xGaxSe2 Thin Films Research Articles

Depth profile crystal orientation determination of Cu(In1−xGax)Se2 thin films by GIXRD method applying skin depth theory

In this study, Cu(In1−xGax)Se2 (CIGS) thin film was deposited on molybdenum (Mo) coated glass substrate by thermal co-evaporation technique and depth profile crystal orientation of the this film was examined by grazing incidence X-ray diffraction (GIXRD) method. The crystal structure of the CIGS thin film was determined within 20° to 80° (2θ) scanning range at 0.5° to 15° incidence angle of X-ray. Cross section images were investigated by scanning electron microscope measurements and these measurements showed that bilayer Mo thickness and CIGS thickness are approximately 480 nm and 2 µm, respectively. The surface roughness of films investigated by atomic force microscopy (AFM) and average roughness Ra was found 11.07 nm. According to GIXRD measurements; the interdiffusion of the constituent elements and their effect on the crystal structure were defined both electromagnetic field penetration (skin depth) and mass attenuation viewpoints, and also these results were supported by energy dispersive spectroscopy measurements. As a result, it was seen that the GIXRD method can be used with ease to define the crystal phase homogeneity and depth profile characterization of thin film volume applying skin depth theory.

The Growth and Structural Characterization in Sol–Gel Derived CuIn1–xGaxSe2 Thin Films by Controlling pH-Value

The Growth and Structural Characterization in Sol–Gel Derived CuIn_1–<i>x</i>Ga_<i>x</i>Se₂ Thin Films by Controlling pH-Value

Investigation of CuIn1−xGaxSe2 thin films prepared by chemical solution-based processes for CIGS solar cells

ABSTRACTIn this study, CuIn1−xGaxSe2 (x = 0∼0.5) thin films were synthesized by using a multi-layer spray (MLS) process with Se powder and then CdS buffer layer deposited by CBD deposition process. The as-deposited CIGS thin films were selenized during the rapid thermal process (RTP) in order to improve the crystallization of films and to avoid their oxidation. The obtained samples were systematically characterized by X-ray diffraction (XRD), scanning electron microscopy (SEM), X-ray photoelectron (XPS) and UV-vis spectroscopy. The physicochemical properties of the prepared films were good enough to be used in CIGS solar cell devices on the basis of analytical results.

Influence of Na on grain boundary and properties of Cu(In,Ga)Se2 solar cells

AbstractWhether added from external sources or supplied via substrate, incorporation of Na has always improved the conversion efficiency of polycrystalline thin‐film Cu(In,Ga)Se2 (CIGS) solar cells. Segregation of Na along grain boundary (GB) is thought to be one of the main reasons for better efficiency, and changing its concentration levels may tune the device properties accordingly. Our previous works have shown that changes in GB chemistry over various Ga/In concentrations are the main reason for poor performance of Ga‐rich (18%) CIGS, besides its optimum band gap (1.4 eV) for energy conversion. It was suspected that changes in GB chemistry or device performance or both could be driven by variation in Na concentration at GB. In this research, atomic level fluctuations of Na atoms at GB for various CuIn1−xGaxSe2 thin films were measured accurately with atom probe tomography technique. Fluctuations observed in Na concentration at GB are restricted to the range 0.5–6.5 atoms per nm2 and is irrespective of x value (GGI) and GB surface density. Results also show that changes in GB chemistry for similar CIGS compositions are not necessary due to changes in Na concentration levels at GB. However, an increased density of GBs per unit volume with GGI may indicate an overall rise in Na in thin films and could explain the drop in CIGS performance. Copyright © 2017 John Wiley &amp; Sons, Ltd.

Effects of substrate temperatures in the three–stage growth of CuIn1−xGaxSe2 thin films and their photovoltaic performances

Abstract CuIn 1 − x Ga x Se 2 (CIGS) polycrystalline thin film solar cells were deposited on 10 cm × 10 cm Mo–coated soda–lime glass (SLG) substrates using the three–stage co–evaporation process. The formation of (In,Ga) 2 Se 3 precursor layer was strongly affected by the Se flux supplied during the first stage of the deposition. In our deposition system, the Se source temperature of 300 °C was sufficient for the growth of precursor and throughout the entire deposition. Then, the influence of the substrate temperatures during the first stage (T 1 ), the second and third stages (T 2 ) of the CIGS thin film depositions were investigated by varying T 1 from 300 °C–410 °C and T 2 from 500 °C–580 °C. The radiation from the surface of the growing films detected by a pyrometer and the output power of the substrate temperature controller were used to monitor the deposition process. For our system, the optimum values of T 1 and T 2 were 370 °C and 520 °C, respectively. The XRD results showed the highest (112):(220)/(204) intensity ratio of the chalcopyrite phase for T 1 = 370 °C and T 2 = 520 °C resulting in the device's maximum efficiency of 13.7%.

Direct comparison of atomic layer deposition and sputtering of In2O3:H used as transparent conductive oxide layer in CuIn1−xGaxSe2 thin film solar cells

In this study thin films of hydrogenated In2O3 (IOH) were fabricated by physical vapor deposition (PVD) with and without a post-annealing step, and by atomic layer deposition (ALD). The electro-optical properties on glass as well as the performance as a transparent conductive oxide (TCO) layer in CuIn1−xGaxSe2 (CIGSe)-based solar cells are compared and related to a ZnO:Al (AZO) baseline TCO. Corresponding TCO film thicknesses were adjusted to a resulting sheet resistance of about Rsh = 20 Ω/sq for all samples. Structural investigations were conducted by X-ray diffraction (XRD) and transmission electron microscopy (TEM), while Hall and optical absorption measurements were performed to analyze the electrical and optical quality of the window layers. It is shown that the fully crystallized IOH layers processed by ALD and PVD show similar microstructural and electro-optical properties, which are superior to the AZO baseline. The finalized solar cells were characterized by current-voltage and reflectance-corrected quantum efficiency measurements. While there is no significant gain in short circuit current density (Jsc) for as-deposited PVD In2O3 layers, the application of crystalline In2O3 TCOs leads to an improvement of more than 2mA/cm2 due to an increase in “optical” band gap energy and less free charge carrier absorption (FCA). The open circuit voltage (Voc) of the best cells is 10–15mV higher as compared to the AZO reference, independent of the crystallinity and process of the In2O3 films. The results indicate that the gain in Voc is due to inherent material properties of the IOH films and does not originate from less sputter damage or an affected i-ZnO/TCO interface. Device simulations show that the higher electron affinity χ of the IOH can explain an increased Voc if the Fermi level (EF) is pinned at the CIGSe/CdS interface and why it might not be possible to see the gain when alternative buffer layers are applied.

The importance of back contact modification in Cu2ZnSnSe4 solar cells: The role of a thin MoO2 layer

Cu2ZnSn(SxSe1−x)4 (CZTSSe) photovoltaic absorbers could be the earth-abundant and low toxicity replacement for the already commercialized CuIn1−xGaxSe2 (CIGS) thin film technology. In order to make this possible, specific research efforts applied to the bulk, front and back interfaces must be performed with the aim of improving CZTSSe performance. In this paper the importance of back contact modification to obtain high efficiency Cu2ZnSnSe4 (CZTSe) solar cells and to increase a paramount and limiting parameter such as VOC is highlighted. Several Mo configurations (monolayer, bi-layer and tri-layer) with different electrical and morphological properties are investigated in CZTSe solar cells. An optimum tri-layer configuration in order to minimize overselenization of the back contact during thermal annealing while keeping reasonable electrical features is defined. Additionally, a thin intermediate MoO2 layer that results in a very effective barrier against selenization and innovative way to efficiently assist in the CZTSe absorber sintering is introduced. The use of this layer enhances grain growth and subsequently the efficiency of solar cells increases via major VOC and FF improvement. An efficiency increase from 7.2% to 9.5% is obtained using a Mo tri-layer with a 20nm intermediate MoO2 layer.

Parameterized complex dielectric functions of CuIn1−xGaxSe2: applications in optical characterization of compositional non‐uniformities and depth profiles in materials and solar cells

AbstractIn‐situ spectroscopic ellipsometry (SE) was employed to extract the complex dielectric functions ε = ε1 + iε2 over the spectral range of 0.75–6.5 eV for a set of polycrystalline CuIn1−xGaxSe2 (CIGS) thin films with different alloy compositions x = [Ga]/{[In] + [Ga]}. For highest possible accuracy in ε for each CIGS thin film, specialized SE procedures were adopted including (i) deposition to a thickness of ~600 Å on smooth native oxide covered crystal silicon wafers, which minimizes the surface roughness on the film and thus the required corrections in data analysis, and (ii) measurement in‐situ, which minimizes ambient contamination and oxidation of the film surface. Assuming an analytical form for each of the ε spectra for these CIGS films, oscillator parameters were obtained in best fits, and these parameters were fit in turn to polynomials in x. With the resulting database of polynomial coefficients, the ε spectra for any composition of CIGS can be generated from the single parameter, x. In addition to enabling accurate contactless determination of bulk and surface roughness layer thicknesses of CIGS films by high speed multichannel SE, the database enables characterization of the composition and its profile with depth into these films, and even how the depth profile varies spatially within the plane of the films. In this study, depth profile parameters were found to correlate spatially with solar cell performance parameters. As a result, SE provides the capability of contactless compositional analysis of production‐scale CIGS photovoltaic modules at high speed. Copyright © 2016 John Wiley &amp; Sons, Ltd.

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...

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.

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.

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.

Influence of substrate and its temperature on the optical constants of CuIn1−xGaxSe2 thin films

We investigate the influence of substrate and its temperature on the optical constants of CuIn1−xGaxSe2 (CIGSe) thin films using the transfer-matrix method. The optical constants of a CIGSe layer on top of a transparent conducting oxide (TCO) layer were calculated considering the realistic optical constants of the TCO layer after CIGSe deposition. It was found that TCO substrates could influence the optical constants of CIGSe layers and that the ITO (Sn doped In2O3) substrate had a greater impact than IMO (Mo doped In2O3) for the CIGSe (x = 0.4) film when compared to a reference on bare glass substrate. Additionally, the varied substrate temperatures did not impact the optical constants of CGSe (x = 1). For CIGSe (x = 0.4), the refractive index n stayed relatively independent although at low temperature the grain size was reduced and the Ga/(Ga+In) profile was altered compared to that at high temperature (610 °C). In contrast, the extinction coefficient k at low temperature showed higher absorption at longer wavelengths because of a lower minimum bandgap (Eg,min) originating from reduced inter-diffusion of Ga–Se at a low substrate temperature.

Growth and structural properties of reactively co-sputtered CIGS films and their solar cell applications

Using reactive sputtering, we fabricated stoichiometric CuIn1−xGaxSe2 (CIGS) thin films. Both Cu0.6Ga0.4 (CuGa) and Cu0.4In0.6 (CuIn) alloy targets were simultaneously sputtered under the delivery of elemental Se produced from a thermal cracker. By changing the sputtering rates of the CuGa and the CuIn, we were able to obtain the composition ratios of Cu/(Ga+In) and Ga/(Ga+In) in the range of 0.71–0.95 and 0.10–0.30, respectively. Both the grain size and the surface roughness of the CIGS film increased as the Cu/(Ga+In) ratio increased. In the X-ray diffraction analysis on CIGS films of 0.9 m, preferential growth with a [112] orientation was found, and reflections from the (211), (220)/(204), (301), (312)/(116), (400)/(008), and (332)/(316) planes were observed. The CIGS films showed the existence of Cu2−xSe phases in the Cu-rich samples and ordered defect compound (ODC) phases in the Cu-poor films, as confirmed in the Raman measurements. A best device performance of η = 8.1%, Voc = 0.442 V, Jsc = 34.3 mA/cm2, and FF = 53.4% was obtained from a cell fabricated with a CIGS layer (t = 0.9 µm) with the Cu/(Ga+In) ratio = 0.71 and the Ga/(Ga+In) ratio = 0.10.

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.

Nanostructured light-absorbing crystalline CuIn(1–x)GaxSe2 thin films grown through high flux, low energy ion irradiation

A hybrid effusion/sputtering vacuum system was modified with an inductively coupled plasma (ICP) coil enabling ion assisted physical vapor deposition of CuIn1−xGaxSe2 thin films on GaAs single crystals and stainless steel foils. With &amp;lt;80 W rf power to the ICP coil at 620–740 °C, film morphologies were unchanged compared to those grown without the ICP. At low temperature (600–670 °C) and high rf power (80–400 W), a light absorbing nanostructured highly anisotropic platelet morphology was produced with surface planes dominated by {112}T facets. At 80–400 W rf power and 640–740 °C, both interconnected void and small platelet morphologies were observed while at &amp;gt;270 W and above &amp;gt;715 °C nanostructured pillars with large inter-pillar voids were produced. The latter appeared black and exhibited a strong {112}T texture with interpillar twist angles of ±8°. Application of a negative dc bias of 0–50 V to the film during growth was not found to alter the film morphology or stoichiometry. The results are interpreted as resulting from the plasma causing strong etching favoring formation of {112}T planes and preferential nucleation of new grains, balanced against conventional thermal diffusion and normal growth mechanisms at higher temperatures. The absence of effects due to applied substrate bias suggests that physical sputtering or ion bombardment effects were minimal. The nanostructured platelet and pillar films were found to exhibit less than one percent reflectivity at angles up to 75° from the surface normal.

The effect of surface roughness on the determination of optical constants of CuInSe2 and CuGaSe2 thin films

To investigate the effect of surface roughness on the calculation of optical constants, e.g., the complex refractive index n+ik or (n,k) of CuIn1−xGaxSe2 (CIGSe) thin films, we took CuInSe2 (CISe) and CuGaSe2 (CGSe) as examples and applied the “Modified Transfer-Matrix (MTM)” method to calculate optical constants with considering the effect of scattering due to surface roughness. Compared to the Transfer-Matrix (TM) method without considering surface roughness, it was revealed that the MTM method could improve the accuracy of calculation. The calculated refractive index values from the MTM method increase by 6.89% for CISe and 2.59% for CGSe in contrast to those from the TM method. In addition, bromine solution was confirmed via Scanning Electron Microscopy and Atomic Force Microscopy to be able to reduce the surface roughness. Calculated results from smoothened samples showed that the accuracy of calculated optical constants was further improved. Finally, optical constants calculated by the MTM method were compared to those from smoothened samples, validating that the MTM method could eliminate the influence of surface roughness on the calculation of optical constants more effectively for CGSe with low surface roughness than for CISe with high surface roughness.

Comparison of thin-film and bulk CuIn1 − xGaxSe2 samples by laser induced breakdown spectroscopy

The results for laser induced breakdown spectroscopy (LIBS) measurements of CuIn1−xGaxSe2 (CIGS) thin film samples deposited on Mo-coated soda–lime glass by co-evaporation technique and those of bulk form CIGS targets are reported. The LIBS intensity calibration results for the thin film and bulk CIGS samples were compared with respect to the concentration ratios. While the concentrations for thin film CIGS samples were measured by X-ray fluorescence, the compositions of bulk samples were provided by the manufacturer. It was found that the calibration results of Ga/(Ga+In) ratio obtained from the thin film and bulk form samples showed a good agreement at optimized measurement conditions. It is demonstrated that bulk samples can provide reliable reference composition data for the elemental analysis of thin CIGS films.