Ga homogenization by simultaneous H2Se/H2S reaction of Cu-Ga-In precursor

The compositional distribution of Ga and S in Cu(InGa)(SeS)₂ films fabricated by a simultaneous selenization and sulfization process was systematically investigated. At low H₂Se/H₂S reaction temperature (490 °C), most Ga remains at the back of the film adjacent to the Mo back contact. However, the Ga/III ratios at the top and bottom of the Cu(InGa)(SeS)₂ layer monotonically increase and decrease with reaction temperatures, respectively. At T>550 °C, homogeneous distribution of elemental Ga and In through film is achieved. Further increase of the reaction temperature (e.g., T>550 °C) causes phase segregation on the surface of the Cu(InGa)(SeS)₂ film confirmed by XRD, GIXRD and EDS analysis.     

Improved compositional flexibility of Cu(In,Ga)Se2-based thin film solar cells by sodium control technique

The effects of sodium on off-stoichiometric Cu(In,Ga)Se₂ (CIGS)-based thin films and solar cells were investigated. The CIGS-based films were deposited with intentionally incorporated Na₂Se on Mo-coated SiO_x/soda-lime glass substrates by a multi-step process. By sodium control technique high-efficiency ZnO : Al/CdS/CIGS solar cells with efficiencies of 10–13.5% range were obtained over an extremely wide Cu/(In + Ga) ratio range of 0.51–0.96, which has great merit for the large-area manufacturing process. The improved efficiency in the off-stoichiometric regions is mainly attributed to the increased acceptor concentration and the formation of the Cu(In,Ga)₃Se₅ phase films with p-type conductvity. A new type of solar cell with p-type Cu(In,Ga)₃Se₅ phase absorber materials is also suggested.     

Preparation of CuGaS2 thin films by two-stage MOCVD method

Copper gallium disulfide (CuGaS₂; CGS) films were deposited on glass or ITO glass by two-stage metal-organic chemical vapor deposition (MOCVD) method, using Cu- and Ga/S-containing precursors without toxic H₂S gas. First, pure Cu thin films were prepared on glass substrates by using a single-source precursor, bis(ethylbutyrylacetato)copper(II) or bis(ethylisobutyrylacetato)copper(II). Second, the resulting Cu films were processed using tris(N,N-ethylbutyldithiocarbamato)gallium(III) at 410-470 °C to produce CuGaS₂ films. The optical band gap of the CGS film grown at 440 °C was about 2.53 eV. In addition, it was found that the elemental ratio of Cu and Ga elements of the CGS films can be elaborately adjusted by controlling deposition conditions on demand.     

Improved CIGS thin-film solar cells by surface sulfurization using In2S3 and sulfur vapor

Surface sulfurization of Cu(In,Ga)Se₂ (CIGS) thin films was carried out using two alternative techniques that do not utilize toxic H₂S gas; a sequential evaporation of In₂S₃ after CIGS deposition and the annealing of CIGS thin films in sulfur vapor. A Cu(In,Ga) (S,Se)₂ thin layer was grown on the surface of the CIGS thin film after sulfurization using In₂S₃, whereas this layer was not observed for CIGS thin films after sulfurization using sulfur vapor, although a trace quantity of S was confirmed by AES analysis. In spite of the difference in the surface modification techniques, the cell performance and process yield of the ZnO:Al/CdS/CIGS/Mo/glass thin-film solar cells were remarkably improved by using both surface sulfurization techniques.     

Improved Cu(In,Ga)(S,Se)2 thin film solar cells by surface sulfurization

Surface sulfurization was developed as a technique for fabricating efficient ZnO : Al/CdS/graded Cu(In,Ga)(S,Se)₂/ Mo/glass solar cells. Prior to the sulfurization, single-graded Cu(In,Ga)Se₂ (CIGS) films were deposited by a multi-stage process. The sulfurization of CIGS films was carried out using a H₂S---Ar mixture at elevated temperatures. The crystallographic and compositional properties of the absorber layers were investigated by XRD, SEM and AES analyses. After sulfurization, sulfur atoms were substituted for selenium atoms at the surface layer of CIGS films to form a Cu(In,Ga)(S,Se)₂ absorber layer. The diffusion of sulfur depends strongly on the grain structure of CIGS film. The cell efficiency of the 8–11% range before sulfurization was improved dramatically to 14.3% with V_oc = 528 mV, J_sc = 39.9 mA/cm² and FF = 0.68 after the sulfurization process.     

Studies on Cu2ZnSnS4 (CZTS) absorber layer using different stacking orders in precursor thin films

Cu₂ZnSnS₄ (CZTS) thin films were deposited by sputtering on glass substrates using stacked precursors. The stacked precursor thin films were prepared from Cu, SnS₂ and ZnS targets at room temperature with different stacking orders of Cu/SnS₂/ZnS/glass (A), ZnS/Cu/SnS₂/glass (B) and SnS₂/ZnS/Cu/glass (C). The stacked precursor thin films were sulfurized using a tubular rapid thermal annealing system in a mixed N₂ (95%)+H₂S (5%) atmosphere at 550 °C for 10 min. The effects of the stacking order in the precursor thin films on the structural, morphological, chemical, electrical and optical properties of the CZTS thin films were investigated. X-ray diffraction, Raman spectroscopy and X-ray photoelectron spectroscopy studies showed that the annealed CZTS thin film using a stacking order A had a single kesterite crystal structure without secondary phases, whereas stacking orders B and C have a kesterite phase with secondary phases, such as Cu_2−xS, SnS₂ and SnS. The annealed CZTS thin film using stacking order A showed a very dense morphology without voids. On the other hand, the annealed CZTS thin films using stacking orders B and C contained the volcano shape voids (B) and Sn-based secondary phases (C) on the surface of the annealed thin films. The direct band gap energies of the CZTS thin films were approximately 1.45 eV (A), 1.35 eV (B) and 1.1 eV (C).     

Increased homogeneity and open-circuit voltage of Cu(In,Ga)Se2 solar cells due to higher deposition temperature

Cu(In,Ga)Se₂-absorber deposition on commonly used soda lime glass is constrained in temperature by the softening of the substrate. To overcome this limitation, a high-temperature resistant glass was employed as a substrate for the growth of Cu(In,Ga)Se₂-absorbers by multi-stage coevaporation at standard (530 °C) and elevated (610 °C) temperatures. Absorbers were investigated using cathodoluminescence and X-ray diffraction and compared to the performance of solar cells fabricated from these absorbers. The higher deposition temperature is shown to lead to an increased homogeneity of the absorber layer both laterally and vertically and strongly enhanced open-circuit voltage. A best certified efficiency of 19.4% is reached.     

TiO2 thin films - Influence of annealing temperature on structural, optical and photocatalytic properties

Nanostructured TiO₂ thin films were deposited on glass substrates by sol-gel dip coating technique. The structural, morphological and optical characterizations of the as deposited and annealed films were carried out using X-ray diffraction (XRD), Raman spectroscopy, atomic force microscopy (AFM), and UV-vis transmittance spectroscopy. As-deposited films were amorphous, and the XRD studies showed that the formation of anatase phase was initiated at annealing temperature close to 400 °C. The grain size of the film annealed at 600 °C was about 20 nm. The lattice parameters for the films annealed at 600 °C were a = 3.7862 ? and c = 9.5172 ?, which is close to the reported values of anatase phase. Band gap of the as deposited film was estimated as 3.42 eV and was found to decrease with the annealing temperature. At 550 nm the refractive index of the films annealed at 600 °C was 2.11, which is low compared to a pore free anatase TiO₂. The room temperature electrical resistivity in the dark was of the order of 4.45 × 10⁶ ohm-cm. Photocatalytic activity of the TiO₂ films were studied by monitoring the degradation of aqueous methylene blue under UV light irradiation and was observed that films annealed above 400 °C had good photocatalytic activity which is explained as due to the structural and morphological properties of the films.     

Effects of substrate temperature on the structural and electrical properties of Cu(In,Ga)Se2 thin films

Polycrystalline Cu(In,Ga)Se₂ (CIGS) thin films were deposited onto soda-lime glass substrates using the three-stage process at the substrate temperature (T_sub) varying from 350 to 550 °C. The effect of T_sub on the structural and electrical properties of CIGS films has been characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and Hall effect measurement. We found that the surface roughness, constituent phases, film morphologies, resistivity (ρ) and carrier concentration (N_P) of as-grown CIGS films indicated different change trends with increase in T_sub. The higher T_sub gives smooth surface, large grain size and single-phase CIGS films. The values of N_P and ρ have two demarcated regions at T_sub of 380 and 450 °C. At lower T_sub of 380 °C, larger N_P and lower ρ were dominated by the existence of secondary-phase Cu_xSe with lower resistivity. In the case of 450 °C, the obvious changes in N_P and ρ can be attributed to the sufficient Na incorporation diffused from the glass substrate. Finally, the correlation of cell parameters with T_sub was analyzed.     

p-Type CuSbS2 thin films by thermal diffusion of copper into Sb2S3

We report the preparation of copper antimony sulfide (CuSbS₂) thin films by heating Sb₂S₃/Cu multilayer in vacuum. Sb₂S₃ thin film was prepared from a chemical bath containing SbCl₃ and Na₂S₂O₃ salts at room temperature (27 °C) on well cleaned glass substrates. A copper thin film was deposited on Sb₂S₃ film by thermal evaporation and Sb₂S₃/Cu layers were subjected to annealing at different conditions. Structure, morphology, optical and electrical properties of the thin films formed by varying Cu layer thickness and heating conditions were analyzed using different characterization techniques. XRD analysis showed that the thin films formed at 300 and 380 °C consist of CuSbS₂ with chalcostibite structure. These thin films showed p-type conductivity and the conductivity value increased with increase in copper content. The optical band gap of CuSbS₂ was evaluated as nearly 1.5 eV.     

Determination of Cu(In1−xGax)3Se5 defect phase in MBE grown Cu(In1−xGax)Se2 thin film by Rietveld analysis

Quantitative phase analysis of Cu(In_1−xGa_x)Se₂ (CIGS) thin film grown over Mo coated soda lime glass substrates was studied by Rietveld refinement process using room temperature X-ray data at θ-2θ mode. Films were found to contain both stoichiometric Cu(In_1−xGa_x)Se₂ and defect related Cu(In_1−xGa_x)₃Se₅ phases. Best fitting was obtained using crystal structure with space group I-42d for Cu(In_1−xGa_x)Se₂ and I-42m for Cu(In_1−xGa_x)₃Se₅ phase. The effects of Ga/III (=Ga/In+Ga=x) ratio and Se flux during growth over the formation of Cu(In_1−xGa_x)₃Se₅ defect phase in CIGS was studied and the correlation between quantity of Cu(In_1−xGa_x)₃Se₅ phase and solar cell performance is discussed.     

Thin films of Cu(In,Ga)Se2 and ordered vacancy compound prepared by thermal crystallization and their photovoltaic applications

Thin films of Cu–In–Ga–Se alloy system with various composition were prepared by thermal crystallization from In/CuInGaSe/In precursor. Electron probe microanalysis and X-ray diffraction study revealed that these samples were assigned to chalcopyrite Cu(In,Ga)Se₂ or ordered vacancy compound Cu(In,Ga)₂Se_3.5. Solar cell with ZnO:Al/i–ZnO/CdS/Cu(In,Ga)Se₂/Mo/soda-lime glass substrate structure was fabricated by using thermal crystallization technique, and demonstrated a 9.58% efficiency without AR-coating.     

Characterization of Cu(In,Ga)Se2 thin films prepared by thermal crystallization on Mo/glass substrate

Thin films of Cu(In,Ga)Se₂ were prepared by thermal crystallization on the sputtered Mo/substrate and characterized. MoSe₂ layer was formed at the interface between Cu(In,Ga)Se₂ and Mo layers after the thermal crystallization. The graded Ga concentration in crystallized Cu(In,Ga)Se₂ thin films was confirmed. Cu(In,Ga)Se₂ thin films prepared on the Mo/soda-lime glass had large and columnar grains rather than those on the Mo/quartz substrate.     

Preparation of CuIn1−xGaxSe2 thin films by sputtering and selenization process

CuIn_1−xGa_xSe₂ polycrystalline thin films were prepared by a two-step method. The metal precursors were deposited either sequentially or simultaneously using Cu–Ga (23 at%) alloy and In targets by DC magnetron sputtering. The Cu–In–Ga alloy precursor was deposited on glass or on Mo/glass substrates at either room temperature or 150°C. These metallic precursors were then selenized with Se pellets in a vacuum furnace. The CuIn_1−xGa_xSe₂ films had a smooth surface morphology and a single chalcopyrite phase.     

Structure, composition and optical properties of Cu2ZnSnS4 thin films deposited by Pulsed Laser Deposition method

Polycrystalline Cu₂ZnSnS₄ (CZTS) thin films have been directly deposited on heating Mo-coated glass substrates by Pulsed Laser Deposition (PLD) method. The results of energy dispersive X-ray spectroscopy (EDX) indicate that these CZTS thin films are Cu-rich and S-poor. The combination of X-ray diffraction (XRD) results and Raman spectroscopy reveals that these thin films exhibit strong preferential orientation of grains along [1 1 2] direction and small Cu_2−xS phase easily exists in CZTS thin films. The lattice parameters and grain sizes have been examined based on XRD patterns and Atom Force Microscopy (AFM). The band gap (E_g) of CZTS thin films, which are determined by reflection spectroscopy varies from 1.53 to 1.98 eV, depending on substrate temperature (T_sub). The optical absorption coefficient of CZTS thin film (T_sub=450 °C) measured by spectroscopic ellipsometry (SE) is above 10⁴ cm⁻¹.     

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.     

Preparation of Cu(In,Ga)Se2 thin films from Cu–Se/In–Ga–Se precursors for high-efficiency solar cells

Improved preparation process of a device quality Cu(In,Ga)Se₂ (CIGS) thin film was proposed for production of CIGS solar cells. In–Ga–Se layer were deposited on Mo-coated soda-lime glass, and then the layer was exposed to Cu and Se fluxes to form Cu–Se/In–Ga–Se precursor film at substrate temperature of over 200°C. The precursor film was annealed in Se flux at substrate temperature of over 500°C to obtain high-quality CIGS film. The solar cell with a MgF₂/ITO/ZnO/CdS/CIGS/Mo/glass structure showed an efficiency of 17.5% (V_oc=0.634 V, J_sc=36.4 mA/cm², FF=0.756).     

The structural and material properties of CuInSe2 and Cu(In,Ga)Se2 prepared by selenization of stacks of metal and compound precursors by Se vapor for solar cell applications

CuInSe₂ films and related alloys were prepared by thermal evaporation of Cu, InSe and GaSe compounds instead of elemental sources. Band-gap tailoring in Cu(In,Ga)Se₂-based solar cells is an interesting path to improve their performance. In order to get comparable results, solar cells with Ga/(In+Ga) ratios x=0 and 0.3 were prepared, all with a simple two-step sequential evaporation process. The morphology of the resulting films grown at 550 °C was characterized by the presence of large facetted chalcopyrite grains, which are typical for device quality material. It is important to note that absorber films with elemental gallium resulted in a significant decrease in the average grain size of the film. The X-ray diffraction (XRD) diffraction pattern of single-phase Cu(In,Ga)Se₂ films depicts diffraction peaks shifting to higher 2θ values compared to that of pure CuInSe₂. The photoluminiscence (PL) spectrum of Cu(In,Ga)Se₂ thin films also depicts the presence of the peak at higher energy that is attributed to the incorporation of gallium into the chalcopyrite lattice. As the band gap of CIGS increases with gallium content, desirable effects of producing higher open-circuit voltage and low current density devices were achieved. A corresponding increase in device efficiency with gallium content caused by a higher fill factor was observed. The best results show passive area efficiencies of up to 10.2% and open-circuit voltage (V_oc) up to 519 mV at a minimum band gap of 1.18 eV.     

Preparation and characterization of Cu(In,Ga)(Se,S)2 thin films from electrodeposited precursors for hydrogen production

Semiconducting Cu(In,Ga)(Se,S)₂ thin films were made from electrodeposited Cu(In,Ga)Se₂ precursors, followed by physical vapor deposition of In₂S₃, Ga, and Se. The bandgaps of these materials were found to be between 1.6 and 2.0 eV, which spans the optimal bandgap necessary for application for the top junction in photovoltaic multijunction devices and for unassisted water photolysis. These films were characterized by electron-probe microanalysis, scanning Auger spectroscopy, X-ray diffraction, and photocurrent spectroscopy.     

Analysis of Cu(In,Ga)(S,Se)2 thin-film solar cells by means of electron microscopy

The present work gives an overview of how electron microscopy and its related techniques are used to analyze individual layers and their interfaces in Cu(In,Ga)(S,Se)₂ thin-film solar cells. Imaging of samples can be performed at scales of down to the (sub)angstroms range. At similar spatial resolutions, information on composition can be gathered by means of energy-dispersive X-ray spectroscopy (EDX) and on spatial distributions of electrostatic Coulomb potentials in the specimen by applying electron holography. Microstructural and compositional properties as well as charge-carrier collection and radiative recombination behavior of the individual layers are accessible by use of electron backscatter diffraction, EDX, electron-beam-induced current (EBIC) and cathodoluminescence measurements, available in scanning electron microscopy. The present contribution gives an overview of the various scanning and transmission electron microscopy techniques applied on Cu(In,Ga)(S,Se)₂ thin-film solar cells, examples from case studies, and also demonstrates how these techniques may be combined in order to improve the analysis. Particularly, EBIC results show a reduced charge-carrier collection at Cu(In,Ga)Se₂ grain boundaries, while no indication was found for a charge accumulation at the grain boundaries by electron holography.