Cu(In,Ga)Se2 Solar Cells Research Articles

Optimizing Wide Band Gap Cu(In,Ga)Se2 Solar Cell Performance: Investigating the Impact of “Cliff” and “Spike” Heterostructures

In recent years, the efficiency of high-efficiency Cu(In,Ga)Se2 (CIGS) solar cells has been significantly improved, particularly for narrow-gap types. One of the key reasons for the enhancement of narrow-gap device performance is the formation of the “Spike” structure at the CdS/CIGS heterojunction interface. Wide-gap CIGS solar cells excel in modular production but lag behind in efficiency compared to narrow-gap cells. Some studies suggest that the “Cliff” structure at the heterojunction of wide-gap CIGS solar cells may be one of the factors contributing to this decreased efficiency. This paper utilizes the SCAPS software, grounded in the theories of semiconductor physics and photovoltaic effects, to conduct an in-depth analysis of the impact of “Cliff” and “Spike” heterojunction structures on the performance of wide band gap CIGS solar cells through numerical simulation methods. The aim is to verify whether the “Spike” structure is also advantageous for enhancing wide-gap CIGS device performance. The simulation results show that the “Spike” structure is beneficial for reducing interfacial recombination, thereby enhancing the VOC of wide-gap cells. However, an electronic transport barrier may form at the heterojunction interface, resulting in a decrease in JSC and FF, which subsequently reduces device efficiency. The optimal heterojunction structure should exhibit a reduced “Cliff” degree, which can facilitate the reduction of interfacial recombination while simultaneously preventing the formation of an electronic barrier, ultimately enhancing both VOC and device performance.

High efficiency wide gap Cu(In,Ga)Se2 solar cells: Influence of buffer layer characteristics

Wide-gap Cu(In,Ga)Se2 (CIGS) solar cells exhibit a superior match to the solar spectrum, resulting in a higher ideal efficiency (Eff). However, in reality, their device Eff is lower than that of narrow-gap CIGS solar cells. This study aims to identify the factors that limit the performance enhancement of wide-gap CIGS solar cells, focusing on the characteristics of the buffer layer. The influence of the thickness and doping concentration of the CdS layer on the built-in electric field and interfacial recombination of the heterojunction has been investigated through simulation. The simulation results indicate that when the doping concentration of the CdS layer is lower than or similar to that of the CGS layer, decreasing the thickness of the CdS layer (e.g., 10 nm) is beneficial for improving device performance. However, if it is higher than that of the CGS layer, increasing the thickness of the CdS layer (e.g., 50 nm) is conducive to improving device performance. The thickness of the CdS layer that maximizes the Eff of the wide-gap CGS device should be approximately 50 nm, and its doping concentration should be higher than that of the CGS layer. This optimization can simultaneously enhance the built-in electric field of the heterojunction and minimize its interfacial recombination, thereby improving the open-circuit voltage and Eff of wide-gap CGS devices.

16.48% Efficient solution-processed CIGS solar cells with crystal growth and defects engineering enabled by Ag doping strategy

Solution-processed Cu(In,Ga)Se2 (CIGS) solar cells suffer from serious carrier recombination and power conversion efficiency (PCE) loss because of the poor film properties and easy formation of defects. Herein, we propose Ag&Se co-selenization strategy to enhance the crystallization and passivate harmful defects of the CIGS films. The formation of Ag-Se phase during the selenization process enables the formation of large grains and suppresses the deep level defects. It is found that Ag doping can enlarge the depletion region width, lower the Urbach energy and prolong the carrier lifetime. As a result, a champion solution-processed CIGS solar cell presents a high efficiency of 16.48% with the highly improved open-circuit voltage (VOC) of 662 mV and fill factor (FF) of 75.8%. This work provides an efficient strategy to prepare high quality solution-processed CIGS films for high-performance CIGS solar cells.

Effect of solution‐processed cesium carbonate on Cu(In,Ga)Se2 thin‐film solar cells

AbstractHeavy alkali‐metal treatments have been the most recent breakthrough in improving the efficiency of Cu(In,Ga)Se2 (CIGS) solar cells. Alkali halides are generally evaporated onto the surface of the CIGS thin film by a vacuum process. Here, we report an alternative, low‐cost solution process for the surface treatment of CIGS thin films using cesium carbonate (CsCO3) as a new route to incorporate cesium (Cs) for improving solar cell performance. CIGS thin films were fabricated using pulsed hybrid reactive magnetron sputtering and the surface treatment was performed by spin‐coating CsCO3 solution on the surface of CIGS at room temperature, followed by vacuum annealing at 400°C. The surface chemistry of the CIGS thin film changed after the treatment and the efficiency of respective solar cells improved by more than 30%, mostly driven by an enhancement in open‐circuit voltage. X‐ray photoelectron spectroscopy revealed the depletion of copper and the presence of Cs on the surface of the CIGS thin film. Ultraviolet photoelectron spectroscopy showed the lowering of the valence band maximum by around 0.25 eV after the treatment, which plays a positive role in reducing interfacial recombination. High‐resolution transmission electron microscopy indicates the presence of Cs and depletion of Cu at the grain boundaries of the CIGS thin film. These findings open a low‐cost route for improving the performance of CIGS solar cells by surface modification using a solution process.

Multiple Impacts of the Aluminum Oxide Passivation Layer on the Properties OF Cu(In,Ga)Se2 Solar Cells

AbstractIn this study, the origins of efficiency gains in Cu(In,Ga)Se2 (CIGS) solar cells are investigated by introducing an Al2O3 passivation layer in terms of the oxidation condition of Mo back contact, alkali‐metal diffusion, minority carrier lifetimes (τ), and charge conditions. The study reveals that introduction of an Al2O3 back‐contact passivation layer into solar cells yields multiple impacts. Al2O3 deposition enhances the oxidation of the Mo back contacts, increasing Na solubility in Mo and Na diffusion from Mo into the CIGS layer, thereby modifying the metastable properties of CIGS. The charge condition at the CIGS/Al2O3 interface is not fixed negative charge but variable, dependent on whether electrons or holes are supplied. During solar cell operation, the interfacial charge condition is expected to be neutral or positive for Al2O3 grown using plasma or thermal atomic layer deposition techniques, respectively. Moreover, the mechanical peeling off of CIGS from Mo back contact enhanced τ in a similar way as with the insertion of Al2O3. Based on this study, the enhancement of alkali metal supply and the removal of direct contact of CIGS to the metal contact (Mo) can play crucial roles in improving the performance of CIGS solar cell.

Understanding of the Relationship between the Properties of Cu(In,Ga)Se2 Solar Cells and the Structure of Ag Network Electrodes

The relation between the structure of the silver network electrodes and the properties of Cu(In,Ga)Se2 (CIGS) solar cells is systemically investigated. The Ag network electrode is deposited onto an Al:ZnO (AZO) thin film, employing a self‐forming cracked template. Precise control over the cracked template's structure is achieved through careful adjustment of temperature and humidity. The Ag network electrodes with different coverage areas and network densities are systemically applied to the CIGS solar cells. It is revealed that predominant fill factor (FF) is influenced by the figure of merit of transparent conducting electrodes, rather than sheet resistance, particularly when the coverage area falls within the range of 1.3–5%. Furthermore, a higher network density corresponds to an enhanced FF when the coverage areas of the Ag networks are similar. When utilizing a thinner AZO film, CIGS solar cells with a surface area of 1.0609 cm2 exhibit a notable performance improvement, with efficiency increasing from 10.48% to 11.63%. This enhancement is primarily attributed to the increase in FF from 45% to 65%. These findings underscore the considerable potential for reducing the thickness of the transparent conductive oxide (TCO) in CIGS modules with implications for practical applications in photovoltaic technology.

Alkali Metal Pretreatment for Precise Na Doping and Voc Improvement in CIGS Thin-Film Solar Cells.

The pretreatment of the Cu(In,Ga)Se2 (CIGS) absorption layer using an alkali element can effectively improve the photoelectric conversion efficiency (PCE) of CIGS solar cells. Here, we propose using NaF layer pretreatment below the CIGS absorption layer deposited by a three-stage process. Sodium ions in NaF can effectively suppress the diffusion of Ga elements and form a steep gradient backscatter layer on the back of the CIGS absorption layer, thereby passivating solar cell defects, inhibiting carrier recombination, promoting carrier transmission and collection, improving open circuit voltage (VOC), short circuit current (Jsc), and filling factor (FF), and further improving the PCE.

CIGS bifacial solar cells with novel rear architectures: Simulation point of view and the creation of a digital twin

The performance of bifacial Cu(In,Ga)Se2 (CIGS) cells does not yet reach the one of monofacial cells, with standard Mo rear contacts. By exploring optoelectronic simulations, this study addresses the impact of a transparent conducting oxide layer used as rear contact in CIGS solar cells. These simulations, which aim to create a digital twin of bifacial CIGS solar cells, offer insight into the impact of novel rear contact architectures. It was observed that with an indium tin oxide (ITO) rear contact the CIGS cell performance lacks in comparison to a Mo one due to an electrical barrier, under rear side illumination these devices may only reach 3.9 % in conversion efficiency. The implementation of dielectric rear passivation schemes have great performance benefits in cells with Mo rear contacts. Therefore, the same passivation strategy in ITO contacts is discussed and additional novel architectures are proposed, from which a design of alternated lines between dielectric and 15 nm Mo layers on ITO showed promising results. Such rear architectures aim to achieve i) a highly transparent rear contact, ii) an ohmic-like behaviour with CIGS, and iii) a low defective interface with rear passivation. The selenization of the complete 15 nm Mo in the proposed novel architecture during the CIGS growth was considered, resulting in transmittance values above 70 % and, notably, reaching the transparency of bare ITO in the infrared range. Such rear architecture may provide for bifacial CIGS solar cells with conversion efficiency values surpassing 20 and 15 % for front and rear side illumination, respectively, allowing to fully exploit bifaciality.

Suppressing interface recombination via element diffusion regulation towards high-efficiency Cd-free Cu(In,Ga)Se2 solar cells

Magnetron-sputtered Zn1-xMgxO (ZMO) films suffer from severe elemental diffusion to Cu(In,Ga)Se2 (CIGS), which limits the further improvement of device performance. Herein, we introduced Zn(O,S) (ZOS) as a barrier layer, and we controlled the diffusion of Zn within an appropriate range by adjusting the annealing temperature of ZOS. The results indicate that Zn can occupy copper vacancies (VCu) in CIGS and easily diffuse through VCu. A moderate amount of Zn at the interface acts as a charge compensator, reducing the band tailing effect and promoting the carrier collection efficiency of photovoltaic (PV) devices. However, excessive Zn doping induces additional defects dominated by ZnCu, ZnGa and ZnIn, leading to severe nonradiative recombination. Consequently, the Cd-free CIGS solar cell produced based on these insights exhibited a 49.4-mV reduction in the Voc deficit (Voc,def) as well as a notable increase in the power conversion efficiency (PCE) to 18.0 %. This study is the first to reveal the mechanism of defects caused by Zn diffusion, and these findings provide theoretical support for the study of Cd-free thin-film solar cells.

Comparison of polycrystalline and epitaxial Cu(In, Ga)Se2 solar cells with conversion efficiencies of more than 21%

Polycrystalline Cu(In,Ga)Se2 (CIGS) solar cells are widely studied for their application in thin-film photovoltaics, but analysis of their crystalline properties is difficult owing to the presence of grain boundaries and defects. Epitaxial CIGS (epi-CIGS) solar cells can potentially address the issues of polycrystalline CIGS (poly-CIGS) solar cells, but their conversion efficiency has not been widely studied. We fabricated epi-CIGS layers using techniques developed for high-efficiency ploy-CIGS solar cells such as Ga grading, Na doping, and heat-light soaking. Here, scanning transmission electron microscopy and secondary ion mass spectrometry were used to compare the structural characteristics of poly- and epi-CIGS solar cells with conversion efficiencies of more than 21%. Both types of solar cells had similar Ga gradient profiles, and abrupt interfaces were observed between the CIGS and Mo, CIGS and GaAs, and CdS and CIGS layers. The poly-CIGS layer had a higher Na concentration than the epi-CIGS layer because the grain boundaries of the former included high concentrations of Na ions. Meanwhile, the Na ions in the epi-CIGS layer promoted the interdiffusion of In and Ga. For both types of solar cells, heat-light soaking increased the carrier concentration by more than 1 × 1017 cm−3. These observations suggest that grain boundaries are not the main factor limiting the conversion efficiency of poly-CIGS solar cells.

Synergistic Defect Management for Boosting the Efficiency of Cu(In,Ga)Se2 Solar Cells

In this study, a feasible strategy is proposed for directly depositing high-quality Cu(In,Ga)Se2 (CIGS) films using Na-doped targets in a selenium-free atmosphere to boost the power conversion efficiency (PCE) of CIGS solar cells. Introducing a small amount of sodium dopant effectively promoted the textured growth of CIGS crystals in the prepared films, resulting in larger grain sizes and a smoother interface. The higher MoSe2 content at the CIGS/Mo interface increased the carrier lifetime in the films. In addition, sodium doping increased the proportion of Se atoms on the film surface and reduced the concentration of defects caused by the direct sputtering of the films in the selenium-free atmosphere. Therefore, the separation and transportation of photo-generated carriers in the devices were effectively enhanced. Using the optimized parameters, a record-high PCE of 17.26% was achieved for the 7.5% Na-doped devices, which represents an improvement of nearly 63%.

Effect of monolithic photovoltaic-photoelectrochemical-integrated Cu(In, Ga)Se2-related co-planar device on water splitting reaction

This study presents the fabrication of a photovoltaic-photoelectrochemical-integrated (PV-PEC) (Cu(In, Ga)Se2 (CIGS)-related co-planar device (integrated CIGS co-planar water splitting device) that resulted in an onset potential (V onset) of over 1.23 V to achieve photoelectrochemical water splitting without external bias. Therefore, the utility of this device was indicated for unassisted water splitting reaction. Moreover, the effects of the open-circuit voltage of the CIGS solar cell part on the photocurrent density and V onset of the fabricated water splitting device were investigated. These results suggest that the applying the reverse bias owing to V oc of the CIGS solar cell part influences the space charge layer at the surface of the CIGS photoelectrode. This effect leads to the formation of an inversion layer, suppressing surface recombination on the CIGS photoelectrode and contributing to an increase in the photocurrent density. The results represent a preliminary step toward realizing potential applications of the CIGS PV-PEC device for the unassisted water splitting reaction.

Radiation resistant chalcopyrite CIGS solar cells: proton damage shielding with Cs treatment and defect healing via heat-light soaking

Cu(In, Ga)Se2 (CIGS) solar cells are recognized as next-generation space technology due to their flexibility, lightweight nature, and excellent environmental stability.

Highly Improved PID Stability for Cu(In,Ga)Se2 Solar Modules Due to a Filled P1 Groove

Two types of Cu(In,Ga)Se2 (CIGS) thin-film solar modules, differing only in the patterning procedures, were exposed to a high voltage (1 kV) across the thickness of the soda-lime glass substrate. Both module types utilized a cell stack, and in particular, a molybdenum back contact, which was optimized for CIGS solar cells with enhanced stability against potential-induced degradation (PID). A standard module with regular patterning lines for monolithic interconnection consists of P1, P2, and P3 lines, with P1 separating the back contact, P2 establishing the contact between the front contact TCO of one cell to the Mo back contact of the next cell, and P3 isolating the front contact from one cell to the next cell. However, modules employing this cell stack with regular patterning lines P1, P2, and P3 suffered considerably from PID while modules with a P1 groove filled with an insulator showed greatly enhanced stability similar to the pure single cell without P1 patterning. PID manifests once a specific quantity of charge has been transmitted through the soda-lime glass substrate, while the molybdenum back contact of the cell functions as the cathode, maintaining a negative bias relative to the substrate’s backside. As a consequence of PID, sodium is increased in the adsorber for susceptible cells. Therefore, inhibiting sodium transport through the P1 groove by filling it with an insulating material enhances the PID stability of the modules considerably. As a result, the modules with a filled P1 groove showed similar stability to the single solar cells with an improved PID stable cell stack, while modules without a filled P1 patterning were much more susceptible to PID, although a cell stack with greatly enhanced PID stability was used. In summary, the presented strategy to fill the P1 groove offers a viable and novel path to improved PID stability of CIGS modules.

Numerical Simulation, Preparation, and Evaluation of Cu(In, Ga)Se2 (CIGS) Thin-Film Solar Cells

This study presents the numerical simulation, optimization, preparation, and characterization of Cu(In, Ga)Se2 (CIGS) thin-film solar cells (TFSCs). Different cell parameters were investigated, including Ga/(Ga+In) (GGI) ratios, the thicknesses of CIGS absorption layers, the fill factor (FF), the open-circuit voltage (Voc), and the short-circuit current (Isc). The effects of the simulated parameters on the power conversion efficiency (η) of each prototype CIGS cells were investigated. The optimal GGI ratio was approximately 0.6. Using COMSOL Multiphysics software, a CIGS layer thickness of 2 μm and an η of 17% was calculated, assuming constant operating temperatures. Moreover, prototype CIGS solar cells with various compositions were prepared via a simple and cost-effective method based on sol–gel, sonication, and spin-coating techniques. The microstructures and electrical and optical properties of the CIGS-based solar cells were evaluated using current–voltage (I-V) characteristics, scanning electron microscopy (SEM), X-ray diffraction, atomic force microscopy (AFM), and UV-vis spectroscopy. The elemental compositions of the solar cell layers were evaluated via energy-dispersive X-ray fluorescence (EDXRF). The obtained results were compared with the experimental results. For example, in a prototype cell with a CIGS absorption layer thickness of 2 μm and a GGI ratio of 0.6, the experimental value of η was about 15%. Our results revealed that the agreement between the simulation results and the experimental findings for most of the simulated parameters is quite good. These findings indicate that a non-destructive analysis based on EDXRF is a versatile tool for evaluating CIGS solar cells in a very short time with excellent repeatability for both layer composition and thickness.

Chemical phases in the solution-grown Zn(O,S) buffer of post-annealed Cu(In,Ga)Se2 solar cells investigated by transmission electron microscopy and electroreflectance

Thin-film solar cells with Cu(In,Ga)Se2 (CIGS) absorber layers have been intensively studied due to their high power conversion efficiencies. CIGS solar cells with Zn(O,S) buffer layers achieved record efficiencies due to their reduced parasitic absorption compared with the more commonly used CdS buffer. Accordingly, we have studied solution-grown Zn(O,S) buffer layers on polycrystalline CIGS absorber layers by complementary techniques. A bandgap energy Eg of 2.9 eV is detected by means of angle-resolved electroreflectance spectroscopy corresponding to Zn(O,S), whereas an additional Eg of 2.3 eV clearly appeared for a post-annealed CIGS solar cell (250 °C in air) compared with the as-grown state. To identify the chemical phase that contributes to the Eg of 2.3 eV, the microstructure and microchemistry of the Zn(O,S) buffer layers in the as-grown state and after annealing were analyzed by different transmission electron microscopic techniques on the submicrometer scale and energy-dispersive x-ray spectroscopy. We demonstrate that the combination of these methods facilitates a comprehensive analysis of the complex phase constitution of nanoscaled buffer layers. The results show that after annealing, the Cu concentration in Zn(O,S) is increased. This observation indicates the existence of an additional Cu-containing phase with Eg close to 2.3 eV, such as Cu2Se (2.23 eV) or CuS (2.36 eV), which could be one possible origin of the low power conversion efficiency and low fill factor of the solar cell under investigation.

Annealing effects on Cu(In,Ga)Se2 solar cells irradiated by high-fluence proton beam

Radiation tolerance of Cu(In,Ga)Se2 (CIGS) solar cells has been investigated using high-fluence proton beam irradiation for application to devices in extremely-high-radiation environments. CIGS solar cells deteriorated after high-energy proton irradiation with non-ionizing energy loss of 1 × 1016 MeVneq cm−2, however, the CIGS solar cells could generate power after high-fluence irradiation. The ideality factors increased from 1.3 to 2.0, and series resistance increased, indicating that the concentration of recombination centers increased in CIGS layers. After heat-light annealing, the conversion efficiencies gradually recovered, and the recombination centers were confirmed to be partly passivated by annealing at 90 °C. The short-circuit currents for 10 μm thick CIGS solar cells were recovered by dark annealing in the same manner as for 2 μm thick CIGS solar cells. Dark annealing on irradiated CIGS solar cells has beneficial effects on passivate the recombination centers, even using thicker CIGS layers.

Reverse-Bias Defect Creation in Cu(In,Ga)Se2 Solar Cells and Impact of Encapsulation

Reverse breakdown in Cu(In,Ga)Se2 (CIGS) solar cells can lead to defect creation and performance degradation. We present pulsed reverse-bias experiments, where we stress CIGS solar cells with a short reverse voltage pulse of ten milliseconds and detect the electrical and thermal response of the cell. This way, we limit the duration of the reverse stress, allowing us to study the initial stages of reverse-bias defect creation in CIGS solar cells and modules. Our results show that permanent damage can develop very fast in under milliseconds. Furthermore, we find the location of defect creation as well as the susceptibility to defect creation under reverse bias depends strongly on whether the cell is encapsulated or not, where encapsulated cells are generally more robust against reverse bias.

A process study of high-quality Zn(O,S) thin-film fabrication for thin-film solar cells

Abstract The Zn(O,S) thin film is considered a most promising candidate for a cadmium-free buffer layer of the Cu(In,Ga)Se2 (CIGS) thin-film solar cell due to its advantages of optical responses in the short-wavelength region and adjustable bandgap. In this paper, the thin-film growth mechanism and process optimization of Zn(O,S) films fabricated using the chemical bath deposition method are systematically investigated. The thickness and quality of Zn(O,S) films were found to be strongly affected by the concentration variation of the precursor chemicals. It was also revealed that different surface morphologies of Zn(O,S) films would appear if the reaction time were changed and, subsequently, the optimum reaction time was defined. The film-growth curve suggested that the growth rate varied linearly with the deposition temperature and some defects appeared when the temperature was too high. In addition, to further improve the film quality, an effective post-treatment approach was proposed and the experimental results showed that the microstructure of the Zn(O,S) thin film was improved by an ammonia etching process followed by an annealing process. For comparison purposes, both Zn(O,S)-based and CdS-based devices were fabricated and characterized. The device with a Zn(O,S)-CIGS solar cell after post-treatment showed near conversion efficiency comparable to that of the device with the CdS-CIGS cell.

Enhancement in Efficiency of CIGS Solar Cell by Using a p-Si BSF Layer

Copper–indium–gallium–diselenide Cu(In,Ga)Se2 (CIGS) is a semiconductor compound belonging to group I-III-VI, with a chalcopyrite crystal structure. CIGS is promising for the development of high-performance photovoltaic applications in terms of stability and conversion efficiency. It is one of the main candidates to rival the efficiency and stability of conventional crystalline silicon cells, due to its high light absorption coefficient, lower material cost, and high stability. The limitation of its use is that CIGS integrates indium (In) and gallium (Ga), which are rare and expensive materials. The amount of these materials in the CIGS cell can be reduced by optimizing the thickness of the absorber. We show that the introduction of a layer of highly doped silicon in the structure of the solar cell between the absorber layer and the back surface field layer effectively allows for decreasing the thickness of the absorber. Within the same objective, we focus on the danger of cadmium in the CdS buffer layer. In the first optimizations, we replaced the n-type CdS buffer layer with a n-type Zn(O,S) buffer layer. For this work, we used a one-dimensional simulation program, named Solar Cell Capacitance Simulator in one Dimension (SCAPS-1D), to investigate this new CIGS solar cell structure. After optimization, a maximum conversion efficiency of 24.43% was achieved with a 0.2 μm CIGS absorber layer and a 1 µm Si BSF layer.