CuIn1-xGaxSe2 Thin Films Research Articles

Toward low-cost large-area CIGS thin film III: Effect of Se concentration on crystal growth and defect formation of sequentially electrodeposited CIGS thin films

Nowadays, large area CuInxGa1−xSe2 (CIGS) thin film solar cells still face difficulties and challenges of the uniformity of composition and structure. This study illustrates how Se concentration affects the crystal growth and the defect formation of CIGS from low cost electrodeposited stacked Cu/In/Cu/Ga/Cu layers selenized in the rapid thermal process. Various crystal growth modes were observed in the film selenized with different Se concentrations. It was found that the crystal growth was strongly dependent on the Se concentration in the early stage of selenization between 250 and 350°C, which can lead to variations of film composition and structure including a phase separation of CuGaSe2 from CuInSe2 and formation of defect holes in a CIGS film at a higher selenization temperature.

Electrophoretic Deposition of CuIn1–xGaxSe2 Thin Films Using Solvothermal Synthesized Nanoparticles for Solar Cell Application

CuIn1–xGaxSe2 (CIGS) thin films are successfully prepared by convenient electrophoretic deposition, using colloidal nanoparticles. This is the first report which focuses on the electrophoretic deposition (EPD) of presynthesized CIGS nanoparticles directly from their colloid for solar cell application. In this research, CIGS nanoparticles are first synthesized via solvothermal process and then dispersed in a media containing a mixture of ethanol as the solvent and triethylenetetramine as the additive to be used for the film deposition via the electrophoretic method. By simple adjustment of the electrophoretic parameters, including applied voltage, pH, deposition time, and composition of nanoparticles, CIGS thin films with controlled thickness and optoelectronic properties can be fabricated. The highest photovoltaic efficiency of 5.57% is obtained in the CuIn0.75Ga0.25Se2 sample. It is believed that this fabrication approach may open up a new gate to reduce the production cost of a highly demanding CIGS abs...

Electrodeposition of CuInxGa1−xSe2 thin films via pulse technique from Ionic liquid containing n-propyl alcohol

Stoichiometric Cu1.00In0.51Ga0.1Se2.04 (CIGS) thin films containing Cu(II), In(III), Ga(III) and Se(IV) are obtained for the first time in ionic liquid/n-propyl alcohol mixtures with controlled pulse electrodeposition parameters at 323K. The effects of deposition current density, deposition time, frequency and duty cycle are studied in detail. The band gap of obtained CuInxGa1−xSe2 (CIGS) thin film are approximately 1.35eV, as measured using UV–visible absorption spectra. The interplanar spacing and average Hall coefficient of the CIGS thin films are approximately 3.315Å and 1.011×10−1cm3/C, respectively.

Deposition and characterization of graded Cu(In1-xGax)Se2 thin films by spray pyrolysis

Cu(In1-xGax)Se2 (CIGS) thin films and their graded (x = 1 to 0) layer were grown on soda lime glass substrates using chemical spray pyrolysis (CSP) at different substrate temperatures (Ts). After optimization of Ts, depositions were carried out at different gallium composition (x) at optimized temperature of 350 °C. All the films deposited at Ts ≥ 350 °C were polycrystalline chalcopyrite structure, with a preferential orientation of (112), including the graded layer. With increase in x, lattice parameters a and c were observed to decrease. Line scan of the CIGS layer showed intersection of gallium and indium concentrations, revealing the graded nature of the film. Composition dependence of Raman peak for CuInSe2 (CIS) deposited by CSP was analyzed. Optical transmittance at a wavelength of 800 nm of the film with x = 0 (CIS) (30%) was found lower than that of the film grown with x = 0.82 (CIGS) (50%). Cusp-shape of the resistivity was observed with an increase of x leading to steep rise in resistivity of the films (1.61–71.68 Ω-cm) till x = 0.42 and then decreased to 4.78 Ω-cm at x = 0.82. Carrier concentrations of the films were evaluated in the order of 1016–1019 cm−3 with p-type conductivity. These results indicate that graded CIGS thin films with modulated gallium composition can be prepared by CSP.

Individual identification of free hole and electron dynamics in CuIn1−xGaxSe2 thin films by simultaneous monitoring of two optical transitions

The photocarrier dynamics of CuIn1−xGaxSe2 (CIGS) thin films were studied using white-light transient absorption (TA) measurements, as an understanding of this behavior is essential for improving the performance of solar cells composed of CIGS thin films. A characteristic double-peak structure due to the splitting of the valance bands in the CIGS was observed in the TA spectra under near-band-gap resonant excitation. From a comparison of the TA decay dynamics monitored at these two peaks, it was found that the slow-decay components of the electron and hole relaxation are on the nanosecond timescale. This finding is clear evidence of the long lifetimes of free photocarriers in polycrystalline CIGS thin films.

Fabrication of Se-rich Cu(In1-XGaX)Se2 quaternary ceramic target

High density Cu(In1-XGaX)Se2(CIGS) quaternary ceramic targets were prepared by hot-pressing sintering from CIGS and Se mixed powders. Influences of Se contents and sintering temperatures on the densities, compositions, structures and morphologies of the CIGS targets were investigated. The results show that the structure of the targets after sintering was single chalcopyrite, and the concentration of Se was higher than 50 at%. CIGS thin films were fabricated by magnetron sputtering followed by an annealing process in N2 instead of toxic H2Se. CIGS films with a Se-rich concentration and appropriate electrical properties for fabricating CIGS solar cells were achieved after annealing.

Macroscopic and microscopic electrical properties of Cu(In,Ga)Se2 thin-film solar cells with various Ga/(In + Ga) contents

CuIn1−xGaxSe2 (CIGS) thin-films were deposited by a three-stage co-evaporation process. We obtained an optimum value for the Ga/(In + Ga) ratio of CIGS solar cells of 0.29, which exhibits a band-gap of 1.14 eV and has the highest conversion efficiency. The Ga/(In + Ga) ratio in CIGS solar cells is one of main characteristics that can improve efficiency, but the optimum value is still uncertain. In this study, we investigated the local electrical properties, which are closely related to the device properties, of CIGS according to the Ga/(In + Ga) ratio. We measured the local current of the films using conductive atomic force microscopy. The local current indicates relatively small values for the current ratio and the average current on the film surface, which has a high shunt resistance and a low series resistance in high-efficiency CIGS thin-films. However, low efficiency CIGS exhibits the opposite electrical behavior. Thus, the macroscopic and microscopic electrical behaviors are closely correlated with the conversion efficiency and with the device factors of CIGS thin-film solar cells with a varying Ga/(In + Ga) ratio. These results suggest that the control of carrier transport over the grains will improve the conversion efficiency of CIGS thin-film solar cells.

The influence of cracked selenium flux on CIGS thin film growth and device performance prepared by two-step selenization processes

Thermal-cracking system has been adopted to produce cracked selenium and the influence of cracked selenium flux on the structure and reaction pathway during the first-step selenization is investigated. High Cracked-Selenium (HC-Se) may facilitate the Cu–Se and In–Se reaction at lower temperatures. The “Polygon grains” observed in the HC-Se samples play a key role in further selenium diffusion into the film since they make the film more incompact. In addition, activation energy analysis indicates that Cu2−xSe and β-In2Se3 formed in the samples prepared in HC-Se atmosphere may result in different growth pathway of Cu(In1−xGax)Se2 (CIGS) thin film compared with that prepared in Low Cracked-Selenium (LC-Se) atmosphere during the first-step selenization, which restrains lamination in CIGS films effectively so that the distribution of Ga is more uniform throughout CIGS films and no small grains of CuGaSe2 (CGS) accumulate near the Mo back-contact. As a result, the shunt conductance in this CIGS thin film device prepared in HC-Se is reduced, and the fill factor, open-circuit voltage, as well as the cell efficiency are improved.

Crystallization Behavior of Solution-Processed CIGSe Thin Film Semiconductor by Stepwise Annealing Process.

CuIn(x)Ga1-xSe2 (CIGS) thin films were prepared by a solution-based CuInGa (CIG) precursor- selenization process. First, we investigated the effect of selenization temperature on the formation of polycrystalline CIGS and grain growth. The CIG precursor films were selenized using a two-step process to investigate the reaction of Se and CIG precursors during the formation of CIGS thin films. Depending on the temperature in the 1st step of the selenization process, the CIG precursor forms a different intermediate phase between the single phase to ternary phase such as Cu, Se, CuSe, InSe, and CuInSe2. In addition, the intermediate phase exerts a significant influence on the final phase obtained after the 2nd step of the selenization process, particularly with regard to characteristics such as polycrystalline structure and grain growth in the CIGS films. The photoelectron conversion efficiency of devices prepared using CIGS thin films was approximately 1.59-2.75%.

Effect of sodium doping on graded Cu(In1−xGax)Se2 thin films prepared by chemical spray pyrolysis

In the present work we have studied the effect of Na on the properties of graded Cu(In1−xGax)Se2 (CIGS) layer. Graded CIGS structures were prepared by chemical spray pyrolysis at a substrate temperature of 350°C on soda lime glass. Sodium chloride is used as a dopant along with metal (Cu/In/Ga) chlorides and n, n-dimethyl selenourea precursors. The addition of Na exhibited better crystallinity with chalcopyrite phase and an improvement in preferential orientation along the (112) plane. Energy dispersive analysis of X-rays (line/point mapping) revealed a graded nature of the film and percentage incorporation of Na (0.86at%). Raman studies showed that the film without sodium doping consists of mixed phase of chalcopyrite and CuAu ordering. Influence of sodium showed a remarkable decrease in electrical resistivity (0.49–0.087Ωcm) as well as an increase in carrier concentration (3.0×1018–2.5×1019cm−3) compared to the un-doped films. As carrier concentration increased after sodium doping, the band gap shifted from 1.32eV to 1.20eV. Activation energies for un-doped and Na doped films from modified Arrhenius plot were calculated to be 0.49eV and 0.20eV, respectively. Extremely short carrier lifetimes in the CIGS thin films were measured by a novel, non-destructive, noncontact method (transmission modulated photoconductive decay). Minority carrier lifetimes of graded CIGS layers without and with external Na doping are found to be 3.0 and 5.6ns, respectively.

Growth and characterization of Cu(In,Ga)Se2 thin films by nanosecond and femtosecond pulsed laser deposition.

In this work, CuIn1 - x Ga x Se2 (CIGS) thin films were prepared by nanosecond (ns)- and femtosecond (fs)-pulsed laser deposition (PLD) processes. Different film growth mechanisms were discussed in perspective of the laser-produced plasmas and crystal structures. The fs-PLD has successfully improved the inherent flaws, Cu2 - x Se, and air voids ubiquitously observed in ns-PLD-derived CIGS thin films. Moreover, the prominent antireflection and excellent crystalline structures were obtained in the fs-PLD-derived CIGS thin films. The absorption spectra suggest the divergence in energy levels of radiative defects brought by the inhomogeneous distribution of elements in the fs-PLD CIGS, which has also been supported by comparing photoluminescence (PL) spectra of ns- and fs-PLD CIGS thin films at 15 K. Finally, the superior carrier transport properties in fs-PLD CIGS were confirmed by fs pump-probe spectroscopy and four-probe measurements. The present results indicate a promising way for preparing high-quality CIGS thin films via fs-PLD.

Structurally tailored Cu(InxGa1 − x) Se2 thin films via RF magnetron sputtering

Cu(InxGa1−x) Se2 (CIGS) thin films were deposited onto soda-lime glass substrates with Mo coating via the one step radio frequency (RF) magnetron sputtering without a post-selenization process at the substrate temperature varying from 550°C to 630°C. The effect of deposition temperature on the structural properties of CIGS thin films has been characterized by X-ray diffraction (XRD), Raman spectroscopy, field emission scanning electron microscopy (FESEM), and energy dispersive X-ray spectroscopy (EDX). It was found that an increased deposition temperature of up to 580°C contributes to produce the smooth surface, large grain size, and increased crystallinity of the thin films. But further increased deposition temperature results in a decrease in smoothness and an increase in grain size. The optimized temperature (580°C) shows the best effect on the composition and formation of the chalcopyrite structure without impurities.

CIGS Thin Films for Cd-Free Solar Cells by One-Step Sputtering Process

Cu(In1−xGax)Se2 (CIGS) thin films were deposited by a one-step radio frequency (RF) magnetron sputtering process using a quaternary CIGS target. The influence of substrate temperature on the composition, structure, and optical properties of the CIGS films was investigated. All the CIGS films exhibited the chalcopyrite structure with a preferential orientation along the (112) direction. The CIGS film deposited at 623 K showed significant improvement in film crystallinity and surface morphology compared to films deposited at 523 and 573 K. To simplify the manufacturing procedure of solar cells and avoid the use of the toxic element Cd, the properties of ZnS films prepared by RF sputtering were also investigated. The results revealed that the sputtered ZnS film exhibits good lattice matching with the sputtered CIGS film with significantly lower optical absorption loss. Finally, all-sputtered Cd-free CIGS-based heterojunction solar cells with the structure SLG/Mo/CIGS/ZnS/AZO/Al grids were fabricated without post-selenization. Furthermore, the results demonstrated the feasibility of using a full sputtering process for the fabrication of Cd-free CIGS-based solar cell.

Ablation and spectroscopic characteristics of thin CuIn1-xGaxSe2 solar cell films fabricated by co-evaporation and co-sputtering processes

The influence of fabrication processes on the LIBS (Laser Induced Breakdown Spectroscopy) spectra of major and minor chemical constituents in CuIn1-xGaxSe2 (CIGS) absorber films produced by co-sputtering and co-evaporation techniques on Mo-coated soda lime glass (SLG) is reported. It was found that the ablation rate per pulse of CIGS layers fabricated by the co-sputtering technique is higher than those fabricated by the co-evaporation technique, resulting in higher LIBS signal intensities of the constituent elements. The examination of surface morphology of irradiated surfaces and changes in LIBS signal intensities revealed evidences of elemental fractionation for the CIGS films fabricated by co-sputtering technique but not for those by co-evaporation technique. From x-ray diffraction measurements, it was confirmed that the differences in the ablation and spectroscopic characteristics of the two different types of CIGS absorber films were contributed to the differences in crystalline properties. Furthermore, it was demonstrated that LIBS can effectively determine a depth profile of sodium concentration in CIGS thin films, diffused from SLG.

Electroreflectance study of CuIn1−xGaxSe2 thin film solar cells

We have investigated the optical properties of CuIn1−xGaxSe2 (CIGS) thin film solar cells using their electroreflectance (ER) at room temperature. The ER spectra exhibited one broad and two narrow signal regions. Using the photoluminescence (PL) and photocurrent (PC) spectra, the peaks in the low-energy region (1.02–1.35 eV) can be assigned to the CIGS thin film. The PC results implied that the peaks in the high-energy region (2.10–2.52 eV) can be assigned to the CdS band-gap energy. Using the applied bias voltage, the broad signals in the 1.35–2.09 eV region can be assigned to the Franz–Keldysh oscillation (FKO) due to the internal electric field. The ER spectra exhibited a distorted CdS signal for the CIGS thin film solar cell with low shunt resistance and efficiency.

Post-annealing effect on the reactively sputter-grown CIGS thin films and its influence to solar cell performance

Using a reactive co-sputtering from Cu0.6Ga0.4 and Cu0.4In0.6 alloy targets, we prepared CuIn1−xGaxSe2 (CIGS) thin films on Mo/soda-lime glass (SLG) in association with a thermal cracker for elemental atomic Se radicals. The film growth was performed at 500 °C for 90 min. To achieve the composition ratio of CIGS absorber layer, Cu0.6Ga0.4 target was set at RF power of 50 W, 60 W, 70 W, and 80 W while keeping at 100 W for Cu0.4In0.6 alloy target. Post-annealing was done for all the CIGS films at 550 °C for 30 min. The composition ratio of [Cu]/[In + Ga] and [Ga]/[In + Ga] was increased with RF power but showed no change after post-annealing. X-ray diffraction analysis revealed all the samples has grown dominantly in the [112] crystal orientation. We found the Cu2−xSe and (InGa)2−xSe3 defect phase both at the surface and in the bulk, and developed with post-annealing. From the devices fabricated in the structure of grid/ITO/i-ZnO/CdS/CIGS/Mo/soda-lime glass (SLG), the external quantum efficiency (EQE) was observed to improve in the wavelength, λ ≥ 550 nm in the samples treated with annealing. In the current–voltage (J–V) measurements, the solar cell showed the best performance of FF = 54.1%, Voc = 0.48 V, Jsc = 33.1 mA/cm2 and η = 8.5% in the sample with [Cu]/[In + Ga] = 0.84 that improved largely from η = 4.6% for the solar cell with an as-grown CIGS films.

CuIn<sub>1-x</sub>Ga<sub>x</sub>Se<sub>2</sub> Thin Films Prepared by Co-Evaporation Technique

The optical properties of Cu-poor CuIn1-xGaxSe2 thin films with different gallium contents grown by co-evaporated technique were studied. Measurements of photoluminescence and photoreflectance were performed on the samples. The photoluminescence and photoreflectance emission peaks observed around 1.1 eV are attributed to donor-acceptor pair luminescence. These donor-acceptor pair emissions are considered to originate from relatively shallow acceptor and donor energy levels. With increasing gallium content, the emission peaks shift towards higher levels of photon energy, and the linewidths of the luminescence spectra for the samples become wider, which we attributes to the greater statistical disorder between indium and gallium. Moreover, the conversion efficiency of the CuIn1-xGaxSe2-based solar cells is obtained. The measured results coincide with the inference given by the photoluminescence spectra.

Diffusion Coefficients of Selenium and Gallium during the Cu(In<sub>1-x</sub>Ga<sub>x</sub>)Se<sub>2</sub> Thin Films Preparation Process

Cu (In1-xGax)Se2 (CIGS) polycrystalline thin films with Ga-gradient structures were prepared by selenization of sputtered Cu-In-Ga precursors. The Ga contents of the as-selenized CIGS thin films were measured by EDS. With greater Ga content, the peaks in the diffraction pattern become broadened. Auger electron spectroscopy was used to measure the composition distribution of the Cu, In, Ga, and Se elements in the CIGS and CuInSe2 films. At 300°C, the diffusion coefficients DSe was approximately (6.7±1.0) x 10-16 m2s-1, and DGa was about (4.5±1.0) x 10-18 m2s-1. DSe are two orders of magnitude greater than DGa, which is also the reason why the selected CIGS film was almost completely selenized, but still able to keep certain Ga-grading profile. The temperature used in this work is within the low temperature range of the two-step selenization approach, which is more suitable for low-cost substrates like flexible substrates.

In-situ growth of a CdS window layer by vacuum thermal evaporation for CIGS thin film solar cell applications

Highly crystalline and transparent CdS films are grown by utilizing the vacuum thermal evaporation (VTE) method. The structural, surface morphological, and optical properties of the films are studied and compared with those prepared by chemical bath deposition (CBD). It is found that the films deposited at a high substrate temperature (200 °C) have a preferential orientation along (002) which is consistent with CBD-grown films. Absorption spectra reveal that the films are highly transparent and the optical band gap values are found to be in a range of 2.44 eV–2.56 eV. CuIn1−xGaxSe2 (CIGS) solar cells with in-situ VTE-grown CdS films exhibit higher values of Voc together with smaller values of Jsc than those from CBD. Eventually the conversion efficiency and fill factor become slightly better than those from the CBD method. Our work suggests that the in-situ thermal evaporation method can be a competitive alternative to the CBD method, particularly in the physical- and vacuum-based CIGS technology.

Electrodeposited CuIn1−xGaxSe2 thin films from non-aqueous medium for solar cell applications

Polycrystalline thin films of group I-III-VI, especially copper indium diselenide and its alloys with gallium (CuInxGa1−xSe2, CIGS) have proven to be a suitable absorbers layer to obtain high-efficiency solar cells. We have deposited CuInxGa1−xSe2 (CIGS) thin films by cost effective electrochemical technique from non-aqueous bath onto fluorine doped tin oxide (FTO) coated glass substrates. The electrochemical parameters have been investigated by cyclic voltammetry in a non-aqueous solution of ethylene glycol, consisting precursors of Cu, In, Ga, and Se. The structural, morphological, compositional, and optoelectronic properties of CIGS thin films were studied. The prominent X-ray diffraction peak (112) is systematically shifted from 2θ = 26.60 to 27.26°, with increase in the contents of Ga in CIGS precursor layer. The atomic percentage of Ga in CIGS thin film was further confirmed by energy dispersive x-ray analysis technique. Preliminary solar cell devices (Glass/FTO/CdS/CIGS/Au) showed photoactive behavior with good diode characteristics in dark and under illumination conditions.