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.     

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).     

CuIn1−xGaxSe2-based photovoltaic cells from electrodeposited and chemical bath deposited precursors

We have fabricated 13.7%- and 7.3%-efficient CuIn_1−xGa_xSe₂ (CIGS)-based devices from electrodeposited and chemical bath deposited precursors. As-deposited precursors are Cu-rich films and polycrystalline (grain size is very small) in nature. Only preliminary data is presented on chemical bath deposited precursors. Additional In, Ga, and Se were added to the precursor films by physical evaporation to adjust the final composition to CuIn_1−xGa_xSe₂. Addition of In and Ga and also selenization at high temperature are very crucial to obtain high efficiency devices. Three devices with Ga/(In+Ga) ratios of 0.16, 0.26, and 0.39 were fabricated from electrodeposited precursors. The device fabricated from the chemical bath deposited precursor had a Ga/(In+Ga) ratio of 0.19. The films/devices have been characterized by inductive-coupled plasma spectrometry, Auger electron spectroscopy, X-ray diffraction, electron-probe microanalysis, current-voltage characteristics, capacitance–voltage, and spectral response. The compositional uniformity of the electrodeposited precursor films both in the vertical and horizontal directions were studied. The electrodeposited device parameters are compared with those of a 17.7% physical vapor deposited device.     

Cu(In,Ga)Se2 based photovoltaic structure by electrodeposition and processing

CuIn_1−xGa_xSe₂ (CIGS) thin films were formed from an electrodeposited CuInSe₂ (CIS) precursor by thermal processing in vacuum in which the film stoichiometry was adjusted by adding In, Ga and Se. The structure, composition, morphology and opto-electronic properties of the as-deposited and selenized CIS precursors were characterized by various techniques. A 9.8% CIGS based thin film solar cell was developed using the electrodeposited and processed film. The cell structure consisted of Mo/CIGS/CdS/ZnO/MgF₂. The cell parameters such as J_sc, V_oc, FF and η were determined from I–V characterization of the cell.     

Study of Cd-free buffer layers using Inx(OH,S)y on CIGS solar cells

The alternative buffer layer material In_x(OH,S)_y was deposited on Cu(In,Ga)Se₂ (CIGS) thin films by chemical-bath-deposition (CBD). The impurities in In_x(OH,S)_y buffer layers and their atomic concentration were characterized by X-ray photoelectron spectroscopy (XPS) and Auger electron spectroscopy (AES) analyses. In addition, AES was used to depth profile the samples. The band-gap energy of the deposited In_x(OH,S)_y was determined from optical absorption data. Both the dark- and photo-current-voltage (I–V) characteristics of the CIGS solar cells with In_x(OH,S)_y buffer layers were measured, and the results were compared to the CIGS cells deposited with CBD CdS buffer layers.     

Formation of CuInSe2 and Cu(In,Ga)Se2 films by electrodeposition and vacuum annealing treatment

Polycrystalline thin films of CuInSe₂ and Cu(In,Ga)Se₂ (CIGS) were grown on both polished Mo substrates and Mo-coated glass substrates by one-step electrodeposition. All the as-deposited films have been annealed in vacuum at 450°C for a short time to improve the crystalline properties. The films have been characterized by X-ray diffraction, scanning electron microscopy and energy dispersive X-ray analysis. The results indicate that the crystallization of the films was greatly improved after annealing. Further more, a CIGS film with 23 at% Ga was obtained.     

CIS and CIGS based photovoltaic structures developed from electrodeposited precursors

In the present study we report the electrodeposition and characterization of CIS and CIGS thin films and a post-deposition thermal processing in vacuum to improve the film stoichiometry by incorporating additional In, Ga and Se. Different kinds of analyses showed that CIS as well as CIGS possess a very thin In-rich surface n-layer. The formation and characterization of solar cell structures from the electrodeposited precursor with the configuration glass/Cr/Mo/CIS(CIGS)/CdS/ZnO/MgF₂ is also reported. The optoelectronic properties such as Voc, Isc, FF, η etc. of the cells are presented.     

Annealing effects on Zn1−xMgxO/CIGS interfaces characterized by ultraviolet light excited time-resolved photoluminescence

Annealed Zn_1−xMg_xO/Cu(In,Ga)Se₂ (CIGS) interfaces have been characterized by ultraviolet light excited time-resolved photoluminescence (TRPL). The TRPL lifetime of the Zn_1−xMg_xO/CIGS film increased on increasing the annealing temperature to 250 °C, whereas the TRPL lifetime of the CdS/CIGS film had little change by annealing at temperatures lower than 200 °C. This is attributed to the recovery of physical damages by annealing, induced by sputtering of the Zn_1−xMg_xO film. The TRPL lifetime abruptly decreased with annealing at 300 °C. The diffusion of excess Zn from the Zn_1−xMg_xO film into the CIGS interface is clearly observed in secondary ion mass spectroscopy (SIMS) depth profiles. These results indicate that excess Zn at the vicinity of the CIGS surface acts as non-radiative centers at the interface. The TRPL lifetime of the Zn_1−xMg_xO/CIGS film annealed at 250 °C reached values to be comparable to that of the as-deposited CdS/CIGS film. Performance of the Zn_1−xMg_xO/CIGS cells varied with the annealing temperature in the same manner as the TRPL lifetime. The highest efficiency of the Zn_1−xMg_xO/CIGS solar cells was achieved for annealing at 250 °C. The results of the TRPL lifetime on annealing show that the cell efficiency is strongly influenced by the Zn_1−xMg_xO/CIGS interface states related to the damages and diffusion of Zn.     

CuIn1−xGaxSe2-based photovoltaic cells from electrodeposited precursor films

We have developed an electrodeposition bath based on a buffer solution so that the stability of the electrodeposition process is enhanced and no metal oxides or hydroxides precipitate out of solution. The buffer-solution-based bath also deposits more gallium in the precursor films. As-deposited precursors are stoichiometric or slightly Cu-rich CuIn_1−xGa_xSe₂. Only a minimal amount of indium was added to the electrodeposited precursor films by physical vapor deposition to obtain a 9.4%-efficient device.     

Characterization and electrochromic properties of CuxNi1−xO films prepared by sol–gel dip-coating

Cu_xNi_1−xO electrochromic thin films were prepared by sol–gel dip coating and characterized by XRD, UV–vis absorption and electrochromic test. XRD results show that the structure of the Cu_x Ni_1−xO thin films is still in cubic NiO structure. UV–vis absorption spectra show that the absorption edges of the Cu_xNi_1−xO films can be tuned from 335 nm (x = 0) to 550 nm (x = 0.3), and the transmittance of the colored films decrease as the content of Cu increases. Cu_xNi_1−xO films show good electrochromic behavior, both the coloring and bleaching time for a Cu_0.2Ni_0.8O film were less than 1 s, with a variation of transmittance up to 75% at the wavelength of 632.8 nm.     

Effect of Ga incorporation in sequentially prepared CuInS2 thin film absorbers

Thin film CuInS₂:Ga solar cell absorber films were prepared by sequential evaporation of Cu–In–Ga precursors and sulfurization in sulfur vapor. The depth distribution of Ga was found to be highly inhomogeneous caused by CuGaS₂ phase segregation at the back contact. Depending on overall Ga content and sulfurization temperature a quaternary CuGa_xIn_1−xS₂ compound formed exhibiting a shift in absorber lattice constant and band gap. Micro Raman measurements showed that crystal quality was also affected by Ga. Open-circuit voltages well above 800 mV were achieved while sustaining high fill factors of 71%.     

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.     

Photoconductivity of Cu(In, Ga) Se2 films

Polycrystalline CuIn_{1 − x}Ga_xSe₂ (0 ≤ x < 0.3) films (CIGS) were deposited by coevaporating the elements from appropriate sources onto glass substrates (substrate temperature 720 to 820 K). Photoconductivity of the polycrystalline CIGS films with partially depleted grains were studied in the temperature range 130–285 K at various illumination levels (0–100 mW/cm²). The data at low temperature (T < 170 K) were analyzed by the grain boundary trapping model with monovalent trapping states. The grain boundary barrier height in the dark and under illumination were obtained for different x-values of CuIn_1−xGa_xSe₂ films. Addition of Ga in the polycrystalline films resulted in a significant decrease in the barrier height. Variation of the barrier height with incident intensity indicated a complex recombination mechanism to be effective in the CIGS films.     

Preparation and characterization of semiconducting Zn1−xCdxSe thin films

The electrodeposition of Zn_1−xCd_xSe polycrystalline semiconducting thin films from aqueous acidic bath without any additives onto tin oxide-coated conducting glass and titanium substrates are described. The influence of deposition parameters on the film formation and deposition mechanism based on cyclic voltammetry is discussed. X-ray diffraction studies showed the polycrystalline wurtzite nature for all the films deposited under the proposed conditions. The optical studies revealed the band gap values in the range between 2.82 and 1.72 eV as the film composition changes from ZnSe to CdSe. It has been observed that the concentration of cadmium salt plays an essential role on the alloy formation. The surface morphological studies and composition analysis were carried out and the results are discussed.     

Control of conduction band offset in wide-gap Cu(In,Ga)Se2 solar cells

The effects of conduction band offset of window/Cu(In,Ga)Se₂ (CIGS) layers in wide-gap CIGS based solar cells are investigated. In order to control the conduction band offset, a Zn_1−xMg_xO film was utilized as the window layer. We fabricated CIGS solar cells consisting of an ITO/Zn_1−xMg_xO/CdS/CIGS/Mo/glass structure with various CIGS band gaps (E_g≈0.97–1.43 eV). The solar cells with CIGS band gaps wider than 1.15 eV showed higher open circuit voltages and fill factors than those of conventional ZnO/CdS/CIGS solar cells. The improvement is attributed to the reduction of the CdS/CIGS interface recombination, and it is also supported by the theoretical analysis using device simulation.     

CuIn(Ga)Se2-based devices via a novel absorber formation process

A novel pathway for the formation of copper–indium (gallium) diselenide has been developed. This two-stage process consists of (a) the formation of Cu–In–(Ga)–Se precursors, and (b) subsequent thermal treatment to form CuIn(Ga)Se₂. The morphology, structure and growth mechanism for several different precursor structures prepared under various conditions were studied and correlated to the deposition parameters as well as the structure and morphology of the annealed films. Photovoltaic devices prepared from CuInSe₂ and CuIn_0.75Ga_0.25Se₂ resulted in efficiencies of 10% and 13%, respectively.     

Investigation of pulsed non-melt laser annealing on the film properties and performance of Cu(In,Ga)Se2 solar cells

Pulsed non-melt laser annealing (NLA) has been used for the first time to modify near-surface defects and related junction properties in Cu(In,Ga)Se₂ (CIGS) solar cells. CIGS films deposited on Mo/glass substrates were annealed using a 25 ns pulsed 248 nm laser beam at selected laser energy density in the range 20–60 mJ/cm² and pulse number in the range 5–20 pulses. XRD peak narrowing and SEM surface feature size increase suggest near-surface structure changes. Dual-beam optical modulation (DBOM) and Hall-effect measurements indicate NLA treatment increases the effective carrier lifetime and mobility along with the sheet resistance. In addition, several annealed CdS/CIGS films processed by NLA were fabricated into solar cells and characterized by photo- and dark-J–V and quantum efficiency (QE) measurements. The results show significant improvement in the overall cell performance when compared to unannealed cells. The results suggest that an optimal NLA energy density and pulse number for a 25 ns pulse width are approximately 30 mJ/cm² and 5 pulses, respectively. The NLA results reveal that overall cell efficiency of a cell processed from an unannealed film increased from 7.69% to 13.41% and 12.22% after annealing 2 different samples at the best condition prior to device processing.     

Structural changes of CIGS during deposition investigated by spectroscopic light scattering: A study on Ga concentration and Se pressure

We have studied the three-stage deposition process of CuIn_1−xGa_xSe₂ (CIGS) thin films using spectroscopic light scattering (SLS), under varied deposition conditions. The structural changes of CIGS films by (1) Ga composition, (2) Se supply and (3) low deposition temperature, were observed in situ by SLS. The largest changes in SLS profiles by the Ga composition was observed between x=0.3 and 0.5. The SLS profiles changed significantly during stage 1 by varying the Se pressure, while the temperature profiles did not.     

The performance of CuIn1−xGaxSe2-based photovoltaic cells prepared from low-cost precursor films

In this paper we report the 15.4%- and 13.4%-efficient CuIn_1−xGa_xSe₂ (CIGS)-based devices from electrodeposited (ED) and electroless deposited (EL) precursors. The efficiency of the device prepared from electroless precursor film has been improved from 12.4% to 13.4%. The dependence of quantum efficiencies on reverse-bias voltage has been measured for a 15.4%-efficient ED device, 18.8%-efficient physical-vapor-deposited device, and 14.2%-efficient Cd-free device. The purpose of this work is to explore and improve the current collection mechanism.     

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).