Buffer Layer In Solar Cells Research Articles

Comparative studies of CdS thin films by chemical bath deposition techniques as a buffer layer for solar cell applications

Cadmium sulfide (CdS) buffer layer that decouples the absorber layer and window layer in thin-film solar cells was synthesized by two different chemical bath deposition (CBD) techniques with varying deposition parameters. X-ray diffraction (XRD) revealed that the CdS thin film crystallizes in a stable hexagonal wurtzite structure having a preferential orientation along (002) reflection plane with a crystallite size varying from 20 to 40 nm. First longitudinal optical phonon mode was identified at Raman shift of 305 cm−1. Uniform, granular, continuous, and smooth surface with an average grain sizes (< 100 nm) as well as small roughness (< 9 nm) was observed by scanning electron microscopy (SEM) and atomic force microscopy (AFM), respectively. The symmetric composition of cadmium and sulfur along with larger grains (20 nm) was observed at higher deposition temperatures and times. The optical band gap of CdS samples obtained from process one was in the range of 2.3–2.35 eV, while the band gap by the second CBD process lay in between 2.49 and 2.65 eV, showing the most stable compound of CdS. The presence of a green emission band in photoluminescence spectra (PL) demonstrated that the CdS material has better crystallinity with minimum defect density. Hall effect studies revealed the n-type conductivity of CdS thin films with a carrier concentration values in the order of 1016 cm−3. Furthermore, CdS thin films fabricated by CBD process exposed better quality that might be more suitable material as a buffer layer for thin-film 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.

An exploration into the quantum confinement of CTS/natural dye core- shell quantum dots

In this work, we have presented a simple way of changing the confinement energies of Copper Tin Sulphide (CTS) quantum dots using natural dyes as shell material. Tetragonal CTS quantum dots in the size range of 1.7 nm–2.2 nm, of bandgaps of 2.48eV and 5.0 eV were prepared by means of a green colloidal synthesis technique. These quantum dots were treated with natural dyes such as onion and beetroot skin dyes. Pelargonidin and Betanin (pigments of onion and beetroot skin dye respectively) formed hydrogen bonding with the capping agent, thus forming a shell around the CTS quantum dots. The change in confinement due to the effect of dye as shell was studied from absorption, photoluminescence and infrared spectroscopic techniques. The transitions occurring were analysed using a theoretical approach. CTS quantum dots, with its high transmittance in a wide range of wavelengths find promising applications in the buffer layer of solar cells.

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.

Гидрохимический синтез пленок халькогенидов металлов. Часть 40. Влияние условий гидрохимического осаждения пленок ZnS тиоацетамидом на их толщину и морфологию

ZnS thin films are promising as a buffer layer in solar cells; photovoltaic cells, photoelectric sensors, light-emitting diodes are being developed on their basis. In this work, the growth kinetics of ZnS films has been studied during hydrochemical precipitation with thioacetamide from acidic (biphthalate, acetate) and alkaline (ammonia) aqueous solutions. From the biphthalate bath at pH = 2.5–3.0 at 343 K and from the ammonia reaction mixture at 293 K on glass substrates, mirror layers of zinc sulfide with a thickness of up to 250 and 330 nm were obtained, respectively. The maximum thickness of the ZnS films from the ammonia system is achieved by increasing the ammonia content in it to 5 mol/l. For all the ammonia concentrations used, an extreme dependence of the thickness of the ZnS films on the concentration of thioacetamide in the system was established. When precipitating ZnS films from the acetate system, lowering the pH of the solution from 5.0 to 4.5 increases their thickness by a factor of 1.5-2.0. On the kinetic curves of changes in the thickness of the ZnS films during deposition from the biphthalate system, an area is distinguished, which is characterized by the presence of a plateau, which presumably corresponds to the sulfidization process of the zinc hydroxide phase formed at the initial stage of synthesis. The optimal composition of the reaction mixtures and the recommended time of deposition of ZnS films from the studied reaction mixtures are proposed. According to electron microscopy, the main structural elements of the zinc sulfide films precipitated with thioacetamide, regardless of the composition of the reaction mixture, are globular aggregates. The layers precipitated from the biphthalate reaction mixture have more uniform microstructure with a predominant particle size of 30-50 nm. Films obtained from the ammonia reaction mixture are formed from globules with a diameter of from 0.5 to 5.0 μm with particles of ~ 100 nm interspersed. The content of the main elements (Zn, S) in the ZnS films obtained from the biphthalate and ammonia reaction mixtures, according to the data of energy dispersive microanalysis, was 49.48, 50.52 and 50.35, 49.65 at.%, respectively.

Гидрохимический синтез пленок халькогенидов металлов. Часть 39. Химическое осаждение тонких пленок ZnS тиоацетамидом

ZnS thin films are promising as a buffer layer in solar cells, which can be basis of photovoltaic cells, photoelectric sensors, and light-emitting diodes. For the preparation of thin ZnS films by chemical bath deposition, thioacetamide or thiourea is used as a chalcogenization agent, and ammonia, triethanolamine and sodium citrate are mainly used as ligands, carrying out the process in an alkaline medium. In the present work, in order to predict the conditions of hydrochemical deposition of ZnS films, we have analyzed ionic equilibria in two reaction systems “ZnCl2 – NH4OH – CH3CNH2” and “ZnCl2 – CH3CSNH2 – KHC8H4O4” that differ in acidity of the medium. An analysis of ionic equilibrium showed that in the first bath ~80% of the metal is in the form of a neutral hydroxo complex Zn(OH)2 at pH &gt; 7, and in the second more than 98% of zinc is present as acetate complexes Zn(CH3COO)+ and Zn(CH3COO)2 in the range of pH from 0 to 7. The thermodynamic evaluation of the boundary conditions for the formation of zinc sulfide made it possible to conclude that a zinc sulfide film can be formed in both systems without the admixture of Zn(OH)2 hydroxide. ZnS films were obtained by hydrochemical deposition with thick about 100 nm from both systems. Using local energy-dispersive elemental analysis, it was found that the average ratio between the main elements of Zn and S in the layers obtained in an alkaline medium is 49.48 and 50.52 at.%, and in the synthesized from acidic solutions – 50.35 and 49.65 at.%. According to the data of electron microscopy, up to 85% of the agglomerates have an average size of 200-450 nm that formed from ZnS particles growing in an alkaline reaction bath. At the same time, there are aggregates whose dimensions reach 700 nm. The layers that deposited from relatively acidic solutions are distinguished by a higher degree of dispersion. Here up to ~90% of the film-forming particles is in the nanoscale range from 50 to 90 nm.

Thin-Film FTO-ZnO-CZTS Solar Cells Fully Deposited by Spray Pyrolysis

Thin films of ZnO, CZTS (Cu2ZnSnS4) and ZnO-CZTS were prepared by spray pyrolysis deposition (SPD). After spraying a precursor solution of ZnO directly onto fluorine-doped tin oxide (FTO) substrats, CZTS thin layers were sprayed onto layers of ZnO to forme a p-n junction. Some CZTS layers were directly spray onto fluorine-doped tin oxide substrats. ZnO is a wide band gap n-type material, consisting of abundant and nontoxic elements, and is thus expected to be a good substitute for CdS buffer layer in solar cells. In this paper, we report the study of CZTS and ZnO thin films synthesized by chemical spray pyrolysis (CSP) method. During the deposition of the ZnO thin films on the FTO substrats the number of sequences was varied from 20 to 40. The influence of the ZnO on the structural, optical, morphological and electrical properties of CZTS films was studied using various techniques. The X-ray diffraction studies showed the formation of kesterite (Cu2ZnSnS4) phases with the peaks corresponding to (112), (220) and (312) planes. SEM study revealed a lack of uniformity of the surface of the CZTS layers sprayed onto the ZnO layers for a lower thickness of ZnO (FTO-ZnO20-CZTS). The band gap values of FTO-CZTS and FTO-ZnO-CZTS thin films were measured and found in the range 2.2 - 2.25 eV which are in good agreement with the results reported (2-2.2 eV). The morphological studies revealed the formation of some clusters randomly distributed on the film surface in the ZnO-containing layers, i.e. the FTO-ZnO20-CZTS and Finally, current-voltage measurements for different PV cells with maximum efficiency were carried out. A conversion efficiency (η) of 5.99% with fill factor (FF) = 18.8%, open circuit voltage (Voc) = 0.54 V, short circuit current density (Jsc) = 59 mA.cm2 were recorded through the FTO-ZnO40-CZTS thin film PV cell.

Pulsed Laser Deposition of Zn(O,Se) Layers for Optoelectronic Application

Zinc oxyselenide—Zn(O,Se)—could become a novel buffer layer in solar cells and a functional layer in different optoelectronic devices. In this study, we systematically investigated the influence of the deposition temperature ranging from room temperature (RT) to 650 °C on the structural and optoelectronic properties of Zn(O,Se) layers grown on photovoltaic (PV) glass substrates by one-step pulsed laser deposition in a high vacuum. All layers were characterized using energy-dispersive X-ray spectroscopy (EDX), scanning electron microscopy (SEM), X-ray diffractometry (XRD), UV–vis spectroscopy, and the Hall and van der Pauw technique. We demonstrated that polycrystalline, uniform, and electrically conductive Zn(O,Se) layers were grown at the substrate temperatures of 500–650 °C, while those layers grown at temperatures below 500 °C were characterized as amorphous and exhibiting a semi-insulating behavior. According to the XRD data, single-phase layers consisting of a ternary Zn(O,Se) phase were formed only ...

VUV-Assisted Photochemical Sol-Gel Processing of Metal Oxide Thin Films

Metal oxides such as TiO2, ZnO, SnO2 are useful in many of energy devices because of their high chemical stability, n-type semiconductivity and photoelectrochemical properties. Various methods of solution processing of their thin films are known, such as chemical bath deposition, electrodeposition and sol-gel reactions. Typically, high temperatures need to be applied for crystallization to achieve good properties. The need of high temperature however limits the choice of substrates to heat-resistant materials. Also, crystallization often results in pinholes and cracks to damage the performance of devices. We have developed an alternative sol-gel method to obtain ultra-smooth thin films of various metal oxides at room temperature, by employing photochemical decomposition of coated precursor by vacuum ultraviolet (VUV) light irradiation. The method is compatible with conductive plastic film substrates, so that multiple applications in thin film devices such as transparent conductive layer, light emitter, buffer layer in solar cells and thin-film encapsulation (TFE) as an example shown in Fig. 1 can be envisioned. Figure 1

Realizing zinc-doping of CdS buffer layer via partial electrolyte treatment to improve the efficiency of Cu2ZnSnS4 solar cells

Cadmium sulfide (CdS) is the most widely used buffer material for a variety of thin-film solar cells including Cu2ZnSnS4. However, reports have shown that CdS film obtained by chemical bath deposition (CBD) is not the ideal buffer layer for pure sulfide CZTS solar cell. Zinc doping is a viable approach to modifying the CdS buffer film, but the present methods are far from satisfactory. Here, we innovatively developed an effective way of Zinc doping using partial electrolyte (PE) treatment, resulting in state-of-the-art buffer layer for solar cells based on high-surface-waviness CZTS light absorber. Our study shows that this Zn PE-treated CdS film improved the properties including coverage, full-range light transmittance and conduction band alignment with CZTS. Ultimately, the resultant modified CdS film boosted the open-circuit voltage of our devices by more than 100 mV, yielding power conversion efficiency (PCE) of 3.30%. We note that this is the highest efficiency that has been reported for all solution-processed CZTS solar cells.

Tunable optical and structural properties of Zn(S,O) thin films as Cd-free buffer layer in solar cells

Zn(S,O) thin films were grown by three step process: (1) the deposition of Zn layer on glass substrates by vacuum thermal evaporation, (2) the oxidation of the as-deposited Zn layer in air atmosphere and (3) the sulfurization of the oxidized samples in N2 atmosphere. The samples were analyzed for their microstructural, morphological and optical properties. XRD analysis revealed that the films were polycrystalline with a highly c-axis orientation. EDX measurements suggested that the sulfur content of the films could be inflected by the sulfurization temperature. The optical constants of the films were calculated from the analysis of the transmittance and reflectance data. The band gap energy varied from 3.27 to 3.08eV depending on the sulfur content. The surface morphological studies revealed that the films had an average roughness between 13.1 and 16.9nm. Zn(S,O) thin films exhibited n-type electrical conductivity.

Numerical Simulation of Varied Buffer Layer of Solar Cells Based on Cigs

Numerical simulation has been used to investigate the effect of different buffer layer components on the performance of CuInGaSe2 solar cells with SCAPS-1D software. The main photovoltaic parameters of simulated devices: open-circuit voltage (Voc), short-circuit current (Jsc), fill factor (FF), and conversion efficiency (h), areanalysed as a function of thickness and temperature in the different buffer layers used. According to numerical simulation the highest conversion efficiency (23%) of CIGS solar cell is reached for the CdS buffer layer. This result is validated by experimental results (20%). At 300 K, when the thickness of the buffer layer (CdS, ZnS, ZnSe, InSe2) increases from 100 nm to 500 nm, with the other parameters maintained constant, the efficiency decreases. When the temperature increases from 300 K to 400 K, with the other parameters maintained constant, both open circuit voltage and conversion efficiency also decrease. The effect of dual buffer layers of ZnS/CdS has also been analysed and his efficiency increases of 3% than a single buffer CdS.

Stoichiometry and local bond configuration of In2S3:Cl thin films by Rutherford backscattering spectrometry

In2S3 thin films deposited using chemical methods always contain residual elements from the precursors, which modify their properties. As buffer layers in solar cells, the residual elements in the In2S3 layer affect the performance of these devices. The stoichiometry of In2S3 thin films deposited by spray ion layer gas reaction (ILGAR) was studied as a function of the residual Cl from InCl3 precursor by varying the deposition parameters. The chemical formula was deduced from the elemental composition determined using Rutherford backscattering (RBS). Incomplete sulfurization of the precursor implies that residual Cl− remains bonded to the In3+ ions while some occupy interstitial and/or antisite positions in the In2S3 matrix. This results in thin films with different stoichiometry, described by the formula In4S6−xCl2x+2y. This changes the local bond configuration and geometry and underpins the influence of residual Cl on the physical properties of In2S3 thin films.

Silicon nanospheres for directional scattering in thin-film solar cells

Reducing active layer thickness of solar cell stresses on efficient light trapping mech- anisms to keep the cell efficiency intact. Directional light scattering and promising refractive index of silicon nanoparticles make them encouraging scattering centers for thin-film silicon solar cells. Finite-difference time-domain simulations are used to study the optical properties of silicon nanospheres embedded in the top and bottom buffer layer of solar cells. Diameter of a silicon nanoparticle plays a crucial role in the forward and backward scattering of incident light into the cell. Silicon nanospheres outperform commonly used metallic and dielectric nano- spheres and trapped the incident light over a broad spectrum. Silicon nanospheres require special attention when placed in both the buffer layers of the solar cell simultaneously, and lateral dis- placement of the silicon nanospheres at the top buffer layer with respect to nanospheres at the bottom buffer layer is beneficial. Lateral displacement of nanospheres provides a total quantum efficiency of 51.49% in comparison to 21.9% of the pristine cell. These exceptional scattering competencies of silicon nanospheres make them a promising candidate for photovoltaic appli- cations. Silicon scatterers may be used with well-established fabrication techniques. © 2016

Influence of oxidization temperature on Zn(O,S) films deposited by electron beam evaporation

Zn(O,S) films were fabricated by oxidizing ZnS thin films deposited by electron beam evaporation method onto glass substrates at temperatures of 350–500℃ for 2 h in an atmosphere of oxygen. The XRD and EDX confirmed that the Zn(O,S) films were obtained successfully. The influence of the oxidization temperature on the optical and electrical properties of the Zn(O,S) thin films was investigated. The experimental results show that the Zn(O,S) thin film oxidized at the temperature of 400℃ exhibits better properties than others, with the transmittance of 86% in the visible region, the band gap energy of 3.36 eV and the resistivity of 3.22 × 103 Ω·cm, which makes it a potential buffer layer of solar cell.

Study of structural, optical and photoluminescence properties of indium-doped zinc sulfide thin films for optoelectronic applications

In the present work, we have deposited indium-doped zinc sulfide (ZnS:In) thin films by chemical bath deposition technique (CBD). The structural properties studied by X-ray diffraction indicate that ZnS:In has a cubic structure with an average crystallite size 4.7–11.0nm. Transmission and reflection spectra reveal the presence of interference fringes indicating thickness uniformity and surface homogeneity of deposited material. All the films were transparent in the visible and infrared regions (⩾60%), which allows us to use this material as an optical window or a buffer layer in solar cells. The obtained band gap energy Eg is in the range of 3.70–3.76eV. The refractive index and thickness of ZnS:In thin films was calculated using envelope method. The variation of the refractive index along the Cauchy distribution was observed in all ZnS:In thin films. The analysis of the refractive index data through the Wemple–DiDomenico model leads to the single oscillator energy (E0) and the dispersion energy (Ed).

Study of the morphology, photoluminescence and photoconductivity of ZnO–CdO nanocrystals

Transparent and highly photoconductive nano-composite ZnO–CdO films were fabricated on glass substrate at 360 °C substrate temperature. X-ray diffraction (XRD) studies indicate that the films are nano-crystalline and contain a combination of hexagonal ZnO and cubic CdO structures. According to scanning electron microscopy (SEM) studies, the surface of the films is found to be rough over large areas favoring photo absorption. From the optical and electrical measurements, it is found that ZnO–CdO composite combines the properties of optical transparency in the visible region with a high electrical conductivity. Based on the characteristics of photoluminescence (PL) and photoconductivity, it seems that Zn/Cd = 3:1 nano-crystalline film is preferable for application as buffer layers in solar cells.

Solution-based Ag-doped ZnSe thin films with tunable electrical and optical properties

Thin film transistors fabricated using ZnSe thin films synthesized using chemical bath deposition.

Fabrication and characterization of nano-structured ZnS thin films as the buffer layers in solar cells

High quality nanocrystalline ZnS thin films were deposited on glass substrates by a chemical bath deposition (CBD) method from an aqueous solution containing triethylamine, as a nontoxic complexing agent for zinc ions, at temperatures ranging from 50 to 90 °C. The effects of the deposition time and temperature on thickness as well as structural, morphological and optical properties of the ZnS thin films were examined by X-ray diffraction (XRD), field emission scanning electron microscopy (FE-SEM) and ultraviolet-visible (UV-vis) spectroscopy. Fourier transform infrared spectroscopy (FT-IR) studies were done to investigate the presence of organic and/or other compounds in films. UV-vis spectroscopy data revealed the obtained ZnS films to have an optical band gap ranging from 4.1 to 3.9 eV. XRD results showed that the prepared films were highly crystalline with a cubic zinc blende structure consisting of very small nanocrystallites, 2.0–2.5 nm in size, showing quantum confinement effects.

Fabrication of ZnS Thin Film Buffer Layer in Solar Cell by Radio Frequency Sputtering Method

The present study aims to investigate the influence of Coring glass substrate temperature on the topography, deposition rate, crystal structure, optical, and electrical properties of ZnS thin films produced by magnetic radio frequency sputtering method. From plain view SEM micrographs, the pebble structure has shown in all ZnS thin films deposited at various substrate temperatures. Through higher substrate temperature, smaller ZnS grains can be obtained in the present study. From XRD analysis, ZnS thin film exhibits hexagonal Wurtzite structure. When thickness of ZnS thin film arrive 300nm, optical transmission rate can be above 85% regardless of substrate temperature and gets optical energy barrier of 3.9 eV. From electrical measurement, the variation of resistivity with temperature exhibits a linear relationship for ZnS thin film.