High Efficiency CIGS Solar Cells Research Articles

Investigation of effective local contact Al2O3 rear passivation for high-efficiency thin CIGS solar cells

Copper indium gallium selenide (Cu(In,Ga)Se2; CIGS) solar cells with small thicknesses active layer have limits to show high efficiency owing to high carrier recombination and low reflection at the electrode/active layer interface. A passivation layer applied to the rear of the CIGS solar cell is regarded as one of the solutions. However, depending on the thickness of the passivation layer, the surface morphology of the thin CIGS absorber layer can be changed, affecting light absorption and photovoltaic properties. In this study, the optical electrical performances of CIGS solar cells with a thin enough layer of the local contact aluminum oxide (Al2O3) (LC-Al2O3) rear passivation were analyzed and demonstrated. The thin passivation layer with tens of nanometers resulted in higher increased efficiency because of improved open circuit voltage and short circuit current density. In addition, from the experiment, the optimal thickness of the thin passivation layer was derived. Too thin a layer causes a degradation of the performance, providing the surface scattering and parasitic resistance. Our results can be used to develop guidelines for designing high-performance CIGS solar cells with optimized passivation layers.

Numerical study of high-efficiency CIGS solar cells by inserting a BSF µc-Si:H layer

Numerical study of high-efficiency CIGS solar cells by inserting a BSF µc-Si:H layer

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.

Optimization of MoSe2 back interface layer for highly efficient CIGS solar cells: Numerical analyses

The effect of the MoSe2 layer has been numerically investigated using SCAPS-1D solar cell modeling software. Using a combination of energy bandgap from 1 to 1.5 eV and thickness from 10 to 130 nm for MoSe2 best configuration was determined for high-efficiency CIGS solar cell. Optimized MoSe2 layer improved the efficiency to 20.4 % with Voc of 0.74 V, Jsc of 3.6 mA/cm2, and fill factor of 0.89 relative to 14.55 % for cell without MoSe2 layer. With the increase in bandgap of MoSe2, the efficiency of the cell increased gradually up to 1.35 eV and then suddenly decreased for the bandgap above 1.35 eV. The decrease in efficiency is observed mainly because of the reduction in Jsc and reduction in FF due to lower maximum voltage output. As the thickness was increased from 10 nm up to 50 nm, Jsc increased due to a reduction in recombination back contact and remains almost constant for thickness above 60 nm. Efficiency was observed to be improved with the thickness of the MoSe2 layer above 60 nm. With the increase of 8 % efficiency, the findings could be used to develop high-efficiency, cost-effective CIGS solar cells.

Above 15% Efficient Directly Sputtered CIGS Solar Cells Enabled by a Modified Back-Contact Interface.

The Schottky back-contact barrier at the Mo/Cu(In,Ga)Se2 (CIGS) interface is one of the critical issues that restrict the photovoltaic performance of CIGS solar cells. The formation of a MoSe2 intermediate layer can effectively reduce this back-contact barrier leading to efficient hole transport. However, the selenium-free atmosphere is unfavorable for the formation of the desired MoSe2 intermediate layer if the CIGS films are prepared by the commonly used direct sputtering process. In this work, high-efficiency CIGS solar cells with a MoSe2 intermediate layer were fabricated by the direct sputtering process without a selenium atmosphere. This is enabled by an intermediate CIGS layer deposited on the Mo substrate at room temperature before being ramped to a high temperature (600 °C). The room-temperature-deposited amorphous CIGS intermediate layer is Se rich, which reacts with the Mo substrate and forms very thin MoSe2 at the interface during the high-temperature process. The formed MoSe2 decreased the CIGS/Mo barrier height for better hole transport. Consequently, the CIGS solar cell with an 80 nm intermediate layer achieved a power conversion efficiency of up to 15.8%, which is a benchmark efficiency for the direct sputtering process without Se supply. This work provides the industry a new approach for commercialization of directly sputtered CIGS solar cells.

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.

Required CIGS and CIGS/Mo Interface Properties for High-Efficiency Cu(In, Ga)Se<SUB>2</SUB> Based Solar Cells

In this work, we have modeled and simulated the electrical performance of CIGS thin-film solar cell using one-dimensional simulation software (SCAPS-1D). Starting from a baseline model that reproduced the experimental results, the properties of the absorber layer and the CIGS/Mo interface have been explored, and the requirements for high-efficiency CIGS solar cell were proposed. Simulation results show that the band-gap, acceptor density, defect density are crucial parameters that affect the performance of the solar cell. The best conversion efficiency is obtained when the absorber band-gap is around 1.2 eV, the acceptor density at 1016 cm−3 and the defect density less than 1014 cm−3. In addition, CIGS/Mo interface has been investigated. It appears that a thin MoSe2 layer reduces recombination at this interface. An improvement of 1.5 to 2.5 mA/cm2 in the current density (Jsc) depending on the absorber thickness is obtained.

High-efficiency CIGS solar cell by optimization of doping concentration, thickness and energy band gap

In this paper, the performance of copper-indium-gallium-diselenide Cu(In,Ga)Se2 solar cell, with ZnO window layer, ZnSe buffer layer, CIGS absorber layer and InGaP reflector layer was studied. The study was performed using the TCAD Silvaco simulator. The effects of grading the band gap of CIGS absorber layer, the various thicknesses and doping concentrations of different layers have been investigated. By optimizing the solar cell structure, we have obtained a maximum open circuit voltage of 0.91901 V, a short circuit current density of 39.89910 mA/cm2, a fill factor (FF) of 86.67040% and an efficiency of 31.78% which is much higher than the values for similar CIGS solar cells reported so far.

Thermal annealing induced physical properties of ZnSe thin films for buffer layer in solar cells

Abstract This article is dedicated to the study on the influence of thermal annealing on the physical properties of ZnSe thin films in order to seek a suitable alternative Cd-free window layer for developing high efficiency CdTe and CIGS solar cells. ZnSe films having thickness 300 nm were deposited onto glass and ITO substrates employing e-beam evaporation technique and then thermal annealing was undertaken in temperature range of 373–573 K with an interval of 100 K. The structural studies show that films are having cubic phase with (111) predominant peak and grain size corresponding to the preferred peak is observed to vary with annealing. The optical parameters like absorbance, transmittance, extinction coefficient, refractive index were also evaluated. The transmittance of films is found to vary and 573 K annealed films have shown maximum transmittance. The optical energy band gap was found to increase with annealing from 1.97 eV to 2.47 eV. Electrical analysis revealed the linear relationship between current and voltage indicating ohmic nature of the contacts. The results are also correlated with the surface morphology of the thin ZnSe films while the presence of Zn and Se peaks in EDAX spectra confirmed the film deposition.

Defect Analysis of Solution-Based Process CIGS Thin-Film Solar Cells Using Technology Computer-Aided Design.

Copper indium gallium sulfur selenide (Cu(In1-xGax)SeS, CIGS) thin film solar cells are fabricated using a solution-based process, and their defect models are studied through a computer-aided design method. Cu(In1-xGax)SeS is structured with a graded bandgap by controlling the ambient gas and precursor composition, during the fabrication process. The defects in the CIGS are modeled as two donor-like defects, which are differently distributed as per the CIGS grain size (large and small grains at upper and bottom layers, respectively), whereas those in the cadmium sulfide (CdS)/CIGS interface are modeled as a complex model of both donor- and acceptor-like defects in the CdS, near the interface. By measuring the external quantum efficiency and current density-voltage characteristics, the best-fitting match of the simulated values with the measured values are obtained. The simulation results demonstrate that the defects (defect density of ~7 × 1018) in the CdS interface are more serious, compared to the CIGS defects (defect density of ~2 × 1015 in the bottom), which were initially expected to be more severe because of grain nonuniformity. For increasing the cell efficiency, we establish that the process and material quality need to be further improved not only during CIGS formation using a multistep spin-coated precursor but also during the initial deposition of the CdS buffer. This numerical approach can enable better understanding of the defect behavior in solar cells, and indicate directions for improvement in the fabrication process and device structure, for developing high-efficiency solution-based CIGS solar cells.

Effect of sodium diffusion on the properties of CIGS solar absorbers prepared using elemental Se in a two-step process

The influence of Na diffusion from various glass substrates during a high-temperature slenization process on the microstructure and morphology of two-step formed CIGS absorber layers is investigated. In order to minimise the CIGS absorber formation time, elemental Se vapour is used to prepare the CIGS absorber. The grain sizes of the CIGS films are found to increase with increasing sodium in the glass substrates (extra clear glass, soda-lime glass, borosilicate glass). TiN and SiN thin films are used as diffusion barrier layers inserted between the glass substrate and the Mo rear conatct to tune the Na diffusion from the soda-lime glass. The interdiffusion between the In-rich CuInSe2 surface layer and the Ga-rich CuGaSe2 layer is promoted by the barrier layer, leading to larger CIGS grains. Efforts are also taken to understand the differences in Na diffusion (from the glass substrates) and their effects on the MoSe2 intermediate layer formation during the high-temperature CIGS absorber formation processes. We find that excess amounts of Na and Se are essential for the MoSe2 growth. The excessive Na in the form of Na2Sex at the CIGS/Mo interface works as a Se source and catalyses the MoSe2 formation. The Se flow in the two-step CIGS formation process must be sufficiently high to obtain high-efficiency CIGS solar cells.

Fabrication of CIGS/CZTS Thin Films Solar Cells by Non-vacuum Process

Inorganic semiconductor compounds, e.g., CIGS and CZTS, are promising materials for thin film solar cells because of their high light absorption coefficient and stability. Research on thin film solar cells using this compound has made remarkable progress in the last two decades. Vacuum-based processes, e.g., co-evaporation and sputtering, are well established to obtain high-efficiency CIGS and/or CZTS thin film solar cells with over 20% of power conversion. However, because the vacuum-based processes need high cost equipment, they pose technological barriers to producing low-cost and large area photovoltaic cells. Recently, non-vacuum based processes, for example the solution/nanoparticle precursor process, the electrodeposition method, or the polymer-capped precursors process, have been intensively studied to reduce capital expenditure. Lately, over 17% of energy conversion efficiency has been reported by solution precursors methods in CIGS solar cells. This article reviews the status of non-vacuum techniques that are used to fabricate CIGS and CZTS thin films solar cells.

Influence of CdS Buffer Layer Thickness on the Photovoltaic Parameters of Solar Cells

The paper presents the results of a study on the optimization of the CdS buffer layer thickness for the production of high efficiency CIGS solar cells by analyzing the photoelectric parameters of elements with different thicknesses of the CdS layer. The difference in the efficiency of elements with CdS layer thicknesses of 20 and 80 nm was 30% (increase), and 50% (decrease) for elements with CdS layer thicknesses of 80 and 120 nm.

Sodium doping mechanism on sol-gel processed kesterite Cu2ZnSnS4 thin films

Alkali metal doping represents one of the main ways to achieve high-efficiency CIGS solar cells. Since Kesterites are a promising alternative to CIGS based thin films, in this paper, we have investigated the impact of heavy sodium-doping on sol-gel processed CZTS thin films. CZTS solutions were prepared using NaCl as sodium source for doping. Different NaCl to CZTS molar ratios – (0%, 10%, 20% and 30%) were employed. The solutions were then spin coated on clean glass substrates and immediately preheated on a hot plate and subsequently sulfurized in a tubular furnace. Based on the obtained results, 10% molar doping was found to have the best structural quality as proved by XRD and Raman spectroscopy. On the other hand, we also observed that the band gap decreases from 1.45eV down to 1.34eV with increasing sodium-doping from 0% to 30% molar doping, respectively. We used heavily Na-doped CZTS in order to compare our experimental results to theoretical calculations given by density functional theory. This comparison leads to evidence of interstitial sodium doping as dominant model rather than substitution, since the doping expands the volume of the unit cell and reduces the optical band gap, contrary to CIS where the sodium substitution doping is the principal mode.

Buffer layer optimization for high efficiency CIGS solar cells

This work presents a study concerning the numerical optimization of a buffer layer for high efficiency CIGS solar cells. The dependence of the solar cell properties on the buffer layer material, the layer thickness, the type and density of defects within the same layer were numerically investigated and analysed. Promising results were obtained with alternative Cd-free buffer layers (ZnSnO, InS and ZnS) in place of the standard CdS.

Optimization of CdS Buffer Layer for High Efficiency CIGS Solar Cells.

In present work, effects of the thickness on the structural and optical properties of chemically deposited CdS thin films were investigated. In addition, we fabricated Cu(In, Ga)Se2 solar cells with various thicknesses of CdS buffer layer and optimized the thickness for a high efficiency. When the CdS thin films were thicker, the crystallinity improved but the transmittance decreased. The short-circuit current density (J(sc)) and the fill factor are the major efficiency limiting factors for the CIGS solar cells. As the thickness of the CdS buffer layer, the open-circuit voltage (V(oc)) and the fill factor increased, whereas the J(sc) slightly decreased. The improvement of the fill factor and thus efficiency resulted from larger shunt resistance. For the solar cells without a high resistive intrinsic ZnO layer, the highest efficiency was acquired at the thickness of 89 nm. With further increasing the thickness, the J(sc) decreased significantly, resulting in poor efficiency.

The effects of pulse repetition rate on the structural, optical, and electrical properties of CIGS films grown by pulsed laser deposition

Well crystallized CuIn1−xGaxSe2 (CIGS, x=0.22–0.26) films were deposited at the temperatures below 300°C by pulsed laser deposition. Increasing pulse repetition rate from 5Hz to 1kHz significantly increased the crystallinity of films. All the films were Cu rich and those deposited with 5Hz showed higher Ga deficiency, but in those deposited with 1kHz the loss of Se was higher. The growth mechanisms under the low and high repetition rates as well as their effects on the grain size and composition were discussed in detail. All the films had p-type conductivity. The resistivity of 1kHz films was two times higher but the carrier concentration was four times lower than the 5Hz films. I–V characteristics were measured on the n-ZnO/p-CIGS heterojunctions. The junction made with 1kHz film had a single ideality factor of 1.3 up to the high current limit, which is often seen in the high efficiency CIGS solar cells, whereas the I–V curve of the junction made with 5Hz film consisted of two sections described by large ideality factors of 3.0 and 4.8, respectively, indicating that there are two recombination mechanisms each of which dominates at a different current region.

Graded Carrier Concentration Absorber Profile for High Efficiency CIGS Solar Cells

We demonstrate an innovative CIGS-based solar cells model with a graded doping concentration absorber profile, capable of achieving high efficiency values. In detail, we start with an in-depth discussion concerning the parametrical study of conventional CIGS solar cells structures. We have used the wxAMPS software in order to numerically simulate cell electrical behaviour. By means of simulations, we have studied the variation of relevant physical and chemical parameters—characteristic of such devices—with changing energy gap and doping density of the absorber layer. Our results show that, in uniform CIGS cell, the efficiency, the open circuit voltage, and short circuit current heavily depend on CIGS band gap. Our numerical analysis highlights that the band gap value of 1.40 eV is optimal, but both the presence of Molybdenum back contact and the high carrier recombination near the junction noticeably reduce the crucial electrical parameters. For the above-mentioned reasons, we have demonstrated that the efficiency obtained by conventional CIGS cells is lower if compared to the values reached by our proposed graded carrier concentration profile structures (up to 21%).

Influence of iron on the performance of CIGS thin-film solar cells

In the last years steel foil has become an attractive candidate for use as a flexible substrate material for Cu(Inx,Ga1−x)Se2 thin-film solar cells (CIGS). It is stable at the high temperatures present during CIGS processing and is also commercially available. However, one disadvantage is the diffusion of substrate elements into the CIGS layer, which are detrimental for the performance of solar cells. We found a direct correlation between the iron (Fe) content in the CIGS layer and corresponding solar cell parameters. A deep defect related to Fe impurities could be detected by admittance spectroscopy measurements. The solar cell parameters could be well fitted by simulation with recombination at an acceptor like deep defect in the bulk of the CIGS layer. The simulated density of deep defect states correlates nicely with the Fe concentration in the CIGS layer. From this we conclude that Fe replaces an In (or Ga) site in the CIGS lattice and creates an FeIn2+ (or FeGa2+) deep acceptor state in the bulk of CIGS layers, which is detrimental already at a low concentration in the sub ppm range. The simulations enabled us to estimate the maximum Fe concentration in CIGS layers which is tolerable without disturbing the performance of high-efficiency CIGS solar cells.

Simulation and performance analysis of superstrate Cu(In,Ga)Se<SUB align="right">2 solar cells using nanostructured Zn<SUB align="right">1−xV<SUB align="right">xO thin films

In this paper, we describe in the first step the structural, electrical and optical properties of the nanostructured Zn 1– x V x O thin films deposited on glass substrates by rf-magnetron sputtering using aerogel nanoparticles synthesised by the sol–gel method. The best properties, satisfying the role of window and buffer layers, were achieved, respectively, for the films of Zn 0.99 V 0.01 O elaborated at room temperature and Zn 0.80 V 0.20 O at 200ºC. In the second step, the nanostructured Zn 0.99 V 0.01 O and Zn 0.80 V 0.20 O layers are, respectively, proposed as alternative to the traditional (ITO) window and (CdS) buffer layers and tested numerically in Cu(In,Ga)Se 2 (CIGS) solar cell using one-dimensional AMPS-1D device simulator. The influence of physical and geometrical parameters of the p -type CIGS absorber layer on the performance of the superstrate SLG/(n+)Zn 0.99 V 0.01 O/(n)Zn 0.80 V 0.20 O/(p)Cu(In,Ga)Se 2 /Mo solar cell was investigated. The calculations assume fixed Zn 1– x V x O input parameters. The carrier concentration and thickness of the absorber layer were found to be a key factor, affecting the solar cell performance. On the basis of the simulation results, a short-circuit current density of about 33 mA/cm 2 has been obtained for 4 µm-CIGS solar cell using n -type Zn 0.80 V 0.20 O buffer layer for 100 nm thick. It is also found that a conversion efficiency of more than 19% AM 1.5 G could be expected for more than 3 µm absorber thickness and acceptor concentration varying between 2 × 10 16 and 10 17 cm –3 . From the results obtained, we suggest the use of Zn 0.80 V 0.20 O and Zn 0.99 V 0.01 O as a buffer and window layers, respectively, to achieve high-efficiency CIGS solar cells with better photovoltaic parameters.