Electrosynthesis and characterization of iron selenide thin films

Iron selenide thin films have been deposited onto stainless steel and fluorine-doped tin oxide (FTO) glass substrates by the electrodeposition process, in potentiostatic mode using ferric chloride (FeCl₃) and selenium dioxide (SeO₂) salts. The deposition mechanism and growth of the films were investigated by cyclic voltammetry and chronoamperometry. The structural, morphological, compositional and optical properties of the deposited films have been studied using X-ray diffraction (XRD), scanning electron microscopy (SEM), EDAX and optical absorption techniques, respectively. XRD studies reveal that the films are polycrystalline with monoclinic crystal structure. The surface morphology study shows that grains are uniformly distributed over the entire surface of the substrate. EDAX study reveals that the iron selenide films are rich in iron. Optical absorption study shows the presence of direct transition with bandgap energy of 1.23 eV.     

CuInSe2 thin film preparation through a new selenisation process using chemical bath deposited selenium

Copper indium selenide thin films were prepared through a novel and an eco-friendly selenisation process. In this method, selenium film required for selenisation was prepared using chemical bath deposition technique, at room temperature. Thus, totally avoided usage of highly toxic H₂Se or selenium vapour. Here, the process involved annealing the Stacked layer, Se/In/Cu in which Cu and In were deposited using vacuum evaporation technique. Investigations on the solid-state reaction between the layers were done by analysing structural and optical properties of films formed at different annealing temperatures. Optimum annealing condition for the formation of copper indium selenide thin film was found to be 673 K for 1 h in high vacuum. Compositional dependence of the growth process was also studied using various Cu/In ratios. Optical band gap was decreased with increase in Cu/In ratio. Carrier concentration and hence conductivity were found to be increased with increase in Cu/In ratio. The films obtained were p-type and highly Cu-rich films were degenerate.     

The formation of CuInSe2 thin films by rapid thermal processing

Formation of polycrystalline thin film CuInSe₂ was achieved by the rapid thermal processing of vacuum-deposited copper, indium, and selenium. Films were fabricated and characterized in three composition regions: copper-poor (approximately 20 at.% Cu). stoichiometric (25 at.% Cu) and copper-rich (approximately 28 at.% Cu). Characterization results including X-ray diffraction analysis, electron probe for microanalysis, scanning electron microscopy, and optical reflection and transmission measurements are presented. The results show that nearly single-phase material has been formed from co-deposited precursors with a post-deposition annealing time of less than 2 min. The films have smooth morphologies amenable for photovoltaic device fabrication, optical absorption coefficients in the high 10⁴ cm⁻¹ range, and an optical band gap of 1.0 eV.     

Precursor modification for preparation of CIS films by selenization technique

CuInSe₂ films have been prepared using the selenization technique. Preparation of the precursor as well as selenization were carried out by the vacuum evaporation technique. The sequence of copper and Indium layer deposition during precursor preparation affects the morphological and structural properties of precursor which directly have effects on the properties of selenized CIS films. A thin layer of amorphous selenium at the substrate/film interface has been used to improve the adherence of the film. The effect of the Se under-layer has been studied on the layers of copper, indium, CuIn precursors and CIS films, using structural, morphological and optical properties. The surface morphology of a single layer of copper and indium, with and without the Selenium under-layer, are quite different and drastically affect the properties of the precursor and selenized films. The Se under-layer does not take part in the chemical reaction of CIS formation during the selenization process. The modified CIS films are uniform, single phase, polycrystalline, chalcopyrite with (1 1 2) preferred orientation showing an energy band gap of 0.99 eV and an absorption coefficient of 10⁵ cm⁻¹, and have good adherence to the substrate for the scotch tape test.     

Investigation of n-type Cu2O layers prepared by a low cost chemical method for use in photo-voltaic thin film solar cells

A low cost and simple chemical method of boiling copper plates in CuSO₄ solution is used to prepare Cu₂O layers. X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron spectroscopy (XPS), glow discharge optical emission spectroscopy (GDOES) and optical absorption have been used to characterise these layers. It has been found that the layers consist of Cu₂O phase with a thickness of about 1.4 μm for 60 minutes boiling in CuSO₄ solution. The largest grain sizes are in the order of 1 μm and the layers contain cubic Cu₂O phases. The layers are n-type in electrical conduction and the optical band gap observed is 2.2 eV.     

The formation of CuInSe2 thin film solar cell absorbers from electroplated precursors with varying selenium content

We present results from real-time X-ray diffraction experiments on the formation of CuInSe₂ solar cell absorbers by annealing precursors, produced by simultaneous electrodeposition of copper, indium and selenium. The investigations reveal, that a reduced amount of electrochemically deposited selenium is the decisive parameter in order to realise a chalcopyrite formation behaviour as observed for sputtered stacked elemental layer (SEL) precursors. A simultaneous electrodeposition of the elements copper, indium and selenium in the molar ratio 1:1:2 of the chalcopyrite CuInSe₂ leads to the formation of binary copper and indium selenides during the electrodeposition process. The existence of binary selenides besides the intermetallic phase Cu₁₁In₉ as initial phases leads to an unfavourable absorber morphology. This can be explained by the observed semiconductor formation mechanism. A reduction of the deposited amount of selenium favours the formation of the intermetallic compound Cu₁₁In₉ and reduces the amount of binary selenides. These precursors show a formation behaviour and resulting absorber morphology as known for sputtered SEL precursors.     

Optical properties and characterization of CuInSe2

CuInSe₂ single crystals grown from stoichiometric melts were studied by differential thermal analysis, X-ray diffraction, optical absorption and photoluminescence measurements. The resulting crystals were In-rich and exhibited n-type conductivity. From the optical measurements at low temperatures, a donor level around 12 meV and an acceptor level at about 40 meV, which have been attributed to indium atoms on copper sites and to copper vacancies respectively, appear as the predominant defect pair in our samples. Another defect level at about 70 meV, probably a donor level, has also been observed. Optical absorption measurements at room temperature show the presence of residual absorption, which has been attributed to direct-phononassisted transitions.     

Growth, structural and optical properties of copper indium diselenide thin films deposited by thermal evaporation method

Copper indium diselenide (CuInSe₂) compound was synthesized by reacting its constituent’s elements copper, indium and selenium in near stoichiometric proportions (i.e. 1:1:2 with 5% excess selenium) in an evacuated quartz ampoule. Synthesized pulverized compound material was used as an evaporant material to deposit thin films of CuInSe₂ onto organically cleaned sodalime glass substrates, held at different temperatures (300-573 K), by means of single source thermal evaporation method. The phase structure and the composition of chemical constituents present in the synthesized compound and thin films have been investigated using X-ray diffraction and energy dispersive X-ray analysis, respectively. The investigations show that CuInSe₂ thin films grown above 423 K are single phase, having preferred orientation of grains along the (112) direction, and having near stoichiometric composition of elements. The surface morphology of CuInSe₂ films, deposited at different substrate temperatures, has been studied using the atomic force microscopy to estimate its surface roughness. An analysis of the transmission spectra of CuInSe₂ films, recorded in the wavelength range of 500-1500 nm, revealed that the optical absorption coefficient and the energy band gap for CuInSe₂ films, deposited at different substrate temperatures, are ∼10⁴ cm⁻¹ and 1.01-1.06 eV, respectively. The transmission spectrum was analyzed using iterative method to calculate the refractive index and the extinction coefficient of CuInSe₂ thin film deposited at 523 K. The Hall effect measurements and the temperature dependence of the electrical conductivity of CuInSe₂ thin films, deposited at different substrate temperatures, revealed that the films had electrical resistivity in the range of 0.15-20 ohm cm, and the activation energy 82-42 meV, both being influenced by the substrate temperature.     

Light absorption efficiencies of photoactive multilayered films: a realistic approach for porous materials

In the present work the authors report a methodology to obtain an accurate calculation of the light absorption efficiency of hybrid photoactive layers. The photoactive layer (PAL) is a semiconductor matrix doped with a molecular species. The chosen matrix was TiO₂ prepared either by sputtering or by sol–gel techniques. Zinc meso-tetraphenylporphyrin and Nile red were selected as light absorbing dopants due to their absorption spectra for solar light. The proposed methodology takes into account a detailed optical characterization of each PAL where the morphology and the porosity of the semiconductor matrix are considered. The role of direct excitation of the semiconductor could also be discriminated. Moreover, the method allows an accurate estimate of the effective light absorption efficiency of complex multilayered materials from the optical characterization of each layer separately.     

X-ray absorption spectroscopic study of photovoltaic solar cell materials of the type AIBIIIC2VI with special reference to CuInSe2

The K absorption discontinuity of copper has been recorded for compounds of the type A^IB^IIIC₂^VI using a Cauchois-type bent crystal X-ray spectrograph. The chemical shift in the positions of the discontinuity in the compounds relative to that in pure copper metal is found to be governed by the effective charge on the copper ion. A linear relationship between the chemical shifts and the band gaps has also been observed. This dependence is useful in determining the values of the band gaps in materials for which they are not known.It is shown that an approximate picture of the band structure of CuInSe₂ can be obtained from X-ray absorption near-edge structure studies.     

Electrocatalytic performance of copper selenide as structural phase dependent for hydrogen evolution reaction

A cost-effective, efficient and stable electrocatalyst is a remarkable and significant prospect for hydrogen evolution reaction. By adjusting the copper contents, the Hexagonal and Tetragonal phases of the copper selenide have been synthesized using the solvo-hydrothermal process. Hexagonal phased-based copper selenide has exhibited a greater current density than the Tetragonal phase. However, the augmented structure, lower Gibbs Free energy, greater electrical conductivity and electrochemical surface promoted the electrocatalytic behavior of Hexa-CuSe-1.04 nanosheets. Hexa-CuSe-1.04 nanosheets exhibit a good overpotential of 61 mV at the state of art current density of 10 mAcm⁻² along with Tafel slope of 59 mV dec⁻¹ in 1 M KOH, which is better than the other Hexagonal phase structure, Hexa-CuSe-1.67 nanosheets and Tetra-Cu₂Se-1.30 phase of the copper selenides. The electrocatalyst Hexa-CuSe-1.04 maintains the current density for a long duration and exhibits a similar linear sweep voltammetry curve in 1 M KOH during the chronoamperometry test. Moreover, Hexa-CuSe-1.04 nanosheets show a good Tafel slope of 26 mV dec⁻¹ with a good turnover frequency of 72.40 m s⁻¹ in 0.5 M H₂SO₄ and follow the Tafel phenomenon. Optimized density functional theory reveals that Hexa-CuSe-1.04 exhibits the lower Gibbs of 1.04 eV, free energy, which promotes the desorption and recombination process of the active hydrogen atoms at the active sites and enhanced the hydrogen evolution process. All results indicate the leading potential application of the Hexagonal phase of Hexa-CuSe-1.04 for hydrogen evolution reaction.     

Band gap measurements in thin films of hematite Fe2O3, pyrite FeS2 and troilite FeS prepared by chemical spray pyrolysis

Thin films of Fe₂O₃, FeS₂ and FeS have been prepared by the method of chemical spray pyrolysis. The absorption and transmission spectra are recorded for different thicknesses of these films. The spectra are analyzed and the direct band gaps, the indirect band gaps, both allowed and forbidden, the forbidden direct band gaps and other optical parameters are all investigated.     

Structural phase change and optical band gap bowing in hot wall deposited CdSexTe1−x thin films

CdSe_xTe_1−x thin films of different compositions have been deposited on glass substrates by hot wall deposition method under conditions very close to thermodynamical equilibrium with minimum loss of material. The structural studies carried out on the deposited films revealed that they are crystalline in nature and exhibit either cubic zinc blende or hexagonal phase or both depending on the composition of the material. The lattice parameter values for both cubic and hexagonal phases have been determined and are observed to vary with composition according to Vegard’s law. The optical properties of the deposited CdSe_xTe_1−x thin films have been studied using transmittance spectra. The spectra shows a sharp fall in transmittance at wavelength corresponding to the band gap of the material. The optical band gap has been determined and found to be direct allowed. The band gap has been observed to strongly depend on film composition. The variation of band gap with composition has been observed to be quadratic in nature exhibiting a bowing behaviour.     

Effect of sodium diffusion on the structural and electrical properties of Cu2ZnSnS4 thin films

Cu₂ZnSnS₄ (CZTS) is a kesterite semiconductor consisting of abundantly available elements. It has a band gap of 1.5 eV and a large absorption coefficient. Hence, thin films made of this material can be used as absorber layers of a solar cell. CZTS films were deposited on soda lime and Na free borosilicate glass substrates through Ultrasonic Spray Pyrolysis. The diffusion of sodium from soda lime glass was found to have a profound effect on characteristics like grain size, crystal texture and conductivity of CZTS thin films. Copper ion concentration also varied during the deposition and it was observed that the carrier concentration was enhanced when there was a deficiency of copper in the films. The effect of sodium diffusion and copper deficiency in enhancing the structural and electrical properties of CZTS films are presented in this paper.     

Electronic and optical analysis of high-efficiency photovoltaic materials based on a GaN semiconductor

An ab initio study using the local spin density approximation of the electronic and optical properties of materials where Cr transition metal substitutes for N in the GaN host semiconductor with an atomic concentration of 1.56% is presented. This material, characterized by an isolated and partially filled intermediate band, is a candidate for high-efficiency solar cells. The atomic and orbital composition of this band has been analyzed showing that is mainly made up of a t-group orbital of the transition metal. The absorption coefficient theoretical results show a sub-gap absorption with respect to the host semiconductor which could lead to an increase in solar conversion efficiency.     

Morphology and Photoelectric Characteristics of the Thin-Film Polycrystalline Structure SnO<Subscript>2</Subscript>-CdS/Cu(InGa)Se<Subscript>2</Subscript>-Ag

The surface morphology and photoelectric characteristics of a thin-film SnO₂-CdS/Cu(InGa)Se₂-Ag heterostructure have been studied. The chemical and phase composition of the Cu(InGa)Se₂ film in the synthesized structure have been investigated. The current transfer mechanism has been studied, and the main parameters of the semiconductor material have been determined. It has been found that there are no oppositely connected barriers in the heterostructure.     

Quantum dots as enhancer in photocatalytic hydrogen evolution: A review

Advanced energy conversion processes like photochemical and photoelectrochemical water splitting now a day plays a very important role in challenging the present energy crisis of our world. The successful utilization of this process depends on development of highly efficient, more stable, low cost and outstanding environmental benign semiconductor materials. From recent advancements, it is revealed that quantum dots (QDs) are very outstanding and promising material for the mentioned processes due to their favorable physical and chemical characteristics like high absorption co-efficient, quantum confinement effect, thermal, chemical, mechanical and optical stability, high conductivity and recyclability. In this review article, we have clearly explained the importance of QDs in water splitting along with the general mechanism involved in the process. Following that the enhancement of different materials like metal oxides, layered double hydroxides (LDH), carbonaceous materials (g-C₃N₄, benzene and benzene like materials) by QDs have discussed in the field of water splitting.     

Real-time investigations on the formation of CuInSe2 thin film solar cell absorbers from electrodeposited precursors

In this article, we present results of a detailed real-time X-ray diffraction (XRD) study on the formation of CuInSe₂ from electroplated precursors. The solid-state reactions observed during the selenisation of three different types of precursors are presented. The first type of precursors (I) consists of the nanocrystalline phases Cu_2−xSe and InSe at room temperature, which react to CuInSe₂ starting at 470 K. The second type of precursor (II) shows an inhibited CuInSe₂ formation out of the initial phases Cu_2−xSe and γ-In₂Se₃ starting at 400 K. The third precursor type (III) shows completely different selenisation behaviour. Starting from the intermetallic compound Cu₁₁In₉ and amorphous selenium, the formation of the binary selenides In₄Se₃ and CuSe is observed after the melting point of selenium at 494 K. After selenium transfer reactions, the compound semiconductor CuInSe₂ is formed out of Cu_2−xSe and InSe. This type (III) reaction path is well known for the selenisation of SEL precursors (stacked elemental layers of sputtered copper and indium and thermally evaporated selenium).     

Effect of bath temperature and annealing on the formation of CuInSe2

CuInSe₂ films of 2 μm thickness were electrodeposited potentiostatically, from aqueous solution containing thiocyanate as a complexing agent, on Mo substrates. For all the experiments, the potential of the potentiostatic deposition of the materials was chosen to be −1 V, whereas the bath temperature of electrolyte was varied from 20 to 80 °C. It was found that the electrodeposited CuInSe₂ was characterized by an amorphous layer and densely-packed nanometric grains with a good homogeneity. After vacuum annealing at 200 °C, glancing angle X-ray diffraction revealed the presence of the CuInSe₂ phase whereas annealing under selenium atmosphere lead to the growth of molybdenum selenide compound MoSe₂, in addition to a better crystallization of the copper indium diselenide compound. Scanning electron microscopic revealed that despite an increase in the grains dimensions, there was no significant change in the films surface morphology when the bath temperature was varied from 20 to 80 °C. At the same time, the composition of the electrodeposited Cu-In-Se layers becomes richer in copper. This increase in copper concentration is mainly compensated by a deficit in selenium atoms.     

Optical,structural and phase transition properties of Cu2O,CuO and Cu2O/CuO: Their photoelectrochemical sensor applications

Cu₂O and CuO provide a unique possibility to tune the band gap into the middle of the efficiency maximum for photoelectrochemical (PEC) and solar cell applications. Photoactive materials containing Cu₂O, CuO and Cu₂O/CuO have been prepared with high quality and stability in various compositions by an economic, simple and reliable electrodeposition (ED) method. These materials based on copper oxide have been characterized and compared using XRD, SEM, EDX, UV–Vis, PL, FTIR, Raman spectroscopy and electrochemical techniques. Based on the electrochemical production conditions; phase changes of photoactive materials and, at which conditions which phase or phases are present, were evaluated in detail. It was carried out that a full phase change from single-phase Cu₂O to single-phase CuO. The crystal dimensions expand as the cube-shaped Cu₂O transforms into CuO, crystal surface areas increase, crystal shapes change and turn gradually into flower-shaped crystals. Here, the band gap of copper oxide material can be altered within a broad scale by adjusting the element ratios. The semiconductors have been found to have direct band gap that is more preferred for solar energy applications. PEC performances of the copper oxide electrodes containing a different phase structure were determined, and the changes of PEC activities were examined comparatively. Copper oxide semiconductors have p-type conductivity and they act as photocathodes.