Close-spaced Sublimation Research Articles

Studies of CdS/CdTe interface: Comparison of CdS films deposited by close space sublimation and chemical bath deposition techniques

The CdS layers were deposited by two different methods, close space sublimation (CSS) and chemical bath deposition (CBD) technique. The CdS/CdTe interface properties were investigated by transmission electron microscope (TEM) and X-ray photoelectron spectroscopy (XPS). The TEM images showed a large CSS-CdS grain size in the range of 70–80 nm. The interface between CSS-CdS and CdTe were clear and sharp, indicating an abrupt hetero-junction. On the other hand, CBD-CdS layer had much smaller grain size in the 5–10 nm range. The interface between CBD-CdS and CdTe was not as clear as CSS-CdS. With the stepwise coverage of CdTe layer, the XPS core levels of Cd 3d and S 2p in CSS-CdS had a sudden shift to lower binding energies, while those core levels shifted gradually in CBD-CdS. In addition, XPS depth profile analyses indicated a strong diffusion in the interface between CBD-CdS and CdTe. The solar cells prepared using CSS-CdS yielded better device performance than the CBD-CdS layer. The relationships between the solar cell performances and properties of CdS/CdTe interfaces were discussed.

High rate deposition of thin film cadmium sulphide by pulsed direct current magnetron sputtering

Cadmium Sulphide (CdS) is an important n-type semiconductor widely used as a window layer in thin film photovoltaics Copper Indium Selenide, Copper Indium Gallium (di)Selenide, Copper Zinc Tin Sulphide and Cadmium Telluride (CdTe). Cadmium Sulphide has been deposited using a number of techniques but these techniques can be slow (chemical bath deposition and Radio Frequency sputtering) or the uniformity and the control of thickness can be relatively difficult (close space sublimation). In this paper we report on the development of a process using pulsed Direct Current magnetron sputtering which allows nanometre control of thin film thickness using time only. The CdS thin films deposited in this process are highly uniform and smooth. They exhibit the preferred hexagonal structure at room temperature deposition and they have excellent optical properties. Importantly, the process is highly stable despite the use of a semi-insulating magnetron target. Moreover, the process is very fast. The deposition rate using 1.5kW of power to a 6-inch circular magnetron was measured to be greater than 8nm/s. This makes the process suitable for industrial deployment.

Effect of CdCl2 annealing treatment on structural and optoelectronic properties of close spaced sublimation CdTe/CdS thin film solar cells vs deposition conditions

CdTe/CdS thin films were deposited by close-spaced vacuum sublimation at substrate temperatures 250–500 °C. The effect of substrate temperature in conjunction with CdCl2 heat treatment on the properties of CdTe/CdS thin films and devices is reported. X-ray diffraction measurements of structural properties of the films were carried out to study their phase composition and texture. Surface morphology, grain size and growth mechanism of the films were determined by the scanning electron microscopy. The photoluminescence measurements allowed us to establish the correlation between the point and extended defect structure on the one hand and the growth conditions on the other. The CdTe/CdS solar cells fabricated at high substrate temperature, exhibited improved electrical characteristics compared to cells fabricated at low substrate temperature, yielding a high efficiency.

Cu 2 S as ohmic back contact for CdTe solar cells

Abstract We prepared a back contact for CdTe solar cells with Cu 2 S as primary contact. Cu 2 S was evaporated on CdCl 2 treated CdTe solar cells in superstrate configuration. The CdTe and CdS layers were deposited by Closed Space Sublimation. Direct interface studies with X-ray photoelectron spectroscopy have revealed a strongly reactive interface between CdTe and Cu 2 S. A valence band offset of 0.4–0.6 eV has been determined. The performance of solar cells with Cu 2 S back contacts was studied in comparison to cells with an Au contact that deposited onto a CdCl 2 -treated CdTe surface that was chemically etched using a nitric-phosphoric etch. The solar cells were analyzed by current–voltage curves and external quantum efficiency measurements. After several post deposition annealing steps, 13% efficiency was reached with the Cu 2 S back contact, which was significantly higher than the ones obtained for the NP-etched back contacts.

Structural and optical properties of AgAlTe2 layers grown on sapphire substrates by closed space sublimation method

AgAlTe2 layers were grown on a- and c-plane sapphire substrates using a closed space sublimation method. Grown layers were confirmed to be single phase layers of AgAlTe2 by X-ray diffraction. AgAlTe2 layers were grown to have a strong preference for the (112) orientation on both kinds of substrates. The variation in the orientation of grown layers was analyzed in detail using the X-ray diffraction pole figure measurement, which revealed that the AgAlTe2 had a preferential epitaxial relationship with the c-plane sapphire substrate. The atomic arrangement between the (112) AgAlTe2 layer and sapphire substrates was compared. It was considered that the high order of the lattice arrangement symmetry probably effectively accommodated the lattice mismatch. The optical properties of the grown layer were also evaluated by transmittance measurements. The bandgap energy was found to be around 2.3 eV, which was in agreement with the theoretical bandgap energy of AgAlTe2.

Effects of the Substrate Temperature on the Structure and Properties of Cd<sub>1-x</sub>Mn<sub>x</sub>Te Films

Cd1-xMnxTe (CdMnTe) can be a good candidate for gamma and X-ray detection because of its wide band-gap, high resistivity, and good electro-transport properties. Polycrystalline CdMnTe films were grown by closed-space sublimation method on glasses at different substrate temperatures. In this paper, substrate temperature dependent surface morphology, chemical composition, structural and electrical properties of CdMnTe films are investigated systematically.

Close-Spaced Sublimation of Organic Materials for Organic Light-Emitting Devices

We have demonstrated organic light-emitting devices (OLEDs) fabricated by using a close-spaced sublimation (CSS) method. We have deposited tris(8-hydroxyquinolinato)aluminum (Alq3) layer using a CSS process for the formation of electron transport layer of the PVK/Alq3 bilayer OLEDs. The distance between the sublimation source and substrate was fixed to be less than 5 mm. The substrate temperature was maintained less than 70°C while the sublimation source temperature was varied from 185 to 200°C during the deposition of Alq3 film. We investigated the photoluminescence characteristic of the CSS processed Alq3 film. We also investigated the emission and electro-optical properties of the PVK/Alq3 bilayer OLEDs with CSS deposited Alq3 film.

Flexible cadmium telluride thin films grown on electron-beam-irradiated graphene/thin glass substrates

We demonstrate the close-spaced sublimation growth of polycrystalline cadmium telluride (CdTe) thin films on a flexible graphene electrode/thin glass substrate structure. Prior to the growth of CdTe films, chemical-vapor-deposited graphene was transferred onto a flexible glass substrate and subjected to electron-beam irradiation at an energy of 0.2 MeV in order to intentionally introduce the defects into it in a controlled manner. Micro-Raman spectroscopy and sheet resistance measurements were employed to monitor the damage and disorder in the electron-beam irradiated graphene layers. The morphology and optical properties of the CdTe thin films deposited on a graphene/flexible glass substrate were systematically characterized. The integration of the defective graphene layers with a flexible glass substrate can be a useful platform to grow various thin-film structures for flexible electronic and optoelectronic devices.

Plasmonic modification of CdTe thin films by gold nanoparticles: Methods, difficulties and solutions

Despite numerous theoretical studies on CdTe absorbers with embedded metal nanoparticles (NPs), no technological reports are found. We present an experimental study of 300-nm-thick CdTe absorber layers deposited by close spaced sublimation with incorporated gold NPs to obtain the effect of surface plasmon resonance (SPR). Gold NPs were formed from a HAuCl4·3H2O solution in ethanol and deposited by spin coating or spray pyrolysis on the top of CdTe film. As alternative, sputtering followed by air annealing of gold thin layer was applied to form Au NPs between the glass substrate and the CdTe layer. The properties of these structures were studied by UV–VIS spectroscopy, scanning electron microscopy, energy dispersive spectroscopy, and X-ray diffraction.The SPR effect was observed in the wavelength region of 570–590nm for CdTe layers, with Au NPs measuring 30–40nm in size. At the increased amount of sprayed HAuCl4·3H2O solution the nanoparticles were agglomerated into grains and the SPR peak shifted to longer wavelengths. In the case of spin coating the HAuCl4 solution decomposes on the CdTe surface and by chemical reaction contributed to the substantial removal of the CdTe. Sputtering and spray pyrolysis methods did not change the bulk CdTe layer and introduced the plasmonically enhanced absorption, but led to the formation of additional inter-metallic phases, such as AuCd, Au0.3Te0.7 or AuTe2. The Au0.3Te0.7 phase is supposed to play the role of a shell for Au NPs shifting the 592nm SPR peak to lower wavelengths. Chemical processes responsible for these effects are discussed.

Single CdTe microwire photodetectors grown by close-spaced sublimation method.

We demonstrate single CdTe microwire field-effect transistors (FETs) that are highly sensitive to ultraviolet (UV) light. Dense CdTe microwires were catalytically grown using a close-spaced sublimation system. Structural, morphological and transport properties in conjunction with the optoelectronic properties were systemically investigated. CdTe microwire FETs exhibited p-type behaviors with field-effect mobilities up to 1.1 × 10(-3) cm2 V(-1) s(-1). Optoelectronic properties of our CdTe microwire FETs were studied under dark and UV-illumination conditions, where photoresponse was highly dependent on the back-gate bias conditions. Our CdTe microwire FET-based photodetectors are promising for high-performance micro-optoelectronic applications.

The effects of high temperature processing on the structural and optical properties of oxygenated CdS window layers in CdTe solar cells

High efficiency CdTe solar cells typically use oxygenated CdS (CdS:O) window layers. We synthesize CdS:O window layers at room temperature (RT) and 270 °C using reactive sputtering. The band gaps of CdS:O layers deposited at RT increase when O2/(O2 + Ar) ratios in the deposition chamber increase. On the other hand, the band gaps of CdS:O layers deposited at 270 °C decrease as the O2/(O2 + Ar) ratios increase. Interestingly, however, our high temperature closed-space sublimation (CSS) processed CdTe solar cells using CdS:O window layers deposited at RT and 270 °C exhibit very similar cell performance, including similar short-circuit current densities. To understand the underlying reasons, CdS:O thin films deposited at RT and 270 °C are annealed at temperatures that simulate the CSS process of CdTe deposition. X-ray diffraction, atomic force microscopy, and UV-visible light absorption spectroscopy characterization of the annealed films reveals that the CdS:O films deposited at RT undergo grain regrowth and/or crystallization and exhibit reduced band gaps after the annealing. Our results suggest that CdS:O thin films deposited at RT and 270 °C should exhibit similar optical properties after the deposition of CdTe layers, explaining the similar cell performance.

CdCl2 Activation Annealing of CdTe Microwires Grown by Close-Spaced Sublimation Method

CdCl2 Activation Annealing of CdTe Microwires Grown by Close-Spaced Sublimation Method

Morphological and chemical study of CdTe thin films annealed in CHClF2–O2 gas mixture

The effect of annealing in CHClF2–O2 gas mixture on morphological and chemical properties of CdTe thin films deposited by conventional close space sublimation (CSS) has been investigated. Annealing was done at 400°C for 5min in a mixture of CHClF2 and O2 with partial pressures of 50 and 160mbar, respectively, and to reach a total pressure of 800mbar the annealing chamber was filled with Ar. After annealing, residuals at the surface of films were not removed in order to be study. High Resolution Scanning Electron Microscopy (HR-SEM) and X-ray Photoelectron Spectroscopy (XPS) were employed for analysis. The annealing promotes the etching of the CdTe grains at faces and edges; as a consequence, the surface became smooth with a compact structure. The main improvement on morphology by the inclusion of O2 during the annealing was the reduction of the grain boundary width of films. Chemical analysis of samples annealed in CHClF2 with and without mixed oxygen showed that the oxygen in the gas mixture acts as a catalyst in the decomposition of the CHClF2. Annealing with the CHClF2–O2 mixture reduces the oxidized compounds at the surface of films and promotes incorporation of Cl in the bulk. The experimental results give a clear demonstration why CdS/CdTe solar cells fabricated using CHClF2–O2 during chlorination treatments have better performance, as referred in a previous work.

Study of the correlation between structural and photoluminescence properties of CdSe thin films deposited by close-spaced vacuum sublimation

CdSe polycrystalline films were deposited by a close-spaced vacuum sublimation method at different substrate temperatures (Ts) using glass slides as substrates. At Ts≤673K the films have a structure with strong dispersion of grain size (d) (from 0.1 to 0.3μm). In this case the layer-by-layer mechanism determines the growth process of the layers. For Ts=873K they have a columnar-like structure with a clear growth texture and the average grain size d=3–4μm. The films obtained at Ts>473K are n-type and only correspond to a single wurtzite phase. The crystallites are preferentially oriented with the (102) planes parallel to the substrate. At lower temperatures the films are bi-phase. The microstress level in CdSe films obtained at Тs=873K (0.5×10−3) is considerably smaller than for the films deposited at Тs=773K (4.0×10−3). Increase of the value of Ts improves the stoichiometry of CdSe films. Analysis of the low-temperature photoluminescence (PL) spectra let us determine the nature and energy of point and extended defects in the investigated films. It was shown that the films contain Na(Li) and P residual impurities. The results of the structural and PL measurements showed that the CdSe polycrystalline films are of fairly good crystal and optical quality for Ts=873K and can be suitable for various applications.

Assessment of photovoltaic junction position using combined focused ion beam and electron beam‐induced current analysis of close space sublimation deposited CdTe solar cells

ABSTRACTA major limitation of the cross‐section electron beam‐induced current method—the use of roughly fractured surfaces to provide the cross‐section—has been overcome with the use of focused ion beam microscope sample preparation. Using this method, it was possible to undertake a study of the relation between junction position and the corresponding external quantum efficiency (EQE) curves. For the case of cadmium telluride (CdTe) cells, it was demonstrated that the EQE curve shape that indicates a buried CdTe homojunction only arises if the junction is buried by more than 0.9 µm from the heterointerface. This highlights the limitations of interpreting EQE curve shape to determine junction position. It was also shown that extended postgrowth annealing degrades the cadmium sulfide by Kirkendall voiding, and this leads to efficiency loss. © 2014 The Authors. Progress in Photovoltaics: Research and Applications published by John Wiley & Sons, Ltd.

Raman investigation on thin and thick CdTe films obtained by close spaced vacuum sublimation technique

AbstractThe CdTe thin and thick films were obtained by the close spaced vacuum sublimation technique on a glass substrate under the following growth conditions: the evaporator temperature was 620 °C; and the substrate temperature was varied in the range from 250 °C to 550 °C. High purity CdTe powder was used as a charge for evaporation. The Raman spectra were measured using TRIAX 320 and TRIAX 550 spectrometers at room temperature. The 488‐nm line and 514.5‐nm line of an Ar+ laser and a 785‐nm diode laser were used as excitation sources. The signal was collected by the liquid nitrogen cooled charge‐coupled‐device (CCD) detector. A number of intense Raman peaks at 140, 167, 190, 271, 332 and 493 cm‐1 were observed and were interpreted as TO (140 cm‐1), 1LO (167 cm‐1), 2LO (332 cm‐1), 3LO (493 cm‐1) phonon modes and plasmon‐phonon mode (190 cm‐1). The presence of several phonon replicas in the Raman spectra confirms high crystal quality of the samples. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Improvement of the physical properties of ZnO/CdTe core-shell nanowire arrays by CdCl2 heat treatment for solar cells.

CdTe is an important compound semiconductor for solar cells, and its use in nanowire-based heterostructures may become a critical requirement, owing to the potential scarcity of tellurium. The effects of the CdCl2 heat treatment are investigated on the physical properties of vertically aligned ZnO/CdTe core-shell nanowire arrays grown by combining chemical bath deposition with close space sublimation. It is found that recrystallization phenomena are induced by the CdCl2 heat treatment in the CdTe shell composed of nanograins: its crystallinity is improved while grain growth and texture randomization occur. The presence of a tellurium crystalline phase that may decorate grain boundaries is also revealed. The CdCl2 heat treatment further favors the chlorine doping of the CdTe shell with the formation of chlorine A-centers and can result in the passivation of grain boundaries. The absorption properties of ZnO/CdTe core-shell nanowire arrays are highly efficient, and more than 80% of the incident light can be absorbed in the spectral range of the solar irradiance. The resulting photovoltaic properties of solar cells made from ZnO/CdTe core-shell nanowire arrays covered with CuSCN/Au back-side contact are also improved after the CdCl2 heat treatment. However, recombination and trap phenomena are expected to operate, and the collection of the holes that are mainly photo-generated in the CdTe shell from the CuSCN/Au back-side contact is presumably identified as the main critical point in these solar cells.

Influence of CdS deposition technique for CdS/CdTe solar cells applications

The manufacturing of an optimal window layer is essential for a high efficiency solar cell. CdS thin films were deposited on FTO commercial substrates by two different techniques: Chemical Bath Deposition (CBD), and Closed Space Sublimation (CSS) using SiC rods as electrical heating elements. The composition, morphology, and crystalline structure of the CdS thin films were analyzed by Energy Dispersive X-ray spectroscopy (EDX), Scanning Electron Microscope (SEM), and X-Ray Diffraction (XRD). CdS layers obtained by CSS have shown good stoichiometries of 51.07(Cd at %), and 48.93(S at %), with the presence of disordered microrods of hexagonal shape. The CdS films obtained by CBD exhibited certain contamination from the bath chemical components and a poor defined structure. In conclusion, the layers deposited by electric heating based CSS present a better morphology and crystal structure than the layers deposited by CBD. The CdTe absorber layer deposited by CSS electric heating also shows good cubic structure and morphology.

Effect of Ag doping on opto-electrical properties of CdS thin films for solar cell applications

Cadmium sulfide (CdS) polycrystalline thin films of different thicknesses (ranging from 370nm to 750nm) were fabricated on corning glass substrates using Close Spaced Sublimation (CSS) technique. Optical and electrical investigation revealed that CdS thin films show an appreciable transparency (50–70% transmission) in visible range and a highly resistive behavior (106Ωcm). Samples were doped by silver (Ag) at different concentrations, using ion exchange technique, in order to reduce the resistivity of CdS thin films and to improve their efficiency as a window layer for solar cell application. The doping of Ag in pure CdS thin films resulted into an increase of surface roughness and a decrease both in electrical resistivity and in transparency. By optimizing annealing parameters, we were able to properly control the optical properties of the present system. In fact, the Ag doping of pure CdS films has led to a decrease of the sample resistivity by three orders of magnitude (103Ωcm) against a 20% cut in optical transmission.

CdTe deposition by successive ionic layer adsorption and reaction (SILAR) technique onto ZnO nanowires

In this study is reported CdTe deposition by Successive Ionic Layer Adsorption and reaction (SILAR) at room temperature onto ZnO nanowires (NWs). The as-deposited CdTe layer exhibits poor crystalline quality and not well defined optical transition which is probably result of its amorphous nature. The implementation of an annealing step and chemical treatment by CdCl2 to the classical SILAR technique improved significantly the CdTe film quality. The XRD analysis showed that the as treated layers are crystallized in the cubic zinc blende structure. The full coverage of ZnO nanowires and thickness of the CdTe shell, composed of small crystallites, was confirmed by STEM and TEM analysis. The layer thickness could be controlled by the number of SILAR cycles. The sharper optical transitions for the annealed and CdCl2 treated heterostructures additionally proves the enhancement of the layer crystalline quality. For comparison CdTe was also deposited by close space sublimation (CSS) method onto ZnO nanowires. It is shown that the SILAR deposited CdTe exhibits equal crystalline and optical properties to that prepared by CSS. These results demonstrate that SILAR technique is more suitable for conformal thin film deposition on nanostructures. CdTe extremely thin film deposited by SILAR method onto ZnO nanowire.