Se2 Thin Film Solar Cells Research Articles

Interplay between strain and charge in Cu(In,Ga)Se2 flexible photovoltaics

Flexible and lightweight Cu(In,Ga)Se2 (CIGS) thin-film solar cells are promising for versatile applications, but there is limited understanding of stress-induced changes. In this study, the charge carrier generation and trapping behavior under mechanical stress was investigated using flexible CIGS thin-film solar cells with various alkali treatments. Surface current at the CIGS surface decreased by convex bending, which occurs less with the incorporation of alkali metals. The formation energy of the carrier generating defects increased in convex bending environments clarifying the degradation of the surface current. Moreover, alkali-related defects had lower formation energy than the intrinsic acceptors, mitigating current degradation in mechanical stress condition. The altered defect energy levels were attributed to the deformation of the crystal structure under bending states. This study provides insights into the mitigating of strain-induced charge degradation for enhancing the performance and robustness of flexible CIGS photovoltaic devices.

Bandgap-graded Cu2Sn1-xGexS3 thin film solar cells prepared by sputtering SnGe/Cu targets

Cu2Sn1-xGexS3 (CTGS) thin films were created through sulfurizing SnGe/Cu precursors deposited by the sputtering method. The effects of different sulfurization conditions on the crystallinity, morphology, composition, optical-electrical properties and chemical information of the CTGS thin films have been investigated in detail. The results demonstrate that the CTGS thin films obtained by using sulfurization temperature at 550 °C for 20min have the best crystal quality and smooth surface morphology with largest grains and the best optical-electrical properties. Meanwhile, CTGS thin films solar cell with the best performance is found that the CTGS film possesses a graded band gap structure, in which the band gap gradually become larger towards the Mo electrode, and the CdS/CTGS hetero-junction interface shows the “type I″ energy band alignment with a “spike-like” structure. As a result, the obtained bandgap-graded CTGS thin film solar cells with the best power conversion efficiency (PCE) of 3.35 (±0.04) % manifest an open-circuit voltage of 389 (±7.78) mV and a short-circuit current density of 26.98 (±0.54) mA cm−2. This work demonstrates a promising route to develop high-efficiency Cu(In,Ga)Se2 or Cu2SnS3-based thin film solar cells.

Highly conductive and transparent metal microfiber networks as front electrodes of flexible thin-film photovoltaics

Simultaneously enhancing the optical transmittance and electrical conductivity of transparent conductors (TCs) applicable in various optoelectronic devices is a long-standing challenge. Herein, we present an innovative approach for fabricating electroplated Ni and Cu microfiber networks (NiMF and CuMF) as highly conductive TCs to realize high efficiency and desired aesthetics in thin-film solar cells and modules. The metal microfibers (MFs) are fabricated using electrospun polyacrylonitrile nanofibers as the template. The large cross-sectional aspect ratio of the metal MF networks remarkably and concurrently improves their electrical conductivity and optical transmittance. Between the NiMF and CuMF TCs, the CuMF sample exhibits a superior figure of merit owing to its exceptionally low electrical resistivity. The metal MF TC is a promising alternative to conventional patterned grids used in flexible Cu(In,Ga)Se2 thin-film solar cells, because it effectively reduces the series resistance, which is advantageous for large-area cells. The CuMF can be successfully employed as a ribbon to maintain the solar cell performance in centimeter-scale cells connected in series. The outstanding performance of the metal MF TCs indicates their potential to eliminate complicated monolithic integration processes or front grids and ribbons in flexible thin-film solar cells and modules.

Influence of oxygen content on the properties of In2(OxS1−x)3 used as buffer material in Cu(In,Ga)Se2 solar cells

We investigate magnetron-sputtered In2(OxS1−x)3 compounds acting as an alternative buffer system to the solution-grown CdS or Zn(O,S) buffer layers in Cu(In,Ga)Se2 (CIGS) thin-film solar cells. The influence of the oxygen content on the solar cell performance, microstructure of the mixed systems, bandgap, and band offsets to CIGS is investigated experimentally and also characterized by calculations based on density functional theory. Samples in a series with different chemical compositions ranging from In2S3 to In2O3 are either directly deposited from ceramic targets or from a pure In2S3 target by reactive sputtering by adding O2 in the Ar sputtering gas. The binary compounds In2S3 and In2O3 sputtered at 220 °C substrate temperature from ceramic targets exhibit a crystalline structure, whereas the ternary In2(O,S)3 compounds are either nanocrystalline in the case of In2(O0.25S0.75)3 or amorphous for In2(O0.5S0.5)3 and In2(O0.75S0.25)3. For [O]/([O] + [S]) ratios above 0.25, the cell efficiencies decrease drastically, mainly due to lower open-circuit voltages (VOC). This behavior can be explained by an increase of the negative conduction band offset between the CIGS absorber and the oxygen-rich In2(OxS1−x)3 or In2O3 buffer, resulting in pronounced VOC losses. Adding oxygen to In2S3 with optical bandgap energies of around 2 eV results in a bowing of the values to below 2 eV and finally reaching values of around 2.7 eV for In2O3 if an indirect band transition is assumed. In summary, our results reveal that pronounced oxygen incorporation in In2S3 is not beneficial in terms of CIGS device efficiency because oxygen is electronically inactive and poorly miscible.

Effects of ZnSO4 concentration on the high-rate deposition of CBD-Zn(O,S) buffer layer for Cu(In,Ga)Se2 thin-film solar cells

To utilize eco-friendly materials in Cu(In,Ga)Se2 (CIGS) thin-film solar cells, a Zn-based buffer layer has been adopted to replace the conventional CdS buffer layer. Buffer layers can be grown using a variety of techniques, with chemical bath deposition (CBD) generally used for the preparation of the Zn(O,S) buffer layer. The growth rate of the CBD-Zn(O,S) buffer layer is determined by the dissociation rate of S during chemical reaction between Zn and S ions. Thiourea (TU) is a conventional precursor of S, but it suffers from poor dissociation rate of S, leading to prolonged layer deposition times. In this study, we replaced TU with thioacetamide (TAA), thereby drastically shortening the deposition time from 80 min (TU) to 7 min (TAA). Furthermore, incorporating Zn with S ions is known to be difficult owing to the high-speed release of S ions from TAA. However, by adjusting the molar concentration of ZnSO4, we discovered a process to effectively form the Zn–S bonds for the CBD-Zn(O,S) buffer layer. The efficiency of the CIGS thin-film solar cell shows the best performance at a ZnSO4 concentration of 0.05 M, which rivals the performance of the TU base process. The physical and chemical properties of CBD-Zn(O,S) grown at a high deposition rate using TAA were examined as a function of ZnSO4 molar concentration. We proposed a growth model that accounts for the changes in film thickness and surface morphology evolution, both of which depend on the ZnSO4 concentration. Additionally, to investigate the p–n junction properties in terms of device operation, we elucidated the band alignment at the CIGS/CBD-Zn(O,S) interface, concentrating on the interfacial barrier height. It is empirically demonstrated that a lower ΔEC value improves the solar cell operation.

Unveiling the optimal selenization temperature for enhancing efficiency in flexible Cu(In,Ga)Se2 thin film solar cells

This work is centered on the growth of metallic precursors (Cu/In/Ga) through sequential evaporation, followed by a controlled ramping step selenization process aimed at synthesizing thin films based on copper indium gallium diselenide (CIGSe) on stainless steel substrates. Selenization temperatures ranging from 475 to 525 °C were employed to optimize the most suitable recrystallization temperature. A comprehensive characterization was conducted, covering structural, morphological, topographical, optical, and electrical aspects, revealing the distinct role of selenization temperature. Subsequently, solar cells were fabricated using the synthesized absorbers, resulting in an impressive efficiency of 15.89 % for the absorber selenized at 525 °C. This observation underscores the pivotal role of selenization parameters in shaping the quality of CIGSe, directly influencing the performance of solar cells. Consequently, this research not only focuses on refining the growth process of CIGSe through a modified approach but also provides valuable insights for the advancement of flexible solar cell technology.

Unravelling the intricacies of selenization in sequentially evaporated Cu(In,Ga)Se2 Thin film solar cells on flexible substrates

This study aimed to fabricate copper indium gallium diselenide (CIGSe) thin films using a novel two-step approach. Firstly, we deposited metallic precursors (Cu/In/Ga) onto a Mo-coated stainless steel substrate using thermal evaporation at unintentional substrate temperature. Subsequently, selenization was carried out in a furnace under the presence of an inert gas. The quality of the CIGSe thin films was analyzed to explore the influence of selenization temperature (450 °C–550 °C) and duration (30 and 60 min), while maintaining an inert atmosphere inside the selenization furnace. The structural analysis revealed the progressive development of additional phases over time, resulting in the formation of a complete chalcopyrite CIGSe structure with the preferred reflection on the (112) plane. The absorber layer exhibited a thickness of 2 μm, with atomic ratios of 0.83 for Cu/(In+Ga) and 0.24 for Ga/(In+Ga) in the film selenized at 550 °C. P-type conductivity was observed in the CIGSe thin film, with a carrier concentration of up to 1017 cm−3, and it displayed a well-defined and uniform morphology characterized by a large grain size of approximately 0.9 μm. Utilizing the optimized conditions, we successfully fabricated solar cells on a flexible substrate, achieving a photoconversion efficiency of up to 9.91%. This research delves into the impact of selenization parameters on the growth of CIGSe absorber layers and introduces a new approach that could significantly influence the feasibility and industrialization of flexible solar cells.

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.

Implementation of various colors in Cu(In,Ga)Se2 thin-film solar cells by diffractive nanostructures

With the increasing use of building-integrated photovoltaic technology, it has become necessary for solar panels to blend in with their surroundings. Diffractive nanostructures can be used to reduce optical losses and create colorful solar cells. To make colors on Cu(In,Ga)Se2 (CIGS) thin-film solar cells, we fabricated diffractive nanostructures by using nanoscale imprinting and transfer lithography methods. And we investigated how the material types and pattern shapes of the nanostructures influence the optical properties of solar cells, such as the short-circuit current density (JSC), coordination of color appearance, and color quality. We used two types of pillars and grating nanostructures of SiO2 and TiO2 layers on CIGS thin-film solar cells to produce various colors. The SiO2 demonstrated an increase in JSC without significant loss of color quality compared to TiO2. By utilizing a hexagonally arrayed pillar pattern, colors are observed at various angles on an axis different from the incident light, unlike colors observed only at a single angle in the grating structure. The nanoimprinting lithography process allowed us to produce high-quality nanostructures on both rigid glass and flexible stainless-steel substrates.

Impact of tellurium as an anion dopant on the photovoltaic performance of wide-bandgap Cu(In,Ga)Se2 thin-film solar cells with rubidium fluoride post-deposition treatment

The development of wide-bandgap Cu(In,Ga)Se2 thin films is crucial in order to reach the theoretical Shockley–Queisser limit values in single-crystal solar cells. However, the performance of solar cells based on wide-bandgap thin film absorbers has lagged significantly compared to that of their narrow-bandgap counterparts. Herein, we develop a feasible strategy to improve the photovoltaic performance of wide-bandgap Cu(In,Ga)Se2 chalcopyrite thin-film solar cells by simultaneously doping with both RbF PDT and Te2− anions as dopants in the absorber layer during the three-stage co-evaporation process. Besides inducing significant change in the GGI gradient, the synergistic effect of the Te2− anion dopant is rather beneficial in terms of controlling grain size, defects in grain boundaries, and charge carrier lifetime for encouraging charge separation and extraction, which contributes to simultaneously boosting short-circuit current density and fill factor. Te-poor devices afford an impressive efficiency of 9.58%, compared to 6.43% for control devices. More importantly, the efficiency and Voc values obtained for wide-bandgap-based thin-film solar cells containing Te anions were the highest compared to their counterparts as reported in the literature. These results demonstrate the role of Te2− anions in wide-bandgap absorber thin films on the photovoltaic performance of thin-film solar cells and the potential of this approach for use in reasonable and effective design of highly efficient wide-bandgap thin-film solar cells.

Reflective color filters deposited via ZnS single-target sputtering for colorful solar cell applications

The use of colorful solar cells is necessary for building-integrated photovoltaic (BIPV) applications in urban areas, where aesthetics play an important role. The conventional deposition process of multilayer color filters utilized in solar cells for BIPV applications is expensive because it requires multiple sputter targets. Herein, to address the issue of high cost, a single sputter target was used to produce a color filter consisting of a Zn-based multilayer structure. The oxygen content in the Zn(O,S) films could be controlled by adjusting the amount of oxygen gas supplied during the sputtering process. This allows for control of the optical properties of the films, including the refractive index and extinction coefficient. A color filter with the Zn(O0.92,S0.08)/ZnS structure repeated three times was applied to Cu(In,Ga)Se2 thin-film solar cells; the resulting solar cell demonstrated a wide range of color expressions and less than 10% reduction in optical performance. The proposed technology is expected to significantly reduce the cost of optical filter processes and their applications.

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.

Implementation of Various Colors in Cu(In,Ga)Se2 Thin-Film Solar Cells by Diffractive Nanostructures

Implementation of Various Colors in Cu(In,Ga)Se2 Thin-Film Solar Cells by Diffractive Nanostructures

Color Variation in Cu(In,Ga) Se2 Thin-Film Solar Cells by Employing Diffractive Nanostructures is Dependent on Patterns and Materials

Color Variation in Cu(In,Ga) Se2 Thin-Film Solar Cells by Employing Diffractive Nanostructures is Dependent on Patterns and Materials

A direct measure of positive feedback loop-gain due to reverse bias damage in thin-film solar cells using lock-in thermography

In this work, we present a method to study thermal runaway effects in thin-film solar cells. Partial shading of solar cells often leads to permanent damage to shaded cells and degrades the performance of solar modules over time. Under partial shading, the shaded cells may experience a reverse bias junction breakdown. In large-area devices such as solar cells, this junction breakdown tends to take place very locally, thus leading to very local heating and so-called “hot-spots”. Previously, it was shown that a positive feedback effect exists in Cu(In,Ga)Se2 (CIGS) thin-film solar cells, where a highly localized power dissipation is amplified, which may lead to an unstable thermal runaway process. Furthermore, we introduced a novel characterization technique, laser induced Hot-Spot Lock-In Thermography (HS-LIT), which visualizes the positive feedback effect. In this paper, we present a modified HS-LIT technique that allows us to quantify directly a loop-gain for hot-spot formation. By quantifying the loop-gain we obtain a direct measure of how unstable a local hot-spot is, which allows the non-destructive study of hot-spot formation under various conditions and in various cells and cell types. We discuss the modified HS-LIT setup for the direct measurement of the loop-gain. Furthermore, we demonstrate the new method by measuring the loop-gain of the thermal runaway effect in a CIGS solar cell as a function of reverse bias voltage.

20%-efficient polycrystalline Cd(Se,Te) thin-film solar cells with compositional gradient near the front junction

Bandgap gradient is a proven approach for improving the open-circuit voltages (VOCs) in Cu(In,Ga)Se2 and Cu(Zn,Sn)Se2 thin-film solar cells, but has not been realized in Cd(Se,Te) thin-film solar cells, a leading thin-film solar cell technology in the photovoltaic market. Here, we demonstrate the realization of a bandgap gradient in Cd(Se,Te) thin-film solar cells by introducing a Cd(O,S,Se,Te) region with the same crystal structure of the absorber near the front junction. The formation of such a region is enabled by incorporating oxygenated CdS and CdSe layers. We show that the introduction of the bandgap gradient reduces the hole density in the front junction region and introduces a small spike in the band alignment between this and the absorber regions, effectively suppressing the nonradiative recombination therein and leading to improved VOCs in Cd(Se,Te) solar cells using commercial SnO2 buffers. A champion device achieves an efficiency of 20.03% with a VOC of 0.863 V.

Efficiency boost of bifacial Cu(In,Ga)Se2 thin-film solar cells for flexible and tandem applications with silver-assisted low-temperature process

Efficiency boost of bifacial Cu(In,Ga)Se2 thin-film solar cells for flexible and tandem applications with silver-assisted low-temperature process

Performance optimization of single graded CIGS absorber and buffer layers for high efficiency: A numerical approach

We present a numerical simulation based study of single graded Cu(In,Ga)Se2 (Copper Indium Gallium Diselenide) thin film solar cell. In this work, initially a basic CIGS single graded cell structure is optimized in terms of thickness, band-gap and doping concentration. CdS is kept as the buffer layer, which is widely used for high efficiency CIGS solar cells. In the next step, CdS is replaced with ZnMgO as the buffer layer in order to exploit its greater photon absorption ability due to its higher band-gap which further enhances the cell efficiency. A thorough analysis is carried out on the solar cell parameters open circuit voltage (Voc), short circuit current density (Jsc), fill factor (FF) and quantum efficiency (η) of the photovoltaic cell structure. An intermediate layer of p-type MoS2 is inserted in between the single graded CIGS absorber layers. The objective is to limit the unintentional Ga inter diffusion and maintain the desired grading during the high temperature annealing for the absorber preparation. The power conversion efficiency of the bilayer device structure with Ga fraction x=(0.31) of the top absorber layer along with Ga fraction y=(0.25) of the bottom absorber layer exhibits an improved efficiency from 24.02% (CdS as the buffer layer) to 25.37% (ZnMgO as buffer layer). An excellent power efficiency of η = 26.78% is reported after adding the intermediate layer of p-type MoS2 and optimizing its thickness and the carrier concentration.

Surface/Interface Effects by Alkali Postdeposition Treatments of (Ag,Cu)(In,Ga)Se2 Thin Film Solar Cells.

Ag alloying and the introduction of alkali elements through a postdeposition treatment are two approaches to improve the performance of Cu(In,Ga)Se2 (CIGS) thin film solar cells. In particular, a postdeposition treatment of an alkali metal fluoride of the absorber has shown a beneficial effect on the solar cells performance due to an increase in the open circuit voltage (VOC) for both (Ag,Cu)(In,Ga)Se2 (ACIGS) and CIGS based solar cells. Several reasons have been suggested for the improved VOC in CIGS solar cells including absorber surface and interface effects. Less works investigated how the applied postdeposition treatment influences the ACIGS absorber surface and interface properties and the subsequent buffer layer growth. In this work we employed hard X-ray photoelectron spectroscopy to study the chemical and electronic properties at the real functional interface between a CdS buffer and ACIGS absorbers that have been exposed to different alkali metal fluoride treatments during preparation. All samples show an enhanced Ag content at the CdS/ACIGS interface as compared to ACIGS bulk-like composition, and it is also shown that this enhanced Ag content anticorrelates with Ga content. The results indicate that the absorber composition at the near-surface region changes depending on the applied alkali postdeposition treatment. The Cu and Ga decrease and the Ag increase are stronger for the RbF treatment as compared to the CsF treatment, which correlates with the observed device characteristics. This suggests that a selective alkali postdeposition treatment could change the ACIGS absorber surface composition, which can influence the solar cell behavior.

Research Progress of Alkali Doped Cu(In,Ga)Se2 Thin Film Solar Cells

Cu(In,Ga)Se2 (CIGS) thin film solar cell has the advantages of high efficiency, good working stability and low manufacturing cost, which is the most promising thin film solar cell. Currently, the efficiency of CIGS thin film cell has reached 23.35% by doping alkali elements. This review summarized the current status of doping alkali elements on flexible CIGS thin film solar cells with a focus on recent advancements intended for higher efficiency and novel applications. First, the structure of CIGS thin film cell was introduced. According to the structural characteristics of CIGS cells, different doping methods of alkali metals were summarized. Then, the recent developments and trends of research in doping methods of alkali elements within the last years were reviewed, and the effect of different alkali elements on CIGS efficiency was emphasized. Finally, the challenges and opportunities of alkali doping CIGS solar cell were prospected.