force for α-CIGS phase formation at a rate of 17 nmol/cm2/min at 500 °C. Because of the thermal expansion of the substrate, cluster analysis and classification algorithms were employed to track compositionally similar regions over time and across samples measured at different temperatures. Beyond the direct application of these results for the optimization of CIGS manufacturing, these in situ experiments demonstrate the feasibility of quantitative compositional evaluation at the nanoscale ...

Pre-incorporation of Na into flexible Cu(In,Ga)Se2 thin film solar cells

Incorporation of sodium into the Cu(In,Ga)Se2 (CIGS) absorber layer is a key step for the growth of high conversion efficiency CIGS solar cells. However, the doping of Na into CIGS layer on a flexible substrate, i.e. Titanium foil, is still a challenge. In this paper, we report our results on comparing two different Na pre-incorporation methods in constructing flexible CIGS solar cells on Ti substrates: i) by sputtering a soda-lime glass thin film (SLGTF) onto Ti substrates prior to the growth of CIGS and Mo back contact layers and ii) by evaporating a NaF precursor layer on the Mo layer before the CIGS growth. By carefully optimizing the growth conditions, the conversion efficiencies of 13.47% and 15.02% are achieved for the SLGTF sample and the NaF sample, respectively. The NaF precursor method shows a better effect. The diffusion of Na and the gradient of Ga play important roles in the performance of the solar cells.

Structural and photoluminescent properties of CuGaxIn1−xSe2 thin films prepared by close-spaced vapor transport technique

CuGaxIn1−xSe2 (CIGS) (x = 0, x = 0.3 and x = 1) absorber layers were deposited onto transparent conducting oxide (SnO2) substrates using close-spaced vapor transport technique (CSVT). The aim of this work was to assess the quality of CIGS absorber films with various gallium contents. Composition found by Energy Dispersive Spectroscopy (EDS) showed that the investigated samples were Cu-poor and CuGa0.3In0.7Se2 thin film had a composition close to 0.3 as planned. The structural properties were investigated by X-ray diffraction analysis (XRD) and Raman spectroscopy, while optical properties were investigated by measuring transmittance and photoluminescence spectroscopy. It was found that all investigated samples crystallize with (112) as a preferential crystallographic direction. The shift of the XRD peaks towards a higher value of 2θ with the increase of Ga composition was observed. The grain size determined from the width of 112 XRD peak was within 40–75 nm range with no clear correlation with Ga content. Raman spectra of samples with x = 0 and 0.3 featured mainly a peak assigned to the A1 mode at about 175 cm−1. In CuGaSe2 the secondary Cu2Se phase was detected both by XRD and Raman. The band gap as evaluated by optical transmission measurement is shifted towards lower than expected values due to substantial defect-related bands broadening. A large concentration of defects in the samples was also confirmed by low or absent PL signal in the vicinity of the bandgap, while the emission due to deep defects was prevailing in the spectrum. The optoelectronic properties as indicated by photoluminescence have to be improved if CSVT method of absorber preparation is to be used more widely in photovoltaics.

Effects of selenization process on densification and microstructure of Cu(In,Ga)Se2 thin film prepared by doctor blading of CIGS nanoparticles

Flaws, such as porous structure and small grains easily occur during Cu(In,Ga)Se2 (CIGS) thin film absorber preparation from nanocrystallites have been the major obstacle to practical application of this technology. The selenization process parameter effects on the densification, microstructure, and electrical properties were investigated in this study using scanning electron microscopy (SEM), energy dispersive spectroscopy (EDS), and Hall-effect analyzer. Grain size was observed nearly independent of the selenization time but increased with increasing selenium mass in the graphite box for selenized CIGS film at 500 °C. Pre-sintering at 2 bar N2 atmosphere overpressure can effectively suppress the in-plane tensile stress generated by the shrinkage mismatch between the CIGS and Mo-coated glass substrate during 500 °C sintering, leading to better densification. The pre-sintered and selenized CIGS film exhibited good electrical properties suitable for solar cell applications.

Structure and Spectral Dependences of the Raman Scattering of Photosensitive CuIn0.95Ga0.05Se2 Films on Glass, Aluminum, and Nanoporous Al/Al2O3 Substrates

Photosensitive polycrystalline CuIn0.95Ga0.05Se2 thin films have been formed on glass, aluminum, and nanoporous Al/Al2O3 substrates by means of two-step selenization in a gas (nitrogen) flow carrying a reaction component (selenium). The structural properties and the Raman scattering spectral dependences have been investigated. The dependence of the main lattice parameters and intensities of the Raman scattering lines on the substrate material is demonstrated.

Cu(In,Sn)Se2 thin films for absorption of energy from infrared radiation

In this study, we sought to lower the bandgap of thin film solar cells by replacing the Ga used in the absorber layer of Cu(In,Ga)Se2 with Sn (bandgap of 0.07 eV) to form Cu(In,Sn)Se2. The proposed scheme was shown to reduce the bandgap of the absorber layer from 1.0 eV to 0.88 eV. Sn films of various thicknesses were deposited using precursors of Sn–In–Cu metal in order to study the effects of Sn/(In + Sn) ratio (SIR) on the structure of the material and photoelectrical characteristics of the Cu(In,Sn)Se2 absorber layer. Experiment results revealed that a higher SIR following selenization increased the grain size and surface roughness of the absorber layer. It increased the quantity of secondary phases of SnSe2 and Cu2SnSe3 and improved the distribution of Cu and In in the absorber layer. A higher SIR was also shown to increase electron mobility while decreasing carrier concentration and conductivity. When SIR≧0.25, the replacement of In3+ with Sn4+in the Cu+ vacancies decreased the electron strength of In. We speculate that an increase in SIR caused a relative increase in the quantity of Sn2+ compared to Sn4+, thereby increasing the electron strength of Sn and switching the absorber layer from a p-type to an n-type semiconductor.

Influence of composition on structural properties and optical parameters of thermally evaporated Ge10-xSe60Te30Inx (0 ≤ x ≤ 6) thin films

Thin films of Se based chalcogenide glassy alloys Ge10-xSe60Te30Inx (x = 0, 2, 4, 6) were deposited on to glass substrate by vacuum evaporation technique. Glassy nature of the film has been confirmed by DSC and XRD technique. Optical characterization of the thin films has been investigated by analyzing the absorption spectra measured at normal incidence in the spectral range 300–1000 nm. The optical constants viz absorption coefficient (α), extinction coefficient (k), optical band gap (Eg), and refractive index (n) have been studied. It has been observed that absorption coefficient (α) increases with increase in photon energy (hν). Extinction coefficient (k) is found to decrease with the increase of wavelength. Both absorption coefficient (α) and extinction coefficient (k) are found to decrease with In concentration for all compositions under considerations. Optical band gap (Eg) is evaluated and found to increase with increasing In content in Ge-Se-Te matrix. Refractive index (n) of the sample investigated has been estimated using Moss equation and is recorded to have a decreasing trend with increasing In incorporation in the ternary alloy. A correlative study of the optical bandgap and electronegativity of alloys indicate that optical band gap has increasing trend with decrease of electronegativity.

Influence of selenization temperatures on the microstructures and photoelectric properties of iron-ion doped Cu(In,Ga)Se2 thin-film solar cells

Cu(In,Ga)Se2 (CIGS) thin films were prepared on molybdenum-coated flexible stainless steel substrates through a non-vacuumspin-coating process followed by selenization at temperatures ranging from 450 °C to 575 °C. The effects of selenization temperatures and concentration of iron ions on the photoelectric properties of the CIGS solar cells was thoroughly investigated. All X-ray diffraction (XRD) patterns appropriately matched with those of CIGS phase. The diffraction peak intensities of undoped CIGS films were increased with an increase in selenization temperatures. However, XRD peak intensities of iron-ion doped CIGS films were decreased with the elevation of selenization temperatures owing to the degradation of thermal stability of CIGS films after iron-ion doping. As the selenization temperatures increased, the grain sizes of particles in CIGS films increased, and the films became densified with flat surfaces. The values of Voc, Jsc, and fill factor for CIGS solar cells prepared at 450 °C were 0.401 V, 21.84 mA/cm2, and 41.12%, respectively. These parameters were enhanced with increasing selenization temperatures due to improved crystallinity of CIGS and densified microstructures. However, iron-ion doping deteriorated the photoelectric properties because iron ions diffused into the absorption layers at high temperatures. The present study indicates that iron-ion doping adversely affected the photoelectric performance and microstructures of CIGS solar cells.

Engineered Cu(InGa)Se2 thin films through CaF2 post-deposition treatment for enhancing solar cell performance

It is considered that alkaline elements (such as Na, K, Rb) introduced through post-deposition treatment are able to modify the near-surface region of Cu(InGa)Se2 thin films. However, research on the effects of Ca element on properties of Cu(InGa)Se2 thin films is hardly reported. In this paper, Cu(InGa)Se2 thin films were prepared on quartz glass by e-beam evaporation via two-step process. Study of structural property showed that a certain amount of CaF2 through post-deposition treatment was beneficial to the uniform composition distribution of Cu(InGa)Se2 thin films. Near-surface region of Cu(InGa)Se2 thin films was more Cu-depleted by increasing introduced amount of Ca element. It was inferred that Ca diffused into the Cu(InGa)Se2 thin films substituted the In sites as CaIn so as to increase the carrier concentration. By calculation, the spike barrier of CdS/Cu(InGa)Se2 heterojunction was shown to decrease with increased thickness of CaF2 layers. As a result, Cu(InGa)Se2 solar cell subjected to 20 nm CaF2 post-deposition treatment, which possessed higher fill factor, open circuit voltage and short circuit current density, achieved the optimal power conversion efficiency. An efficiency of Cu(InGa)Se2 solar cell was enhanced by 2.9% after 20 nm CaF2 post-deposition treatment.

Piezo-phototronic and pyro-phototronic effects to enhance Cu(In, Ga)Se2 thin film solar cells

Cu(In, Ga)Se2 (CIGS)-based materials have gained remarkable attention for thin-film photovoltaic applications due to their high absorption coefficient, tunable bandgap, compositional tolerance, outstanding stabilities, and high efficiency. A small increase in the efficiency of CIGS solar cells has huge economic impact and practical importance. As such, we fabricated a flexible CIGS solar cell on amica substrate and demonstrated the enhanced device performance through the piezo- and pyro-phototronic effects based on a ZnO thin film. The device showed enhanced energy conversion efficiency from 13.48% to 14.23% by decreasing the temperature from 31 to 2 °C at a rate of ∼ 0.6 °C·s−1 via the pyro-phototronic effect, and further enhanced from 14.23% to 14.37% via the piezo-phototronic effect by further applying a static compressive strain. A pyro-electric nanogenerator effect was also found to promote the performance of the CIGS solar cell at the beginning of the cooling process. The manipulated energy band of the CIGS/CdS/ZnO heterojunction under the influence of the inner pyroelectric and piezoelectric potentials is believed to contribute to the sephenomena. Applying the piezo- and pyro-phototronic effects simultaneously offers a new opportunity for enhancing the output performance of commercialthin film solar cells.

Effects of spot size variation on the laser induced breakdown spectroscopy analysis of Cu(In,Ga)Se2 solar cell

The effects of spot size variation on the results of laser induced breakdown spectroscopy (LIBS) analysis of Cu(In,Ga)Se2 (CIGS) thin film solar cell using a scanning laser beam (λ = 532 nm, τ = 5 ns, top-hat spatial profile) are reported. The spot diameter of laser beam was reduced from 150 μm to 35 μm, below the laser P2 scribing pattern width, at the fixed laser fluence of about 4.5 J/cm2 and overlap ratio of 79%. The results showed that the measured LIBS signal intensity ratio rather than intensity could be nearly independent of laser spot diameter if the spectral lines are properly selected. It is demonstrated that the concentration ratio of CIGS layer can be predicted within 5% error from the average concentration ratio measured by inductively coupled plasma optical emission spectroscopy by LIBS analysis with a spot diameter of 50 μm.

The Influence of Sodium on the Characterization of Cu(In,Ga)Se2 Thin Films Prepared by Three-Stage Deposition Process

The Influence of Sodium on the Characterization of Cu(In,Ga)Se2 Thin Films Prepared by Three-Stage Deposition Process

Analysis of Thermodynamic Properties of Cu(In,Ga)Se2 Thin-Film Solar Cells for Viable Space Application

The use of thin film solar cells for power generation when colonizing space stations is an interesting idea. Resisting harsh space environments and low power to mass ratio shows potential for the future application of thin film solar cell in future space application such as roll able solar blankets. Cu(In,Ga)Se2 thin film solar cell are analyzed to determine their viability in space focusing on two modular aspects of emissivity and buffer lays. Findings and calculations showed how temperature affects the efficiency of solar cells and it could also be found how increasing their emissivity with different kinds of coatings can offset the loss in efficiency. Through the study of buffers, it was found that lower band gaps reduce the quantum efficiency of a solar cell. Therefore, ZnS with an energy band gap of 3.5eV will have the least amount of spectral absorption of higher wavelengths.

Advanced characterization and in-situ growth monitoring of Cu(In,Ga)Se2 thin films and solar cells

The continuous improvement of Cu(In,Ga)Se2 (CIGSe) solar cells relies considerably on advanced characterization of individual layers in the solar-cell stacks as well as of completed CIGSe devices. The present contribution provides an overview of corresponding efforts performed by various research groups at Helmholtz-Zentrum Berlin für Materialien und Energie GmbH. In-situ growth monitoring of CIGSe absorber layers by means of energy-dispersive X-ray spectrometry and light scattering is described, as well as structural analyses by means of X-ray and neutron diffraction. In addition, the characterization of surfaces and interfaces by soft X-ray and electron spectroscopy, the microscopic analysis by means of correlative electron microscopy, and optoelectronic characterization by optical spectroscopy are highlighted. The present contribution shows which substantial efforts in a research network are necessary in order to obtain deeper insight into materials properties and potentially limiting factors for the device performance, as well as to be able to control these factors during the solar-cell production.