CuGaSe2 Films Research Articles

2‐step process for 5.4% CuGaSe2 solar cell using fluorine doped tin oxide transparent back contacts

AbstractAs single‐junction solar cells are approaching theoretical limits, multijunction solar cells are becoming increasingly relevant, and low‐cost wider bandgap light harvesters in tandem with silicon are the next frontier in thin film photovoltaic research. Cu‐based chalcogenide compounds have achieved great success as standard absorbers, but performance for bandgaps above 1.5 eV is still lacking. Additionally, the use of transparent back contacts remains challenging for this class of materials. In this work, we report on the fabrication of wide bandgap CuGaSe2 absorbers by a combination of metallic sputtering and reactive thermal annealing grown on transparent fluorine‐doped tin oxide‐coated glass substrate. The annealing temperature is carefully tuned in regard to material and photovoltaic device properties. The introduction of an ultrathin Mo interlayer at the CuGaSe2/back interface favors a higher contact's ohmicity and results in an important improvement of all figures of merit. A record conversion efficiency of 5.4% is obtained, which is the highest value reported for this class of absorber on transparent back contact. Fundamental material characterization of the as‐grown CuGaSe2 films reveals a better homogeneity in Cu distribution throughout the absorber's thickness when using a Mo interlayer, along with an enhanced crystalline quality. The sub‐bandgap transparency of the final device remains perfectible, and improvement pathways are proposed using transfer matrix‐based optical modeling, suggesting to use more specular interfaces to enhance optical transmission.

Effect of the Temperature of Selenization on the Microstructure and Chemical State of CuGaSe2 Films

Effect of the Temperature of Selenization on the Microstructure and Chemical State of CuGaSe2 Films

Physical and chemical aspects at the interface and in the bulk of CuInSe2-based thin-film photovoltaics.

Chalcopyrite CuInSe2 (CISe)-based thin-film photovoltaic solar cells have been attracting attention since the 1970s. The technologies of CISe-based thin-film growth and device fabrication processes have already been put into practical applications and today commercial products are available. Nevertheless, there are numerous poorly understood areas in the physical and chemical aspects of the underlying materials science and interfacial and bulk defect physics in CISe-based thin-films and devices for further developments. In this paper, current issues in physical and chemical studies of CISe-based materials and devices are reviewed. Correlations between Cu-deficient phases and the effects of alkali-metals, applications to lightweight and flexible solar minimodules, single-crystalline epitaxial Cu(In,Ga)Se2 films and devices, differences between Cu(In,Ga)Se2 and Ag(In,Ga)Se2 materials, wide-gap CuGaSe2 films and devices, all-dry processed CISe-based solar cells with high photovoltaic efficiencies, and also fundamental studies on open circuit voltage loss analysis and the energy band structure at the interface are among the main areas of discussion in this review.

Silver-alloyed wide-gap CuGaSe2 solar cells

Crystal growth promotion for CuGaSe2 (CGSe) photovoltaic absorber by silver (Ag)-addition is demonstrated, enhancing device performance for wide-gap CGSe solar cells. By introducing Ag precursor, the grain size in the polycrystalline CGSe films deposited by the three-stage process is increased, and a residual Ga2Se3 phase during the growth is suppressed. Photoluminescence intensity is enhanced with Ag-addition, indicating reduced non-radiative recombination, which is related to the suppression of Ga2Se3 phase and reduction of undesirable deep defect levels. For the Ag-alloyed CGSe solar cells, Urbach energy defined from the sub-gap tail in exponential external quantum efficiency is monotonically reduced from 25.5 to 21.1 meV, suggesting the improvement of CGSe bulk quality by Ag-alloying. Open-circuit voltage is enhanced since carrier recombination in Ag-alloyed CGSe bulk is suppressed. Short-circuit current density is also enhanced as carrier collection efficiency is improved by widened depletion width due to the reduction of carrier concentration by Ag-alloying. On the other hand, excess Ag-addition leads to increased series resistance and deterioration of the built-in potential in the solar cells due to the reduced carrier concentration in the Ag-alloyed CGSe absorber. Finally, the conversion efficiency is maximized to 5.3% at [Ag] / ([Ag] + [Cu]) ratio of 0.07 for the Ag-alloyed CGSe solar cells. These findings will help the performance enhancement for the wide-gap CGSe solar cells.

Enhancement of the photoelectrochemical properties of a CuGaSe2-based photocathode for water reduction induced by loading of a Cu-deficient layer at the p–n heterointerface

Photoelectrochemical activity for water reduction (H2 liberation) over a co-evaporated CuGaSe2 compact thin film modified with a CdS layer and Pt deposits under simulated sunlight (AM 1.5G) radiation was evaluated, specifically focusing on the impact of a Cu-deficient layer (CDL) loaded on the top part of the CuGaSe2 film. It was found that the intentional loading of the CDL with an appropriate thickness was effective for achieving a large current flow and relatively positive photocurrent onset. The half-cell solar-to-hydrogen efficiency reached 6.6% over the best photocathode used. Moreover, the highest photocurrent onset potential of more than 0.9 V vs reversible hydrogen electrode was achieved over the photocathode based on the CuGaSe2 film having an extremely thick CDL (200 nm) with a relatively thick CdS layer (90 nm) due to efficient spatial separation of photogenerated carriers.

Polycrystalline CuGaSe2 thin film growth and photovoltaic devices fabricated on alkali-free and alkali-containing substrates

Abstract Polycrystalline thin films of ternary CuGaSe2 (CGS), which are one of the potential materials for wide-gap top cells in tandem structure photovoltaic devices, are grown on alkali-free and alkali-containing substrates. The effects of alkali-metals on CGS film morphology, grain size, and photovoltaic device properties are studied. High temperature (>600 °C) growth of CGS films is also investigated with a focus upon variations in alkali-metal distribution profiles in CGS films and the formation of a surface Cu-deficient layer (CDL), which plays an important role in p-n junction formation. The use of a high growth temperature is effective in realizing large grain sizes due to enhanced elemental migration during growth, whereas the formation of a surface CDL is diminished and the alkali-distribution profiles in CGS films are modified resulting in a decrease in carrier density. It is suggested that large CGS grain sizes contribute to obtaining high photocurrent values, but coping with modifications in surface/interface and carrier density with alkali-metals is important to enhance overall photovoltaic performance.

CuGaSe2 Thin Film Solar Cells: Challenges for Developing Highly Efficient Wide‐Gap Chalcopyrite Photovoltaics

CuGaSe2, with a band‐gap energy of 1.7 eV, is expected to be a practical material useful for wide‐gap top cells in tandem structure solar cells. In contrast to the success of narrow‐gap Cu(In,Ga)Se2 devices which are already in commercial production, there is room for further improvement before considering practical applications for CuGaSe2 devices. In this review, the developments in CuGaSe2 devices reported to date are surveyed from the perspective of wide‐gap chalcopyrite photovoltaics with a focus upon ternary CuGaSe2 films and solar cells. Differences observed between CuGaSe2 and CuInSe2 or Cu(In,Ga)Se2 films and device properties and correlations with the alkali‐effects are also discussed.

Deep level emission in polycrystalline CuGaSe2 thin-films observed by micro-photoluminescence

Polycrystalline CuGaSe2 thin-films with various Cu-contents were deposited by three-stage co-evaporation method. Radiative transitions in the CuGaSe2 were studied by micro-photoluminescence technique in relation to the Cu/Ga ratio in the films. In addition to a peak DAP1 around 1.60 eV, a peak DAP2 at relatively lower energy of 1.35 eV and a broadened deep-level emission DL roughly centered at 1.05 eV were observed in the photoluminescence (PL) spectra of the CuGaSe2 films. Excitation-power dependence along with temperature dependence of the PL suggests that dominant recombination mechanism due to donor–acceptor pair recombination at lower temperature becomes impurity-to-band transition at higher temperature for all the three peaks. A deep donor-level, roughly 630 meV below the conduction band, which corresponds to DL-emission has been confirmed, and was investigated further in relation to the compositional variation in the CuGaSe2 films.

Synthesis of in-gap band CuGaS2:Cr absorbers and numerical assessment of their performance in solar cells

CuGaS2 thin films were obtained by sulfurization of CuGaSe2. CuGaSe2 thin films were first electrodeposited from aqueous solutions containing CuCl2, GaCl3, and H2SeO3 and subsequently annealed at 400°C for 10min in forming gas atmosphere and in the presence of molecular sulfur. This sulfurization process resulted in the complete conversion of CuGaSe2 into CuGaS2. The formation of CuGaS2 was proven by X-Ray diffraction and optical spectroscopy. Diffraction peaks of CuGaS2 shifted to higher angles than those observed for CuGaSe2 films, and the optical band gap shifted to blue rising from 1.66eV for CuGaSe2 to 2.2eV for CuGaS2. When Cr ions were added to the initial electrolyte, the final CuGaS2 films exhibited a broad in-gap absorption band centred at 1.63eV that can be attributed to Cr atoms at the Ga sites. The performance of solar cells based on CuGaS2:Cr absorbers containing an in-gap absorption band was then estimated by numerical simulation using Solar Cell Capacitance Simulator Software. Both quantum efficiency and short circuit current of simulated Mo/CuGaS2:Cr/CdS/ZnO solar cells rose proportionally to the amount of Cr present in the CuGaS2:Cr absorbers. As a result, the photo conversion efficiency of the simulated devices changed from 14.7% for CuGaS2 to 34% for CuGaS2:Cr absorbers. Nevertheless, when neutral defects related to Cr-doping were introduced in the absorber layer, the positive effect of the enhancement of photon harvesting due to IGB was compensated by a decline in the carrier collection and the overall efficiency of the device diminished considerably.

Effect of Cu/Ga ratio on deep-level defects in CuGaSe2 thin films studied by photocapacitance measurements with two-wavelength excitation.

The effect of the Cu/Ga ratio on properties of deep-level defects in CuGaSe2 thin films were studied, using photocapacitance methods with two-wavelength excitation. The transient photocapacitance method, using a monochromatic probe light, determined two kinds of defects located at 0.8 eV and 1.5 eV above the valence band, respectively, the positions of which kept almost constant regardless of Cu/Ga ratio. In addition to the probe light, laser light with a wavelength of 1550 nm corresponding to 0.8 eV was then used to study the saturation effect of the deep-level defect at 0.8 eV above the valence band. The results suggest that the defect level at 0.8 eV acts as a recombination center at room temperature, and it becomes more effective in CuGaSe2 films with a lower Cu/Ga ratio.

Epitaxial growth of CuGaSe2 thin-films by MBE—Influence of the Cu/Ga ratio

By molecular beam epitaxy (MBE) CuGaSe2 (CGS) thin-films with varying Cu/Ga ratios were grown epitaxial on GaAs (100) and stepped GaAs (111)A substrates. Cu/Ga ratios from Cu-poor to Cu-rich were obtained. In this work the appearance of Cu crystallites on the surface of epitaxial CGS (001) layers are observed and strategies to avoid these precipitations are presented. High quality thin CGS films of around 100nm thickness are obtained, enabling a detailed analysis of the electronic and chemical properties as well as of the crystal structure of the CGS surfaces. The electronic structure with respect to the Cu/Ga ratio was characterized in-situ by XPS and UPS. By LEED a (4×1) (Cu-poor and near stoichiometric) and a (4×2) (Cu-rich) reconstruction of a zinc blende structure were obtained. For CuGaSe2 (112) the LEED pattern showed a (3×1) chalcopyrite reconstruction for Cu/Ga ratios<1. A (1×1) reconstruction of the chalcopyrite structure was observed for Cu-rich (112) samples. The observed dependence of the surface reconstruction on the stoichiometry for CGS grown on GaAs has not been reported in literature so far. Additionally, for Cu-rich stoichiometries a binary phase of Cu2-xSe appeared independently of orientation. The film morphology was investigated ex-situ by SEM.

Electrodeposition of CuGaSe2 and CuGaS2 thin films for photovoltaic applications

CuGaSe2 and CuGaS2 polycrystalline thin film absorbers were prepared by one-step electrodeposition from an aqueous electrolyte containing CuCl2, GaCl3 and H2SeO3. The pH of the solution was adjusted to 2.3 by adding HCl and KOH. Annealing improved crystallinity of CuGaSe2 and further annealing in sulphur atmosphere was required to obtain CuGaS2 layers. The morphology, topography, chemical composition and crystal structure of the deposited thin films were analysed by scanning electron microscopy, atomic force microscopy, energy dispersive spectroscopy and X-ray diffraction, respectively. X-Ray diffraction showed that the as-deposited CuGaSe2 film exhibited poor crystallinity, but which improved dramatically when the layers were annealed in forming gas atmosphere for 40 min. Subsequent sulphurization of CuGaSe2 films was performed at 400 °C for 10 min in presence of molecular sulphur and under forming gas atmosphere. The effect of sulphurization was the conversion of CuGaSe2 into CuGaS2. The formation of CuGaS2 thin films was evidenced by the shift observed in the X-ray diffraction pattern and by the blue shift of the optical bandgap. The bandgap of CuGaSe2 was found to be 1.66 eV, while for CuGaS2 it raised up to 2.2 eV. A broad intermediate absorption band associated to Cr and centred at 1.63 eV was observed in Cr-doped CuGaS2 films.

Growth and Structural Characteristics of CuGaSe2 Films Fabricated from Metallic Precursors Followed by an Elemental Se Reaction Process.

We investigated the growth, structural and optical characteristics of CuGaSe2 thin films prepared with the selenium reaction. Metallic precursor layers from Cu0:5Ga0.5 and Cu0.8Ga0.2 alloy targets were prepared on a sodalime glass substrate by using DC magnetron sputtering, and then annealed to form CuGaSe2 in a rapid thermal process (RTP) with selenium radicals generated by a thermal cracker. The base and sputtering pressures were < 5 x 10(-7) Torr and 30 mTorr, respectively. At ambient temperature, the precursors from the Cu0.5Ga0.5 and Cu0.8Ga0.2 targets were deposited at the rates of 42 nm/min. and 45 nm/min., respectively. The film thicknesses were about 300 nm. Selenization was carried out at different annealing temperatures of T(a) = 450 degrees C, 500 degrees C, 550 degrees C, and 600 degrees C for time periods of 15 min., 30 min., and 60 min. We found that high quality CuGaSe2 films of crystal grains (-1 μm in dia.) fabricated with a reaction using elemental Se at temperatures as low as 450 degrees C for 30 min. When T(a) ≤ 350 degrees C, the Se reaction was insufficient to form CuGaSe2. However, the annealing time had little effect on the formation of CuGaSe2 at T(a) ≥ 450 degrees C. For all the samples, the photoluminescence (PL) emission was only from the donor-acceptor interband transition D1A1 for all the composition ratios of the films [Ga]/[Cu] -1.

Probing diffusion of In and Ga in CuInSe2/CuGaSe2 bilayer thin films by x-ray diffraction

The bilayer thin films of CuInSe2 (CIS) and CuGaSe2 (CGS) are fabricated by a molecular beam deposition (MBD) technique by sequential depositions of CGS followed by CIS and vise versa on Mo-coated soda-lime glass (SLG) and Mo/Al2O3-coated (Na blocking) SLG substrates. The thicknesses of CIS/CGS layers are adjusted to have the overall x=[Ga]/([In]+[Ga]) at approximately 0.4 similar to what is generally used in CIGS thin film solar cells. The growth temperature and the Cu-ratio y=[Cu]/([In]+[Ga]) of the CIS and CGS layers are systematically varied for Cu-rich (y>1) and Cu-poor (y<1) conditions. X-ray diffraction (XRD) technique is used to analyze the shift of diffraction peaks of the preferred orientations of the bilayers compared to those of the single-layer CIS, CGS and CIGS thin films. The XRD results show that higher Ga diffusion is observed in the Cu-rich CIS/CGS bilayer rather than others. The results suggest that the diffusion of Ga is enhanced by the excess Cu–Se phase that is dependent upon the substrate temperature. The Na from the SLG substrate is one of the important factors affecting Ga diffusion. The results show alloying CIGS patterns rather than separated CIS and CGS patterns as observed in the bilayers with Na.

Interfacial alkali diffusion control in chalcopyrite thin-film solar cells.

Alkali elements, specifically sodium (Na), are key materials to enhance the energy conversion efficiencies of chalcopyrite and related thin-film photovoltaic solar cells. Recently, the effect of potassium (K) has also attracted attention because elemental K has unique effects different from Na as well as a similar beneficial effect in improving device performance. In this study, the control of selective alkali K and Na diffusion into chalcopyrite thin-films from soda-lime glass substrates, which serve as the monolithic alkali source material and contain both K and Na, is demonstrated using ternary CuGaSe2. Elemental K is found to be incorporated in the several ten nanometer thick Cu-deficient region, which is formed on the CuGaSe2 film surface, while Na is ejected, although both K and Na diffuse from the substrate to the CuGaSe2 film surface during growth. The alkali [K]/[Na] concentration ratio in the surface region of CuGaSe2 films strongly depends on the film structure and can be controlled by growth parameters under the same substrate temperature conditions. The results we present here offer new concepts necessary to explore and develop emerging new chalcopyrite and related materials and optimize their applications.

Structural tuning of wide-gap chalcopyrite CuGaSe2 thin films and highly efficient solar cells: differences from narrow-gap Cu(In,Ga)Se2

Simultaneous realization of high values of open circuit voltage (Voc), fill factor (FF), and energy conversion efficiency (η) in wide-gap CuGaSe2 (CGS) solar cells has long been one of the most challenging issues in the realm of chalcopyrite photovoltaics. In this communication, structural tuning of CGS thin films by means of controlling the amount of Se flux used during CGS film growth and improvements in solar cell performance (Voc > 0.9 V, FF > 0.7, and η > 10%) are demonstrated. Systematic variations in CGS film properties with the Se flux and correlation with device properties are shown. The unique CGS thin-film growth kinetics, which are different from narrow-gap Cu(In,Ga)Se2, are also presented and discussed. This development of double digit efficiency for CGS solar cells opens a new frontier for the broad application of a new class of chalcopyrite-based devices. Copyright © 2014 John Wiley & Sons, Ltd.

Impact of a binary Ga2Se3 precursor on ternary CuGaSe2 thin-film and solar cell device properties

CuGaSe2 (CGS) thin-films grown by the three-stage coevaporation process using various Se-fluxes during each growth stage exhibited distinctive properties different from those grown with a constant Se-flux during all three stages. CGS grain size, growth orientation, and the depletion width in solar cell devices were found to depend strongly on the Ga2Se3 precursor formed during the first stage, whereas photo-absorption spectra and device parameters were largely determined by the Se-flux used during the second and third stages when the CGS film near-surface region was formed. A certified efficiency exceeding 10% demonstrated for a ternary CGS solar cell is also reported.

Formation of CuInSe2 and CuGaSe2 Thin‐Films Deposited by Three‐Stage Thermal Co‐Evaporation: A Real‐Time X‐Ray Diffraction and Fluorescence Study

AbstractThin film solar cells based on co‐evaporated Cu(In,Ga)Se2 absorber films present the highest efficiencies among current polycrystalline thin‐film technologies. Thanks to the development of a novel experimental setup for in situ growth studies, it was possible to follow the formation of the crystalline phases during such deposition processes for the first time. This synchrotron‐based energy‐dispersive X‐ray diffraction and fluorescence setup is suited for real‐time studies of thin film vapor deposition processes. Focusing on the growth of CuInSe2 and CuGaSe2 fabricated by three‐stage processing, we find that the phase transitions in the Cu‐In‐Se system follow the reported pseudo‐binary In2Se3‐Cu2Se phase diagram. This requires a transformation of the Se sublattice during the incorporation of Cu‐Se into the In2Se3 precursor film from the first process stage. In the Cu‐Ga‐Se system, besides an increase in the lattice spacings, we observe no transformation of the Se sublattice. Furthermore, the structural defects of the Ga‐Se precursor film are preserved until the CuGaSe2 stoichiometry is reached. By means of model calculations of the fluorescence signals, we confirm in both systems the segregation of Cu2Se at the surface near a concentration of 25 at.% Cu shortly after the recrystallization of the films. The modeling also reveals that Cu2Se penetrates into the CuInSe2 film, whereas it remains at the surface of the CuGaSe2 film.

Dynamics of evaporation from CuGaSe2 targets in pulsed electron deposition technique

The evaporation mechanisms from a solid target in pulsed electron deposition (PED) technique have been investigated by analysing the chemical composition and the thickness distribution of CuGaSe2 (CGS) films grown at different discharge voltages on glass substrates. The behaviour of the plasma plume generated from the target can be described by a linear combination of two coexisting processes: incongruent thermal evaporation and congruent ablation, which exhibit different weights depending on the PED voltage. The first component arises from the thermodynamic liquid-to-vapour transition involving the very first layers of the target surface, while the second one is due to the subsurface target penetration of the pulsed e-beam. The chemical composition of the thermally evaporated cloud, according to the CuGaSe2 phase diagram, exhibits an incongruent Cu depletion during the solid-to-gas phase transition with respect to the target, thus forming the ordered vacancy compound CuGa3Se5, while the sublimation of ablated species is perfectly stoichiometric. The thermally evaporated plasma follows a typical surface source spatial distribution, while the expansion of the ablation products exhibit a forward-peaked angular behaviour proportional to cos pθ (4 < p < 7). The incongruent component becomes negligible by enhancing the discharge voltage, where the e-beam is able to more deeply penetrate the target, and the electron power density exceeds the threshold ablation value of 1 × 108 W cm−2. The proposed mechanism for PED process is compared to other models describing the plume generation in pulsed high-energy-induced growth technique. This study represents a remarkable result to better understand and control the PED process.

Effects of substrates on structural and optical properties of Cu-poor CuGaSe2 thin films prepared by in-situ co-evaporation

We investigated the structural and optical properties of Cu-poor CuGaSe2 (CGSe) films depending on the use of different substrates: indium-doped tin oxide (ITO) coated soda-lime glass (SLG) and fluorine-doped tin oxide (FTO) coated SLG as back contacts, widely used Mo-coated SLG, and pure SLG. The Cu-poor phase is chosen as a counterpart of Cu-poor Cu(In,Ga)Se2 to show the highest efficiency in this class of materials, and also give a test board for parasitic phases which might influence on device properties. Although the Cu-poor CGSe thin-films were deposited on the four substrates at the same time in an identical condition, they showed differences in the morphology and grain size due to different CGSe/substrate interfaces and growth mechanisms depending on the substrates. These surface properties of the CGSe films were identified clearly by atomic force microscopy (AFM) measurements. X-ray diffraction (XRD) measurement also supported the result of the AFM analysis and showed that the preferred orientation of CGSe is (112), independent of the substrates. The existence of parasitic phases was examined by Raman and photoluminescence spectroscopic techniques. While defect compounds such as CuGa3Se5 and CuGa5Se8 were identified for all films, the signals related to these parasitic phases are strongest for the films on the pure SLG substrate. Furthermore, the absorption property was investigated by spectroscopic ellipsometry in a photon energy range of 0.7–5 eV. We found that the absorption coefficient values for the CGSe films are about 104–105 cm−1 in the visible region. The absorption coefficient is also changed according to the use of different substrates. This difference comes from the parasitic phase formation, which leads to an increase of the bandgap and suppression of the optical absorption strength. Our systematic study suggests clearly that the difference in distribution of parasitic phases in the CGSe films could originate primarily from the different substrates used for the film deposition.