Composition and Morphology of Electrodeposited CuInSe2 Precursor Films

CuInSe₂ (CIS) precursor films have been prepared by electrodeposition in aqueous solution. The electrodeposited films were characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS) for structural, morphological and componential properties. The influence of deposition potential and Na-citrate concentration on composition and morphology of electrodeposited films was studied in detail. It is found that the film morphology is strongly influenced by deposition potential and Na-citrate concentration. Films with large and homogenous grain size and ratio of Cu/In approaching 1 were obtained at deposition potentials of -0.7 and -0.75 V vs the saturated calomel electrode (SCE) and Na-citrate concentration of 500 mmol/L. Chalcopyrite phase CuInSe₂ is contained in precursor films that have poor crystallinity.     

CuInSe2 monograin growth in the liquid phase of potassium iodide

The results of monograin CuInSe₂ synthesis from Cu-In alloy and Se in liquid KI are presented. The amounts of CuInSe₂ and KI were nearly equal to fulfil the criterion for the monograin growth (all free volume between the particles has to be filled with liquid). All the grown powder materials with narrow-disperse granularity were chalcopyrite CuInSe₂. The grown crystallites had tetrahedral shapes and homogeneous composition. Particle size distribution was used to describe the growth process. The activation energy of linear growth of crystals was E_d = 0.25 ± 0.05 eV, and the power of time dependence of the crystal growth was l/n = 0.26 ± 0.06. The solubility of CuInSe₂ in KI at 990 K was 0.17 ± 0.05 wt. %. The solubility of potassium and iodine in CuInSe₂ at 990 K was 0.094 wt. %, and 0.0086 wt. %, respectively. As a result, homogeneous p-type CuInSe₂ monograin materials were synthesised in KI solvent.     

Characterization of stepwise flash-evaporated CuInSe2 films

CuInSe₂ (CIS) films were deposited by stepwise flash evaporation from polycrystalline powder source onto glass substrates held at various temperatures ranging from 100 to 560 K. The phase purity and microstructure were analyzed by transmission electron microscopy. The investigations show that films grown at 300 K and below were amorphous, whereas those grown at 370 K and above were polycrystalline in nature. The grain size in polycrystalline films were found to improve with increase in substrate temperature and during post-deposition annealing. The films had near stoichiometric composition as revealed by Rutherford backscattering spectrometry. Analysis of the optical transmittance spectra of CIS films deposited at 520 K yielded a value of ∼0.97 eV for the fundamental band gap.     

Phase evolution during CuInSe2 electrodeposition on polycrystalline Mo

Using structural analyses means of ex-situ Raman spectroscopy and X-ray diffraction combined with electrical measurements, we study the phase evolution in the growth by electrodeposition technique of CuInSe₂ on polycrystalline Mo. For this purpose the growth was stopped at different stages, and then the different layers were analysed. First growth steps seem to be controlled by the deposition of secondary phases, like elemental Se and Cu₂Se binary. After the deposition of approximately 300 nm of material, CuInSe₂ ternary and ordered vacancy compounds start to adequately form. At a thickness close to 2000 nm, the formation of binary Cu_xSe is observed, remaining up to the final growth process (4350 nm). All these results are compared with the kinetic model of the system under the consideration of the experimental composition evolution.     

The effect of sodium doping to CuInSe2 monograin powder properties

The effect of sodium doping to the electrical and photoluminescence properties of CuInSe₂ monograin powders was studied. Sodium was added in controlled amounts from 5 × 10¹⁶ cm^− 3 to 1 × 10²⁰ cm^− 3. The photoluminescence spectra of Na-doped stoichiometric CuInSe₂ powders had two bands with peak positions at 0.97 and 0.99 eV. The photoluminescence bands showed the shift of peak positions depending on the Na doping level. Peak positions with maximum energy were observed if added sodium concentration was 1 × 10¹⁹ cm^− 3. This material had the highest carrier concentration 2 × 10¹⁷ cm^− 3. In the case of stoichiometric CuInSe₂ (Cu:In:Se = 25.7:25.3:49.0), Na doping at concentrations of 3 × 10¹⁷ cm^− 3 and higher avoided the precipitation of Cu-Se phase. Solar cells output parameters were dependent on the Na doping level. Sodium concentration 3 × 10¹⁸ cm^− 3 resulted in the best open-circuit voltage.     

The performance of CuInSe2 monograin layer solar cells with variable indium content

The recent results on the characterization of CuInSe₂ monograin layer solar cells are presented. The influence of different Cu/In ratio on solar cell characteristics was studied. It was determined that device-quality CuInSe₂ monograin powder could be grown from CuIn precursor alloys with a composition between 0.9 < Cu/In < 1. For absorber material, these powders were post-treated in sulphur vapour. The cells based on these absorbers showed efficiencies up to 9.5%. However, the quantum efficiency measurements revealed a significant loss in the long-wavelength range of photons λ > 800 nm. The derivative of QE with respect to wavelength showed two peaks at the energy values of ∼ 1.4 eV and ∼ 1.03 eV, which proves that these sulphurised absorber materials consist of two different phases.     

Instantaneous preparation of CuInSe2 films from elemental In, Cu, Se particles precursor films in a non-vacuum process

CuInSe₂ (CIS) films are successfully prepared by means of non-vacuum, instantaneous, direct synthesis from elemental In, Cu, Se particles precursor films without prior synthesis of CIS nanoparticle precursors and without selenization with H₂Se or Se vapor. Our precursor films were prepared on metal substrates by spraying the solvent with added elemental In, Cu, and Se particles. Precursor films were instantaneously sintered using a spot welding machine. When the electric power was fixed to 0.6 kVA, elemental In, Cu, or Se peaks were not observed and only peaks of CIS are observed by X-ray diffraction (XRD) on the film sintered for 7/8 s. We can observe XRD peaks indicative of the chalcopyrite-type structure, such as (101), (103) and (211) diffraction peaks. We conclude that the synthesized CIS crystals have chalcopyrite-type structure with high crystallinity.     

Solvothermal preparation and characterization of sheet-like CuInSe2 with hierarchically mesoporous structures

By using a solvothermal reaction in mixed solvent of ethylenediamine and ethylene glycol, nanoparticle-assembled sheet-like CuInSe₂ with hierarchically mesoporous structures were successfully fabricated with the absence of any template or structure-directing agent. The as-synthesized products were characterized by XRD, EDX, FESEM, TEM, HRTEM, BET nitrogen adsorption and NIR absorption spectrum. The possible formation mechanism was simply discussed. The size of mesopores is hierarchically distributed in the range of 2-30 nm and the specific surface area was estimated to be 10.15 m²/g. The value of band gap (E_g) was calculated to be 1.00 eV based on its NIR absorption spectrum.     

Study of composition reproducibility of electrochemically co-deposited CuInSe2 films onto ITO

CuInSe₂ thin films were electrodeposited onto ITO substrates from aqueous solution containing 4.5 mM CuSO₄, 10 mM In₂(SO₄)₃ and 10 mM SeO₂ isopotentionally and by two-step deposition technique — changing the deposition potential during the deposition process step-wise: from lower to higher and vice versa in the potential range of − 0.6 to − 1.0 V (vs. Ag/AgCl). Solution pH was varied from 1.3 to 1.9 at temperatures 293 and 313 K to clarify the region for co-deposition of In-rich CuInSe₂ films on ITO with high composition reproducibility. Morphology and composition of CuInSe₂ layers depended not only on deposition temperature and potential, but also on the direction of potential change in the case of two-step process. The potential change from lower negative values to higher ones resulted in layers with low adhesivity. Alternatively, from higher potential to lower potential, the homogeneous, well-structured, adhesive and smooth layers were deposited. Modifying deposition periods in a two-step process, the chemical composition of films can be tailored. At pH < 1.9 reproducibility of the film's composition was low. ITO electrodes were found not to be stable at pH < 1.9.     

One-step electrodeposited CuInSe2 thin films studied by Raman spectroscopy

CuInSe₂ thin films one-step electrodeposited under different conditions were studied by MicroRaman spectroscopy to identify and quantify the individual phases present in the films.From the analysis of the Raman spectra, the main ternary phase (CuInSe₂) and elementary selenium Se⁰ were clearly identified. Specific chemical etches confirm the presence of elementary selenium Se⁰ and copper selenide binary phases Cu_xSe in selenium rich film.The amounts of these two phases were evaluated from X-ray Fluorescence measurements and confirmed using phase selective chemical treatments.     

Formation of rod-crystals on CuInSe2 thin films by SILAR method using CH3-(CH2)11-C6H4-SO3Na surfactant

The formation of rod-crystals was observed on CuInSe₂ thin films prepared by successive ionic layer adsorption and reaction (SILAR) method using sodium dodecylbenzene sulfonate (SDBS) as directing agent. Rod-crystals appeared on the surface of CuInSe₂ thin film when adding SDBS into cationic precursor solution. FESEM, EDS, XRD and HRTEM were used to characterize the rod-crystals. The length of rod-crystals has a proportional relationship with SDBS amounts in the given scope of 0.001-0.01 mol/L. The stoichiometry of rod was close to 1:1:2 of CuInSe₂, and rod growth of partially preferential orientation along [112] was observed. The growth of rod could be explained by steric hindrance effects of SDBS adsorbed on the inorganic deposit surface.     

Structural and optical characterization of CuInSe2 films deposited by hot wall vacuum evaporation method

Copper indium diselenide (CuInSe₂) compound was prepared by direct reaction of high-purity elemental copper, indium and selenium. CuInSe₂ thin films were deposited onto well-cleaned glass substrates by a hot wall deposition technique using quartz tubes of different lengths (0.05, 0.07, 0.09, 0.11 and 0.13 m). X-ray diffraction studies revealed that all the deposited films are polycrystalline in nature and exhibit chalcopyrite structure. The crystallites were found to have a preferred orientation along the (1 1 2) direction. Micro-structural parameters of the films such as grain size, dislocation density, tetragonal distortion and strain have been determined. The grain sizes in the films were in the range of 65-250 nm. As the tube length increases up to 0.11 m the grain size in the deposited films increases, but the strain decreases. The film deposited using the 0.13 m long tube has smaller grain size and more strain. CuInSe₂ thin films coated using a tube length of 0.11 m were found to be highly crystalline when compared to the films coated using other tube lengths; it has also been found that films possess the same composition (Cu/In=1.015) as that of the bulk. Scanning electron microscope analysis indicates that the films are polycrystalline in nature. Structural parameters of CuInSe₂ thin films deposited under higher substrate temperatures were also studied and the results are discussed. The optical absorption coefficient of CuInSe₂ thin films has been estimated as 10⁴ cm⁻¹ (around 1050 nm). The direct band gap of CuInSe₂ thin films was also determined to be between 1.018 and 0.998 eV.     

Characteristics of stacked CuInS2 and CuGaS2 layers as determined by the growth sequence

CuInS₂ and CuGaS₂ thin films have been prepared sequentially from elemental evaporation sources onto conventional soda lime glass substrates heated at 350 °C during the deposition process. The gradient in the structure and composition of the stacked layers has been investigated for the two possible growth sequences. Structural depth profiling and crystallographic phase analysis were performed by grazing incidence X-ray diffraction. The atomic distribution in the films depth was analyzed by X-ray photoelectron spectroscopy combined with sputter etching. Formation of the quaternary compound CuIn_1 − xGa_xS₂, with a high Ga content x > 0.80, has been detected with different distribution depending on the growth sequence.     

Growth and characterization of CuInSe2 thin films prepared by successive ionic layer adsorption and reaction method with different deposition temperatures

CuInSe₂ films were prepared at different deposition temperatures (T_D) by successive ionic layer adsorption and reaction method with chelating solutions. Influence of T_D on film growth, morphology, crystal structure, and band gap energy was investigated. Results showed that elevation of T_D mainly enhanced reaction kinetics and ionic diffusion velocity, resulting in fast growth rate of CuInSe₂ films, and maximum 20-30 nm/cycle depended upon T_D were acquired. Films with 60 dip-cycles could grow from 180 nm to 1000 nm by elevating T_D from 30 °C to 90 °C. Surface roughness of CuInSe₂ films was closely related to dip-cycle times and T_D. Accelerated growth rate by T_D could reduce the dip-cycle times for a required film thickness, which improved quality of film morphology.     

Key role of Cu-Se binary phases in electrodeposited CuInSe2 precursors on final distribution of Cu-S phases in CuIn(S,Se)2 absorbers

Using micro-Raman spectroscopy, we demonstrate that the formation of the CuS binary phase in CISEL™ cells absorbers is highly determined by the presence of a Cu-Se binary phase in the precursors and depends on the Cu/In ratio. A selective sulphurization mechanism of the Cu-Se phase is proposed as the origin of CuS platelets. For low Cu/In ratio both binaries are associated to the surface and do not affect the device performance. Conversely, when the binary phase in the precursor is present close to the back contact (for high Cu/In ratio), the CuS binary phase is also formed at this region after sulphurization. Strongly associated to this Cu-rich binary phase, a spinel-type phase is also observed in the sulphurized samples. This phase has n-type conductivity, which has a strong impact on the characteristics of the solar cells. The relationship between the presence of these phases and the electrical properties of the final devices is described, showing that the reduction of the V_oc and the increase of the R_s are related to the formation of a back diode by the spinel-type layer, as consequence of an undesirable n-p-n structure.     

Thermal stability of sputtered Mo/polyimide films and formation of MoSe2 and MoS2 layers for application in flexible Cu(In,Ga)(Se,S)2 based solar cells

Molybdenum (Mo) films with a thickness of about 800 nm were room temperature sputtered onto flexible polymeric substrates. Upilex® films were chosen as substrates on the basis of their high thermal endurance and reduced coefficient of thermal expansion. Thermal stability of Mo films has been proved by heat treatment of the Mo/Upilex® structures at a temperature comparable to that used in the preparation of the Cu(In,Ga)(Se,S)₂ absorber layer. A combination of high optical reflectance (maximum values of 75-80%), low electrical resistivity (about 30 μΩ cm) and a smooth surface free of cracks for heated films highlights their good thermal stability. The formation of MoSe₂ and MoS₂ layers, after selenization/sulfurization of the Mo/Upilex® structures, has been further investigated in view of their application as back contact layers in flexible CIGS based solar cells.     

Cu2ZnSnS4 solar cells prepared with sulphurized dc-sputtered stacked metallic precursors

In the present work we report the details of the preparation and characterization results of Cu₂ZnSnS₄ (CZTS) based solar cells. The CZTS absorber was obtained by sulphurization of dc magnetron sputtered Zn/Sn/Cu precursor layers. The morphology, composition and structure of the absorber layer were studied by scanning electron microscopy, energy dispersive spectroscopy, X-ray diffraction and Raman scattering. The majority carrier type was identified via a hot point probe analysis. The hole density, space charge region width and band gap energy were estimated from the external quantum efficiency measurements. A MoS₂ layer that formed during the sulphurization process was also identified and analyzed in this work. The solar cells had the following structure: soda lime glass/Mo/CZTS/CdS/i-ZnO/ZnO:Al/Al grid. The best solar cell showed an open-circuit voltage of 345 mV, a short-circuit current density of 4.42 mA/cm², a fill factor of 44.29% and an efficiency of 0.68% under illumination in simulated standard test conditions: AM 1.5 and 100 mW/cm².     

Study of Cu2ZnSnSe4 monograin formation in molten KI starting from binary chalcogenides

The present study deals with the possible chemical interactions of CuSe, SnSe, and ZnSe in molten KI as a flux material in vacuum ampoules. The aim is to find suitable preparation conditions for the synthesis of Cu₂ZnSnSe₄ monograin powders as an absorber material for solar cells. Impurities in the precursor materials and in the product powders were determined by inductively coupled plasma mass spectrometry. Differential thermal analysis was used to determine thermal effects. Phase composition in the mixtures of binary precursors and KI was determined by X-ray diffraction and Raman spectroscopy. It was found that no new compound was formed in the CuSe/KI, ZnSe/KI and SnSe/KI mixtures. The solubility of the binary chalcogenides in KI at 740 °C was determined.     

Structure and temperature dependence of conduction mechanisms in hot wall deposited CuInSe2 thin films and effect of back contact layer in CuInSe2 based solar cells

Copper indium diselenide (CuInSe₂) was prepared by direct reaction of high purity elemental Copper, Indium and Selenium. CuInSe₂ thin films were prepared on well-cleaned glass substrates by a hot wall deposition technique. The X-ray diffraction studies revealed that all the deposited films are poly crystalline in nature, single phase and exhibit chalcopyrite structure. The crystallites were found to have a preferred orientation along the (112) direction. Structural parameters of CuInSe₂ thin films coated with higher substrate temperatures were also studied. As the substrate temperature increases the grain size increases. The resistivity is found to decrease with increase in temperature. Two types of conduction mechanisms are present in the hot wall deposited CuInSe₂ films. In the temperature region below 215 K the conduction is due to a variable range hopping mechanism and in the temperature region above 215 K the conduction is due to a thermally activated process. It is observed that the solar cell with molybdenum as back contact has low series resistance (R_s), high shunt resistance (R_sh) and large fill factor (FF) when compared with CuInSe₂ based solar cells with other back contact material layers.     

Addition of Na into CuInS2 thin film via co-evaporation

While most of studies focus on the addition of Na into CuInGaSe₂ as well as CuInGaS₂ thin films, this study examines the addition of Na into CuInS₂ (CIS) thin films. Moreover, an alternative approach has been used to incorporate Na into CIS thin films. Two source evaporation (Cu and In) was first performed to obtain Cu-In layers with desired thicknesses. Three source evaporation (Cu, In, and NaF) then followed subsequently to produce Na-doped Cu-In precursor films having different Na concentrations. The precursor films were immediately sulfurized in the same evaporation chamber to form CIS thin films. The addition of Na was found to enhance (112) preferred orientation and reduce the grain size. Raman spectra show that the addition of Na does not alter the needed phase transformation from CuInS₂-CuAu structure to CuInS₂-chalcopyrite structure during the sulfurization. Blue shift of the CIS Raman CH mode occurs as a result. The doping of Na was also found to decrease the film resistivity or increase the hole concentration in the films.