CIGS Solar Cells Research Articles

Analyzing the causes of limited performance improvement in cigs devices after potassium fluoride post-deposition treatment using SCAPS

Recently, potassium fluoride post-deposition treatment (KF-PDT) attracts more attention for which has beneficial effect on open circuit voltage (Voc) with the CIGS solar cell conversion efficiency increased from 20.3 % to 22.9 %. However, the short circuit current density (Jsc) and fill factor (FF) are not always increased after KF-PDT. In this study, we investigate the absorber characteristics after KF-PDT and analyze the major factors which limit the improvement of Jsc and FF with SCAPS simulation. According to the simulation results, we find that the declining of Jsc and FF are due to the high electronic barrier formed at the interface of absorbe1(KInSe2)/absorber2 after KF-PDT. However, the high electronic barrier can be lowered by increasing of ZnCu or CdCu concentration near absorber surface and by reducing the defects concentration on the buffer/absorber interface, thereby the improvement of Jsc and FF. In addition, the device performance of conduction band offset ΔAB≥0 eV at buffer/absorber interface is enhanced obviously after potassium post treatment. Therefore, increasing the ZnCu or CdCu concentration and maintaining the ΔAB≥0 eV are key points to further improve the device performance after KF-PDT.

The role of metastability and concentration on the performance of CIGS solar cells under Low-Intensity-Low-Temperature conditions

Commercially available lightweight-flexible CIGS solar cells are investigated under the Low-Intensity-Low-Temperature (LILT) conditions that exist at Mars, Jupiter, and Saturn. Current density-voltage measurements, concentrated solar, and external quantum efficiency measurements are performed under varying temperatures and illumination intensities to determine the applicability and performance of flexible CIGS in outer planetary conditions. The well-known metastability of the CIGS absorber is observed as a result of a barrier to minority carrier extraction at the CIGS/CdS interface under higher intensity illumination. However, despite the low temperatures and low intensities experienced in deeper space, the presence of this barrier does not significantly affect the performance of the solar cells under LILT conditions. This is attributed to the lower photogeneration rate of carriers particularly at conditions relative to Saturn and Jupiter, which appears to be less than the thermionic emission rate across the barrier and therefore the carrier extraction is relatively unaffected under these illumination conditions. At elevated temperatures and/or intensities such as at AM0 and conditions relative to Mars, however, the higher carrier generation rate results in the appearance of large series resistance and a significant loss of fill factor at irradiation levels greater than 1-sun AM0. Proton irradiation of the solar cells systematically reduces the performance, predominately through the formation of defect states in the absorber layer, the presence of which is increasingly more prohibitive in LILT conditions due to the low thermal energy of the minority carriers and the subsequent increased effect of SRH recombination.

Device modeling approach and simulation of the effect of the ODC thin layer on bifacial solar cells based on CuIn1-xGaxSe2 thin films absorbers

In this work, thin films of In2Se3, which are employed for bifacial CIGS solar cell applications, are grown onto coated soda lime glass (SLG) substrates using physical vapor deposition (PVD) technique. Their structural, optical, and electrical properties are also illustrated. In addition, the device of bifacial CIGS thin film solar cells is studied. To employ the one-dimensional modeling tool analysis of microelectronic and photonic structures, we use the simulator (AMPS-1D) to numerically analyze the performances of bifacial CIGS solar cells. We show the role of the formation of the ordered defect compound (ODC) layer between the buffer layer and the absorber part on the performance of the device. The obtained results show that this defect layer is important in the photovoltaic performance of CIGS device. To optimize the bifacial cells, the simulated results show that the optimal thickness of ODC and the defect density are in the ranges 400–600 nm and 1 × 1014 - 1015 cm−3, respectively. Thus, the results indicate that the ODC had an optimal bandgap above 1.3 eV. The conversion efficiencies exceeding 16% and 14% were obtained with illumination from the front and back contacts, respectively.

Investigations of potassium-induced surface treatment of Cu(In,Ga)Se2 (CIGSe) thin film solar cells prepared by two-stage process using elemental selenium

The influences of alkaline doping of CIGS absorbers by a potassium fluoride post-deposition treatment (KF-PDT) were investigated systematically. Three factors that influence the CIGS solar cell performances were investigated: (i) deposition temperature, (ii) post-deposition annealing, and (iii) the thickness of the potassium fluoride (KF) layer. It is deduced that Cu vacancies were more effectively occupied by K instead of In, which prevents the formation of InCu donor defects and facilitates the subsequent Cd doping into the CIGS region. We confirmed large densities of deep defects induced by KF-PDT, leading to significant FF losses of CIGS solar cells. The potassium diffusion and redistribution during post-annealing is critical to micro-structure and crystallinity of CIGS absorber, as well as the defect properties. We pointed out the effects of surface treatment extend to the bulk of CIGS, dependent on the chemical compositions. With modified post-annealing process towards homogeneous potassium distribution and high-quality CGIS crystals, room temperature KF-PDT remains an effective strategy to enhance the performances of CIGS solar cells prepared by two-stage process.

Numerical analysis of CdS-CIGS interface configuration on the performances of Cu(In,Ga)Se2 solar cells

One dimension solar Cell Simulation package (SCAPS) is used to analyze the impact of the CdS-CIGS interface configuration on the performances of CIGS solar cells. We simulated the current-voltage characteristic of two models of the cell: one with a donor type defect (OVC model) and the other with acceptor type defect (P+ model) at the CdS-CIGS interface. The advantages and disadvantages of these CIGS surface configuration on the electrical parameters were discussed according to their thicknesses, defect density and carrier lifetime. The simulation results show that the model with the P+ layer has poor performance when its thickness and defect density increase, due to a huge distortion on the J-V characteristic. On the other hand, the OVC layer plays a fundamental role in the performance of CIGS solar cells. Better performances are obtained with the OVC model when the density of donor defect is in the range 1013 - 1015 cm−3, the charge carriers lifetime in the range 0.02 - 1 ns, and the thickness of the OVC layer in the range 200 - 400 nm.

Efficiency enhancement of ultrathin CIGS solar cells by optimal bandgap grading: erratum.

Typographical errors in a few equations in [Appl. Opt.58, 6067 (2019)APOPAI0003-693510.1364/AO.58.006067] are corrected.

Metastable Behavior on Cesium Fluoride‐Treated Cu(In1–x,Gax)Se2 Solar Cells

Metastable behavior of cesium fluoride (CsF)‐treated Cu(In1–x,Gax)Se2 (CIGS) solar cells is investigated under heat‐light soaking (HLS) and heat‐soaking (HS) treatments. HLS increases open‐circuit voltage, fill factor, efficiency, and net carrier concentration and decreases short‐circuit current density, whereas heat‐soaking treatment acts oppositely. The performance of a CsF postdeposition treatment to CIGS thin film in selenium vapor, and closer to stoichiometry copper content, did not mitigate the open‐circuit voltage improvement after HLS. These results argue the traditional concept of the VSe–VCu divacancy complex for the total beneficial effect of HLS in alkali‐treated CIGS solar cells. The metastable behavior observed in the CsF‐treated devices due to the HLS and HS treatments is explained by the specific behavior of alkali‐containing new compounds at the surface and/or the migration of alkali metals at the surface and bulk regions.

Efficiency Enhancement of Ultra-thin CIGS Solar Cells Using Bandgap Grading and Embedding Au Plasmonic Nanoparticles

The objective of this study is to enhance the efficiency of copper indium gallium selenide (CIGS) solar cells. To accomplish that, composition grading of absorber layer was carried out by using SILVACO’s technology aided computer design (TCAD) ATLAS program. Results showed a meaningful improvement of output parameters including open-circuit voltage (Voc), short-circuit current (Isc), fill factor (FF), and power conversion efficiency (η). For further performance improvement of the cell, Au plasmonic scattering nanoparticles were loaded on the top of the ZnO window layer. Plasmonic nanoparticles can restrict, absorb, navigate, or scatter the incident light. By using the spherical Au nanoparticles, a very good increase in the light absorption in the cell over the reference planar CIGS solar cell was observed. The highest η = 19.01% was achieved for the designed ultra-thin bandgap-graded CIGS solar cell decorated by Au nanoparticles.

Cd-free Cu(InGa)Se2 solar cells with eco-friendly a-Si buffer layers

CdS, In2S3 and ZnO-based materials are typically adopted as buffer layers in high efficiency CIGS solar cells. However, the drawbacks involved toxicity, complicated synthesis process and environmental damage of aforementioned materials would someday limit the development themselves. Here, for the first time, eco-friendly and low-cost a-Si thin films prepared by e-beam evaporation were used as buffer layers in CIGS solar cells. Thickness of a-Si films was changed to optimize the performance of CIGS solar cells through investigating their optical, structural and electrical properties. It revealed that the conduction band alignment of CIGS and a-Si is a favorable spike and a 60 nm a-Si film is optimal for CIGS/a-Si interface properties. Following the SCAPS simulation, bandgaps and conduction band energy level of CIGS films were facilely modulated through altering the Ga/(Ga + In) ratios. The conversion efficiency of solar cell with modified CIGS film was relatively improved by over 30% compared to the pristine one. The present work provides a new and eco-friendly strategy for fabrication of CIGS solar cells and expands the application of a-Si thin films as buffer layers for solar cells.

Defect activation and annihilation in CIGS solar cells: an operando x-ray microscopy study

The efficiency of thin-film solar cells with a Cu( Gax)Se2 absorber is limited by nanoscopic inhomogeneities and defects. Traditional characterization methods are challenged by the multi-scale evaluation of the performance at defects that are buried in the device structures. Multi-modal x-ray microscopy offers a unique tool-set to probe the performance in fully assembled solar cells, and to correlate the performance with composition down to the micro- and nanoscale. We applied this approach to the mapping of temperature-dependent recombination for Cu( Gax)Se2 solar cells with different absorber grain sizes, evaluating the same areas from room temperature to . It was found that poor performing areas in the large-grain sample are correlated with a Cu-deficient phase, whereas defects in the small-grain sample are not correlated with the distribution of Cu. In both samples, classes of recombination sites were identified, where defects were activated or annihilated by temperature. More generally, the methodology of combined operando and in situ x-ray microscopy was established at the physical limit of spatial resolution given by the device itself. As proof-of-principle, the measurement of nanoscopic current generation in a solar cell is demonstrated with applied bias voltage and bias light.

Towards an optimized gallium gradient for Cu(In,Ga)Se2 thin film via an improved constant low-temperature deposition process

The gallium gradient of the CIGS film plays a key role for device performance of solar cells. However, a three-stage co-evaporation deposition process at low substrate temperature often produces a pronounced steeper gallium (Ga) gradient, which is detrimental to the transport of carriers and leads to lower conversion efficiency. In this work, we propose a modified low-temperature deposition process by introducing an overlapping process between the 1st stage and the 2nd stage. This modified low-temperature process mitigates the diffusion of In and Ga and facilitates the formation of ideal back gradient. Meanwhile, we have demonstrated the phase transition process of the back gradient during the overlapping process by characterization techniques. With the design of new reaction models during the overlapping process, growth mechanisms of the CIGS film in the modified low-temperature process could be visually displayed. This modified low-temperature process has improved the device efficiency of CIGS solar cells from 14.1% to 16.7%.

Influence of Absorption Layer Thickness on the Performance of CIGS Solar Cells

As solar light was mainly absorbed by absorb layer in thin film solar cells, the quality of absorb layer has a great effect on the performance of solar cells. Based on the diffusion drift model of carrier in copper indium gallium selenide (CIGS) solar cells, and then the key parameters are analyzed. Combining with the experimental results of others, this paper discusses the absorption layer’s thickness influence on the open circuit voltage, short circuit current, filling factor and conversion efficiency of the copper indium gallium selenium based solar cells. The results show that the absorption layer’s thickness always affects the performance parameters of the battery. Open circuit voltage increased with increasing thickness and reached saturation above 2μm. Short circuit current increased first and then decreased, then reaches its maximum value at about 2μm thickness. Conversion efficiency is similar to the short circuit current properties. While the fill factor decreased with the increase of thickness above 1^m. This can provide theoretical support for the design of better device structures.

Incorporation of copper–indium back-end layers in the solution-based Cu(In, Ga)Se2 films: enhancement of photovoltaic performance of fabricated solar cells

The morphology and photovoltaic properties of the solution-based Cu(In, Ga)Se2 films are effectively improved via the incorporation of copper-indium back-end layers in the precursor films. The effects on the concentrations of bimetal-ions solutions to prepare copper-indium back-end layers are investigated in this study. The incorporation of copper-indium back-end layer in the precursor film enhances the internal diffusion between gallium-ions and indium-ions during selenization reaction. Hence, the porous structure in the back-contact region of prepared CIGS films becomes densified, and the bandgap distribution of films shows a gradient profile. The densified morphology and gradient bandgap reduce the carrier recombination and improve the carrier collection of solar cells. In contrast to the pristine precursor film, the precursor film with a copper-indium back-end layer increase the conversion efficiency of prepared solar cells from 8.34% to 11.13%. The enhancement of conversion efficiency is attributed to the improvement of short-circuit current density and fill factor from 25.70 mA cm−2 to 31.79 mA cm−2 and 57.65% to 65.70%, respectively. This study reveals that the photovoltaic properties of solution-based CIGS solar cells can be improved significantly via the incorporation of copper-indium back-end layers into the precursor films.

The Effect of ALD-Zn(O,S) Buffer Layer on the Performance of CIGSSe Thin Film Solar Cells

In this paper, we report the development of Cd-free buffers using atomic layer deposition (ALD) for Cu(In,Ga)(S,Se)2-based solar cells. The ALD process gives good control of thickness and the S/S +O ratio content of the films. The influence of the growth per cycle (GPC) and the S/(S+O) ratio, and the glass temperature of the atomic layer deposited Zn(O,S) buffer layers on the efficiency of the Cu(In,Ga)(S,Se)2 solar cells were investigated. We present the first results from our work on cadmium-free CIGS solar cells on substrates with an aperture area of 0.4 cm2. These Zn(O,S) layers were deposited by atomic layer deposition at 120 °C with S/Zn ratios of 0.7, and layers of around 30 nm. The Zn(O,S) 20% (Pulse Ratio: H2S/H2O+H2S) process results in a S/Zn ratio of 0.7. We achieved independently certified aperture area efficiencies of 17.1% for 0.4 cm2 cells.

Comprehensive Analysis of CuIn<sub>1-x</sub>Ga<sub>x</sub>Se<sub>2</sub> Based Solar Cells with Zn<sub>1-y</sub>Mg<sub>y</sub>O Buffer Layer

The development of cadmium-free CIGS solar cells with high conversion efficiency is crucial due to the toxicity of cadmium. Zinc-based buffer layers seem to be the most promising. In this paper, a numerical analysis using SCAPS-1D software was used to explore the Zn(Mg,O) layer as an alternative to the toxic CdS layer. The effect of several properties such as thickness, doping, Mg concentration of the Zn(Mg,O) layer on the current-voltage parameters was explored and their optimal values were proposed. The simulation results reveal that the optimal value of the ZMO layer thickness is approximately 40 nm, the doping at 1017 cm-3 and an Mg composition between 0.15 and 0.2. In addition, the effect of Gallium (Ga) content in the absorber as well as the Zn(Mg,O)/CIGS interface properties on the solar cell’s performance was examined. The results show that contrary to the CdS buffer layer, the best electrical characteristics of the ZMO/CIGS heterojunction are obtained using a Ga-content equal to 0.4 and high interface defect density or unfavorable band alignment may be the causes of poor performances of Zn(Mg,O)/CIGS solar cells in the case of low and high Mg-contents.

Rear surface passivation of ultra-thin CIGS solar cells using atomic layer deposited HfOx

In this work, hafnium oxide layer is investigated as rear surface passivation layer for ultra-thin (550 nm) CIGS solar cells. Point contact openings in the passivation layer are realized by spin-coating potassium fluoride prior to absorber layer growth. Contacts are formed during absorber layer growth and visualized with scanning electron microscopy (SEM). To assess the passivating qualities, HfOx was applied in a metal-insulator-semiconductor (MIS) structure, and it demonstrates a low interface trap density in combination with a negative density of charges. Since we used ultra-thin devices that are ideal to probe improvements at the rear, solar cell results indicated improvements in all cell parameters by the addition of 2 nm thick HfOx passivation layer with contact openings.

How the absorber thickness influences the formation of reverse bias induced defects in CIGS solar cells

When a PV module is partially shaded, the shaded solar cells operate in a reverse bias condition. For Cu(In,Ga)Se2 cells this condition can cause defects that irreversibly reduce the output of these cells and the full module. In order to design robust shade-tolerant CIGS modules details need to be known of the conditions at which these defects will be formed. In this study a large number of cells were exposed to different reverse bias conditions. By using simple statistics the probability of the occurrence of defects as a result of reverse bias at any given voltage has been determined. Based on our experiments we have found that the absorber thickness is one of the main parameters that affects the shade-tolerance: the thicker the absorber, the more shade tolerant the CIGS module will be.

Increased efficiency of 23% for CIGS solar cell by using ITO as front contact

In this paper, a CIGS material based solar cell structure is proposed. The efficiency of the designed structure is improved by considering ITO (Indium Tin Oxide) as a front contact. The detailed analysis of the device is carried out thoroughly using SILVACO ATLAS. Various design parameters such as open-circuit voltage (Voc), the voltage at maximum power density, short circuit current density (Jsc), maximum output power (Pm), and fill factor (FF), are calculated. CIGS solar cell with ITO as a front contact gives an improved efficiency of 23.074%. Thus, the proposed solar cell is a suitable candidate for ultra-high efficiencies.

Correction: Application of a Sn4+ doped In2S3 thin film in a CIGS solar cell as a buffer layer

Correction for ‘Application of a Sn4+ doped In2S3 thin film in a CIGS solar cell as a buffer layer’ by SeongYeon Kim et al., Sustainable Energy Fuels, 2020, 4, 362–368.

Luminescent down-shifting CsPbBr3 perovskite nanocrystals for flexible Cu(In,Ga)Se2 solar cells

To overcome the parasitic absorption of ultraviolet (UV) light in the transparent conductive oxide (TCO) layer of flexible Cu(In,Ga)Se2 (CIGS) thin film solar cells, a CsPbBr3 perovskite nanocrystal based luminescent down-shifting (LDS) layer was integrated on CIGS solar cells fabricated on a stainless steel foil. The CsPbBr3 perovskite nanocrystal absorbs solar irradiation at wavelengths shorter than 520 nm and emits photons at a wavelength of 532 nm. These down-shifted photons pass the TCO layer without parasitic absorption and are absorbed in the CIGS absorber layer where they generate photocurrent. By minimizing the parasitic absorption in the TCO layer, the external quantum efficiency (EQE) of the CIGS solar cell with the CsPbBr3 perovskite nanocrystal layer is highly improved in the UV wavelength range between 300 and 390 nm. Additionally, in the wavelength range between 500 and 1100 nm, the EQE is improved since the surface reflectance of the CIGS device with the CsPbBr3 perovskite LDS layer was reduced. This is because the CsPbBr3 perovskite nanocrystal layer, which has an effective refractive index of 1.82 at a wavelength of 800 nm, reduces the large refractive index mismatch between air (nair = 1.00) and the TCO layer (nZnO = 1.96 at a wavelength of 800 nm). Both the short circuit current density and power conversion efficiency of the flexible CIGS solar cell integrated with the CsPbBr3 perovskite are improved by 4.5% compared with the conventional CIGS solar cell without the CsPbBr3 perovskite LDS layer.