Layer In Thin Film Solar Cells Research Articles

3D and Multimodal X-Ray Microscopy Reveals the Impact of Voids in CIGS Solar Cells.

Small voids in the absorber layer of thin-film solar cells are generally suspected to impair photovoltaic performance. They have been studied on Cu(In,Ga)Se2 cells with conventional laboratory techniques, albeit limited to surface characterization and often affected by sample-preparation artifacts. Here, synchrotron imaging is performed on a fully operational as-deposited solar cell containing a few tens of voids. By measuring operando current and X-ray excited optical luminescence, the local electrical and optical performance in the proximity of the voids are estimated, and via ptychographic tomography, the depth in the absorber of the voids is quantified. Besides, the complex network of material-deficit structures between the absorber and the top electrode is highlighted. Despite certain local impairments, the massive presence of voids in the absorber suggests they only have a limited detrimental impact on performance.

Photosensitive analysis of spin coated Cu doped ZnO thin film synthesized by hydrothermal method

In this study, the effects of the incorporation of Cu into the crystal structure of the ZnO semiconductor on its structural and optoelectrical properties were investigated. The hydrothermal method for the synthesis of Cu-ZnO compounds and the spin-coating technique were used to prepare Cu-ZnO thin films. Copper is observed to have a major influence on the physical properties of the ZnO compound by changing the morphology of the nanoparticles, from rod-like ZnO nanoparticles to flower-like nanoparticles. This effect changes and improves the light sensitivity of the ZnO compound, making it suitable for use as a Cu-ZnO thin window layer in thin-film solar cells. All the crystal structure parameters and optoelectrical properties are further enhanced upon incorporation of Cu into the ZnO compound due to the increased surface defects of the zinc oxide which in turn shifts the optical absorption coefficient towards the visible region.

Comprehensive Analysis of CdS Ultrathin Films Modified by the Substrate Position inside the Reactor Container Using the CBD Technique.

The cadmium sulfide (CdS) n-type semiconductor is one of the most used as a window layer in thin-film solar cells, such as CdTe, CIS, CIGS, and CZTS. Optoelectronic properties are the most important characteristics for window materials. CdS thin films obtained using the chemical bath deposition technique (CBD) have been reported; however, large amounts of precursor solutions are used, which generate considerable amounts of toxic waste. The aim of this work is to reduce the amount of precursor solutions used for CdS growth; for this, it is necessary to consider an efficient position for the substrate inside the reactor container and at the same time allow obtaining CdS thin films with adequate physical properties to be applied in the photovoltaic solar cell development. CdS thin films were deposited on soda-lime/SnO2:F substrates (FTO) using the CBD technique; the substrates were placed in three different arrangements [rack system, step system (up), and step system (down)]. CdS samples with areas of 4 cm2 and a thickness of 27-48 nm were obtained; the X-ray diffraction patterns show CdS thin films with different polycrystalline structures. The morphological measurements reveal different surface formations depending on the substrate position, and resistivity values of around 105 Ω*cm were measured. UV-vis spectra show transmittance values of around 45-94% in the visible region with band gap energy values of around 2.1-2.36 eV. The best physical properties of CdS thin films and an efficient CBD process were obtained when the FTO substrates were located near the bottom of the reactor container with the FTO side down, leading to an optimal configuration that allows reducing the amount of precursor solutions and in this way reduces the toxic waste generated. These results are important in the photovoltaic technology process and environmental impact.

Radiative property investigation of metal nanoparticles embedded in absorption layer for thin-film solar cell with the consideration of medium absorption and dependent scattering

Spectral radiative transfer between particles of dispersed particulate medium occurs widely in many fields, which may be affected by host medium absorption (HMA) and dependent scattering effect (DSE) between particles. However, few studies on the radiative properties of dispersed particulate medium have considered both the effects of medium absorption and dependent scattering, which can result in significant errors. Meanwhile, metal nanoparticles (NPs) are used generally in absorption layer of thin-film solar cells due to their localized surface plasmon resonance (LSPR) effect. The absorption layer embedded with metal NPs could be seen as the special dispersed particulate medium. To calculate the radiative properties of metal NPs embedded in absorption layer for thin-film solar cell accurately, the idea of combining the LSPR of metal NPs, DSE between particles, and HMA is proposed by the authors. Multiple sphere T-matrix (MSTM) method and generalized Mie scattering theory are used to calculate the radiative properties of metal NPs embedded in absorption layer for thin-film solar cell with the consideration of HMA and DSE. Compared to the conventional method without the consideration of HMA and DSE, the method can decrease the maximum relative error and average error between experimental data and calculation data from 62.75% to 8.09%, and from 14.22% to 3.72%, respectively.

Structural features of Cu2MgxZn1−xSnS4 solid solution films for third-generation solar cells

The paper studies the influence of the ratio of Mg/Zn molar concentrations in the precursor (0 ≤ y ≤ 0.5) on the structural, substructural, optical characteristics and chemical composition of Cu2MgxZn1−xSnS4 films deposited by the spray pyrolysis method from molecular solutions. The data of X-ray structural studies indicate the formation of single-phase films with the structure of kesterite. The single-phase structure is also confirmed by studies of Raman spectra for samples obtained at y ≤ 0.3. Chemical analysis of the films confirms the successful inclusion of Mg in thin layers up to the level of (5.5–6.7) at%. An increase in the total amount of Mg has little effect on the concentration of all elements except zinc. The introduction of magnesium changes the lattice constants of the material, the sizes of coherent scattering regions, the level of microdeformations, and the density of dislocations in the films. At the same time, the layers obtained at y = 0.1 have the minimum total concentration of dislocations (0.73·1017 lin/m−2). Thus, replacing zinc with magnesium increases the quality of the solid solution at concentrations y < 0.3. The band gap of the undoped Mg compound was found to be 1.47 eV, and for the Mg-doped samples it decreases from 1.28 eV (y = 0.1) to 1.22 eV (y = 0.5). Thus, based on the obtained results, the Cu2MgxZn1−xSnS4 solid solution can be considered as an inexpensive material for creating absorption layers of thin-film solar cells.

Simple thermal vapor deposition process for and characterization of n-type indium oxysulfide thin films

The search continues for alternative nontoxic n-type electron transport layers in optoelectronic thin-film devices. Indium oxysulfide, In2(O,S)3, represents one promising material for this application, especially when paired with chalcogenide absorber layers. The ternary nature of the composition allows for electrical conductivity and optical bandgap tuning by tailoring the sulfur to oxygen ratio in the oxysulfide alloy. However, thin films of In2(O,S)3 are typically deposited only by chemical bath deposition or plasma-enhanced atomic layer deposition. We report deposition of thin films of In2(O,S)3 in a custom-built thermal reactor using only water vapor and hydrogen sulfide as the coreactants. This advance is enabled by the use of a recently reported, highly reactive indium formamidinate precursor. As shown by x-ray photoelectron spectroscopy, the composition can be tuned from pure In2O3 to pure In2S3 by varying the ratio of cycles employing water or hydrogen sulfide. The oxygen to the sulfur ratio in the film can be controlled by altering the dose sequence, although films typically contain more sulfur than would be expected naively from the percentage of hydrogen sulfide doses in the deposition recipe. Rutherford backscattering spectrometry confirms the composition is sulfur-rich relative to the dosing ratio. Structural characterization indicates films are relatively amorphous in nature. Electrically, these films offer reasonably constant electron mobility at different O:S ratios, with an electron concentration tunable over 4 orders of magnitude. These oxysulfide films possess a higher indirect bandgap than their oxygen-free indium sulfide counterparts, indicating higher transmittance to blue light. These indium oxysulfide films may be suitable candidates for electron transport layers in thin-film solar cells where their wider bandgap might result in higher optical transparency and thus short circuit current density, while the tunability of their conduction band offset with an absorber layer may result in higher open circuit voltage.

Study of wide bandgap n-type CdS window layers for thin-film solar cells

Study of wide bandgap n-type CdS window layers for thin-film solar cells

Efficiency improvement of CIGS solar cells using RF sputtered TCO/Ag/TCO thin-film as prospective buffer layer

In this paper, TCO (Transparent Conductive Oxide) incorporating ultrathin Ag intermediate film is proposed as a new buffer layer to enhance the efficiency of CIGS thin-film solar cells (TFSCs). In this regard, versatile multilayer thin-films based on ZnO/Ag/ZnO and ITO/Ag/ITO structures were deposited on glass using RF magnetron sputtering technique to determine the optoelectronic parameters of the multilayer structures. The elaborated samples were then characterized using SEM, EDS, XRD, and UV–Visible absorption spectroscopy techniques to investigate the structure morphological, optical, and electronic properties. The deposited multilayer thin-films showed amorphous-like structure and exhibited a broadband absorbance over the visible and even NIR spectrum ranges, indicating its potential application as alternative buffer layers for thin-film solar cells. In this context, TCO/Ag/TCO/CIGS solar cells have been numerically investigated using the deposited multilayer optoelectronic properties. It was revealed that the estimated efficiency of the ZnO/Ag/ZnO/CIGS-based solar cell could reach 18.5% with an open circuit voltage of 0.7 V and a short-circuit current density of 34.8 mA/cm2. The performances exhibited by the investigated solar cell demonstrated that ZnO/Ag/ZnO multilayer can be used as an alternative to the conventional CdS buffer layer for developing high-performance non-toxic CIGS solar cells.

Role of Surface Coverage and Film Quality of the TiO2 Electron Selective Layer for Optimal Hole-Blocking Properties.

Titanium dioxide (TiO2) is a commonly used electron selective layer in thin-film solar cells. The energy levels of TiO2 align well with those of most light-absorbing materials and facilitate extracting electrons while blocking the extraction of holes. In a device, this separates charge carriers and reduces recombination. In this study, we have evaluated the hole-blocking behavior of TiO2 compact layers using charge extraction by linearly increasing voltage in a metal–insulator–semiconductor structure (MIS-CELIV). This hole-blocking property was characterized as surface recombination velocity (SR) for holes at the interface between a semiconducting polymer and TiO2 layer. TiO2 layers of different thicknesses were prepared by sol–gel dip coating on two transparent conductive oxide substrates with different roughnesses. Surface coverage and film quality on both substrates were characterized using X-ray photoelectron spectroscopy and atomic force microscopy, along with its conductive imaging mode. Thicker TiO2 coatings provided better surface coverage, leading to reduced SR, unless the layers were otherwise defective. We found SR to be a more sensitive indicator of the overall film quality, as varying SR values were still observed among the films that looked similar in their characteristics via other methods.

Boron Doped ZnO Films Deposited by DC Reactive Sputtering Using Zn:B Target: Influence of the Deposition Temperature on the Structural, Electrical and Optical Properties

ZnO-based transparent and conducting oxides (TCOs) are commonly used as a window layer in thin-film solar cells. However, TCOs with high transparency in the visible and near-infrared parts of the electromagnetic spectrum, plus excellent electrical properties are required in this application. In this study, TCOs based on ZnO:B films deposited by DC reactive sputtering using Zn:B alloy target were investigated. The impact of deposition temperature on the growth and physical properties of the films was examined. Structural, optical and electrical properties of these films were investigated by means of x-ray diffraction (XRD), Ultraviolet-Visible-Near Infrared (UV-VIS-NIR) spectroscopy, and Hall effect measurement, respectively. The XRD analysis revealed that all films are of hexagonal wurtzite structures, with a preferred orientation along the c-axis. The optical spectroscopy results indicated that all the ZnO:B films had optical transparency above 90% in the visible region which then slightly decreased in the near-infrared region. The highest carrier concentration, conductivity, and mobility were obtained at the deposition temperature of 300 °C–due to improvement in crystal growth–while higher temperatures slightly deteriorated the electrical properties, possibly due to a slight decrease in the crystallite size.&#x0D; Keywords: ZnO; Transparent and conducting oxides; ZnO:B, DC reactive sputtering; deposition temperature

Photo responsive transport properties of hydrothermally synthesized SnO2 layered nanoflakes/Au Schottky junction

Stannic oxide (SnO2) nano-materials has been drawing attentions for applications in transparent electrodes, hazardous-gas sensors, electron transport layers in thin-film solar cells, microelectronics, etc. In this present report, we have analyzed the junction characteristics of the hydrothermally synthesized SnO2 layered nano-flakes (LNFs). The two-dimensional flake like structure of the SnO2 samples was verified from the scanning electron microscopy (SEM) micrographs with a mean thickness of 11.70 nm. The chemical composition of the LNFs was found to be 1:3.51 (Sn:O), from the energy dispersive x-ray spectroscopy (EDS) study. The UV–Vis absorption analysis validates the quantum confinement effect from the widening (blue shifting) of the bandgap to 3.75 eV. The junction characteristics of the indium tin oxide (ITO)/ SnO2 LNFs/gold (Au) device depict the Schottky type properties with barrier potentials, 0.85 eV and 0.63 eV in dark and illumination conditions respectively. However, in both conditions, the ideality factor remains unaltered (0.93). The light induced diminution of the barrier potential and confinement induced widening of band gap make the LNFs suitable for photo-sensors and solar cells.

Atomic-layer-deposited ZnSnO buffer layers for kesterite solar cells: Impact of Zn/(Zn+Sn) ratio on device performance

Atomic layer deposition (ALD) has been used to synthesize earth-abundant and non-toxic ZnSnO thin films as potential replacements for CdS buffer layers in thin-film solar cells. In this study, ALD ZnSnO films with various Zn/(Zn+Sn) ratios ranging from 0.66 to 0.96 were synthesized and their application as buffer layers in CZTSSe solar cells was investigated. The solar cells fabricated with ZnSnO buffer layers exhibit a significant improvement in short-circuit current density (Jsc) compared to the CdS-buffered reference device, primarily due to the wide band gap and high transparency of ZnSnO films. The CZTSSe/ZnSnO solar cell results also demonstrated that the device parameters, particularly the open-circuit voltage (Voc) and fill factor (FF), were significantly affected by the Zn/(Zn+Sn) ratio. A cell efficiency of 8.54% with a Voc of 0.436 V, Jsc of 32.98 mA cm−2, and FF of 0.59 was obtained with ALD ZnSnO buffer layer with an optimal Zn/(Zn+Sn) ratio of 0.76. Increasing or decreasing the Zn/(Zn+Sn) ratio leads to a gradual decrease in the FF and Voc values, which eventually deteriorates device performance. Additionally, the uniform and conformal ALD process results in superior cell-to-cell uniformity for ZnSnO-buffered devices compared to chemical-bath-deposited CdS buffer layers.

Evolution of structural and optoelectronic properties in fluorine–aluminum co-doped zinc oxide (FAZO) thin films and their application in CZTSSe thin-film solar cells

The window layer is an important layer that transmits sunlight and generates electric current as the carriers move. Aluminum (Al) -doped zinc oxide (AZO) has been widely applied as the window layer in thin-film solar cells (TFSCs) owing to its favorable optoelectronic properties, as its properties can be controlled and further improved with suitable dopants. In this study, we strategically co-dope Al and fluorine (F) in ZnO thin films (FAZO) to improve the transmittance with controlled carrier concentration. To study the influence of F dopant on the structural, optical, and electrical properties of FAZO thin films, the F doping concentration is varied (0–1.5%) while keeping the Al content fixed (2%). With increasing F doping concentration, the carrier concentration, transmittance, and bandgap values increase, whereas, the sheet resistance and resistivity of the single-layer FAZO thin films decrease, compared to the reference AZO film. The addition of a small amount of F alters the surface roughness with increasing concentration. The properties of the single-layer FAZO thin films show deteriorated properties at the highest doping concentration (1.5%). Under the optimum condition of 1% F, FAZO thin films exhibit the lowest sheet resistance of 5.84 Ω/sq, in addition to the highest transmittance of 89.3% in the visible light region. Furthermore, the FAZO thin films applied in Cu2ZnSn(S,Se)4 (CZTSSe) device shows significant enhancements in the short-circuit current density (Jsc) and fill factor (FF), resulting in improved device performance from ~8 to 9.57%.

Application of Quantum Dot Down-Conversion Layer in Thin-Film Solar Cells to Increase Short-Wavelength Spectral Response

The short-wavelength optical loss in the Cu(In,Ga)Se2 (CIGS) thin-film solar cells is inevitable owing to the substantial light absorption in the front layers such as the buffer layer and transparent conducting oxide (TCO) layer. Quantum dots (QDs) with CdSe/ZnS core–shell structure is utilized to increase the short-wavelength spectral response of the CIGS thin-film solar cells. The QDs absorbs photons in the short-wavelength region (<540 nm) and re-emits the photons at approximately 540 nm; these photons penetrate the front layers and reach the CIGS absorber layer. The thickness of the QD layer was varied via drop coating with different QD concentrations, thereby facilitating the application of the optimized QD layer as a down-conversion layer in the CIGS thin-film solar cells. The photoelectric parameters of the CIGS thin-film solar cells were dependent on the QD thickness, and they were characterized using quantum efficiency measurements, spectrophotometric analysis, and current–voltage measurements. The CIGS thin-film solar cells with a 0.7 μm-thick QD layer exhibited the highest increase of 1.86 mA cm−2 and 0.75% in the short-circuit current density and efficiency, respectively.

Challenges in the deposition of (Ag,Cu)(In,Ga)Se2 absorber layers for thin-film solar cells

The partial replacement of Cu by Ag in Cu(In,Ga)Se2 thin-film solar cells is strategically interesting to achieve smooth devices with high conversion efficiencies. Yet, the industrial exploitation requires further understanding of the deposition process and control of the absorber layer properties. In this study, three-stage co-evaporation of (Ag,Cu)(Ga,In)Se2 films with [Ag]/([Ag] + [Cu]) contents up to 0.2 was investigated. Deep crevices and voids, sometimes extending down to the rear contact, were found. They mainly occur for high Ag contents and excessive group-I richness during the second stage of the deposition. The formation of cavities is attributed to the segregation of Ag–Se phases and slow Ag diffusion into the chalcopyrite during the deposition. Another identified challenge is the flattening of the desired bandgap grading which is correlated with the Ag content. Optimized process conditions allow fabrication of smooth (Ag,Cu)(Ga,In)Se2 films in a manufacturing-like inline deposition with cell efficiencies up to 20.5%.

Structural, surface morphological and optical properties and their correlation with the thickness of spin coated superior quality CdS thin film synthesized using a novel chemical route

Cadmium sulfide (CdS) is considered as a superior buffer/window layer in thin-film solar cells apart from its various applications. The thickness of the buffer/window layer that defines the interface between the absorber layer is very important and needs to be thoroughly investigated for optimized device performance, particularly when a novel synthesis route and deposition technique is used. This manuscript presents the investigation on the various structural, morphological, and optical parameters and their correlation with the thickness of CdS thin film synthesized using a novel thiol-amine cosolvent with the help of Triton-X 100 surfactant. Spin coating technique is used to deposit CdS thin-films having thicknesses ranging from 100 to 300 nm on a glass substrate. X-ray diffraction (XRD) exposed the polycrystalline nature of the CdS films. All the diffraction peak intensities were found to increase with the thickness. Structural parameters like dislocation density, lattice strain, and crystallites per unit area calculated from the XRD data were found superior to those parameters for CdS films synthesized through various chemical routes, and the parameters indicated a positive correlation of the degree of crystallinity on the thickness. Similarly,surface morphology and optical properties of CdS films that were investigated by the scanning electron microscope (SEM) and UV–VIS spectrophotometer, were also dependent on film thickness. These results show that CdS thin film with superior crystallinity can be synthesized using thiol-amine cosolvent along with the Triton-X 100 surfactant and deposited by low-cost spin-coating on glass substrate. Moreover, the thickness variation appeared to be a single tool for obtaining application-specific various properties of CdS thin-film.

Preparation of AlSb thin films on stainless steel flexible substrates and preventive measures of its deliquescence

Flexible thin-film solar cells are important for their advantages in many aspects, and aluminum antimonide (AlSb) can be used as an absorbent layer in thin-film solar cells. However, its deliquescence limits its practical use. In this work, AlSb films were prepared by radio frequency (RF) magnetron sputtering on stainless steel flexible substrates. A flexible conformal coating membrane using acrylic resin was deposited on the AlSb films and its deliquescence prevention effect was verified. It was found that after conformal coating treatment, the deliquescence of the AlSb film was effectively stopped. The bare AlSb films and the Ni/AlSb/acrylic resin/Ni multilayer films were exposed to ambient air at the same time under the same conditions for lengths of time ranging from 1 min to 60 days. After 60 days, the bare AlSb film experienced significant deliquescence, such that the film was completely destroyed. However, the Ni/AlSb/acrylic resin/Ni multilayer films did not exhibit deliquescence.

Impact of film thickness on optical properties and optoelectrical parameters of novel CuGaGeSe4 thin films synthesized by electron beam deposition

The authors in this article present the synthesis of good quality CuGaGeSe4 thin films of different thicknesses using electron beam deposition on well pre-cleaned glass substrates. X-ray diffraction patterns displayed the amorphous nature of as-prepared CuGaGeSe4 thin films. In addition, the elemental compositional analysis of these films was examined by the energy-dispersive X-ray spectroscopy technique, which showed that there is good matching between the selected and detected percentages. Transmittance and reflectance spectra of these CuGaGeSe4 samples were measured to experimentally determine the absorption coefficient and some related optical parameters. Optical band-gap energy values of samples were determined via Tauc’s Plots; they are arisen owing to the indirect allowed transition. They are decreased from 1.43 to 1.29 eV by increasing the film thickness from 250 to 445 nm. The skin depth, absorption index, and refractive index of CuGaGeSe4 thin films were also obtained and extensively studied. As well as, some optoelectrical parameters of these investigated films were discussed, like optical resistivity, optical mobility, optical conductivity, the lattice dielectric constant, and the ratio of the charge carrier concentrations to the effective mass (Nopt/m*). Along with, some nonlinear optical parameters of CuGaGeSe4 films were studied employing Miller’s formulas. The values of the dispersion energy, static refractive index, the static dielectric constant, the oscillator strength and others increase, while the oscillator energy and the relaxation time decrease as the film thickness increased. The obtained results showed that these CuGaGeSe4 film samples can be successfully used as absorption layers in thin-film solar cells.

A systematic study on chemically deposited cadmium sulfide (CdS) thin film

In this work, the conventional chemical bath deposition (CBD) technique and its growth parameters were systematically studied to deposit CdS thin films on the glass substrate. The samples were prepared using a simple aqueous solution containing cadmium sulfate (CdSO4·8H2O), thiourea [SC(NH2)2], ammonium sulfate [(NH4)2SO4] as a source of cadmium, sulfur, and a complexing agent, respectively. The deposited films were characterized to study the surface morphology, crystallographic structure, chemical composition, optical and electrical properties. Through this study, the structure was determined to cubic with (111) preferential orientation and the crystallinity of the films was improved with the increase of film thickness. The transmission spectra were recorded in the range of wavelength 300–600 nm. The optical bandgap of the optimized CdS films was varied from 2.43 to 2.74 eV. All the films exhibit n-type of conductivity which was found in the order of 10–4 to 10–5 (1/Ω-cm), suggesting that it would be suitable for the buffer layer in thin-film solar cells.

Investigation of Optimum Conditions for Synthesis of Cu(In,Ga)Se2 Nanoparticles by Refluxing

Cu(In,Ga)Se2 (CIGSe) has been proven to be a better candidate as a light absorber layer in thin-film solar cells. However, most processes require high vacuum and high temperature during deposition, which results in significant loss of materials and is not applicable to a flexible substrate. Solution processes often involve low processing temperature and cheap precursor, can be used with flexible substrates, and offer the possibility of roll-to-roll manufacturing, potentially reducing manufacturing costs for the module. Here, we have experimentally investigated the optimum synthesis conditions for CIGSe nanoparticles fabricated by using a facile and a non-vacuum reflux method for low-temperature solution processes. By employing various reflux conditions by changing the temperature of heating mantle, single-phase CIGSe nanoparticles were synthesized at 200 °C. On the other hand, synthesized products with an impure multi-phase were formed at heating mantle temperatures lower than 200°C. XRD measurements confirmed that the Ga content of the CIGSe nanoparticles increased with increasing heating mantle temperature. In addition, the average diameter of the CIGSe nanoparticles increased with increasing reaction time from 5 min to 30 min at a fixed heating mantle temperature of a 200 °C. The optical band gap is calculated by using ultraviolet-visible (UV-Vis) absorption spectra, decreased from 1.69 eV to 1.29 eV with increasing reaction time due to the increased CIGSe nanoparticles size. From our results, we can conclude that the characteristics of the CIGSe nanoparticles can be effectively controlled by using simple growth conditions, thereby providing many advantages for the fabrication of absorber layers for use in CIGSe solar cells.