Cu(In,Ga)Se2 Layer Research Articles

Role of Mo:Na layer on the formation of MoSe2 phase in Cu(In,Ga)Se2 thin film solar cells

This study investigated the influence of Mo:Na on the formation of MoSe2 in Cu(In, Ga)Se2 (CIGS) thin film solar cells and its overall effect on the performance of cells with a structure of Ti/Mo:Na/Mo/CIGS/CdS/i-ZnO/ZnOAl/Al. Varying the thickness of the Mo:Na layer enabled the systematic control of the diffusion of Na into the CIGS layer. Experimental results demonstrate that the thickness of MoSe2 phase decreases with an increase in the thickness of the Mo:Na layer. Reducing the thickness of the MoSe2 layer enhanced cell efficiency. Additionally, Ga distribution in the CIGS layer may vary with a change in the thickness of the Mo:Na layer. When Mo:Na/Mo=500/500nm, the Ga content and the thickness of the MoSe2 layer were close to optimal with regard to cell performance. This paper proposes a model to explain the impact of Na doping on the formation of MoSe2 phase formation in polycrystalline CIGS. It appears that the addition of Na leads to the formation of Na2Sex at the grain boundaries in the absorber layer, which reduces the diffusion of Se to form MoSe2.

Advantageous light management in Cu(In,Ga)Se2 superstrate solar cells

The superstrate configuration of Cu(In,Ga)Se2 (CIGS) photovoltaic devices exhibits the potential for improved light harvesting compared to the standard substrate configuration. Due to the limited power conversion efficiency of superstrate devices caused by the poor hetero-interface between the p-type CIGS and the n-type component, these advantages have not been fully considered so far. This work shows that the superstrate configuration can significantly improve the light management within CIGS devices. Experimentally, it is shown that the electric and optical properties of ZnO window layer strongly improve during the CIGS deposition, allowing the use of structured ZnO layers for efficient light-trapping. A highly reflective and scattering back contact is realized by using the high work function material MoO3−x for the hole-transport layer in combination with a low work function Ag metal layer. Combined with the unique Ga gradient achievable in the superstrate configuration, simulation results show that a strong reduction of the CIGS layer thickness is possible while increasing the power conversion efficiency. This proof of concept highlights the appeal of the superstrate configuration for the future of cost-effective CIGS solar cell production.

Design of the TCO (ZnO:Al) thickness for glass/TCO/CdS/CIGS/Mo solar cells

Modelling solar cells has, in recent years, become a fundamental tool for their optimum design. Here, we present an optical study of CuInGaSe2 (CIGS) solar cells in order to improve them by selecting the appropriate transparent conductive oxide (TCO) layer thickness. The system of interest in our case was glass/TCO/CdS/CIGS/Mo, considering radiation incident perpendicular to the cell from the glass and taking into account the absorbent nature of the TCO, CdS and CIGS layers. The reflectance of this layered system was calculated using the optical matrix method considering the three absorbent layers and the reflectance at the Mo substrate. In addition, the transmittance of the TCO layer was also calculated. Then, having the constant thickness of the CIGS (1 μm) and CdS (0.1 μm) layers, the weighted average of the cell reflectance and the TCO transmittance in the AM1.5 solar spectrum were calculated for different TCO thickness in the range from 0.1 to 1 μm. Finally, we defined a figure of merit that involves the total cell reflectance, the transmittance and the sheet resistivity of the TCO as a function of the TCO layer thickness. Our results indicate that the ZnO film thickness can be increased to 1 μm with a possible improvement of the cell performance compared to cells with typical TCO thickness around 500 nm.

Improved growth of solution-deposited thin films on polycrystalline Cu(In,Ga)Se2

CdS and Zn(O,S) grown by chemical bath deposition (CBD) are well established buffer materials for Cu(In,Ga)Se2 (CIGS) solar cells. As recently reported, a non-contiguous coverage of CBD buffers on CIGS grains with {112} surfaces can be detected, which was explained in terms of low surface energies of the {112} facets, leading to deteriorated wetting of the chemical solution on the CIGS surface. In the present contribution, we report on the effect of air annealing of CIGS thin films prior to the CBD of CdS and Zn(O,S) layers. In contrast to the growth on the as-grown CIGS layers, these buffer lay- ers grow densely on the annealed CIGS layer, even on grains with {112} surfaces. We explain the different growth behavior by increased surface energies of CIGS grains due to the annealing step, i.e., due to oxidation of the CIGS surface. Reference solar cells were processed and completed by i-ZnO/ZnO:Al layers for CdS and by (Zn,Mg)O/ZnO:Al for Zn(O,S) buffers. For solar cells with both, CdS and Zn(O,S) buffers, air-annealed CIGS films with improved buffer coverage resulted in higher power-conversion efficiencies, as compared with the devices containing as-grown CIGS layers. (© 2016 WILEY-VCH Verlag GmbH &Co. KGaA, Weinheim)

Effect of process pressure and substrate temperature on CdS buffer layers deposited by using RF sputtering for Cu(In,Ga)Se2 solar cells

The characteristics of CdS films deposited on Cu(In,Ga)Se2(CIGS)/Mo/glass and glass substrates by using RF magnetron sputtering were investigated. The deposition pressure and the substrate temperature were selected as key parameters to examine the electrical, compositional and optical properties of the films. As the deposition pressure was increased, the resistivity increased while the carrier concentration decreased owing to a stoichiometric change and Cd-O incorporation at high pressure. Field-emission scanning electron microscopy(FE-SEM) revealed that the CdS films on CIGS/Mo became denser as the pressure was increased, which was responsible for the high transmittance of the film deposited at high pressure. As the substrate temperature was increased, the deposition rate decreased, which could be explained by using Langmuir theory. As the temperature was increased from room temperature to 573 K, the resistivity increased and the carrier concentration decreased, which was attributed to an increase in [S]/[Cd] ratio. In addition, as the temperature was increased, the small grains were agglomerated to form larger grains due to the increase in the activity of grains at high temperature. CdS films were confirmed to be uniformly deposited on the CIGS layer by using RF sputtering. The large amount of interdiffusion between the CIGS and the CdS films deposited at a high substrate temperature were observed by using X-ray photoelectron spectroscopy.

Simulation analysis of heterojunction ZnO/CdS/Cu(In,Ga)Se2 thin-film solar cells using wxAMPS

Solar cells based on Cu(In,Ga)Se2 (CIGS) absorber layers have owned the highest energy conversion efficiency among thin film solar cells and it is reported that the efficiency of CIGS solar cells is above 21% at present. In this paper, the performances of monolayer CIGS thin film solar cells with conventional ZnO/CdS/CIGS structure are calculated by using wxAMPS simulative software. The optimum values of the thicknesses of CdS/CIGS layers and carrier density of CIGS layer have been calculated and it is shown that the performances of solar cells have been improved by optimizing the parameters of solar cells. The temperature dependence of performances for CIGS solar cells has been simulated to guide the fabricated devices to obtain higher efficiency in the different temperature conditions. As a result of calculation, it is shown that excellent performance can be obtained by adjusting the parameters, and that the highest efficiency of solar cell can be achieved under certain conditions, where the thicknesses of ZnO, CdS and CIGS are 100nm, 30nm and 3μm, respectively.

Characterization of Cu(In,Ga)Se2 (CIGS) films with varying gallium ratios

Cu(In1−x,Gax)Se2 (CIGS) absorber layers were deposited on molybdenum (Mo) coated soda-lime glass substrates with varying Ga content (described as Ga/(In + Ga) ratios) with respect to depth. As the responsible mechanisms for the limitation of the performance of the CIGS solar cells with high Ga contents are not well understood, the goal of this work was to investigate different properties of CIGS absorber films with Ga/(In + Ga) ratios varied between 0.29 and 0.41 (as determined by X-ray florescence spectroscopy (XRF)) in order to better understand the role that the Ga content has on film quality. The Ga grading in the CIGS layer has the effect causing a higher band gap toward the surface and Mo contact while the band gap in the middle of the CIGS layer is lower. Also, a wider and larger Ga/(In + Ga) grading dip located deeper in the CIGS absorber layers tend to produce larger grains in the regions of the films that have lower Ga/(In + Ga) ratios. Moreover, it was found that surface roughness decreases from 51.2 nm to 41.0 nm with increasing Ga/(In + Ga) ratios. However, the surface roughness generally decreases if the Ga grading occurs deeper in the absorber layer.

Fabricating Cu(In,Ga)Se2 solar cells on flexible substrates by a new roll-to-roll deposition system suitable for industrial applications

In this work, a new hybrid sputtering–evaporation system providing a scalable process for deposition of Cu(In,Ga)Se2 (CIGS) layers is presented. The growth apparatus has been designed and realized to fit a size suitable for direct industrial transfer. In this process the metal precursors are first of all sputtered on rotating transfer devices, then evaporated on the substrate by local heating in a Se atmosphere. The desired thickness and composition of the CIGS film are obtained by repeated sputtering–evaporation cycles. The cylindrical geometry of the deposition chamber has been designed to accommodate different types of flexible substrates with a maximum size of 20 × 120 cm2 in a roll-to-roll configuration. Several techniques, including secondary ion mass spectrometry, Raman and photoluminescence spectroscopies, x-ray diffraction, scanning electron microscopy, external quantum efficiency, and I–V under 1 Sun illumination, have been used to test both the as-grown CIGS layers and the solar cell devices based on them. A significant performance and good control of Ga grading and Na content were obtained for solar cells grown at 450 °C on polyimide substrates with high deposition rates. In spite of the fact that the present efficiency record for CIGS solar cells on polyimide substrates is 20.4%, the 10.1% obtained using the hybrid method presented in this work is significant because the growth apparatus meets the requirements for direct industrial transfer. In fact, this process is being transferred in a 1 MW production line, where standard CIGS layers are deposited at low temperature on flexible substrates in a single-step process with a 1 mm sec−1 substrate velocity.

Features of KF and NaF Postdeposition Treatments of Cu(In,Ga)Se2 Absorbers for High Efficiency Thin Film Solar Cells

The introduction of a KF postdeposition treatment (KF PDT) of Cu(In,Ga)Se2 (CIGS) thin films has led to the achievement of several consecutive new world record efficiencies up to 21.7% for the CIGS solar cell technology. The beneficial effect of the KF PDT on the photovoltaic parameters was observed by several groups in spite of differing growth methods of the CIGS layer. For CIGS evaporated at lower temperature on alkali-free, flexible plastic substrates, a postdeposition treatment to add Na was already successfully applied. However, with the introduction of additional KF under comparable conditions, distinctly different influences on the final absorber alkali content as well as surface properties are observed. In this work we discuss in more detail the intrinsically different role of both alkali-treatments by combining several microstructural and compositional analysis methods. The ion exchange of Na by K in the bulk of the absorber is carefully analyzed, and further evidence for the formation of a K-co...

Na-Dependent Ultrafast Carrier Dynamics of CdS/Cu(In,Ga)Se2 Measured by Optical Pump-Terahertz Probe Spectroscopy

To investigate the origin of the Na effect on photovoltaic (PV) devices, Cu(In,Ga)Se2 (CIGS) and CdS/CIGS layers were grown on borosilicate (BS) and soda-lime glass (SLG), respectively. The defect states and nonequilibrium carrier dynamics of the samples were measured using photoluminescence (PL) and optical pump-THz probe (OPTP) spectroscopy. From the PL results, we discovered that different shallow donor–acceptor levels were formed in the CIGS layer grown on BS and SLG, respectively. In the OPTP results, relaxation times of photocarriers excited from the CdS/CIGS layer were clearly distinguishable, and are explained by the formation of different defect states depending on substrates. In BS, deep defect level ‘DX states’ were formed in the Eg near the p–n junction, which induce trapping photocarriers, resulting in shortening relaxation time. In SLG, there was no “DX state”, which clearly demonstrates the positive effect of Na atoms at the p–n junction on performance of PV devices.

Optical constants of Cu(In, Ga)Se2 for arbitrary Cu and Ga compositions

The optical constants of Cu(In, Ga)Se2 (CIGS)-based polycrystalline layers with different Cu and Ga compositions are parameterized completely up to a photon energy of 6.5 eV assuming several Tauc-Lorentz transition peaks. Based on the modeled optical constants, we establish the calculation procedure for the CIGS optical constants in a two-dimensional compositional space of (Cu, Ga) by taking the composition-induced shift of the critical point energies into account. In particular, we find that the variation of the CIGS optical constants with the Cu composition can be modeled quite simply by a spectral-averaging method in which the dielectric function of the target Cu composition is estimated as a weighted average of the dielectric functions with higher and lower Cu compositions. To express the effect of the Ga composition, on the other hand, an energy shift model reported earlier is adopted. Our model is appropriate for a wide variety of CIGS-based materials having different Cu and Ga compositions, although the modeling error increases slightly at lower Cu compositions [Cu/(In + Ga) < 0.69]. From our model, the dielectric function, refractive index, extinction coefficient, and absorption coefficient for the arbitrary CIGS composition can readily be obtained. The optical database developed in this study is applied further for spectroscopic ellipsometry analyses of CIGS layers fabricated by single and multi-stage coevaporation processes. We demonstrate that the compositional and structural characterizations of the CIGS-based layers can be performed from established analysis methods.

Photocurrent multiplication in Ga2O3/CuInGaSe2 heterojunction photosensors

Photocurrent multiplication was observed in gallium oxide (Ga2O3)/Cu(In,Ga)Se2 (CIGS) heterojunction photosensors. In this study, we investigated the electrical and optical properties of Ga2O3/CIGS heterojunction photodiodes for possible use as high-sensitivity visible-light sensors. To achieve low current density J under dark conditions, a thin layer of Ga2O3, which is an n-type wide band gap (4.7–4.9eV) semiconductor, was grown on the CIGS layer. This Ga2O3 layer functioned as a hole-blocking layer and buffer layer. The effect of oxygen partial pressure during the deposition of the Ga2O3 layer was also examined. With no applied voltage, the quantum efficiency of a photodiode with a Ga2O3 layer deposited with no oxygen partial pressure was considerably high. On the other hand, when the Ga2O3 layer was deposited with oxygen partial pressure, the depletion region almost spread into the Ga2O3 layer with no applied voltage. Considering J, under low oxygen deposition conditions, dark current J increased due to more oxygen vacancies, and with higher oxygen partial pressure during deposition, dark current J increased due to oxygen plasma damage at the Ga2O3/CIGS interface during the sputtering deposition. The photodiode with the lowest dark current J of 10−9A/cm2 at applied voltages less than 3V had a Ga2O3 layer deposited at an optimum oxygen partial pressure of 6.0×10−2Pa. The average quantum efficiency for this photodiode in the visible light range (400–700nm) was 85% at an applied voltage of 3V. Moreover, at higher applied voltages, carrier multiplication phenomena were observed, and the average quantum efficiency (400–700nm) was over 100% at applied voltages over 4V.

Damp heat and thermal cycling-induced degradation mechanism of AZO and CIGS films in Cu(In,Ga)Se2 photovoltaic modules

This paper characterizes and compares the degradation mechanisms of Cu(In,Ga)Se2 (CIGS)-based thin film photovoltaic (PV) modules during exposure to damp heat (85 °C/85% RH) for 1000 h and thermal cycling (−40 °C/85 °C) for 1000 cycles. After damp heat (DH) exposure, the efficiency of the PV modules was reduced from the initial value of ∼12.5 ± 0.1% to 10.5 ± 0.2% due to increase of the resistivity in the AZO and CIGS layers. The optical band gap was also decreased from the initial value of 3.60 eV–3.54 eV after 1000-h DH exposure. This behavior was associated with oxygen adsorption and the generation of hydroxides in the AZO layer. The efficiency of the PV modules after subjection to thermal cycling (TC) was decreased to 11.4 ± 0.2% due to increase of the resistivity of the AZO and CIGS layers. The increase in the resistivity was interpreted as being due to oxygen adsorption, as well as the formation of micro-cracks in the AZO films.

CIGS 태양전지용 Cd-Free 버퍼층 제조

Indium hydroxy sulfide (In(OH)xSy) as a cadmium (Cd)-free buffer layer for CuInGaSe2 (CIGS) solar cells was prepared by the chemical bath deposition (CBD) and the reaction time was optimized. The band gap energy and transmittance data alongside the thickness results from the direct observation with focused ion beam system (FIB) could be a powerful tool for optimizing the conditions. In addition, X-ray diffractometer (XRD), X-ray photoelectron microscopy (XPS), and scanning electron microscope (SEM) were also employed for the layer characterization. The results indicated that the optimum reaction time for In(OH)xSy buffer layer deposition by CBD was 20 min at 70 ℃ under the conditions employed. At the optimum conditions, the buffer layer thickness was near 57 nm and the band gap energy was 2.7 eV. In addition, it was found that there was no XPS peak shift in between the buffer layers deposited on molybdenum (Mo)/glass and that on CIGS layer.

Evaluation of junction quality of buffer-free Zn(O,S):Al/Cu(In,Ga)Se2 thin-film solar cells

Cu(In,Ga)Se2 (CIGS) solar cells without buffer layers, namely, buffer-free solar cells, and those with conventional CdS buffer layers were fabricated. Zn(O,S):Al (AZOS) was used as a transparent conductive oxide layer. The AZOS/CIGS interface in the buffer-free solar cells and the AZOS/CdS/CIGS interface in the CdS-buffered solar cells were investigated by bright-field scanning transmission electron microscopy, high-angle annular dark-field scanning transmission electron microscopy, and fast Fourier transform. It was found that the open-circuit voltage and fill factor of the buffer-free solar cells are lower than those of the CdS-buffered solar cells, which is attributed to the nonepitaxial growth of AZOS on the CIGS layer, forming defects and deteriorating the junction quality of the buffer-free solar cells.

Investigation of the effect of potassium on Cu(In,Ga)Se2 layers and solar cells

We investigate the influence of potassium (K) on the Cu(In,Ga)Se2 (CIGS) growth kinetics on alkali-free alumina substrates and the electrical parameters of the CIGS solar cell by intentional K doping of the CIGS layer by a KF-precursor layer and KF-post deposition treatment (PDT). Secondary ion mass spectroscopy measurements revealed that K can be incorporated into the CIGS layer by both processes. The CIGS composition of the KF-precursor sample shows a stronger [Ga]/([Ga] + [In]) (GGI) profile. By analysing the samples with scanning electron microscopy we observed smaller CIGS grains for the KF-precursor sample compared to the K-free reference and KF-PDT sample. jV-measurements of the KF-PDT and the KF-precursor sample show an increase in the cell efficiency η from 10.7% to 13.6% and 13.7%, respectively, compared to the K-free reference sample. The external quantum efficiency measurements of the KF-precursor sample show an increased absorption in the infrared region. Capacitance–voltage measurements reveal an increase in the net doping concentration of both samples treated with K. We assume that the enhancement is caused by passivation of grain boundaries and donor-like defects by K, as previously demonstrated for Na.

Surface modification of CIGS film by annealing and its effect on the band structure and photovoltaic properties of CIGS solar cells

The composition of Cu(In,Ga)Se2 (CIGS) films employed in CIGS solar cells is Cu deficient. There can be point defects, including Cu vacancies, Se vacancies, and metal anti-site defects. The surface composition and defects are not well controlled right after CIGS film fabrication with a three-stage co-evaporation process. This fabrication technique can result in a large variation in cell efficiency. In order to control the CIGS film in a reproducible way, we annealed the CIGS film in air, S, or Se. With this annealing procedure, the Cu content of the CIGS surface was significantly reduced and Ga content was strongly increased. An intrinsic CIGS layer with a lower valence-band maximum and a wider ban gap was formed at the surface. By annealing the CIGS film, the open-circuit voltage and fill factor were significantly improved, which indicates that the surface intrinsic layer acts as a hole-blocking layer so that the surface recombination rate is suppressed. In addition to CIGS film annealing, with subsequent annealing of the completed devices using rapid thermal annealing, the efficiency and reproducibility of CIGS solar cells were markedly improved.

Failure analysis of Cu(In,Ga)Se2 photovoltaic modules: degradation mechanism of Cu(In,Ga)Se2 solar cells under harsh environmental conditions

AbstractHigh‐temperature‐induced and humidity‐induced degradation behaviors were investigated through the failure analysis of encapsulated Cu(In,Ga)Se2 (CIGS) modules and non‐encapsulated CIGS cells. After being exposed to high temperature (85 °C) for 1000 h, the efficiency loss of CIGS modules and the resistivities of the aluminum‐doped zinc oxide (AZO) layer, CIGS layer, and Mo layer were slightly increased. After damp heat (DH) testing (85 °C/85% RH), the efficiency of some modules decreased significantly accompanied by discoloration, and in these areas, the resistivity of the AZO layers increased markedly. The causes of degradation of CIGS cells after high temperature and DH tests were suggested through X‐ray photoelectron spectroscopy analysis. The high‐temperature‐induced degradation behaviors were revealed to be increases in series resistance of the CIGS cells, due to the adsorption of oxygen on the AZO, CIGS, and Mo layers. The degradation behavior after DH (85 °C/85% RH) exposure was caused by the adsorption of oxygen, as well as the generation of Zn(OH)2 due to water molecules. In particular, the humidity‐induced degradation behavior in discolored CIGS modules was ascribed to the generation of Zn(OH)2 and carboxylic acids in the AZO layer, due to a chemical reaction between the AZO, ethylene‐vinyl acetate copolymer, and water. Copyright © 2014 John Wiley & Sons, Ltd.

Influence of Ni and Cr impurities on the electronic properties of Cu(In,Ga)Se2 thin film solar cells

AbstractDeposition of Cu(In,Ga)Se2 (CIGS) thin film solar cells on metallic substrate is an attractive approach for development of low cost solar modules. However, in such devices, special care has to be taken to avoid diffusion of impurities, such as Fe, Ni, and Cr, from the substrate into the active layers. In this work, the influence of Ni and Cr impurities on the electronic properties of CIGS thin film solar cells is investigated in detail. Impurities were introduced into the CIGS layer by diffusion during the CIGS deposition process from a Ni or Cr precursor layer below the Mo electrical back contact. A high temperature and a low temperature CIGS deposition process were applied in order to correlate the changes in the photovoltaic parameters with the amount of impurities diffused into the absorber layer. Solar cells with Ni and Cr impurities show a reduction in the device performance, whereas the effect was most pronounced in Ni containing devices. The presence of deep defect levels in the absorber layer was identified with admittance spectroscopy and can be related to Ni and Cr impurities, which diffused into the CIGS layer according to secondary ion mass spectroscopy depth profiles and inductively coupled plasma mass spectrometry. Copyright © 2014 John Wiley & Sons, Ltd.

Investigation of the diffusion behavior of sodium in Cu(In,Ga)Se2 layers

Sodium diffusion in Cu(In,Ga)Se2 (CIGS) layers was investigated over a temperature range from 157 °C to 400 °C. The diffusion profiles were measured by secondary ion mass spectrometry. Sodium ions diffused from a sodium fluoride (NaF) layer on the CIGS surface into the CIGS layer. From Na diffusion profiles, the diffusion along grain boundaries could be distinguished from the diffusion into the grain interior. Atom-probe tomography measurements reveal that even at a low temperature of 157 °C bulk diffusion of sodium into CIGS occurs. Based on this data, the slower diffusion coefficient in the volume can be described by the Arrhenius equation DNaV = 9.7 × 10−9 exp(−0.36 eV/kBT) cm2 s−1 and the fast diffusion along the grain boundaries by DNaGB = 6.5 × 10−9 exp(−0.21 eV/kBT) cm2 s−1. Hence, we propose that sodium ions do not only passivate grain boundaries, but also act as dopants in the CIGS bulk.