Dependence of carrier lifetime on Cu-contacting temperature and ZnTe:Cu thickness in CdS/CdTe thin film solar cells

Cu diffusion from a ZnTe:Cu contact interface can increase the net acceptor concentration in the CdTe layer of a CdS/CdTe photovoltaic solar cell. This reduces the space-charge width (W_d) of the junction and enhances current collection and open-circuit voltage. Here we study the effect of Cu concentration in the CdTe layer on carrier lifetime (τ) using time-resolved photoluminescence measurements of ZnTe:Cu/Ti-contacted CdTe devices. Measurements show that if the ZnTe:Cu layer thickness remains constant and contact temperature is varied, τ increases significantly above its as-deposited value when the contacting temperature is in a range that has been shown to yield high-performance devices (~ 280° to ~ 320 °C). However, when the contacting temperature is maintained near an optimum value and the ZnTe:Cu thickness is varied, τ decreases with ZnTe:Cu thickness.     

A detailed study of the series resistance effect on CdS/CdTe solar cells with Cu/Mo back contact

CdS/CdTe thin film solar cells with an area of 1 cm² were obtained and studied in detail. A ZnO buffer layer was deposited by reactive RF-sputtering on commercial ITO substrates. The CdS layer was grown on ZnO also by using RF-sputtering and CdTe thin film was deposited by conventional CSS technique. The chlorination of the solar cells is performed into Freon atmosphere at 400 °C. The CdTe thin film surface was chemically etched by using Br-Methanol solution. The back contact was deposited using RF-sputtering from a pure Cu and Mo targets. The procedure developed in this work led us to make systematically solar cells with good efficiency. However, the series resistance has a high value for an area of 1 cm² (22 Ω cm²). In order to make more detailed study, the solar cell with an area of 1 cm² was divided in a 3 × 3 matrix. A good homogeneity in cell properties is observed and the efficiency increases to more than 11%, fundamentally through decreasing series resistance.     

The use of aluminium doped ZnO as transparent conductive oxide for CdS/CdTe solar cells

CdTe/CdS and CdTe/ZnO thin film solar cells were grown with a high vacuum evaporation based low temperature process (≤ 420 °C). Aluminium doped zinc oxide (AZO) was used as transparent conducting oxide (TCO) material. AZO exhibited excellent stability during the solar cell processing, and no significant change in electrical conductivity or transparency was observed. The current density loss due to absorption in the 1 μm thick AZO layer with 5 Ω per square sheet resistance was found to be 1.2 mA/cm². We investigated the influence of an intrinsic ZnO layer (i:ZnO) in combination with various CdS thicknesses. The i:ZnO layer was found to significantly increase the open circuit voltage of the solar cells with very thin CdS layer. Increasing thickness of the i:ZnO layer leads to UV absorption losses, narrowing of the depletion layer width and hence reduced collection efficiency in the long wavelength (685-830 nm) part. With AZO/i:ZnO bi-layer TCO we could achieve cell efficiencies of 15.6% on glass and 12.4% on the flexible polyimide film.     

Synthesis and characterization of electrolyte-grade 10%Gd-doped ceria thin film/ceramic substrate structures for solid oxide fuel cells

In the present research, spray pyrolysis technique is employed to synthesize 10%Gd-doped ceria (GDC) thin films on ceramic substrates with an intention to use the "film/substrate" structure in solid oxide fuel cells. GDC films deposited on GDC substrate showed enhanced crystallite formation. In case of NiO-GDC composite substrate, the thickness of film was higher (~ 13 μm) as compared to the film thickness on GDC substrate (~ 2 μm). The relative density of the films deposited on both the substrates was of the order of 95%. The impedance measurements revealed that ionic conductivity of GDC/NiO-GDC structure was of the order of 0.10 S/cm at 500 °C, which is a desirable property for its prospective application.     

CdTe solar cell with industrial Al:ZnO on soda-lime glass

One avenue to enhance CdTe cell performance is to improve the optical transmission of the transparent conductive oxide (TCO)/window layer stack. In this paper, we examine soda-lime float glass coated with an Al-doped ZnO layer and a buffer layer. The possible advantages of using a ZnO-based TCO include reduced surface roughness, improved transparency, and an integrated buffer layer that can be optimized for use in a CdTe PV device. Device processing was modified to address the chemical and thermal differences between the ZnO-based TCO stack produced by Saint-Gobain and the TCOs previously used at the National Renewable Energy Laboratory (NREL). These process modifications produced ~ 8% efficiency for devices without a buffer layer. Incorporation of buffer layers has already produced devices with ~ 11% and > 12% efficiency for CdTe deposition temperatures of 570° and 500°C, respectively.     

Preparation and characterization of pulsed laser deposited CdTe thin films at higher FTO substrate temperature and in Ar + O2 atmosphere

Pulsed laser deposition (PLD) is one of the promising techniques for depositing cadmium telluride (CdTe) thin films. It has been reported that PLD CdTe thin films were almost deposited at the lower substrate temperatures (<300 °C) under vacuum conditions. However, the poor crystallinity of CdTe films prepared in this way renders them not conducive to the preparation of high-efficiency CdTe solar cells. To obtain high-efficiency solar cell devices, better crystallinity and more suitable grain size are needed, which requires the CdTe layer to be deposited by PLD at high substrate temperatures (>400 °C). In this paper, CdTe layers were deposited by PLD (KrF, λ = 248 nm, 10 Hz) at different higher substrate temperatures (T_s). Excellent performance of CdTe films was achieved at higher substrate temperatures (400 °C, 550 °C) under an atmosphere of Ar mixed with O₂ (1.2 Torr). X-ray diffraction analysis confirmed the formation of CdTe cubic phase with a strong (1 0 0) preferential orientation at all substrates temperatures on 60 mJ laser energy. The optical properties of CdTe were investigated, and the band gaps of CdTe films were 1.51 eV and 1.49 eV at substrate temperatures of 400 °C and 550 °C, respectively. Scanning electron microscopy (SEM) showed an average grain size of 0.3–0.6 μm. Thus, under these conditions of the atmosphere of Ar + O₂ (15 Torr) and at the relatively high T_s (500 °C), an thin-film (FTO/PLD-CdS (100 nm)/PLD-CdTe (~1.5 μm)/HgTe: Cu/Ag) solar cell with an efficiency of 6.68% was fabricated.     

Optimization of titanium dioxide film prepared by electrophoretic deposition for dye-sensitized solar cell application

Nanocrystalline TiO₂ films were deposited on a conducting glass substrate by the electrophoretic deposition technique. It was found that the thickness of TiO₂ film increased proportionally with an increase in deposition time and deposition voltage. However, as the deposition duration or deposition voltage increased, the film surface was more discontinuous, and microcracks became more evident. The characteristic of the dye-sensitized solar cell using TiO₂ film as a working electrode was analyzed. The results of the energy conversion efficiency and the photocurrent density exhibited a relationship dependent on the TiO₂ thickness. Curve fitting of energy conversion efficiency vs. TiO₂ thickness revealed the optimum solar cell efficiency ~ 2.8% at the film thickness of ~ 14 μm.     

CdTe thin film solar cells: Interrelation of nucleation, structure, and performance

The performance of CdTe solar cells as prototype of thin film solar cells strongly depends on film morphology. The needs for high solar cell performance using thin film materials will be addressed covering nucleation and growth control of thin film materials. In order to understand the basic growth mechanisms and their impact on cell performance, we have systematically investigated the growth of CdTe thin films by Close Spaced Sublimation (CSS) using the integrated ultra-high vacuum system DAISY-SOL. CdTe thin films were deposited on TCO/CdS substrates (transparent conductive oxide) held at 270 °C to 560 °C. The properties of the films were determined before and after CdCl₂ treatment using X-ray diffraction and electron microscopy. In addition, solar cells were prepared to find correlations between material properties and cell efficiency. At low sample temperature the films tend to form compact layers with preferred (111) orientation which is lost at elevated temperatures above 450 °C. For CdS layers without (0001) texture there is in addition a low temperature regime (350 °C) with (111) texture loss. After activation treatment the (111) texture is lost for all deposited layers leading to strong recrystallisation of the grains. But the texture still depends on the previous growth history. The loss of (111) texture is evidently needed for higher performance. A clear correlation between cell efficiency and the texture of the CdTe film is observed.     

Plastic substrate with gas barrier layer and transparent conductive oxide thin film for flexible displays

A novel plastic substrate for flexible displays was developed. The substrate consisted of a polycarbonate (PC) base film coated with a gas barrier layer and a transparent conductive thin film. PC with ultra-low intrinsic birefringence and high temperature dimensional stability was developed for the base film. The retardation of the PC base film was less than 1 nm at a wavelength of 550 nm (film thickness, 120 µm). Even at 180 °C, the elastic modulus was 2 GPa, and thermal shrinkage was less than 0.01%. The surface roughness of the PC base film was less than 0.5 nm. A silicon oxide (SiO_x) gas barrier layer was deposited on the PC base film by a roll-to-roll DC magnetron reactive sputtering method. The water vapor transmission rate of the SiO_x film was less than 0.05 g/m²/day at 40 °C and 100% relative humidity (RH), and the permeation of oxygen was less than 0.5 cc/m² day atm at 40 °C and 90% RH. As the transparent conductive thin film, amorphous indium zinc oxide was deposited on the SiO_x by sputtering. The transmittance was 87% and the resistivity was 3.5 × 10^− 4 ohm cm.     

Flexible cadmium telluride/cadmium sulphide thin film solar cells on mica substrate

Polycrystalline thin film II–VI compound semiconductors of cadmium sulfide (CdS) and cadmium telluride (CdTe) are the leading materials for the development of cost effective and reliable photovoltaic systems. The two important properties of these materials are its nearness to the ideal band gap for photovoltaic conversion efficiency and they have high optical absorption coefficients. Usually thin film solar cells are made by hetero-junction of p-type CdTe with n-type CdS partner window layer. In this article, we have deposited CdTe films on mica substrates using thermal evaporation technique and CdTe/CdS junction were developed by depositing a thin layer of CdS on to the CdTe substrate from chemical bath deposition method. The device was characterized by current voltage and photocurrent spectroscopy technique prior to the deposition of the transparent conducting layer. The devices were annealed in air at different temperatures and found that the device annealed at 673?K had better photovoltaic parameters. The efficiency of a typical device under 50?mW?cm^?2 illumination was estimated as 4%.     

Coercivity variation in exchange-coupled Fe/FePt bilayer with perpendicular magnetization

The soft/hard Fe/FePt film with perpendicular magnetization has been deposited on a glass substrate. The (001) oriented L1₀ FePt film was obtained when annealed by rapid thermal process at 800 °C and a Fe layer was deposited at room temperature with thicknesses of 2 nm to 20 nm. Controlling the Fe layer thickness allowed modification of the hysteresis loops from out-of-plane rigid magnet to in-plane exchange-spring like magnet due to the nanometer scale interface coupling. When the Fe layer thickness increased to 2 nm, the out-of-plane coercivity is reduced to 5.9 kOe but the remanence ratio (0.98) is still high. The Fe (2 nm)/FePt film shows perpendicular magnetization with linear in-plane hysteresis loop. The remanence ratio is reduced to 0.85 when the Fe layer thickness increased to 5 nm. When the Fe layer thickness was varied up to 10-20 nm, the in-plane hysteresis loop shows exchange-spring like behavior with two-step magnetization reversal processes. The films with perpendicular coercivity were moderated by the thickness of soft magnetic layer.     

Growth and characterization of germanium epitaxial film on silicon (001) using reduced pressure chemical vapor deposition

High quality germanium (Ge) epitaxial film is grown directly on silicon (001) substrate using a “three-step growth” approach in a reduced pressure chemical vapor deposition system. The growth steps consist of sequential low temperature (LT) at 400 °C, intermediate temperature ramp (LT-HT) of ~ 6.5 °C/min and high temperature (HT) at 600 °C. This is followed by post-growth anneal in hydrogen at temperature ranging from 680 to 825 °C. Analytical characterizations have shown that the Ge epitaxial film of thickness ~ 1 μm experiences thermally induced tensile strain of 0.20% with a threading dislocation density of < 10⁷ cm^− 2 under optical microscope and root mean square roughness of ~ 0.9 nm. Further analysis has shown that the annealing time at high temperature has an impact on the surface morphology of the Ge epitaxial film. Further reduction in the RMS roughness can be achieved either through chemical mechanical polishing or to insert an annealing step between the LT-HT ramp and HT steps.     

Thickness dependence of the amorphous-cubic and cubic-hexagonal phase transition temperatures of GeSbTe thin films on silicon nitride

The crystallization temperature of GeSbTe thin films with thicknesses between 11 and 87 nm on silicon nitride was studied through resistance versus temperature measurements. The amorphous-cubic phase transition occurs at ~ 150 °C for all films thicknesses, whereas the cubic-hexagonal phase transition temperature increases with film thickness, from ~ 200 °C for the 20 nm film to ~ 250 °C for the 87 nm film. The cubic-hexagonal transition occurs gradually for the 11 nm film. Implications for phase-change memory devices are discussed.     

Fluorescent tags to visualize defects in Al2O3 thin films grown using atomic layer deposition

Defects and cracks in thin film barriers that are coated on polymers allow the leakage of reactive species through the polymer substrate. Fluorescent tags have been developed to visualize defects and cracks in thin film barriers and to inspect rapidly the barrier quality with minimal sample preparation. For Al₂O₃ films with a thickness of 25 nm deposited on polyethylene naphthalate polymer substrates using atomic layer deposition techniques, the fluorescent tags have identified cracks ~ 20 nm in width after applied strain and have observed individual defects as small as ~ 200 nm in diameter.     

Epitaxial growth of Ni films sputter-deposited on a GaAs(001) surface covered with a MgO film

We studied the epitaxial growth of a Ni film prepared on a GaAs(001) substrate covered with a thin epitaxial MgO buffer film, assuming that this buffer film plays a key role in the epitaxial growth of the Ni film. The MgO and Ni films were deposited by radio-frequency magnetron sputtering of the MgO and Ni targets in pure Ar gas. First, a MgO film of thickness ranging from 78 to 4.4 nm was deposited on the GaAs(001) substrate at a temperature ranging from ambient temperature to 700 °C, and then, a 136-nm-thick Ni film was deposited on the MgO/GaAs substrate at a temperature range 300-500 °C. Using transmission electron microscopy and X-ray diffractometry, we showed that the MgO film grows with the epitaxial relationship MgO(001)[001]//GaAs(001)[001] on GaAs(001) at 500 °C, and that the structure of the Ni film depends on three factors: the MgO/GaAs substrate temperature, the MgO thickness, and the annealing condition of the MgO/GaAs substrate before the Ni deposition. In conclusion, we proved that the Ni film grows with the epitaxial relationship Ni(001)[001]//MgO(001)[001]//GaAs(001)[001] on MgO/GaAs with the 4.4-nm-thick MgO film when the MgO/GaAs substrate is annealed in situ at room temperature before the Ni deposition and maintained at 300 °C during the Ni deposition.     

Zinc cadmium oxide thin film transistors fabricated at room temperature

Zinc cadmium oxide (ZnCdO) transparent thin film transistors (TFTs) have been fabricated with a back-gate structure using highly p-type Si (001) substrate. For the active channel, 30 nm, 50 nm, and 100 nm thick ZnCdO thin films were grown by pulsed laser deposition. The ZnCdO thin films were wurtzite hexagonal structure with preferred growth along the (002) direction. All the samples were found to be highly transparent with an average transmission of about 80%~ in the visible range. We have investigated the change of the performance of ZnCdO TFTs as the thickness of the active layer is increased. The carrier concentration of ZnCdO thin films has been confirmed to be increased from 10¹⁶ to 10¹⁹ cm⁻³ as the film thickness increased from 30 to 100 nm. Base on this result, the ZnCdO TFTs show a thickness-dependent performance which is ascribed to the carrier concentration in the active layer. The ZnCdO TFT with 30 nm active layer showed good off-current characteristic of below ~ 10¹¹, threshold voltage of 4.69 V, a subthreshold swing of 4.2 V/decade, mobility of 0.17 cm²/V s, and on-to-off current ratios of 3.37 × 10⁴.     

Dependence of efficiency of thin-film CdS/CdTe solar cell on parameters of absorber layer and barrier structure

Dependences of the open-circuit voltage, short-circuit current, fill factor, and efficiency of a CdS/CdTe solar cell on the resistivity and thickness of the p-CdTe absorber layer, the noncompensated acceptor concentration N_a-N_d, and carrier lifetime τ in CdTe, are investigated, and optimization of these parameters in order to improve the solar cell efficiency is performed. It has been shown that the observed low efficiency of CdS/CdTe solar cells is caused by the too short electron lifetime in the range of 10^− 10-10^− 9 s and too thin (3-5 µm) CdTe layer currently used for fabrication of CdTe/CdS solar cells. To achieve an efficiency of 28-30%, the resistivity and thickness of the CdTe absorber layer, the noncompensated acceptor concentration, and carrier lifetime should be ∼ 0.1 Ω·cm, ≥ 20-30 µm, ≥ 10¹⁶ cm^− 3, and ≥ 10^− 6 s, respectively.     

A three-dimensional electrode for photoelectrochemical cell: TiO2 coated ITO mesoporous film

In this letter, TiO₂ coated ITO mesoporous film was prepared by dipping doctor-blade ITO mesoporous film in TiO₂ sol, followed by sintering at 500 °C for 30 min. The CdS quantum dots (QDs) were deposited on TiO₂ coated ITO mesoporous film using sequential chemical bath deposition (S-CBD) method to form a three-dimensional (3D) electrode. The photo-activity of ITO mesoporous film/TiO₂/CdS electrode was investigated by forming a photoelectrochemical cell, which indicated that the ITO mesoporous film/TiO₂/CdS electrode was efficient in photoelectrochemical cell as a working electrode. The 3D electrode showed lower performance than the conventional electrode of TiO₂ mesoporous film/CdS, and more works are needed to improve the performance of 3D electrode.     

Electrical mechanism analysis of Al2O3 doped zinc oxide thin films deposited by rotating cylindrical DC magnetron sputtering

Cost efficient and large area deposition of superior quality Al₂O₃ doped zinc oxide (AZO) films is instrumental in many of its applications, including solar cell fabrication due to its numerous advantages over indium tin oxide (ITO) films. In this study, AZO films were prepared by a highly efficient rotating cylindrical direct current (DC) magnetron sputtering system using an AZO target, which has a target material utilization above 80%, on glass substrates in argon (Ar) ambient. A detailed analysis on the electrical, optical, and structural characteristics of AZO thin films was performed for the solar cell, as well as display applications. The properties of films were found to critically depend on deposition parameters, such as sputtering power, substrate temperature, working pressure, and film thickness. A low resistivity of ~ 5.5 × 10^− 4 Ω cm was obtained for films deposited at 2 kW, keeping the pressure, substrate temperature and thickness constant at 3 mTorr, 230 °C and ~ 1000 nm respectively. This was due to an increase in carrier mobility and large grain size. Mobility is found to be controlled by ionized impurity scattering within the grains, since the mean free path of carriers is much smaller than the grain size of the films. The AZO films showed a high transparency of ~ 90% in the long wavelength region. Our results offer a cost-efficient AZO film deposition method that can fabricate films with significant low resistivity and high transmittance that can be applied in thin-film solar cells, as well as thin film transistor (TFT) and non-volatile memory (NVM).     

Preparation and quality assessment of CuS thin films encapsulated in glass

Copper sulfide (CuS)-based thin films with different thickness have been prepared by thermal co-evaporation of the elemental constituents. Morphological and microstructural properties were shown to vary with film thickness. Optical properties of films encapsulated in a double-glazed configuration and containing an air gap have been measured. Encapsulated CuS films of thickness 100 and 150 nm showed high transmittance peak values of 48% and 36%, respectively, and a low reflectance below 20% and 15%, respectively, in the visible region. A low transmittance of ~ 10% (~ 15%) and a high reflectance of ~ 45% (~ 40%) in the near-infrared region were obtained for the film with 150 nm (100 nm). Thermal stability of the films has been proved by annealing of the films in air and Argon at 150 °C.