Properties Of Cu(In,Ga)Se2 Thin Films Research Articles

The influence of selenium amount added into the graphite box during the selenization of solution deposited CIGSe thin films.

Cu(In,Ga)Se2 (CIGSe) semiconductor is an efficient light absorber material for thin-film solar cell technology. The sequential evaporation of precursor solution, followed by the selenization process, is a promising non-vacuum and low-cost approach for CIGSe thin-film fabrication. The main properties of CIGSe thin films are strongly affected by the post-selenization step. Hence, thorough control of selenization parameters is essential for achieving pure crystalline, large grain films needed for high-performance solar cell devices. In this study, the impact of selenium (Se) amount added during the selenization step was evaluated. The structural, morphological, and compositional properties of the selenized thin films were investigated. The CIGSe precursor film was deposited by a spin-coating technique using a thiol/amine-based solution, followed by annealing with different Se amounts (100, 200, and 300 mg) within a partially closed small round graphite container. In all cases, uniform films of 1.2–1.5 µm thickness with a well-defined single chalcopyrite phase were obtained. It was observed that the grain size and Se content increased with increasing Se mass added. Moreover, the sample selenized with 200 mg Se resulted in higher surface coverage, thinner fine-grained layer, and less MoSe2 formation than the excess Se samples.

Annealing time effect on CIGS thin films

Chalcopyrite copper indium gallium (di)selenide (Cu(In, Ga)Se2 (CIGS)) thin film has been examined as an absorber layer for solar cells because of its suitable absorption value, stability and economy in manufacture. CIGS thin films belong to the I–III–VI2 group of the periodic table with the appropriate direct bandgap (1.5 eV). In this study, CIGS thin films were annealed at ∼200°C for four different annealing times (15, 30, 45 and 60 min) to investigate the effect of the annealing time on the crystalline structure and optical properties of CIGS thin films prepared by using the sol–gel dip-coating technique. CIGS thin films annealed at ∼200°C for 60 min were found to have the best structural and optical properties in this study. As the crystallite size increased with the rise in the annealing time, the lattice strain decreased, indicating the elimination of crystallite defects in the CIGS thin-film structure. Hence, the structural changes affected the optical properties slightly and the rise in the optical absorbance (A%) resulted in a decrease in the optical transmittance (T%).

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%.

Effect of substrate temperature and post-annealing on the properties of CIGS thin films deposited using e-beam evaporation

Cu(InGa)Se2 (CIGS) thin films were prepared using e-beam evaporation on a soda-lime glass substrate. The effect of substrate temperature and the difference between substrate temperature and post-annealing on the properties of the CIGS thin films and solar cells were studied. X-ray diffraction (XRD) and Raman spectroscopy, energy-dispersive spectroscopy, scanning electron microscopy, UV–Vis–NIR and the Hall effect were used to characterize the structural properties, composition, morphology, optical properties and electrical properties of the as-prepared CIGS thin films, respectively. The results demonstrated that the photoelectric properties of CIGS thin films prepared at a substrate temperature of 300 °C were optimal, with an efficiency of 7.1%. As the substrate temperature increased to over 300 °C, element gallium tended to evaporate from the substrate, which resulted in the variation of the Cu/(In + Ga) and Ga/(In + Ga) ratios of the films. The post-annealing process with in situ annealing temperatures of 300 °C and 400 °C was also studied. The results indicated that the post-annealing process, unlike the process of direct deposition at certain substrate temperatures, was able to avoid the element loss. At 300 °C in situ post-annealing temperature, there formed a ‘polygon grains’ Cu2−xSe phase, which disappeared when the in situ post-annealing temperature rose to 400 °C. The XRD patterns revealed that the post-annealing process made the element diffusion in films more uniform. The post-treated sample with an in situ post-annealing temperature at 400 °C, as a result, showed the highest efficiency of 9.0%, accompanied by the highest open-circuit voltage, short circuit current and fill factor.

Effect of post deposition annealing on the structure, morphology, optical and electrical properties of CuInGaSe2 thin films

Polycrystalline copper indium gallium diselenide (CIGS) thin film is a favourable candidate for solar cell applications. In the present work the effect of post-deposition annealing on the structure, surface morphology, optical and electrical properties are discussed. Initially, gallium rich CIG thin films were deposited by RF magnetron sputtering followed by an indium rich CIG layer and subjected to selenization to realize CIGS stoichiometry. X-ray diffraction (XRD) results revealed the polycrystalline nature of the films with chalcopyrite structure having preferential orientation along <112> direction normal to the substrate. Optical properties of CIGS thin films were studied using UV–vis spectrophotometry and the band gap of CIGS was found to be around 1.15 eV. Hall Effect studies carried out on the CIGS thin films showed a linear dependence of conductivity with post deposition annealing. The elemental composition of the films was quantified using X-ray photoelectron spectroscopy (XPS) and the results are discussed.

Substrate temperature optimization for Cu(In, Ga)Se2 solar cells on flexible stainless steels

Cu(In, Ga)Se2 (CIGS) thin films are deposited on flexible stainless steel (SS) substrates using the so called 3-stage co-evaporation process at different substrate temperatures ranging from 440°C to 640°C during the 2nd stage and the 3rd stage (TS2). The effects of TS2 on the properties of CIGS thin films are systematically investigated. It is found by secondary ion mass spectrometry measurement that CIGS thin films deposited at different TS2 show different Ga/(Ga+In) ratio (GGI) profiles along the growth direction. High TS2 facilitates the grain growth and leads to larger grain size. However, high TS2 worsens the spectral response of CIGS solar cells in the long wavelength range, which is partly attributed to the too much iron atom diffusion from the SS substrates into the CIGS thin films. All CIGS thin films show (112) preferred orientations with a shift to higher angle due to variation of compositions. A shoulder-like two-peak structure of (112) and (220/204) peaks appears for CIGS thin films deposited at lower TS2. Conversion efficiency of 11.3% is obtained for CIGS thin film solar cells deposited at the TS2 of 500°C.

The Optical Properties of CIGS Thin Films Derived by Sol-gel Dip Coating Process at Different Withdrawal Speed

The copper indium gallium diselenide Cu(In,Ga)Se2 (CIGS) thin-films deposited on soda lime silica glass substrates were dip-coated five times and annealed at 180oC. The advantages of CuIn(Ga)Se2 thin films were its wide compositional tolerance, high conversion efficiency, flexibility, low cost of materials and high optical absorption in the visible spectrum. The effect of the withdrawal speed, on the optical properties of the CIGS thin films derived by sol-gel process detailed clearly in this study by transmission and reflection measurements with controlling the withdrawal speed. The prepared CIGS thin films had p-type conductivity. The significant blue shift of absorption edge as well as tuning of optical band gap was observed with the withdrawal speed. The CIGS thin films at four different withdrawal speeds were such as 60, 120, 200 and 300mm min-1 prepared from colloidal suspensions to control the nature of nanospheres in the CIGS texture, because the colloidal solution was utilised as a template to control the particle shape.

Characterization of Cu(In,Ga)Se2 Solar Cells Grown on Na-Free Glass with an NaF Layer on a Mo Film

Cu(In,Ga)Se2 (CIGS) solar cells fabricated on soda-lime glass (SLG) exhibited high efficiency due to the supply of Na from the SLG substrate. As a simple doping method, Na can be supplied into CIGS films using a Na compound deposited on Mo electrodes. In this study, the authors compared the properties of CIGS thin films grown on a NaF layer deposited on Mo-coated Na-free glass with those of CIGS films on Mo-coated SLG by a standard through high-resolution transmission electron microscopy and low-temperature photoluminescence analyzes. After NaF deposition on the Mo film, an amorphous interlayer was detected at the CIGS/MoSe2 interface, and the MoSe2 layer that formed on the Mo surface was thin. The photoluminescence study showed that NaF doping did not effectively prevent the formation of deep donors in the CIGS film, whereas Na supplied by SLG effectively prevents their formation. We concluded that the poor performance of CIGS solar cells incorporating a NaF precursor is due to the amorphous layer at the CIGS/Mo interface and different luminescence characteristic with standard CIGS films, resulting in a low minority carrier collection.