Cu-doped ZnTe Research Articles

Resulting Effect of the p-Type of ZnTe: Cu Thin Films of the Intermediate Layer in Heterojunction Solar Cells: Structural, Optical, and Electrical Characteristics.

The microstructural, electrical, and optical properties of Cu-doped and undoped ZnTe thin films grown on glass substrates are covered in this article. To determine the chemical makeup of these materials, both energy-dispersive X-ray (EDAX) spectroscopy and X-ray photoelectron spectroscopy were employed. The cubic zinc-blende crystal structure of ZnTe and Cu-doped ZnTe films was discovered using X-ray diffraction crystallography. According to these microstructural studies, the average crystallite size increased as the amount of Cu doping increased, whereas the microstrain decreased as the crystallinity increased; hence, defects were minimized. The Swanepoel method was used to compute the refractive index, and it was found that the refractive index rises as the Cu doping levels rises. The optical band gap energy was observed to decrease from 2.225 eV to 1.941 eV as the Cu content rose from 0% to 8%, and then slightly increase to 1.965 eV at a Cu concentration of 10%. The Burstein-Moss effect may be connected to this observation. The larger grain size, which lessens the dispersion of the grain boundary, was thought to be the cause of the observed increase in the dc electrical conductivity with an increase in Cu doping. In structured undoped and Cu-doped ZnTe films, there were two carrier transport conduction mechanisms that could be seen. According to the Hall Effect measurements, all the grown films exhibited a p-type conduction behavior. In addition, the findings demonstrated that as the Cu doping level rises, the carrier concentration and the Hall mobility similarly rise, reaching an ideal Cu concentration of 8 at.%, which is due to the fact that the grain size decreases grain boundary scattering. Furthermore, we examined the impact of the ZnTe and ZnTe:Cu (at Cu 8 at.%) layers on the efficiency of the CdS/CdTe solar cells.

Effect of thermal annealing on physical properties of Cu-doped ZnTe thin films: Functionality as interface layer

Achieving a high quality interface is of great importance in CdTe based solar cells, as it appreciably inhibits interfacial recombination and consequently improves the photovoltaic performance. The low carrier density of CdTe absorber contributes to formation of back contact Schottky barrier which acts as diode setup in opposition to the CdS/CdTe junction. To overcome this problem, the Copper doped Zinc Telluride (ZnTe:Cu) material is estimated to be a good nominee for such interface layer due to its high chemical immovability and appropriate band gap alignment with CdTe absorber. Hence, the present work demonstrates thermal annealing evolution to 200 nm thin resistive heating grown ZnTe:Cu 1 % films to find their feasibility as back/rear contact material in the CdTe based superstrate solar cell devices. The structural analysis revealed that as deposited and annealed ZnTe:Cu 1 % films have (103) preferred reflection with cubic phase and crystallite size is enhanced from 22 nm to 42 nm with annealing. The optical transmittance is varied with annealing and the direct optical energy band gap is found within the range of 2.10–2.74 eV. The pristine and annealed films showed ohmic behavior and the AFM images displayed hill like topographies. Smaller to larger size spherical shaped grains and non-uniform cylindrical shaped grains morphology is obtained in FESEM images whereas the EDS patterns confirmed successful deposition of ZnTe:Cu 1 % thin films. The obtained results signify that the 300 °C annealed ZnTe:Cu 1 % films might be considered as interface layer material for the fabrication of CdTe based solar cell devices. • Annealing induced properties of ZnTe:Cu interface layer are investigated for solar cells. • The ZnTe:Cu films showed crystal growth along (103) preferred orientation. • Optical band gap of ZnTe:Cu films is found in 2.10–2.74 eV range with annealing. • AFM images showed hill-like topographies and EDS patterns confirmed films deposition. • SEM images revealed spherical grains and I-V curves ensured ohmic nature.

Pronounced Impact of p-Type Carriers and Reduction of Bandgap in Semiconducting ZnTe Thin Films by Cu Doping for Intermediate Buffer Layer in Heterojunction Solar Cells.

Stabilized un-doped Zinc Telluride (ZnTe) thin films were grown on glass substrates under vacuum using a closed space sublimation (CSS) technique. A dilute copper nitrate solution (0.1/100 mL) was prepared for copper doping, known as an ion exchange process, in the matrix of the ZnTe thin film. The reproducible polycrystalline cubic structure of undoped and the Cu doped ZnTe thin films with preferred orientation (111) was confirmed by X-rays diffraction (XRD) technique. Lattice parameter analyses verified the expansion of unit cell volume after incorporation of Cu species into ZnTe thin films samples. The micrographs of scanning electron microscopy (SEM) were used to measure the variation in crystal sizes of samples. The energy dispersive X-rays were used to validate the elemental composition of undoped and Cu-doped ZnTe thin films. The bandgap energy 2.24 eV of the ZnTe thin film decreased after doping Cu to 2.20 eV and may be due to the introduction of acceptors states near to valance band. Optical studies showed that refractive index was measured from 2.18 to 3.24, whereas thicknesses varied between 220 nm to 320 nm for un-doped and Cu doped ZnTe thin film, respectively, using the Swanepoel model. The oxidation states of Zn+2, Te+2, and Cu+1 through high resolution X-ray photoelectron spectroscopy (XPS) analyses was observed. The resistivity of thin films changed from ~107 Ω·cm or undoped ZnTe to ~1 Ω·cm for Cu-doped ZnTe thin film, whereas p-type carrier concentration increased from 4 × 109 cm−2 to 1.4 × 1011 cm−2, respectively. These results predicted that Cu-doped ZnTe thin film can be used as an ideal, efficient, and stable intermediate layer between metallic and absorber back contact for the heterojunction thin film solar cell technology.

The effects of Cu-doped ZnTe intermediate layer on the Ohmic contact to CdZnTe films

Polycrystalline CdZnTe films with thickness about 300μm were deposited by close spaced-sublimation (CSS) method. The as-deposited CdZnTe samples were polished by mechanical polishing (MP) process. Cu-doped ZnTe films were sputtered by RF magnetron sputtering as an intermediate layer to improve the performance of Ohmic contact of Au electrodes to CdZnTe films. The effect of Cu-doped ZnTe intermediate layers on the electrical characteristics of Au/CdZnTe and Au/ZnTe:Cu/CdZnTe was investigated by the circular transmission line model (CTLM). The results indicate a good Ohmic contact of Au/ZnTe:Cu/CdZnTe with low contact resistivity of about 0.79Ω·cm2.

Self-Assembled Formation of Well-Aligned Cu-Te Nano-Rods on Heavily Cu-Doped ZnTe Thin Films

Cu doping of ZnTe, which is an important semiconductor for various optoelectronic applications, has been successfully achieved previously by several techniques. However, besides its electrical transport characteristics, other physical and chemical properties of heavily Cu-doped ZnTe have not been reported. We found an interesting self-assembled formation of crystalline well-aligned Cu-Te nano-rods near the surface of heavily Cu-doped ZnTe thin films grown via the molecular beam epitaxy technique. A phenomenological growth model is presented based on the observed crystallographic morphology and measured chemical composition of the nano-rods using various imaging and chemical analysis techniques. When substitutional doping reaches its limit, the extra Cu atoms favor an up-migration toward the surface, leading to a one-dimensional surface modulation and formation of Cu-Te nano-rods, which explain unusual observations on the reflection high energy electron diffraction patterns and apparent resistivity of these thin films. This study provides an insight into some unexpected chemical reactions involved in the heavily Cu-doped ZnTe thin films, which may be applied to other material systems that contain a dopant having strong reactivity with the host matrix.Electronic supplementary materialThe online version of this article (doi:10.1186/s11671-016-1741-x) contains supplementary material, which is available to authorized users.

Effects of Cu incorporation on physical properties of ZnTe thin films deposited by thermal evaporation

The present paper reports on a systematic study of the Cu doping effect on the optical, electrical and structural properties of ZnTe:Cu (Cu=0, 6, 8, and 10at%) thin films. Polycrystalline Cu-doped ZnTe thin films were deposited on glass substrates at room temperature by thermal evaporation. A detailed characterization of the Cu-doped ZnTe films were performed by X-ray diffraction (XRD), Spectrophotometry, Fourier transform infrared spectroscopy (FT-IR) and Raman spectroscopy. XRD of the as-deposited Cu-doped ZnTe films belong to single-phase cubic structure of ZnTe with preferential orientation along (111) planes revealed minor effect of Cu content. The interference pattern in optical transmission spectra was analyzed to determine energy band gap, refractive index, extinction coefficient and thickness of the films. Wemple–DiDomenico and Tauc's relation were used for the determination and comparison of optical band gap values. The formation of ZnTe and Cu-doped ZnTe phase was confirmed by FT-IR. AC conductivity in a frequency range of 0–7MHz has been studied for investigation of the carriers hoping dynamics in the films. Raman spectra indicated merely typical longitudinal optical (LO) phonon mode of the cubic structure ZnTe thin film at 194cm−1 because the excitation energy is well above of the optical band-gap of the material and exhibited a blue-shift from 194 to 203cm−1 with Cu which could be associated to the substitution of Zn atom with Cu at the lattice sites.

Electronic structure and optical properties of Cu-doping and Zn vacancy impurities in ZnTe

The geometric structures of perfect ZnTe, that with Zn vacancy (Zn0.875Te), and Cu-doped ZnTe (Zn0.875Cu0.125Te) were optimized using the pseudopotential plane wave (PP-PW) method based on the density functional theory (DFT) within generalized gradient approximation (GGA). The cohesive energy, band structure, density of states, and Mulliken populations were calculated and discussed in detail. On the other hand, an accurate calculation of linear optical functions (the dielectric function, refraction index, reflectivity, conductivity function, and energy-loss spectrum) was performed. The results demonstrated that compared to the perfect ZnTe, the lattice parameters of Zn0.875Te and Zn0.875Cu0.125Te were changed and the cell volumes decreased to some extent due to the vacancy and introduction of impurity. A vacancy acceptor level and an acceptor impurity level were produced in Zn0.875Te and Zn0.875Cu0.125Te, respectively. By comparison, Cu doping in the ZnTe system is relatively stable while the monovacancy system is not.

Carrier density in p-type ZnTe with nitrogen and copper doping

AbstractIn this paper, we demonstrate deposition methods and conditions that allow the control of the electrical properties of doped ZnTe grown by RF magnetron sputtering using both nitrogen and copper as dopants. The carrier density of the films was characterized using a van der Pauw Hall effect measurement method. We demonstrate how the concentration of nitrogen in the plasma during the growth of the film impacts the conductivity of the ZnTe films. Films with hole concentrations in excess of 1018 cm-3 and a high degree of crystallinity were successfully grown. Similarly, we demonstrate that the hole concentration in the Cu-doped ZnTe can be varied by varying the amount of copper introduced in the films. We also observe that annealing the copper doped ZnTe films increases the carrier density, whereas annealing the nitrogen doped ZnTe films causes a decrease in carrier concentration and conductivity.

Preparation and characterization of ZnTe as an interlayer for CdS/CdTe substrate thin film solar cells on flexible substrates

ZnTe thin films have been prepared on Mo-coated stainless steel substrates by the close spaced sublimation (CSS) technique. Cu-doped ZnTe (ZnTe:Cu) films are prepared by immersing the as-deposited samples in a CuCl–H2O solution. The structural and morphological properties of the ZnTe films as a function of substrate temperature are investigated using X-ray diffraction and scanning electron microscopy. CdS/CdTe solar cells with and without the ZnTe and ZnTe:Cu interlayers are prepared and characterized with current–voltage (J–V), spectral response, and capacitance–voltage measurements. The structural and morphological properties of the ZnTe films prepared by CSS are found to be strongly influenced by the substrate temperature. The ‘roll-over’ behavior in the J–V characteristics of the CdS/CdTe solar cells due to a Schottky barrier at the back contact can be eliminated using ZnTe as the interlayer. Preliminary results on the application of ZnTe:Cu films suggest that the open circuit voltage and fill factor can be further improved, depending on processing conditions.

Recent Results for All-Dry-Processed CdTe/CdS Solar Cells

AbstractSeveral wet-processing steps are used in fabricating high-efficiency CdTe/CdS solar cells. These steps can hinder in-line processing; thus, developing an all-dry processing option is attractive for a manufacturing-friendly process. In this study, we systematically modified the baseline process used in our laboratory to replace CdS deposited by chemical-bath deposition (CBD) with sputter-deposited CdS and Cu-doped graphite paste back-contact with Cu-doped ZnTe deposited by radio-frequency sputtering. In addition to CdTe deposited by close-spaced sublimation, we also used conventionally evaporated CdTe. The results show that replacing only CBD CdS with oxygenated CdS deposited by sputtering produces devices with performance comparable to baseline devices if the front bilayer SnO2 is replaced by a Cd2SnO4/ZnSnO alloy. Replacing the graphite paste back-contact with sputter-deposited Cu-doped ZnTe resulted in device performance comparable to baseline devices. Incorporating both dry processing steps gave performance comparable to the devices with sputtered CdS with a SnO2 front contact. We used capacitance-voltage and minority-carrier lifetime measurements to analyze the factors affecting device performance and we present the results here.

Development of ZnTe Contacts for Cd1-xMgxTe Thin-Film Solar Cells for Tandem Applications

AbstractPolycrystalline Cd1-xMgxTe (CMT) thin films are a potential absorber material for two-junction thin-film tandem solar cell applications because the desired top cell bandgap range of 1.6 to 1.8 eV is readily obtained using CMT with only small resultant changes in the lattice constant from that of CdTe. Tandem devices require the top cell to have a transparent back contact to transmit the sub-bandgap spectrum to the bottom cell. Sputtered Cu-doped ZnTe (ZnTe:Cu) thin films, which offer potential as a transparent back contact interface layer, have been used successfully in CdTe devices. We apply ZnTe:Cu back contacts to CMT devices to continue development toward a transparent top cell. We describe the effects of depositing ZnTe:Cu at different temperatures and with different Cu contents on the net acceptor concentration, minority carrier lifetime, and device performance. We present here the highest reported CMT device efficiency of 9.6% at a bandgap of 1.57 eV.

Properties of Cu-doped Zn Te thin films prepared by closed space sublimation (CSS) techniques

Properties of Cu-doped Zn Te thin films prepared by closed space sublimation (CSS) techniques

Properties of RF Sputtered ZnTe:N films for Back Contact to CdS/CdTe Solar Cells

ABSTRACTMost of the low resistance back contacts formed on high efficiency CdS/CdTe solar cells involve copper either in elemental form (such as Cu/Au back contacts) or as dopant in other material (such as Cu-doped ZnTe). But copper is also suspected to be a cause of degradation of devices in long-term stability tests due to its high diffusion coefficient in polycrystalline CdTe. In this paper, we present results on the development of nitrogen-doped ZnTe back contacts for CdS/CdTe solar cells. Reproducible N-doped p-ZnTe films were prepared using reactive RF magnetron sputtering with Ar/N2 gas mixtures. The conductivity of the doped ZnTe films was more than five orders of magnitude higher than that of intrinsic films. We find that annealing in air can further increase the conductivity. Efficiencies near 10% have been achieved with a ZnTe:N/Ni back contact.

Electrochemical synthesis of cu-doped znte films as back contacts to cdte solar cells

Electrochemical deposition of Cu-doped ZnTe films was investigated as a back contact material for CdTe solar cells. The deposition system consisted of a potentiostat, a substrate (cathode), a Pt counter electrode, and an Ag/AgCl reference electrode. Experimental conditions were pH=2.5∽4, deposition temperature=70 ∽80°C, electrolyte concentration=0.02 M Zn2, 10-4 M TeO2, and deposition potential (Va) = -0.85 ∽ -1.0 V vs Ag/AgCl A cyclic voltammogram obtained from an ITO substrate showed a ZnTe reduction peak at Va = -0 7 ∽ -0 9 V A film obtained at pH=2.5, Va=-0.975 V showed the strongest ZnTe X-ray diffraction peak, but its surface morphology was not as smooth as the films made under the condition of pH=3∽4, Va=-0.950V. Triethanolamine did not form a complex with cu2 in acidic solution as needed for uniform Cu-doping, but oxalate anion and 1,10-phenanthrolme did at pH=2∽3. The efficiency of CdTe solar cells increased from 3.4% to 7.1% when Cu-doped ZnTe film was used as the back contact.

Effect of Cu-doping on the morphology of ZnTe films electrodeposited from nonaqueous bath

Electrodeposition of Cu-doped ZnTe has been studied in a bath containing ZnCl 2, TeCl 4, KI and CuCl 2 at 160°C using cyclic voltammetry in the dark and under illumination. X-ray diffraction as well as scanning electron microscopy techniques have been employed to investigate the influence of deposition potential on the structure and surface morphology of the electrodeposit. It is shown that a mixed cubic as well as hexagonal ZnTe film is electrodeposited even at a potential lower than the equilibrium potential of Zn electrodeposition. Intermediate electrodeposition potential (−0.7 V to −0.75 V) favours growth of a mixed Te and ZnTe phase. Compact, globular and single phase ZnTe can be grown at −0.95 V. The atomic force microscopic examination of the ZnTe films deposited at −0.95 V indicated compact and close packed granular growth.

The structural, optical, and electrical properties of vacuum evaporated Cu-doped ZnTe polycrystalline thin films

We have studied the structural, optical, and electrical properties of thermally evaporated, Cu-doped, ZnTe thin films as a function of Cu concentration and post-deposition annealing temperature. X-ray diffraction measurements showed that the ZnTe films evaporated on room temperature substrates were characterized by an average grain size of 300A with a (111) preferred orientation. Optical absorption measurements yielded a bandgap of 2.21 eV for undoped ZnTe. A bandgap shrinkage was observed for the Cu-doped films. The dark resistivity of the as-deposited ZnTe decreased by more than three orders of magnitude as the Cu concentration was increased from 4 to 8 at.% and decreased to less than 1 ohm-cm after annealing at 260°C. For films doped with 6–7 at.% Cu, an increase of resistivity was also observed during annealing at 150–200°C. The activation energy of the dark conductivity was measured as a function of Cu concentration and annealing temperature. Hall measurements yielded hole mobility values in the range between 0.1 and 1 cm2/V·s for both as-deposited and annealed films. Solar cells with a CdS/CdTe/ZnTe/metal structure were fabricated using Cudoped ZnTe as a back contact layer on electrodeposited CdTe. Fill factors approaching 0.75 and energy conversion efficiencies as high as 12.1% were obtained.

Development of Cu-doped ZnTe as a back-contact interface layer for thin-film CdS/CdTe solar cells

The full potential of thin-film, CdS/CdTe photovoltaic solar cells will not be realized until issues relating to the fabrication of environmentally stable, low-resistance, and easily manufactured contacts to the p-CdTe layer are addressed. One alternative that provides the required contact parameters employs a Cu-doped ZnTe(ZnTe:Cu) interface layer between the p-CdTe and the outer metal contact. Thin films of ZnTe:Cu containing various concentrations of metallic Cu are produced by rf-magnetron sputtering. Additionally, the effect of incorporating small amounts of excess Zn into the sputtering target is studied. We find that the electrical resistivity of ZnTe:Cu films deposited at 300 °C, and prepared with Cu concentrations of ∼0.45 at. %, is much higher than would be expected from studies of films doped with higher Cu concentrations (∼6 at. % Cu). We also find that postdeposition heat treatment significantly reduces the electrical resistivity of the films containing ∼0.45 at. % Cu. However, compositional analysis indicates that the surface of the films become increasingly enriched in Zn at annealing temperatures ≳350 °C. Analysis of the hole effective mass (mh) for films containing ∼6 at. % Cu indicates a value for mh of 0.35 me, and a high-frequency dielectric constant (ε∞) of 8.2.

Study of ZnTe:Cu Back Contacts on CdTe/CdS Thin Film Solar Cells

AbstractVacuum-evaporated Cu-doped ZnTe films have been studied as the intermediate layer between CdTe and metal contacts in CdTe/CdS thin-film solar cells for the formation of low resistance back contacts. Different metals (Au, Ni, Co) have been investigated as the contact material to the ZnTe layer. The effects of Cu concentration, ZnTe:Cu layer thickness, and ZnTe post-deposition annealing temperature on the cell performances have been investigated. We found that different metal contacts on the ZnTe layer lead to different doping densities in the CdTe layer and different open-circuit photovoltages of the solar cells. The possible formation of a back contact diode at the CdTe/ZnTe interface was explored, based on capacitance-voltage analysis. The series resistance of the CdTe/CdS cells was reduced significantly by the introduction of the ZnTe layer. Fill factors greater than 0.76 and an energy conversion efficiency of 12.9% have been achieved using ZnTe back contacts on electrodeposited CdTe.

Development of rf sputtered, Cu-doped ZnTe for use as a contact interface layer to p-CdTe

Cu-doped ZnTe films deposited by rf-magnetron sputtering have been analyzed with the intention to use this material as a contact interface in CdS/CdTe thin-film photovoltaic solar-cell devices. It is observed that unless careful attention is made to the pre-deposition conditioning of the ZnTe target, the electrical resistivity of thin films (∼70 nm) will be significantly higher than that measured on thicker films (∼1.0 μm). It is determined that N contamination of the target during substrate loading is likely responsible for the increased film resistivity. The effect of film composition on the electrical properties is further studied by analyzing films sputtered from targets containing various Cu concentrations. It is determined that, for targets fabricated from stoichiometric ZnTe and metallic Cu, the extent of Zn deficiency in the film is dependent on both sputtering conditions and the amount of metallic Cu in the target. It is observed that the carrier concentration of the film reaches a maximum value of ∼3 × 1020 cm−3 when the concentrations of Te and (Zn+Cu) are nearly equal. For the conditions used, this optimum film stoichiometry results when the concentration of metallic Cu in the target is ≈6 at.%.

Formation of a stable ohmic contact to CdTe thin films through the diffusion of P from Ni-P

Thin films of CdTe used in electronic devices often have problems with electrical contacts. This is due to the non-availability of a contacting material with a large work function for proper matching with p-CdTe. Moreover, the method of doping the thin film is the real problem. There are several possible solutions in this connection, one of which is the formation of a p+ layer on the CdTe surface by the reaction or indiffusion of dopant materials. Another approach is to engineer the barrier height prior to the metal contact deposition by depositing a layer of Cu-doped ZnTe onto the CdTe. However, all of the above methods have their own limitations. In this paper a method is presented which can overcome these limitations. Using an autocatalytic reduction process, NbP composite material has been deposited successfully on the p-CdTe surface. On annealing at an optimum temperature of 250 degrees C, the contact resistivity comes down to 0.1-0.08 Omega cm2. XRD and EDAX studies reveal that the lowering of the contact resistance is due to the diffusion of P into the CdTe with the formation of a p+ layer. A model for this has also been presented in the text.