CdTe Layers Research Articles

Close spaced sublimation deposition of CdTe layers with process gas oxygen for thin film solar cells

Close spaced sublimation (CSS) is one of the leading technologies for deposition of the absorber layer in CdTe solar cells. For large scale production CSS is a dynamic process where the substrates are moved above the crucible at a close distance. In this study oxygen as a process gas is injected during dynamic CSS deposition. Solar cells are prepared from these layers by CdCl2 activation, copper doping and back contact deposition. The samples are investigated using current voltage measurement, quantum efficiency, scanning electron microscopy of cross sections and deep level transient spectroscopy.The results show that the precise introduction of oxygen can improve the solar cell efficiency. The reasons for this effect are discussed in this study.

Study of MgCl2 activation treatment on the defects of CdTe solar cells by capacitance-voltage, drive level capacitance profiling and admittance spectroscopy techniques

CdTe solar cells have recently reached its highest conversion efficiencies, comparable to CuInGaSe2 and Si devices. One of the key factors for the success of these devices is the so-called “activation” treatment, which typically consists in depositing a CdCl2 film on the CdTe absorber layer and in a subsequent annealing in air or nitrogen atmosphere. Nevertheless CdCl2 is a carcinogenic and water-soluble compound; for this reason recent studies have found MgCl2 as a good alternative.In this work we have studied devices (exceeding 14% efficiency) prepared by low-substrate temperature CdTe deposition and activated with MgCl2 treatment (applied by wet deposition). The devices were characterized by means of current-voltage, capacitance-voltage, drive level capacitance profiling and admittance spectroscopy techniques. Carrier concentration and also concentration and distribution of shallow and deep defects are presented and compared with data obtained from analogous devices but activated with the standard CdCl2. Despite CdTe layers treated with the two different processes exhibit similar structural properties, the distribution of deep and shallow defects results to be different and moreover admittance spectroscopy technique reveals the presence of different defects, at 344 and 358meV, which are present only in MgCl2 treatment case.

STRUCTURE AND OPTICAL PROPERTIES OF CdTe FILMS OBTAINED BY MAGNETRON SPUTTERING

The aim of this study was to determine the influence of physical and technological condensation modes of cadmium telluride (CdTe) films obtained by direct current magnetron sputtering on their crystal structure and optical properties.The structure of CdTe films was investigated by X-Ray diffractometry methods with automatic recording of X-ray spectra in scanning θ-2θ mode and “oblique” filming. Optical properties of CdTe layers were studied using a SF-2000 spectrophotometer. The values of layer thickness and band gap of CdTe films were determined.It is found that the method of DC magnetron sputtering with preheating target provides a rate of CdTe film deposition of 150-200 nm/min. It is shown that the conditions of DC magnetron sputtering condensation such as pressure of an inert gas Parg=1-0.9 Pa, plasma discharge current I=80 mA and the magnetron voltage V=600 V during 25 minutes allow to obtain textured hexagonal CdTe layers with a thickness of 4900-5100 nm. The band gap of the obtained CdTe films was 1.52-1.54 eV. Such films of CdTe intensively absorb light in the infrared spectrum and can be used to create optical sensors. After the chloride treatment followed by annealing in air at a temperature 4300C for 25 minutes studied CdTe films due to wurtzite-sphalerite phase transition contain only the stable cubic phase| with lattice parameter a=6,4905 Å, which differs from the tabular valueless than 0.2%.

Comparison of high efficiency flexible CdTe solar cells on different substrates at low temperature deposition

In this paper we present for the first time flexible CdTe solar cells deposited on ultra-thin glass using a low temperature process and compared the results to CdTe cells deposited on polyimides. The effects of the different substrates on the morphology and crystal structure of CdTe absorber layers, as well as on the efficiency of solar cells were investigated by means of different characterization techniques. Atomic force microscopy measurements showed that the CdTe layers deposited on different substrates have a similar morphology. XRD analysis was used to study the structure and the orientation of the crystallite of the CdTe layers on the different substrates. Micro-Raman spectroscopy provided evidence of both the TO phonon of CdTe and the A1 phonon of Te, whose peak position is somehow dependent from the substrate material. CdCl2 treatments change the crystal orientation of the CdTe layers, with the loss of preferential orientation. In the case of the polyimide substrate it has been seen that its transparency decreases after CdCl2 activation treatment, resulting in a parasitic light absorbance and in lower photo-current values of the devices. Moreover strain of the CdTe surface is observed for cells deposited on polymers. Cells on ultra-thin glass do not show these issues, confirming the glass to be an interesting alternative substrate for flexible CdTe solar cells.

Electrodeposition and characterization of as-deposited and annealed CdTe thin films

Thin films of CdTe semiconductor materials were grown on fluorine doped tin oxide (FTO) conducting glass substrates using the technique of electrodeposition. CdSO 4 at high concentrations and CdCl 2 , TeO 2 at low concentrations were used as precursor salts for electrodeposition. The range of deposition potentials was estimated using cyclic voltammetric measurements. The electrical, optical, structural and morphological characteristics of as-deposited and annealed CdTe thin films were characterized using photo-electrochemical (PEC) cell studies, UV-Vis spectrophotometry, X-ray diffraction (XRD) and scanning electron microscopy (SEM). These particular samples were converted from n-type into p-type after heat treatment. UV-Vis spectrometric measurements for CdTe layers indicated that, the energy band gaps of 1.45±0.02 eV for both as-deposited and annealed samples which exhibited the required optical property for fabricating CdS/CdTe solar cells. Little increase in (220) and (311) peaks of XRD spectra were observed for annealed layers compared to the as-deposited material. However, annealing exhibited a small reduction of cubic phase preferential orientation (111). The optical transmission for both as-deposited and annealed CdTe samples were about 60% for wavelengths longer than about 850 nm.

Modeling the charge density profile in thin film devices using SIMS profiling data

To obtain an insight into the degradation kinetics of thin film solar cells, we developed a theory to model the variation of total charge density across the absorber layer of CdS/CdTe thin film solar cells. The variation in total charge density has been generally ignored in the literature despite being the original reason of instability in device parameters. We used SIMS profiling data reported in the literature to track the Cu concentration across CdTe layer and it's variation under short- & open-circuit conditions. Cu migration under the stress of elevated temperature and prolonged irradiation is known to be the major degradation source in CdTe layer by increasing the charge accumulation at the junction and creating a barrier to carrier passivation. We obtained the Cu distribution functions before/after stressing the cell and imported these fitting functions in our model to calculate the real total charge density and electric field variation at both front/back regions. Our simulation indicates that stressing the cell cause charge accumulation at interface/junction of CdTe/metal and CdS/CdTe and reduces the electric field and leads to the relevant degradation trend. The depletion width is also reduced by increased Cu concentration under the stress. This model is capable to be developed to include the real distribution of vacancies, defects, doping and ions (obtained by SIMS or other spectroscopy methods) to calculate the total charge density, electric field, resistivity of solar cells stressed under various light, temperature and biasing conditions. The worsen stress condition is predicted to be the open-circuit condition.

Erratum to: MBE-Grown CdTe Layers on GaAs with In-assisted Thermal Deoxidation

Erratum to: MBE-Grown CdTe Layers on GaAs with In-assisted Thermal Deoxidation

Thin film growth of a topological crystal insulator SnTe on the CdTe (111) surface by molecular beam epitaxy

We report molecular beam epitaxial growth of a SnTe (111) layer on a CdTe template, fabricated by depositing it on a GaAs (111)A substrate, instead of BaF2 which has been conventionally used as a substrate. By optimizing temperatures for the growth of both SnTe and CdTe layers and the SnTe growth rate, we could obtain SnTe layers of the single phase grown only in the (111) orientation and of much improved surface morphology from the viewpoint of the extension and the flatness of flat regions, compared to the layers grown on BaF2. In this optimal growth condition, we have also achieved a low hole density of the order of 1017cm−3 at 4K, the lowest value ever reported for SnTe thin films without additional doping. In the magnetoresistance measurement on this optimized SnTe layer, we observe characteristic negative magneto-conductance which is attributed to the weak antilocalization effect of the two-dimensional transport in the topological surface state.

Strain relaxation of CdTe on Ge studied by medium energy ion scattering

We have used the medium energy ion scattering (MEIS) technique to assess the strain relaxation in molecular-beam epitaxial (MBE) grown CdTe (211)/Ge (211) system. A previous X-ray diffraction study, on 10 samples of the same heterostructure having thicknesses ranging from 25nm to 10μm has allowed the measurement of the strain relaxation on a large scale. However, the X-ray diffraction measurements cannot achieve a stress measurement in close proximity to the CdTe/Ge interface at the nanometer scale. Due to the huge lattice misfit between the CdTe and Ge, a high degree of disorder is expected at the interface. The MEIS in channeling mode is a good alternative in order to profile defects with a high depth resolution. For a 21nm thick CdTe layer, we observed, at the interface, a high density of Cd and/or Te atoms moved from their expected crystallographic positions followed by a rapid recombination of defects. Strain relaxation mechanisms in the vicinity of the interface are discussed

Phase transitions in two tunnel-coupled HgTe quantum wells: Bilayer graphene analogy and beyond.

HgTe quantum wells possess remarkable physical properties as for instance the quantum spin Hall state and the “single-valley” analog of graphene, depending on their layer thicknesses and barrier composition. However, double HgTe quantum wells yet contain more fascinating and still unrevealed features. Here we report on the study of the quantum phase transitions in tunnel-coupled HgTe layers separated by CdTe barrier. We demonstrate that this system has a 3/2 pseudo spin degree of freedom, which features a number of particular properties associated with the spin-dependent coupling between HgTe layers. We discover a specific metal phase arising in a wide range of HgTe and CdTe layer thicknesses, in which a gapless bulk and a pair of helical edge states coexist. This phase holds some properties of bilayer graphene such as an unconventional quantum Hall effect and an electrically-tunable band gap. In this “bilayer graphene” phase, electric field opens the band gap and drives the system into the quantum spin Hall state. Furthermore, we discover a new type of quantum phase transition arising from a mutual inversion between second electron- and hole-like subbands. This work paves the way towards novel materials based on multi-layered topological insulators.

Solution-Processed, Ultrathin Solar Cells from CdCl3(-)-Capped CdTe Nanocrystals: The Multiple Roles of CdCl3(-) Ligands.

Solution-processed CdTe solar cells using CdTe nanocrystal (NC) ink may offer an economically viable route for large-scale manufacturing. Here we design a new CdCl3(-)-capped CdTe NC ink by taking advantage of novel surface chemistry. In this ink, CdCl3(-) ligands act as surface ligands, sintering promoters, and dopants. Our solution chemistry allows obtaining very thin continuous layers of high-quality CdTe which is challenging for traditional vapor transport methods. Using benign solvents, in air, and without additional CdCl2 treatment, we obtain a well-sintered CdTe absorber layer from the new ink and demonstrate thin-film solar cells with power conversion efficiency over 10%, a record efficiency for sub-400 nm thick CdTe absorber layer.

Effect of CdCl2 heat treatment on ZnTe back electron reflector layer in thin film CdTe solar cells

The effect of CdCl2 heat treatment on the ZnTe layer used as the back electron reflector layer in polycrystalline CdTe solar cells was investigated. A thin film of ZnTe was grown on top of polycrystalline CdTe layer using metalorganic vapor phase epitaxy, which was then subjected to CdCl2 heat treatment. The effect was examined using various characterization techniques such as X-ray diffraction, energy dispersive X-ray spectroscopy and scanning electron microscope images and it was found that the ZnTe layer transformed into CdTe and ZnO due to the treatment. This was verified by performing CdCl2 heat treatment on pure ZnTe thin layer directly grown on glass which also showed the conversion of ZnTe to CdTe and ZnO. To study the effect of this transformation on final device performance, thin film CdTe solar cell structures were fabricated with ZnTe electron reflector layer with and without the CdCl2 heat treatment. The device without the ZnTe layer seemed to perform better than the devices with ZnTe layer and the CdCl2 heat treatment.

Effect of microwave (24 GHz) radiation treatment on impurity photoluminescence of CdTe:Cl single crystals

Effect of microwave radiation (24GHz) on transformation of impurity-defect complexes in CdTe:Cl single crystals within the spectral range 1.3–1.5eV was studied using the low-temperature (T=2K) photoluminescence (PL) technique. The shapes of donor–acceptor pairs (DAP) and Y PL bands were studied in detail. The Huang–Rhys factor was calculated for the DAP luminescence depending on microwave radiation treatment. The increase of the distance between the DAP components responsible for emission at 1.455eV and the quenching of Y-band due to microwave irradiation were observed. The method to decrease the amount of extended defects in near-surface layers of CdTe:Cl single crystals has been proposed.

MICROSTRUCTURE ANALYSIS OF CDTE/CDS THIN-FILM SOLAR CELLS FOR OPTIMIZATION OF DEVICES FABRICATED AT LOW TEMPERATURE WITH PULSED LASER DEPOSITION

Low temperature fabrication of CdTe (1.25 μm)/CdS (120 nm) thinfilm solar cells was explored using in situ pulsed laser deposition (PLD) in order to control the CdTe grain morphology, crystallinity, and CdS/CdTe interface, which have proven to be crucial in CdTe/CdS solar cell performance. A comparative study was carried out on solar cells fabricated using PLD with the CdTe layer either grown directly at 400°C or at200°C followed with in situ annealing at 400°C for 5-40 minutes in 20 Torr Ar. The post annealed CdTe films exhibit better crystalline characteristics confirmed by optical transmittance and atomic force microscopy as well as improved CdTe/CdS p-n junction, which results in increasingly higher open circuit voltage and fill factor with increasing anneal times up to 20 minutes. Quantum efficiency measurements revealed that the post-annealed cells have more efficient electron collection, particularly below the CdS band edge, and a correlation is drawn between PLD annealing time and CdCl2 annealing time to optimize device efficiency. The results suggest that the in situ post PLD annealing process developed in this work is advantageous because of much improved crystallinity of CdS and CdTe, a considerably lower deposition thermal budget for better control of the CdS and CdTe interface, plus flexibility for in situ materials mixing for energy band gap engineering.

HgCdTe p+‐n structures grown by MBE on Si (013) substrates for high operating temperature SWIR detectors

AbstractElectrophysical properties of multilayered heteroepitaxial structures of Hg1‐xCdxTe with x=0.3‐0.4 grown by molecular beam epitaxy on silicon substrates are presented. The passivating effect of thin CdTe layers grown on top of the structures in single process is demonstrated. Comparison between experimental and theoretical temperature dependencies of reverse currents in n‐on‐p and p‐on‐n diodes fabricated by boron and arsenic ion implantation in vacancy‐doped p‐type and In‐doped n‐type Hg1‐xCdxTe films, respectively, are presented. The influence of p‐n junction position in double‐layer heterostructure on temperature dependencies of reverse currents is examined. (© 2016 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Optimisation of CdTe electrodeposition voltage for development of CdS/CdTe solar cells

Cadmium telluride (CdTe) thin films have been deposited on glass/conducting glass (FTO) substrates using low-cost two electrode system and aqueous electrodeposition method. The glass/FTO substrates were used to grow the CdTe layers at different deposition voltages. The structural, electrical, optical and morphological properties of the resulting films have been characterized using X-ray diffraction (XRD), Photoelectrochemical (PEC) cell measurements, optical absorption spectroscopy and Scanning Electron Microscopy (SEM). The XRD results indicate that at voltages less than or higher than 1.576 V, crystallinity is poor due to presence of two phases. When CdTe is grown at 1.576 V, the composition is stoichiometric, and the (111) peak has the highest intensity in the XRD diffractogram indicating a high degree of crystallinity. SEM studies showed that all layers had pin-holes and gaps between the grains. These openings seem to be more common in the samples grown at voltages away from the stoichiometric voltage (1.576 V). The linear I–V curves of glass/FTO/CdS/CdTe/Au structures fabricated using stoichiometric CdTe showed efficiency of 10.1 % under AM 1.5 illumination.

The role of back surface field SnS layer in improvement of efficiency of CdTe thin film solar cells

A one-dimensional simulator Analysis of Microelectronic and Photonic Structures in 1 Dimension (AMPS-1D) has been used so as to investigate cadmium telluride (CdTe) based solar cells properties. The aim of the current study is to explore the possibility of improving the efficiency of the solar cells. For that purpose, a novel back surface field (BSF) SnS layer has been inserted after the back contact (Ni) in CdTe solar cells constituted by a pile of thin layers that contains in particular a fine layer called buffer layer (CdS) between the absorber layer CdTe and the windows layer ZnO-Al. The optimal values ​that​ give the maximum performance of the structure ZnO/CdS/CdTe/SnS/Ni, with and without a BSF layer (SnS) has been determined. Our results show an improvement of the efficiency of the solar cell when using the BSF. The proposed cell with SnS layer is found to give an efficiency of 21.83% using only thicknesses of 0.6μm and 0.4μm for CdTe and SnS, respectively. However, the efficiency of the cell reference (without BSF) is 17.40% with a thickness of 1μm for the absorber layer CdTe. Moreover, the normalized efficiency of the proposed cells with BSF layer decreases linearly with increasing the temperature, suggesting a better stability of the ultra thin cells.

Electric and magnetic resonances in optical metamaterial for efficient absorption enhancement in ultrathin CdTe-based photovoltaic cells

Light absorption plays a key role in photovoltaic devices, especially in thin-film CdTe-based solar cells. Here we study the enhancement of optical absorption in a ultrathin CdTe layer by embedding it into optical magnetic metamaterials. In a periodic metal-CdS-CdTe-metal sandwiched structure, the absorption of transverse magnetic polarized light has been significantly enhanced. The maximum enhancement is around 3 times, which is attributed to the strong local field enhancement in the CdTe thin film. Importantly, due to the coexistence of large electric resonance and magnetic resonance in magnetic metamaterials, the enhancement of light absorption has been obtained in a broadband from 400 nm to 800 nm, which is almost 1.5 times of that of the conventional CdTe-based photovoltaic device and covers the strongest part of the solar spectrum. Moreover, such absorption enhancement is also valid for a wide range of incident angles. We believe that our finding can lead to a variety of important applications in ultrathin CdTe-based solar cell technology.

How the Chlorine Treatment and the Stoichiometry Influences the Grain Boundary Passivation in Polycrystalline CdTe Thin Films

The absorption coefficient of CdTe is large enough to assure that all of the visible light is absorbed in a thickness on the order of 1 µm. High efficiency devices are fabricated by using close-spaced sublimation (CSS)-deposited CdTe films with a thickness in the range of 6–8 µm. In order to decrease the thickness of the CdTe film, a novel approach has been used. On top of the CdTe film, whose thickness is reduced to 2–3 μm, another CdTe layer is deposited by RF sputtering, with a thickness of 100–200 nm. The purpose of this approach is to fill up the voids, which tend to form when a low thickness-CdTe film is deposited by close-spaced sublimation. Using this CdTe double layer, solar cells, with an efficiency greater than 15%, were reproducibly obtained. Since the CdTe layer deposited by the CSS technique shows a p-type behavior, whereas the layer deposited by sputtering is n-type, it is supposed that the formation of a p-n junction into the grain boundaries, which makes a mirror for the charge carriers, increases their mean lifetime. In order to also have this system after the essential chlorine treatment of the CdTe layer, a special cadmium-free halogen treatment was developed. This process was especially tuned for very thin (≤3 µm) CdTe film thickness and for not making use of cadmium-based chlorine salt while, producing high efficiency devices, meets a better economic and environmental sustainability.

Physical device modeling of CdTe ultrathin film solar cells

The device physics of CdS/CdTe Ultrathin Film solar cells (UTF-SCs) is modeled using Sah–Noyce–Shockley (SNS) theory for the generation/recombination in the Space Charge Region (SCR) of an asymmetric pn+ junction. This theory is based on Shockley–Read–Hall (SRH) recombination mechanism related to midgap defects in the band gap. The maximum effect of SRH is within SCR where the carriers are mainly collected by the electric field. Since the depletion width in a UTF extends to the back region, a UTF resembles an asymmetric pn+ junction, which is most commonly modelled by SNS theory. Several materials and electrical parameters dominate the device physics of a fully depleted CdTe layer with dCdTe⩽1μm. The uncompensated defect density, recombination lifetime, and CdTe thickness are parameters which effectively control the current–voltage characteristics and quantum efficiency of UTF devices. Simulation results indicate that any parameter that narrows the SCR of a UTF will reduce the open-circuit voltage whereas only thinning the CdTe layer lowers the short-circuit current density. The electrical resistivity and quantum efficiency are a measure of defect density/level and dCdTe or band bending at the junction, respectively. A practical takeaway of our simulation results is to measure the electrical resistivity of the CdTe layer before metallization of UTF solar cells. Band bending is a measure of junction quality. The latter parameters are usually ignored in device fabrication and modeling. However, any parameter related to defect density in UTF devices must be considered in the modeling since the front and back contacts are closer here and the degradation rate is rather probable. The degradation of UTF devices based on carrier collection mechanisms dominated by drift component is briefly discussed.