Ultrathin CdTe Research Articles

Ultrathin CdS/CdTe solar cells by sputtering

We have prepared thin-film CdS/CdTe cells with CdTe thickness from 0.25μm to 2.1μm on commercial SnO2:F-coated soda-lime glass with a high resistivity transparent SnO2 buffer layer. With careful optimization of the magnetron sputtering conditions, CdS and Cu thicknesses, CdCl2 activation, and back-contact diffusion processes, we obtain 8% efficiency with only 0.25μm of CdTe and 11% efficiency with 0.5μm. The results confirm the advantages of magnetron sputtering for ultra-thin CdTe and show some light trapping and excellent carrier collection even for the thinnest devices.

Ultralong CdTe Nanowires: Catalyst‐Free Synthesis and High‐Yield Transformation into Core–Shell Heterostructures

AbstractA novel catalyst‐free synthetic strategy for producing high‐quality CdTe nanowires in solution is proposed. A special reaction condition is intentionally constructed in the reaction system to induce the formation of nanowires through oriented in situ assembly of tiny particles. To establish such special synthetic conditions in the CdTe system, not only are its typical features and possible solutions deeply analyzed, but also related factors, such as the ligand environment, injection and growth temperature, and Cd‐to‐Te precursor ratio, are systemically investigated. High‐quality ultralong (up to 10 μm) and ultrathin (less than 10 nm) CdTe nanowires are produced in solution under optimal reaction conditions. Morphological, spectral, and compositional analyses are performed to examine the products formed at different reaction stages in order to clarify the formation mechanism of the CdTe nanowires. Furthermore, the transformation of the CdTe nanowires into CdTe/CdSe core–shell heterostructures is intensively explored, and the CdSe epitaxial growth process is specially tracked by morphological and spectral characterization techniques. Finally, CdTe nanowires coated with a continuous and dense CdSe shell are successfully fabricated by using a proper coating protocol.

MOCVD of Cd(1−x)Zn(x)S/CdTe PV cells using an ultra-thin absorber layer

Ultra-thin Cd(1−x)Zn(x)S/CdTe devices were produced by atmospheric pressure metal organic chemical vapour deposition (AP-MOCVD) with varying CdTe absorber thicknesses ranging from 1.0 to 0.2μm and compared to baseline cells with total CdTe thickness of 2.25μm. The ultra-thin CdTe layers (≤1μm) were intentionally doped with As to induce p-type conductivity in the absorber. Cell performance reduced with CdTe thickness, with the magnitude of photo-current generation loss becoming more significant for the very thin CdTe layers. The decline in cell performance was lower than the optically limited performance relating to a decrease in shunt resistance, Rsh, especially for the thinnest cells due to areas of incomplete CdTe coverage and large presence of pin-holes leading to micro-shorts. Incorporation of Zn into the CdS window layer improved cell performance for all devices except when 0.2μm thick CdTe was used. This improvement was markedly in the blue region owing to enhanced optical transparency of the window layer. External quantum efficiency (EQE) measurements showed a red-shift of the window layer absorption edge due to leaching out of Zn during the CdCl2 treatment. Reduction of the CdCl2 deposition time was demonstrated to recover the blue response of the ultra-thin cells.

MoOx as an Efficient and Stable Back Contact Buffer for Thin Film CdTe Solar Cells

ABSTRACTA low-resistance back contact for n-CdS/p-CdTe solar cells has been developed, which utilizes a thermally evaporated MoOx thin film as the buffer layer between the p-CdTe and the back electrode. The low-resistance behavior of back contact is attributed to the high work function of MoOx, which reportedly is as high as 6.8 eV, and thus adequately matches that of p-CdTe. With MoOx as the buffer, a variety of common metals, even those with a low work function such as Al, have been found to be useful as the electrode in forming the back contact. Other advantages of the MoOx buffer include dry application by vacuum deposition, and thus it is particularly suitable for the fabrication of ultra-thin CdTe solar cells without introducing additional shorting defects. Surface cleaning of CdTe films prior to MoOx deposition has also been studied. The cell stability has been evaluated through thermal annealing tests. Thermal degradation has been explained in terms of oxidation of the metal electrodes. CdTe cells with high efficiency and good stability have been demonstrated with MoOx as the back contact buffer and Ni as the electrode.

Effects of high-temperature annealing on ultra-thin CdTe solar cells

High-temperature annealing (HTA), a process step prior to vapor cadmium chloride (VCC) treatment, has been found to be useful for improving the crystallinity of CdTe films and the efficiency of ultra-thin CdTe solar cells. Scanning electron microscopy, optical absorption, photoluminescence measurements and analyses on photoluminescence results using spectral deconvolution reveal that the additional HTA step produces substantial grain growth and reduces grain boundary defects. It also reduces excessive sulfur diffusion across the junction that can occur during the VCC treatment. The HTA step helps to produce pinhole-free CdTe films and reduce electrical shorts in ultra-thin CdTe solar cells. An efficiency of about 11.6% has been demonstrated for ultra-thin CdS/CdTe solar cells processed with HTA step.

The role of transparent conducting oxides in metal organic chemical vapour deposition of CdTe/CdS Photovoltaic solar cells

A systematic study is made between the relationship of Cd 0.9Zn 0.1S/CdTe photovoltaic (PV) device properties for three different commercial transparent conducting oxide (TCO) materials and some experimental CdO to determine the role of the TCO in device performance. The resistance contribution from the TCO was measured after depositing the gold contact architectures directly onto the TCOs. These were compared with the Cd 0.9Zn 0.1S/CdTe device properties using the same contact arrangements. Series resistance for the commercial TCOs correlated with their sheet resistance and gave good agreement with the PV device series resistance for the indium tin oxide (ITO) and fluorine doped tin oxide (FTO) 15 Ω/Sq. superstrates. The devices on the thicker FTO 7 Ω/sq superstrates were dominated by a low shunt resistance, which was attributed to the rough surface morphology causing micro-shorts. The device layers on the CdO substrate delaminated but devices were successfully made for ultra-thin CdTe (0.8 μm thick) and compared favourably with the comparable device on ITO. From the measurements on these TCOs it was possible to deduce the back contact resistance and gave an average value of 2 Ω.cm 2. The correlation of fill factor with series resistance has been compared with the predictions of a 1-D device model and shows excellent agreement. For high efficiency devices the combined series resistance from the TCO and back contact need to be less than 1 Ω.cm 2.

Ultra-thin bifacial CdTe solar cell

Developing a high-quality transparent back contact, while maintaining efficient light transmission through the top absorber layer, are key components for achieving high-efficiency II–VI polycrystalline thin-film tandem solar cells. Combining these two elements, we fabricated ultra-thin bifacial CdTe solar cells (0.68 μm) with ZnTe:N/ITO transparent back contact and achieved efficiencies of 5.7% and 5.0% with illumination from the glass and the contact side, respectively. Device analysis, using ( J– V) and QE measurements, show that the loss in efficiency is due to higher R S and J 0 as well as lower, side-dependent, photons absorption.

High efficiency ultra-thin sputtered CdTe solar cells

Abstract Large scale manufacturing of CdTe PV modules at the GW/yr level may be constrained due to the limited availability of the relatively rare (Te) element and the volume of potentially hazardous (Cd) material being used in the typically 3–8 μm thick CdTe absorber layer. However, we find that it is possible to reduce the CdTe layer thickness without much compromise in efficiency. The CdS/CdTe solar cells were fabricated using magnetron sputtering with ultra-thin CdTe layers in the range of 0.5–1.28 μm. The ultra-thin films and cells were characterized using X-ray diffraction (XRD), optical transmission, scanning electron microscopy (SEM), current–voltage and quantum efficiency measurements. These results were compared with those of standard 2.3 μm thick CdTe sputtered cells. Different post-deposition processing parameters were required for cells with ultra-thin and standard CdTe thicknesses to achieve high efficiency. Ultra-thin CdTe cells showed crystallographic texture and CdTe1−xSx alloy formation after CdCl2 treatment very similar to standard CdTe cells. Optimization of the post-deposition CdCl2 treatment and back-contact processing yielded cells of 11.2% efficiency with 0.7 μm CdTe compared to 13.0% obtained with standard 2.3 μm CdTe cells.

Growth of ultrathin films of cadmium telluride and tellurium as studied by electrochemical atomic force microscopy

Time dependent, cathodic electrodeposition of ultrathin CdTe and Te films has been studied in 50 mM H 2SO 4 + 1 mM CdSO 4 + 0.1 mM TeO 2 solutions at room temperature under potential control using electrochemical atomic force microscopy (EC-AFM). The films were also characterized electrochemically and with X-ray diffraction. The growth mechanism and the composition of the films depends on the applied potentials. Island-like growth mode was observed for CdTe films when the deposition potential was − 0.35 V (SHE). At a more positive deposition potential of 0.138 V (SHE), Cd was not codeposited into the film but affected the dynamic growth mode of the deposit. At this voltage smooth Te films were obtained. Depending on the applied potential, Cd acts either as a codeposition element for CdTe film growth, or as a mediator for layer-by-layer growth of Te films.

Tight Binding Approach to the Electronic Properties of MnTe/CdTe Structures

Ultrathin MnTe and CdTe films as well as MnTe/CdTc/MnTe sandwich structures are investigated. Band structure, local densities of states corresponding to Mn(Cd) and Te ions are calculated. Local magnetic moments are also found. Strong modifications of the density of states in the boundary MnTe and CdTe layers are obtained. A small negative splitting of the valence band edge is found at the interface, though no splitting is obtained in the MnTe barrier. The result is consistent with magnetooptical measurements performed for MnCdTe/CdTc heterostructures giving the enhanced Zeeman splitting at the interfaces.

Antiferromagnetic spin ordering and interlayer magnetic correlations in MnTe/CdTe superlattices

Results of neutron scattering studies on MnTe/CdTe superlattices with ultrathin non-magnetic CdTe “barriers” are presented and compared with data from earlier studies on MnSe/ZnSe, MnTe/ZnTe, and MnSe/ZnTe multilayers with thick non-magnetic spacers. The experiments revealed two qualitatively new effects—namely, (i) the existence of pronounced interlayer magnetic correlations in the case of the CdTe thickness corresponding to two single monolayers and (ii) the coexistence of two magnetic phases that never occurred simultaneously in the previously studied systems.

Molecular beam epitaxial growth of ultrathin CdTe–CdMnTe quantum wells and their characterization

We report the growth and optical characterization of CdTe/CdMnTe single quantum wells with well thicknesses ranging from 60 down to 6 Å. The single quantum wells were grown by standard molecular beam epitaxy without growth interruption and investigated by reflection, photoluminescence (PL), and excitation PL. All structures including the 6-Å-thick quantum well exhibit extraordinarily narrow photoluminescence lines. From an analysis of linewidth and Stokes shift of the photoluminescence lines informations on the structure of the CdTe/CdMnTe interfaces are derived. The good quality of those structures made it possible to identify for the first time recombination of two-dimensional free exciton magnetic polarons.

Direct conversion of solar energy through photovoltaic cells

The software of Solar Cell Capacitance Simulator (SCAPS) is used to investigate the performance of ultra-thin CdTe solar cells in the backwall configuration (glass/ITO/MoOx/CdTe/CdS/SnO2/Ag). The backwall structure utilizes ultra-thin CdTe absorber layer instead of CdS film facing light illumination, which eliminates the absorption of CdS in short-wavelength region and improves the blue response of CdTe. A buffer layer of MoOx is added to modify the contact between CdTe and ITO, reducing the valence band barrier height and simultaneously forming an electron reflector, which can reduce electron-hole recombination at this contact. When the thickness of MoOx is 2 nm, the simulation results show that an efficiency can reach up to 25.5% with high ITO work function and ideal interface recombination velocity.