CuInS2 thin films deposited by sol-gel spin-coating method

CuInS₂ thin films were prepared using a sol-gel spin-coating method. Copper acetate monohydrate (Cu(CH₃COO)₂·H₂O) and indium acetate (In(CH₃COO)₃) were dissolved into 2-propanol and 1-propanol, respectively. The two solutions were mixed into a starting solution. The solution was dropped onto glass substrate, rotated at 1500 rpm, and dried at 300 °C for Cu-In as-grown films. The as-grown films were sulfurized inside a graphite container box. A clear chalcopyrite phase was observed without a secondary phase. Surface roughness of the films sulfurized at 500 °C was 19.1 nm. A Raman spectra measurement confirmed that no Cu-S or In-S compounds were created in the thin films.     

Optical constants of Zn-doped CuInS2 thin films

Optical properties of Zn-doped CuInS₂ thin films grown by double source thermal evaporation method have been studied. The amount of the Zn source was determined to be 0%-4% molecular weight compared with CuInS₂ source. After that, samples were annealed in vacuum at the temperature of 450 °C in quartz tube. The optical constants of the deposited films were obtained from the analysis of the experimental recorded transmission and reflexion spectral data over the wavelength range 300-1800 nm. It is observed that there is an increase in optical band gap with increasing Zn % molecular weight. It has been found that the refractive index and extinction coefficient are dependent on Zn incorporation. The complex dielectric constants of Zn-doped CuInS₂ films have been calculated in the investigated wavelength range. It was found that the refractive index dispersion data obeyed the single oscillator of the Wemple-DiDomenico model, from which the dispersion parameters and the high-frequency dielectric constant were determined. The electric free carrier susceptibility and the carrier concentration on the effective mass ratio were estimated according to the model of Spitzer and Fan.     

Grain dependence of ferroelectric domain switching on SrBi2Ta2O9 thin films observed by Kelvin probe force microscope

We have studied grain-shape dependence of Kelvin probe force microscopy of SrBi₂Ta₂O₉ thin films on epitaxial La_0.5Sr_0.5CoO₃/LaAlO₃ substrates. By changing the growth condition in pulsed laser deposition, we have grown the SrBi₂Ta₂O₉ thin films with various grain shapes. The shape and the orientation of SrBi₂Ta₂O₉ the thin films with various growth conditions have been analyzed by X-ray diffraction and scanning electron microscope. The large number of the long rectangular grains was observed accompanied with relatively larger (220) peaks than other peaks. From the Kelvin probe force microscope study, it has been observed that the long rectangular grains showed characteristics of easy ferroelectric domain switching at a low writing bias and weaker influence of surface charges.     

AFM study of the surface morphology of L-CVD SnO2 thin films

In this paper we present the results of Atomic Force Microscopy (AFM) characterisation of the surface morphology of the L-CVD SnO₂ thin films prepared by L-CVD technology and studied after exposure to air, dry air oxidation, and ion beam profiling. The L-CVD SnO₂ thin films after air exposure have a very smooth surface morphology with an average surface roughness (RMS) smaller than 0.5 nm, and average and maximal grain heights of about 1 and 2 nm, respectively. After dry air oxidation the L-CVD SnO₂ thin films exhibit an average surface roughness (RMS), as well as the average and maximal grain height, increased by one order of magnitude. Finally, after the ion beam profiling the L-CVD SnO₂ thin films exhibit an evidently disordered structure with a lot of craters. These experiments showed that the L-CVD SnO₂ thin films exhibit a very high quality surface morphology, what can be useful for solar cells and gas sensors application.     

Oxidation effect on densification of CuInS2 absorber layer by paste coating

Copper indium disulfide (CuInS₂) absorber layers were prepared by using a paste coating method. CuInS₂ and CuS were chosen as precursor powders. Ethylcellulose and phosphate ester were used respectively as a resin and a dispersant. Dihydroterpineol was used as a solvent. The paste was distributed on an Mo-coated soda–lime glass substrate by screen-printing method. It was difficult to obtain the densification of the CuS-doped CuInS₂ layer with only sulfurization in a graphite container box. However, when an oxidation process was applied to the CuS-doped CuInS₂ layer before the sulfurization process, quite dense CuInS₂ layers were obtained. Even though the chalcopyrite phase was nearly broke down with the oxidation, the phase was restructured with the sulfurization.     

In-situ studies of the recrystallization process of CuInS2 thin films by energy dispersive X-ray diffraction

Recrystallization processes during the sulfurization of CuInS₂ (CIS) thin films have been studied in-situ using energy dispersive X-ray diffraction (EDXRD) with synchrotron radiation. In order to observe the recrystallization isolated from other reactions occurring during film growth, Cu-poor, small grained CIS layers covered with CuS on top were heated in a vacuum chamber equipped with windows for synchrotron radiation in order to analyze the grain growth mechanism within the CIS layer. In-situ monitoring of the grain size based on diffraction line profile analysis of the CIS-112 reflection was utilized to interrupt the recrystallization process at different points. Ex-situ studies by electron backscatter diffraction (EBSD) and energy dispersive X-ray spectroscopy (EDX) performed on samples of intermediate recrystallization states reveal that during the heat treatment Cu and In interdiffuse inside the layer indicating the importance of the mobility of these two elements during CuInS₂ grain growth.     

Properties of Cu(In,Ga)(S,Se)2 thin films prepared by selenization/sulfurization of metallic alloys

Single-phase Cu(In,Ga)(S,Se)₂ (CIGSS) thin films have been prepared using a two-step process consisting of annealing of Cu-In-Ga precursors in S/Se ambient. Full characterizations have been carried out using XRD, SEM, EDS, Raman spectroscopy and optical absorption measurements. The depth profiles of constituent elements Cu, In, Ga, S and Se were almost constant throughout the film. Depending on overall Ga content and recrystallization temperature CIGSS thin films exhibited a shift in band gap from 1.04 to 1.19 eV.     

One-step electrodeposition of CuGaSe2 thin films

CuGaSe₂ thin films have been prepared by one-step electrodeposition and rapid thermal annealing process. According to composition and morphology analysis, deposition potential of − 0.6 V vs. SCE is considered to be optimum for electrodeposition. From the X-ray diffraction and Raman studies, the as-deposited film exhibits poor crystallinity without the evidence of CuGaSe₂ or other Ga-containing phases, while the rapid thermal annealing-treated film shows chalcopyrite structure CuGaSe₂ phase containing MoSe₂ phase between the Mo substrate and the absorber and minor second phase Cu_2 − xSe. The obtained CuGaSe₂ thin film has a band gap of about 1.68 eV and p-type conductivity.     

Epitaxial and polycrystalline CuInS2 thin films: A comparison of opto-electronic properties

Epitaxial and polycrystalline thin CuInS₂ (CIS) layers were grown by means of molecular beam epitaxy (MBE) on single crystalline silicon substrates of 4 inch diameter. Photoluminescence (PL) studies were performed to investigate the opto-electronic properties of these layers. For the epitaxial CIS, low-energy-hydrogen implantation leads to the passivation of deep defects and several donor-acceptor (DA) pair recombinations (from 1.034 eV to 1.439 eV) and two free-to-bound (FB) transitions (at 1.436 eV and 1.485 eV) become observable at low temperatures (5 to 100 K). Excitonic luminescence is completely absent for all investigated epitaxial CIS layers. This contrasts sharply with the PL of the polycrystalline films which is dominated by excitonic luminescence (1.527 eV). Also a donor-to-valence band transition at 1.465 eV (BF-1) and one donor-acceptor recombination at 1.435 eV (DA-1) were observed, while luminescence from deep levels is not present at all. Based on these data, a refined defect model for CuInS₂ with two donor and two acceptor states is presented. Under comparable growth conditions, the electronic quality of polycrystalline CIS is superior to epitaxially grown material.     

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.     

Studies on chemically deposited CuInSe2 thin films

Nanocrystalline thin films of CuInSe₂ have been prepared by chemical bath deposition technique at temperatures below 60 °C. X-ray diffraction of the films confirmed the identity of CuInSe₂ and with largely broadened peaks indicated the nanocrystalline nature of the films. Images from scanning electron microscope represented spherical nanoparticles.     

Characterization of CuInS2 thin films for solar cells prepared by spray pyrolysis

We have made a study of the chemical composition, the electrical, the optical and the structural properties of polycrystalline CuInS₂ thin films prepared by spray pyrolysis to be used for thin film solar cells. These films were deposited starting from aqueous solutions with different chemical compositions ([Cu]/[In] and [S]/[Cu] ratios) and at different substrate temperatures. In all cases, the material is p-type with grains preferentially oriented in the (112) direction of the sphalerite structure. The electro-optical properties show a very strong dependence on the [Cu]/[In] ratio in the solution. Films with copper excess have smaller resistivity and better crystallinity than those which are stoichiometric or have indium excess. The results obtained in this work show the possibility of having CuInS₂ thin films with a wide range of resistivity, a fact that could be important for making solar cells based on this material.     

Microscopic characterization of individual grain boundaries in Cu-III-VI2 chalcopyrites

The role of grain boundaries in polycrystalline Cu-III-VI₂ absorber material for thin film photovoltaics has not been fully understood and is currently under discussion. Recently, intensive efforts have been devoted to the characterization of the properties of individual grain boundaries using microscopic techniques, including Kelvin probe force microscopy (KPFM). KPFM provides local electronic information by measuring the surface potential in addition to the topography. We introduce the KPFM method and present simulations assessing the technique's limitations with respect to spatial resolution regarding the measurement of grain boundary properties. KPFM studies of individual GBs in the Cu(In,Ga)Se₂ materials system are reviewed and critically discussed, considering also results from other microscopic characterization techniques.     

CuInS2-based thin film solar cells with sputtered (Zn,Mg)O buffer

In a standard chalcopyrite-based thin film solar cell, the formation of the heterojunction is mediated by a thin buffer layer. This buffer is prepared by chemical bath deposition of CdS. The cell is completed by an undoped ZnO film followed by the highly doped transparent front contact. An alternative structure without any buffer layer where the undoped ZnO is replaced by (Zn,Mg)O has been suggested previously by Minemoto et al. [T. Minemoto, Y. Hashimoto, T. Satoh, T. Negami, H. Takakura, Y. Hamakawa, J. Appl. Phys. 89 (2001) 8327]. By adapting and applying this approach to CuInS₂ absorbers from industrial pilot production we achieved efficiencies of 8% for small area cells (as compared to 9.4% for a reference cell with CdS buffer) as well as monolithically integrated test structures.     

Annealing effects on the photoresponse properties of CdSe nanocrystal thin films

The photoresponse properties of the as-prepared and annealed close-packed CdSe nanocrystal (NC) films were investigated under laser illumination by Kelvin probe force microscopy. The annealing process improved the photoresponse speed of the CdSe NC films. The work function of the annealed CdSe NC films changed more rapidly than that of the non-annealed film in air at room temperature. Combined with X-ray photoelectron spectroscopy measurements and thermogravimetric analysis, the observed phenomena can be interpreted that annealing process removed the organic capping agents completely and eliminated oxide on the CdSe surface, which formed the tunnel barrier between NCs in the CdSe NC films. Consequently, it improved the separation rate of photoelectric charges and thus provided high speed photoresponse.     

Non-destructive evaluation of carrier transport properties in CuInS2 and CuInSe2 thin films using photothermal deflection technique

Photothermal deflection technique (PTD) is a non-destructive tool for measuring the temperature distribution in and around a sample, due to various non-radiative decay processes occurring within the material. This tool was used to measure the carrier transport properties of CuInS₂ and CuInSe₂ thin films. Films with thickness < 1 μm were prepared with different Cu/In ratios to vary the electrical properties. The surface recombination velocity was least for Cu-rich films (5 × 10⁵ cm/s for CuInS₂, 1 × 10³ cm/s for CuInSe₂), while stoichiometric films exhibited high mobility (0.6 cm²/V s for CuInS_2, 32 cm²/V s for CuInSe₂) and high minority carrier lifetime (0.35 µs for CuInS₂, 12 µs for CuInSe₂).     

Size effects of polycrystalline lanthanum modified Bi4Ti3O12 thin films

The film thickness dependence on the ferroelectric properties of lanthanum modified bismuth titanate Bi_3.25La_0.75Ti₃O₁₂ was investigated. Films with thicknesses ranging from 230 to 404 nm were grown on platinum-coated silicon substrates by the polymeric precursor method. The internal strain is strongly influenced by the film thickness. The morphology of the film changes as the number of layers increases indicating a thickness dependent grain size. The leakage current, remanent polarization and drive voltage were also affected by the film thickness.     

Spatially resolved photoelectric performance of axial GaAs nanowire pn-diodes

The spatially resolved photoelectric response of a single axial GaAs nanowire pn-diode has been investigated with scanning photocurrent and Kelvin probe force microscopy. Optical generation of carriers at the pn-junction has been shown to dominate the photoresponse. A photocurrent of 88 pA, an open circuit voltage of 0.56 V and a fill factor of 69% were obtained under AM 1.5 G conditions. The photocurrent followed the increasing photoexcitation with 0.24 A/W up to an illumination density of at least 90 W/cm², which is important for potential applications in concentrator solar cells.     

Photoconductivity effects in CuInS2, CuInSe2 and CuInTe2 thin films

Photoconductivity response spectra for CuInS₂, CuInSe₂ and CuInTe₂ thin films are reported. The temperature dependences of the spectra are utilized to determine the bandgap energies as functions of temperature. Photoconductive response times are measured for these I–III–VI₂ ternary semiconductors and are discussed in terms of deposition parameters and post-deposition treatments. The photoconductive decay times are in the range 10^-3 – 10¹ ms.     

Structural and morphological characterization of Pr and Er-containing SiO2-P2O5 sol-gel thin films

Present work is focused on obtaining and characterization of sol-gel thin films belonging to SiO₂-P₂O₅-Er₂O₃ (I) and SiO₂-P₂O₅-Pr₂O₃ (II) systems. The films have been obtained by spin coating technique for three rotation speeds: 2000, 3500 and 5000 rpm. The deposition of the films was performed at different periods of time, i.e. 24 h, 96 h, 120 h, 144 h and 168 h after instant preparation of the precursor sols. FTIR (Fourier transform infrared spectroscopy) and Raman characterization aimed at investigating the structural changes that occurred in silicophosphate network in dependence on the spin rate of the substrate as well as on the time period elapsed since the sol preparation till the deposition day. FTIR spectra recorded in the 400-4000 cm⁻¹ range revealed Si-O-Si, P-O-P and Si-O-P vibration modes and optical phonons specific for OH units. Raman spectra collected in the 100-4000 cm⁻¹ range put in evidence stretching, bending and mixed vibration modes specific for silicophosphate network as well as rare-earth ion peaks specific to certain electronic transitions. Morphological investigation made by AFM (atomic force microscopy) on Er and Pr-doped silicophosphate sol-gel films evidenced specific features depending on the parameters mentioned above and SEM (scanning electron microscopy) analysis revealed micron sphere structural units, exfoliation of the films and micro cracks.