CuGaSe2 Solar Cells Research Articles

Preparation of wide bandgap CuGaSe2 absorbers and solar cells by sputtering a selenium-rich ceramic target and annealing in a selenium-free atmosphere

CuGaSe2 (CGSe) material possesses a wide bandgap of approximately 1.7 eV, making it a promising prospect for top cells in tandem solar cells. Depositing CGSe precursor films by magnetron sputtering of a selenium-rich ceramic target is easy to operate and control. After annealing the precursor films at different temperatures in a selenium-free argon atmosphere, avoiding the use of toxic H2Se. To achieve an absorber exhibiting appropriate performance, the morphologies, phase composition, absorption characteristics, roughness, lattice structure, and work function of CGSe absorbers annealed at different temperatures were investigated. The CGSe absorber annealed at 550 °C showed uniform grain, a single-phase composition, high absorbance, flat surface, and compact crystal structure. The absorber annealed at excessive temperature led to the formation of Cu2-xSe, inducing defects and carrier recombination. The work function of CGSe absorber annealed at 590 °C increases, resulting in the misalignment of its band with the Mo back contact. The enhancement of absorber contributes positively to solar cell device performance. CGSe solar cells prepared using the absorber annealed at 550 °C exhibited excellent photoelectric properties. The increase in minority carrier lifetime and recombination resistance improves the transport characteristics of carriers.

Over 11% Efficient CuGaSe2 Solar Cells Without Using KCN Treatment

CuGaSe2 (CGS) has a bandgap energy of 1.68 eV and is theoretically very suitable to be in tandem with silicon or Cu(In,Ga)Se2 solar cells. However, due to the high quantity of surface defects, high‐performance CGS usually relies on a KCN surface treatment, which is a high‐toxic process and restricts its further development. Herein, by reducing the exposure time to air of the CGS layer as far as possible and developing a new absorber growth procedure to reduce the surface Cu x Se generation, grain size is successfully increased and interface recombination is reduced without the use of KCN. Combined with a simple annealing process, the defect concentration is successfully decreased and the depletion width is broadened dramatically and the power conversion efficiency is promoted to 11.05% without any traditionally used KCN treatment. This work provides a nontoxic and low‐pollution way to fabricate high‐efficient CGS solar cells.

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.

A Comprehensive Perspective on the Fabrication of CuGaSe2/Si Tandem Solar Cells

This review points out the current need for further research on tandem solar cells as the commercial single‐junction solar cell technologies report close to their maximum performance limit. First, the conceptual insight and the desired interconnection schemes for tandem solar cells are briefly presented. Then, a comprehensive discussion on the structural, optical, and electronic properties of CuGaSe2, evaluating its use as a top cell absorber material, is presented. Furthermore, the current status and the critical analysis of CuGaSe2 solar cells, which finds that the adapted conventional device architecture possesses severe limitations, are presented. Based on the drawbacks of the existing materials in use, new materials to fabricate CuGaSe2 solar cells are proposed. Also, tandem configurations with newly proposed CuGaSe2 solar cell architecture by choosing Si photovoltaics as bottom cells are devised. In the end, further challenges in the devised tandem configurations with a focus on their band alignments are elucidated.

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.

Fabrication of wide band-gap CuGaSe2 solar cells for tandem device applications by sputtering from a ternary target and post selenization treatment

Fabrication of wide band-gap (Eg) CuGaSe2 (CGSe) thin film solar cells with large area uniformity and good reproducibility is difficult, which is crucial for tandem cells. In this work, a method was proposed and investigated to overcome this problem. CGSe solar cells were fabricated by sputtering from a ternary CGSe target and post selenization treatment at different temperatures. After selenization, the CGSe grains grow up gradually, and the copper-poor and selenium-rich CGSe absorbers are obtained successfully. The X-ray diffraction peaks are all corresponded to CGSe phase. As the temperature increases from 530 to 570 °C, the average efficiency increases from 1.57% to 3.11%. When the temperature reaches 590 °C, the efficiency drops to 2.38%. The champion device obtained at 570 °C shows an Eg of 1.66 eV and an efficiency of 3.21%, which indicates the potential for the top cell in tandem solar cells.

Group III Elemental Composition Dependence of RbF Postdeposition Treatment Effects on Cu(In,Ga)Se2 Thin Films and Solar Cells

The effects of RbF postdeposition treatment (RbF-PDT) on Cu(In,Ga)Se2, CuInSe2, and CuGaSe2 thin films and solar cell devices are comparatively studied. Similar to the effect of the KF postdeposition treatment (KF-PDT), Cu(In,Ga)Se2 and CuInSe2 film surfaces show significant pore formation resulting in a rough surface morphology with RbF-PDT, whereas this is not the case for In-free CuGaSe2. The device properties of the In-containing and In-free Cu(In,Ga)Se2 solar cells also show contrasting results, namely, Cu(In,Ga)Se2 or CuInSe2 devices show an increase in the open circuit voltage (Voc) and fill factor (FF) values and almost constant or a slight decrease in the short-circuit current density (Jsc) values with RbF-PDT, whereas CuGaSe2 devices show no significant improvements in the Voc and FF values but a substantial increase in the Jsc values. These results suggest that the alkali effects on the Cu(In,Ga)Se2 film and device properties strongly depend on the group III elemental composition in the Cu(In,G...

Revisiting radiative deep-level transitions in CuGaSe2 by photoluminescence

Recent defect calculations suggest that the open circuit voltage of CuGaSe2 solar cells can be limited by deep intrinsic electron traps by GaCu antisites and their complexes with Cu-vacancies. To gain experimental evidence, two radiative defect transitions at 1.10 eV and 1.24 eV are characterized by steady-state photoluminescence on epitaxial-grown CuGaSe2 thin films. Cu-rich samples are studied, since they show highest crystal quality, exciton luminescence, and no potential fluctuations. Variations of the laser intensity and temperature dependent measurements suggest that emission occurs from two deep donor-like levels into the same shallow acceptor. At 10 K, power-law exponents of 1 (low excitation regime) and 1/2 (high excitation regime) are observed identically for both transitions. The theory and a fitting function for the double power law is derived. It is concluded that the acceptor becomes saturated by excess carriers which changes the exponent of all transitions. Activation energies determined from the temperature quenching depend on the excitation level and show unexpected values of 600 meV and higher. The thermal activation of non-radiative processes can explain the distortion of the ionization energies. Both the deep levels play a major role as radiative and non-radiative recombination centers for electrons and can be detrimental for photovoltaic applications.

A comparative study of the effects of sputtering deposition conditions for ZnO surface electrode layers on Cu(In,Ga)Se2 and CuGaSe2 solar cells

The effects of post p-n junction formation processes, namely transparent conducting oxide (TCO) layer deposition conditions on Cu(In,Ga)Se2 (CIGS) and CuGaSe2 (CGS) solar cell device properties were comparatively studied. It was found that CIGS devices were relatively insensitive to the presence of oxygen and heat during TCO layer deposition, whereas CGS devices were extremely sensitive to oxygen and heat, resulting in a degradation of device performance in the presence of them. The use of the relatively low TCO deposition temperature (Ts) of 80°C led to significant improvements in the fill factor values of CGS devices compared to the case for Ts ~180°C, though the open circuit voltage was reduced. These results suggest that TCO deposition conditions can be an important parameter which critically affects ternary CGS solar cell device properties. It is, therefore, suggested that TCO layer deposition conditions for CGS devices should be optimized independently of that for conventional narrow-gap CIGS devices.

Interface oxygen and heat sensitivity of Cu(In,Ga)Se2 and CuGaSe2 solar cells

Combined oxygen and heat exposure processes after p-CuGaSe2/n-CdS junction formation degrade CuGaSe2 solar cell efficiency, whereas such annealing processes can improve high In content Cu(In,Ga)Se2 device performance. This result is chiefly attributable to different interface structures consisting of oxygen-sensitive CuGaSe2 or relatively oxygen-insensitive Cu(In,Ga)Se2. To reduce CuGaSe2 interfacial recombination, reduction of the process temperature of the front contact layer deposition process is found to be the key. In this work, fill factor values exceeding 0.7 are reproducibly obtained from CuGaSe2 solar cells, though such high fill factor values have been very challenging to demonstrate to date using CuGaSe2 photoabsorber layers.

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.

Cationic point defects in CuGaSe2 from a structural perspective

The chalcopyrite semiconductors show large tolerances to deviations from stoichiometry by keeping the crystal structure. Such deviations always cause structural inhomogeneities and charge mismatches which influence the material properties. We studied the type and concentration of cationic point defects on Cu-poor CuGaSe2 powders by the complementary use of neutrons and photons. It is demonstrated that the main existing defects in this Cu-poor wide gap semiconductor are copper-vacancies (VCu) and gallium on interstitial sites (Gai). The latter may explain why tailoring a highly efficient CuGaSe2 solar cell is an even more challenging task than previously expected.

Controlled electrodeposition of Cu–Ga from a deep eutectic solvent for low cost fabrication of CuGaSe2 thin film solar cells

The electrochemical deposition of Ga and Cu-Ga alloys from the deep eutectic solvent choline chloride/urea (Reline) is investigated to prepare CuGaSe(2) (CGS) semiconductors for their use in thin film solar cells. Ga electrodeposition is difficult from aqueous solution due to its low standard potential and the interfering hydrogen evolution reaction (HER). Ionic liquid electrolytes offer a better thermal stability and larger potential window and thus eliminate the interference of solvent breakdown reactions during Ga deposition. We demonstrate that metallic Ga can be electrodeposited from Reline without HER interference with high plating efficiency on Mo and Cu electrodes. A new low cost synthetic route for the preparation of CuGaSe(2) absorber thin films is presented and involves the one-step electrodeposition of Cu-Ga precursors from Reline followed by thermal annealing. Rotating disk electrode (RDE) cyclic voltammetry (CV) is used in combination with viscosity measurements to determine the diffusion coefficients of gallium and copper ions in Reline. The composition of the codeposited Cu-Ga precursor layers can be controlled to form Cu/Ga thin films with precise stoichiometry, which is important for achieving good optoelectronic properties of the final CuGaSe(2) absorbers. The morphology, the chemical composition and the crystal structure of the deposited thin films are analysed by scanning electron microscopy/energy dispersive X-ray spectroscopy (SEM/EDX) and X-ray diffraction (XRD). Annealing of the Cu-Ga films in a selenium atmosphere allowed the formation of high quality CuGaSe(2) absorber layers. Completed CGS solar cells achieved a 4.1% total area power conversion efficiency.

The effect of composition on structural and electronic properties in polycrystalline CuGaSe2 thin film

Polycrystalline CuGaSe2 thin films on Mo-coated soda-lime glass substrates have been synthesized by co evaporation process from Cu, Ga and Se sources. Structural and electrical properties of the as-grown CuGaSe2 films strongly depend on the film composition. Stoichiometric CuGaSe2 is fabricated, as indicated by x-ray dirraction spectroscope (XRD) and x-ray fluorescence (XRF). A two-phase region is composed of CuGaSe2 and Cu2-xSe phases for Cu-rich films, and CuGaSe2 and CuGa3Se5 phases for Ga-rich films respectively. Morphological properties are detected by scanning electron microscope (SEM) for various compositional films, the grain sizes of the CuGaSe2 films decrease with the extent of deviation from stoichiometric composition. Raman spectroscopy of Cu-rich samples shows that there exist large Cu-Se particles on the film surface. The results from Hall effect measurements for typical samples indicate that CuGaSe2 films are always of p-type semiconductor from Cu-rich to Ga-rich. Stoichiometric CuGaSe2 films exhibit relatively large mobility than any other compositional films. Finally, polycrystalline CuGaSe2 thin film solar cell with a best conversion effciency of 6.02% has been achieved under the standard air mass (AM)1.5 spectrum for 100 mW/cm2 at room temperature (aperture area, 0.24 cm2). The open circuit voltage of the CuGaSe2 solar cells is close to 770 mV.

Microstructures and Photovoltaic Properties of Wide-Gap CuGaSe2 Solar Cells Fabricated by Using a Co-Evaporation Process

Microstructures and Photovoltaic Properties of Wide-Gap CuGaSe2 Solar Cells Fabricated by Using a Co-Evaporation Process

Electroluminescence mapping of CuGaSe2 solar cells by atomic force microscopy

The authors report on the observation of electroluminescence (EL) in CuGaSe2 solar cells using tapping-mode atomic force microscopy based on tuning-fork sensors. Individually injected current pulses are seen during intermittent contact driven by an external bias applied to the conducting tip. It follows that EL can be stimulated when the solar cell is forward biased during the contact cycle. Local L-V characteristics show evidence for EL, with a threshold voltage of 3.0–3.7V. Mapping of the photon emission suggests that grain boundaries effectively isolate grain interiors, which behave as individual light-emitting diodes.

Lift-off process and rear-side characterization of CuGaSe2 chalcopyrite thin films and solar cells

An alternative approach to the so-called “lift-off” technology is presented, in which a CuGaSe2 solar cell absorber film is detached from a Mo-coated glass substrate. The proposed lift-off takes advantage of an interfacial MoSe2 layer, acting as a sacrificial layer, which forms at the rear contact during the growth of the CuGaSe2 film. No additional processing step is thus required to proceed with the lift-off. The lift-off was carried out in ultrahigh vacuum for quality assessment, and the rear CuGaSe2 and top MoSe2 surfaces were characterized by means of surface-sensitive techniques, namely, Kelvin probe force microscopy and photoelectron spectroscopy. The cleanness of the CuGaSe2 rear surface was confirmed by the absence of Mo remnants, thus demonstrating the suitability of the proposed method for further processing of the absorber film onto alternative substrates. In addition, a quantitative analysis of surface photovoltage, doping concentration, and interface charge at grain boundaries on the absorber’s rear surface is presented, exploiting the convenience of the procedure for characterization purposes. Preliminary results regarding the device performance and identification of limiting factors are reported.

Electronic Properties of Modified CuGaSe2 Solar Cells

AbstractWe report on the electronic properties of the state-of the art surface-modified CuGaSe2 solar cells. We compare between the 10.2%-efficient surface-modified and the 9.53%-efficient unmodified CuGaSe2 solar cells. We examined our cells using deep level transient spectroscopy (DLTS) and drive level capacitance profiling (DLCP). The DLTS data for the modified CGS exhibits minority traps with activation energies ranging from Ec — 0.3 to Ec — 0.6 eV, whereas that for the pure CGS exhibits minority traps with activation energies ranging from Ec — 0.06 to Ec — 0.1 eV (where Ec is the energy of electrons at the conduction band minimum). While varying the filling pulse duration, we observed the gradual increase in the amplitude of the DLTS signal for these states until it apparently saturates at a pulse duration of ˜1s for the 10.2% cell, and 0.05 s for the 9.53% cell. Increasing the duration of the filling pulse also results in broadening of the DLTS signals and shifting of the maximum of these signals towards lower temperature, whereas the high-temperature sides coincide. Using a model that allows us to distinguish between bandlike states and localized ones based on the dependence of the shape of their DLTS-signal on the filling-pulse duration, we relate the electron trap to bandlike states. The DLCP data shows that the 10.2% cell has a lower carrier concentration, a more uniform defect density profile, a larger depletion width, and a higher drift collection length for photo-generated carriers as compared to our 9.53% CuGaSe2 cell. We also recorded the transient capacitance versus time and found that the 10.2% has responded differently compared with the 9.53% one. The transient capacitance decay curves for these two cells are different.

Performance of CuGaSe2 Solar Cells Grown By Co-Evaporation Process

AbstractCuGaSe2 absorber layers were prepared by evaporating elemental Cu, Ga and Se in three stage on Molybdenum coated soda lime glass. The composition of the resultant film was studied by monitoring the substrate temperature, which decreased when a Cu-Se secondary phase was formed. As the Ga supplement increased during the third stage the void that formed in the beginning of the third stage was removed, while a small grain Ga-rich layer was formed on the surface, resulting in a Cu deficient surface. Therefore, the Voc was improved because of the enhanced surface morphology and the Jsc was reduced, due to the Ga rich layer on of the surface. Under optimal conditions, we achieved a cell performance of Voc = 780 mV, Jsc = 12.9 mA/cm2, ff = 62.5 and ν = 7.3 %.