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).     

Microstructure evolution of CuInSe2 thin films prepared by single-bath electrodeposition

The composition and the microstructure evolutions of CuInSe₂ thin films under single-bath electrodeposition processes were investigated. It was found that the film composition was mainly determined by the [Se⁴⁺]/[Cu²⁺] ratios in solution, but the film microstructure is strongly dependent on the initial concentrations of Se⁴⁺, Cu²⁺, and In³⁺ precursors. Higher initial concentrations of Cu²⁺ and In³⁺ in solution are beneficial for the fabrication of compact CuInSe₂ thin films with highly crystallized and large grain sized chalcopyrite phase. The microstructure evolution suggests that prior adsorption and reduction of Cu²⁺ ions and the formation of Cu₂Se compound on the substrate can promote the nucleation, growth, and coarsening of CuInSe₂ crystal to form a high quality thin film during the electrodeposition processes.     

Electrodeposition of CuInSe2 from ethylene glycol at 150 °C

Polycrystalline chalcopyrite thin films were potentiostatically electrodeposited from ethylene glycol solution onto SnO₂-coated glass substrates at 150 °C. The thickness of the layers was estimated using talysurf at 1.0 μm after deposition for 60 min. X-ray diffraction (XRD), X-ray fluorescence (XRF), scanning electron microscopy (SEM) and energy dispersive X-ray (EDX) analyses were used to identify and characterise compounds formed at different potentials. It was found that Cu_1.75Se formation was dominant at −0.80 V vs Se and indium assimilation increased at more negative voltages forming a mixture of compounds including numerous Cu-Se binary phases and copper indium diselenide (CuInSe₂) at the cathode. As-deposited materials showed poor crystallinity and therefore films were annealed in Ar/5%H₂ in the presence of Se to improve the material quality for all investigations. Although the films were deposited at 150 °C, no noticeable improvement of the CuInSe₂ was observed, suggesting growth from aqueous media at room temperature to be preferable.     

Progress in electrodeposited absorber layer for CuIn(1−x)GaxSe2 (CIGS) solar cells

Thin film solar cells with chalcopyrite CuInSe₂/Cu(InGa)Se₂ (CIS/CIGS) absorber layers have attracted significant research interest as an important light-to-electricity converter with widespread commercialization prospects. When compared to the ternary CIS, the quaternary CIGS has more desirable optical band gap and has been found to be the most efficient among all the CIS-based derivatives. Amid various fabrication methods available for the absorber layer, electrodeposition may be the most effective alternative to the expensive vacuum based techniques. This paper reviewed the developments in the area of electrodeposition for the fabrication of the CIGS absorber layer. The difficulties in incorporating the optimum amount of Ga in the film and the likely mechanism behind the deposition were highlighted. The role of deposition parameters was discussed along with the phase and microstructure variation of an as-electrodeposited CIGS layer from a typical acid bath. Related novel strategies such as individual In, Ga and their binary alloy deposition for applications in CIGS solar cells were briefed.     

Thermal diffusion of Cu into In2Se3: A better approach to SEL technique in the deposition of CuInSe2 thin films

This paper reports the modifications made in the preparation techniques of getting CuInSe₂ thin films starting with chemical bath deposited (CBD) selenium films. In the present study, CBD Se film was converted into CuInSe₂ by stacked elemental layer (SEL) technique and also by thermal diffusion of Cu into In₂Se₃. In both the cases CBD Se films were used to avoid toxic Se vapor and H₂Se gas. Improvements were made in these techniques through a detailed study, varying the composition of the films over a wide range by changing the Cu/In ratio. Structural, optical and electrical characterizations were performed. On comparing the material properties of CuInSe₂ deposited by these two techniques, it was found that photosensitivity was better for samples prepared by thermal diffusion of Cu into In₂Se₃. So the technique of thermal diffusion of Cu into In₂Se₃ was found to be better than SEL technique in the preparation of CuInSe₂ using CBD Se. Cu-rich, In-rich and nearly stoichiometric samples could be prepared by thermal diffusion of Cu into In₂Se₃. These samples were analyzed using energy dispersive spectroscopy, Raman spectroscopy and atomic force microscopy also.     

Sulfurization of Cu–In electrodeposited precursors for CuInS2-based solar cells

Cu–In electrodeposited layers were annealed using rapid thermal processing (RTP) in a reactive atmosphere containing sulfur vapors. The CuInS₂ formation mechanism during sulfurization of electrodeposited precursors proceeds mainly through direct sulfurization of the metallic Cu–In alloy, forming spinel CuIn₅S₈ and chalcopyrite CuInS₂ ternary phases. During the heating step, the Cu–In metallic alloy gets richer in copper as the temperature increases and transforms from CuIn₂ to Cu₁₁In₉, then Cu₁₆In₉ and finally to Cu₇In₃. The use of rapidly cooled samples stopped after different durations of the process along with ex-situ XRD analysis enabled us to differentiate the Cu₁₆In₉ and Cu₇In₃ phases. Finally, the efficiency of the solar cells made with the two-step electrodeposition and RTP low-cost process reaches 11% (active area 0.421 cm²), which is close to the results obtained for cells made with PVD precursors.     

Comparison of point defects in CuInSe2 and CuGaSe2 single crystals

Hall-effect and photoluminescence measurements have been carried out on as-grown and In/Ga-annealed CuInSe₂ and CuGaSe₂ single crystals grown by chemical vapor transport. Various defect levels in these related compounds have been identified and compared. V_Cu and V_Se show similar properties and activation energies in both materials. A tremendous difference is observed in the behavior of III_Cu antisite defects. Ga_Cu levels in CuGaSe₂ are much deeper than In_Cu in CuInSe₂, and furthermore, the formation of In_Cu is much easier compared to Ga_Cu. This is related to the higher formation energy of Ga_Cu in CuGaSe₂. Due to this difference in the defect chemistry of both compounds, it has not been possible until now, to prepare n-type CuGaSe₂ crystals, whereas CuInSe₂ is easily transformed from p- to n-type by annealing in vacuum or In-atmosphere.     

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.     

An investigation into effect of cationic precursor solutions on formation of CuInSe2 thin films by SILAR method

SILAR deposition of CuInSe₂ films was performed by using Cu²⁺–TEAH₃ (cupric chloride and triethanolamine) and In³⁺–CitNa (indium chloride and sodium citrate) chelating solutions with weak basic pH as well as Na₂SeSO₃ solution at 70 °C. A separate mode and a mixed one of cationic precursor solutions were adopted to investigate effects of the immersion programs on crystallization, composition and morphology of the deposited CuInSe₂ films. Chelating chemistry in two solution modes was deducted based on IR measurement. The XRD, XPS and SEM results showed that well-crystallized, smoothly and distinctly particular CuInSe₂ films could be obtained after annealing in Ar at 400 °C for 1 h by using the mixed cationic solution mode.     

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.     

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.     

Complexing agent effect on the stoichiometric ratio of the electrochemically prepared CuInSe2 thin films

The electrodeposition of CuInSe₂ is investigated to improve the stoichiometric properties of CuInSe₂ layers on indium tin oxide (ITO)-coated glass substrates and to develop one-step electrodeposition method for solar cell applications. XPS was utilized for the characterization of the surface properties of CuInSe₂ layers. The influence of the complexing agent, e.g. benzotriazole, bulk concentration of Cu and Se and deposition potentials on the stoichiometric properties, are discussed.     

Cu/In deposited at room temperature: morphology,phases and reactions

Bilayers of a slightly copper-rich composition of copper and indium deposited (in that order) at room temperature by vacuum evaporation, electroplating, and r.f. sputtering, were analyzed by MeV ⁴He backscattering spectrometry, scanning electron microscopy and X-ray diffraction before and after thermal annealing at 400 °C in vacuum. The surface morphology of as-deposited samples is roughest for evaporated samples and smoothest for samples deposited by sputtering with low r.f. power. All as-deposited samples contain copper and indium phases and the metastable CuIn compound. After 1 h at 400 °C, this metastable phase and that of copper disappear and are replaced by some copper-rich compound (Cu₉In₄, Cu₇In₄) in all cases. Simultaneously, the surface morphology smooths out considerably for the initially rough samples. Bilayers of various Cu/In compositions were also prepared. The results are consistent with the absence of stable compounds outside the 26–38 at.% indium range, as indicated by published phase diagrams.     

Electrodeposition of CuIn alloys for preparing CuInS2 thin films

Copper-indium alloys were prepared by electroplating from citric acid (C₆H₈O₇·H₂O) baths onto Ti substrate. Formation of the alloys was carried out by direct codeposition of the elements and by sequential electrodeposition of copper and indium. Studies of the alloy formation by electrochemical measurements and X-ray diffraction were performed. The presence of Cu₇In₄ in direct deposit as well as in sequentially electrodeposited material was observed during the alloy formation. The as-deposited layers were heated in H₂S. X-ray diffraction showed the annealed layers to be CuInS₂ with the chalcopyrite structure, where the CuIn₅S₈ phase was included during the annealing process.Photoelectrochemical characterization of the samples allowed us to determine the photoconductivity which is related with the Cu/In ratio in the samples. The energy gap for CuInS₂ photoelectrodes in polysulphide solution was 1.57 eV.     

The phase behavior of evaporated copper and indium precursors for selenization

In this paper we discuss the phase behavior and microstructure of sequentially evaporated indium-on-copper layer stacks deposited onto molybdenum-coated glass substrates. It was determined that both equilibrium and non-equilibrium phases can exist depending on processing. Indium evaporated onto copper, without intentional substrate heating and no subsequent annealing, resulted in the formation of the CuIn (JCPDS card 35-1100) alloy phase and an unreported f.c.c. phase of undetermined composition. Auger spectrometry and X-ray diffraction strongly suggested this to be a new CuIn alloy. Indium evaporated onto copper with substrate heating at 200 °C followed by uncontrolled cooling to room temperature strongly favored the formation of Cu₁₁In₉ in addition to the two phases previously mentioned. The lack of elemental indium in both cases, the presence of which is expected from phase diagram analysis, was attributed to kinetic limitations. Annealing of Cu/In layer stacks for 1 h at 200 °C immediately following indium deposition promoted the formation of elemental indium and Cu₁₁In₉ while simultaneously reducing the concentration of both CuIn and the new f.c.c. alloy phase. The implications of these observations are discussed with regard to future selenization experiments.     

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.     

Characterization of co-sputtered Cu-In alloy precursors for CuInSe2 thin films fabrication by close-spaced selenization

Sputtering technique for Cu–In precursor films fabrication using different Cu and In layer sequences have been widely investigated for CuInSe₂ production. But the CuInSe₂ films fabricated from these precursors using H₂Se or Se vapour selenization mostly exhibited poor microstructural properties. The co-sputtering technique for producing Cu–In alloy films and selenization within a close-spaced graphite box resulting in quality CuInSe₂ films was developed. All films were analysed using SEM, EDX, XRD and four-point probe measurements. Alloy films with a broad range of compositions were fabricated and XRD showed mainly In, CuIn₂ and Cu₁₁In₉ phases which were found to vary in intensities as the composition changes. Different morphological properties were displayed as the alloy composition changes. The selenized CuInSe₂ films exhibited different microstructural properties. Very In-rich films yielded the ODC compound with small crystal sizes whilst slightly In-rich or Cu-rich alloys yielded single phase CuInSe₂ films with dense crystals and sizes of about 5 μm. Film resistivities varied from 10⁻²–10⁸ Ω cm. The films had compositions with Cu/In of 0.40–2.3 and Se/(Cu+In) of 0.74–1.35. All CuInSe₂ films with the exception of very Cu-rich ones contained high amount of Se (>50%).     

Structure, morphology and photoelectrochemical activity of CuInSe2 thin films as determined by the characteristics of evaporated metallic precursors

CuInSe₂ thin films have been obtained by the sequential evaporation of Cu and In layers, and subsequent reaction at 400°C with elemental selenium vapor. The individual metallic film thickness and the substrate temperature during evaporation have been varied in order to promote intermixing and alloy formation before the selenization. The structure, morphology and photoelectrochemical activity of the CuInSe₂ films have been determined by the characteristics of the evaporated metallic precursors. An improvement in the CuInSe₂ quantum efficiency, related mainly to the increased homogeneity and smoothing of the sample surface, can be gained by using as precursors multiple stacked Cu–In bilayers evaporated onto unheated substrates.     

Polycrystalline n-CuInSe2 photoelectrochemical solar cells — Electrochemistry and deposition of copper and indium at the photoanode and the accompanying efficiency enhancement

The electrochemical behaviour of polycrystalline n-CuInSe₂ (CIDS) with respect to removal of oxygen, anodic prepolarization, and deposition and stripping of copper and indium in aqueous KC1 solution has been examined. Oxygen contributed to cathodic currents, while anodic prepolarization caused the appearance of a Cu²⁺/Cu couple. The reduction of the copper ion (Cu²⁺) to metallic copper at CIDS was found to be quasi-reversible and diffusion-controlled. Initially, deposited copper facilitated indium deposition at the CIDS electrode. Aged CIDS as well as CIDS deposited with copper underwent stripping (oxidation) with low currents at more negative potentials which could be prevented by codeposition of copper and indium. Further, deposition of copper, or copper and indium on CIDS was observed to improve the charge transfer at the CIDS/ electrolyte interface. These results were applied for the fabrication of CIDS based photoelectrochemical solar cells employing polysulphide redox electrolyte. It was found that codeposition of copper and indium on CIDS electrode led to significant enhancement in the cell efficiency (rises to 11.5% from 3.4%) and slowed down the short-circuit current decay. A part (70–80%) of the decayed short-circuit current could be regained by renewing the CIDS electrode with codeposition treatment for the second time (redeposition). The possibility of applying deposition-redeposition treatment to regulate the output current was suggested. Previously unexplained cooperative effects of Cu⁺ and In³⁺ in polysulphide electrolyte on the cell efficiency were traced to the facilitated deposition of indium on the copper deposited CIDS electrode from the appropriate salt solutions.     

Scanning tunneling microscopy of electrodeposited CuInSe2 nanoscale multilayers

We have investigated the electrochemical deposition of modulated thin films based on the Cu_xIn_2−xSe₂ system. CuInSe₂ is a leading alternative to silicon for use in thin film photovoltaic solar cells due to its optical absorption and electrical characteristics. Alternating layers of two different compositions based on the Cu_xIn_2−xSe₂ system were potentiostatically deposited. These nanometer-scale layers are used to form reduced-dimensionality structures such as superlattices that can be used in concentrator solar cells. We have used X-ray diffraction, energy-dispersive spectroscopy, and scanning tunneling microscopy to characterize our asdeposited thin films. The ability of the scanning tunneling microscope to resolve the individual nanoscale layers of our multilayered thin films is shown and is used to determine modulation wavelengths.