D. Mao

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 300Å 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.

Interdiffusion in polycrystalline thin-film CdTe/CdS solar cells

We have investigated the interdiffusion between CdTe and CdS resulting from post‐deposition annealing treatment. X‐ray diffraction (XRD) measurements reveal the presence of CdTe1−xSx in the CdTe layer and CdS1−y Tey in the CdS layer. x and y values are estimated to be 0.03 and 0.08, respectively, based on the measured lattice spacing. CdCl2 coated on the CdTe/CdS structure prior to annealing enhances significantly the degree of interdiffusion. The S diffusion profile in the CdTe layer is explored. A 10 meV decrease of CdS bandgap is observed as a result of interdiffusion.

Properties of ZnTe:Cu thin films and CdS/CdTe/ZnTe solar cells

The effects of Cu doping and post-deposition annealing on the properties of ZnTe thin films and CdS/CdTe solar cells with the ZnTe:Cu/Au back contact were investigated. The structural, compositional and electrical properties of ZnTe films were studied systematically using X-ray diffraction (XRD), electron microprobe analysis, atomic force microscopy (AFM), electrical resistivity and Hall effect measurements. ZnTe films with Cu concentrations of 1-6 at. % were used successfully as a back contact layer, providing fill factors over 0.70. The stability of CdTe/ZnTe/Au cells under accelerated temperature stress tests was investigated. I-V and C-V measurements, scanning Auger electron spectroscopy (AES) and X-ray photoemission spectroscopy (XPS) were used to clarify possible degradation mechanisms. Significant Cu and Te diffusion into Au layer and Au diffusion into ZnTe were observed after annealing.

Photoconductive lifetime of CdS used in thin-film solar cells

Thin-film CdS is used as a window layer in most of the current thin-film polycrystalline device technologies including absorbers based on cadmium telluride (CdTe) and copper indium selenide (CIS) and copper indium gallium selenide (CIGS). Device performance has been linked to the deposition technique and post-treatment of the CdS layer. Here we used the radio frequency photoconductive decay (RFPCD) technique to measure the photoconductive lifetime of CdS that had been deposited by various techniques. This includes chemical bath deposition (CBD) and laser deposition. CdCl/sub 2/ treatment increases the photoconductive response and reduces shallow trapping effects. In addition, tellurium doping was found to significantly decrease the photoconductive response, as well as the hole lifetime, below that of the undoped material. The impact of these CdS properties on device performance is discussed.

Sulfur Diffusion In Polycrystalline Thin-Film CdTe Solar Cells

X-ray diffraction and photoluminescence measurements have been used to characterize the diffusion of S into CdTe during post growth annealing of CdTe solar cells. For anneals at 410°C in the presence of CdCl2, evidence that both a CdTe1-xSx phase and nearly-pure CdTe are present near the back contact is observed. The ternary phase becomes more prominent and the S concentration increases with depth reaching roughly 4-5% near the CdS interface. Much less diffusion is observed at 350°C while for a 460°C anneal, CdTe1-xSx with a S concentration near 5% is found throughout the layer. The presence of CdCl2 during the anneal enhances the interdiffusion.

Effect of CdCl 2 Treatment of CdS Films on CdTe/CdS Solar Cells

We investigated the effect of CdCl 2 treatment of CdS films on the photovoltaic performance of polycrystalline CdTe/CdS solar cells. X-ray diffraction studies indicated that the diffusion of S into CdTe is qualitatively the same for CdTe/CdS films fabricated with both as-deposited and CdCl 2 -treated CdS. A major difference was observed in the extent of Te diffusion into CdS for the two types of CdS films. Full conversion of CdS into CdS 1-y Te y ; was observed for films prepared with asdeposited CdS, while the formation of the ternary phase was below the detection limit for films prepared with CdCl 2 -treated CdS. Photoluminescence measurements confirmed this result. The difference in interdiffusion leads to differences in optical transmission of CdS films and spectral response of CdTe/CdS solar cells. An increase of 2.7 mA/cm 2 in short-circuit current density was observed as a result of improved spectral response in the wavelength range of 500–600 nm for the CdCl 2 -treated CdS.

The properties and optimization of ZnTe:Cu back contacts on CdTe/CdS thin film solar cells

Vacuum-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) were used 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 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. Preliminary studies showed good stability of Au/ZnTe-contacted cells under illumination and different bias conditions.

Effect of Cu doping on the properties of ZnTe:Cu thin films and CdS/CdTe/ZnTe solar cells

The effects of Cu doping concentration and post-deposition annealing treatment on the properties of ZnTe thin films were investigated in an effort to decrease the Cu doping concentration and improve the long-term stability of CdS/CdTe/ZnTe solar cells. The structural, compositional, and electrical properties were studied systematically using x-ray diffraction (XRD), electron microprobe, Hall effect and conductivity measurements. XRD measurements indicated that the crystalline phase of as-deposited and low-temperature annealed ZnTe films is dependent on Cu doping concentration. Low-Cu-doped films exhibited zincblende phase, whereas high-Cu-doped films showed wurtzite phase. After annealing at high temperature (⩾350 °C), all films exhibited zincblende structure. Electron probe microanalysis revealed a deficiency of cations in low-Cu-doped films and an excess of cations in high-Cu-doped films. Hall effect measurements revealed a dependence of hole mobility on Cu doping concentration with the highest mobility...

Effect and optimization of CdS/CdTe interdiffusion on CdTe electrical properties and CdS/CdTe cell performance

We have investigated the effect of the CdS/CdTe interdiffusion on the properties of the CdTe films and the CdS/CdTe cell performance. Sulfur (S) diffusion into the CdTe films leads to a decreased defect density in the films, improvement of cell performance, and possibly to the increase of the carrier lifetime in the films. Cell performance is improved with the increase of the amount of S in the CdTe films. S diffusion into CdTe also deteriorates the uniformity of the CdS window layers, resulting in worse cell performance. Based on this study, we propose a processing method to improve cell performance.

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

Vacuum-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. The authors 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.