Nowshad Amin

Highly efficient 1 μm thick CdTe solar cells with textured TCOs

Thickness reduction of CdTe absorption layer down to 1 μm has been achieved by controlling the temperature profile used during the close-spaced sublimation (CSS) growth. Transparent conducting oxides, such as indium tin oxide (ITO) and textured fluorine doped tin oxide (SnO 2 :F) films have been investigated as transparent electrodes for such 1-μm-thick CdTe absorption layers to increase the incident light confinement and thus to achieve higher conversion efficiency. The contribution in solar cell performance has been found in the case of textured TCOs with optimum haze ratio (roughness). Conversion efficiencies of 10.6% (V oc : 0.75 V, J sc :22.02 mA/cm 2 , FF: 0.64, area: 1 cm 2 ) and 11.2% (V oc :0.78 V, J sc : 22.6 mA/cm 2 , FF: 0.63) have been achieved for only 0.6-μm-thick CdTe absorption layers with SnO 2 :F-TCO of 11% and 3% of haze ratios, respectively.

Characterization of Highly Efficient CdTe Thin Film Solar Cells by Low-Temperature Photoluminescence

Highly efficient CdTe thin film solar cells prepared by close-spaced sublimation (CSS) method with a glass/ITO/CdS/CdTe/Cu-doped carbon/Ag structure were characterized by low-temperature photoluminescence (PL) measurement. A broad 1.42 eV band probably due to VCd–Cl defect complexes appeared as a result of CdCl2 treatment. CdS/CdTe junction PL revealed that a CdSxTe1-x mixed crystal layer was formed at the CdS/CdTe interface region during the deposition of CdTe by CSS and that CdCl2 treatment promoted the formation of the mixed crystal layer. Furthermore, in the PL spectra of the heat-treated CdTe after screen printing of the Cu-doped carbon electrode, a neutral-acceptor bound exciton (ACu0, X) line at 1.590 eV was observed, suggesting that Cu atoms were incorporated into CdTe as effective acceptors after the heat treatment.

Effect of ZnTe and CdZnTe alloys at the back contact of 1-μm-thick CdTe thin film solar cells

N2-doped ZnTe was introduced onto 1-µm-thick CdTe absorbers in order to reduce the carrier recombination at the back contact of CdS/CdTe/C/Ag configuration solar cells. ZnTe films were grown by molecular beam epitaxy (MBE) on GaAs and Corning glass substrates to investigate the characteristics of the films. Epitaxial growth of ZnTe was realized on GaAs substrates and a hole concentration of 8 ×1018 cm-3 with a resistivity of 0.045 Ωcm was achieved as a result of nitrogen doping. In contrast, polycrystalline ZnTe films were grown on Corning glass and CdTe thin films. Dark and photoconductivity of ZnTe films increased to 1.43 ×10-5 S/cm and 1.41 ×10-4 S/cm, respectively, while the Zn to Te ratio was decreased to 0.25 during MBE growth. These ZnTe films with different thicknesses were inserted into close-spaced sublimation (CSS)-grown 1-µm-thick CdTe solar cells. A conversion efficiency of 8.31% (Voc: 0.74 V, Jsc: 22.98 mA/cm2, FF: 0.49, area: 0.5 cm2) was achieved for a 0.2-µm-thick ZnTe layer with a cell configuration of CdS/CdTe/ZnTe/Cu-doped-C/Ag. Furthermore, to overcome the problem of possible recombination loss in the interface layer of CdTe and ZnTe, the intermediate ternary CdZnTe is investigated. The compositional factor in Cd1-xZnxTe:N alloy is varied and the dependence of the conductivity is evaluated. For instance, Cd0.5Zn0.5Te:N, with dark and photoconductivity of 2.13 ×10-6 and 2.9 ×10-5 S/cm, respectively, is inserted at the back contact of a 1-µm-thick CdTe solar cell. A conversion efficiency of 7.46% (Voc: 0.68 V, Jsc: 22.60 mA/cm2, FF: 0.49, area: 0.086 cm2) was achieved as the primary result for a 0.2-µm-thick Cd0.5Zn0.5Te:N layer with the cell configuration of CdS/CdTe/Cd0.5Zn0.5Te:N/Au.

Prospects of Thickness Reduction of the CdTe Layer in Highly Efficient CdTe Solar Cells Towards 1 µm

This study focuses on the technique for the stable growth of CdTe (1.44 eV) with thickness near its absorption length, 1 µm, by close spaced sublimation (hereafter CSS) process, in order to achieve high conversion efficiency. X-ray diffraction (XRD) spectroscopy was carried out to examine the microstructure of the films. Current-voltage (I–V) characteristics, spectral response and other features of the solar cells using these CdTe films were investigated to elucidate the optimum conditions for achieving the best performance in such thin (1 µm) CdTe solar cells. Thickness was found to be reduced by controlling the temperature profile used during CSS growth. The temperature profile was found to be an important factor in growing high-quality thin films. By controlling the growth parameters and optimizing the annealing temperature at different fabrication steps, we have succeeded, to date, in achieving cell efficiencies of 14.3% (open-circuit voltage (Voc): 0.82 V, short-circuit current (Jsc): 25.2 mA/cm2, fill factor (F.F.): 0.695, area: 1 cm2) with 5 µm, 11.4% (Voc: 0.77 V, Jsc: 23.7 mA/cm2, F.F.: 0.63, area: 1 cm2) with 1.5 µm and 11.2% (Voc: 0.77 V, Jsc: 23.1 mA/cm2, F.F.: 0.63, area: 1 cm2) with only 1 µm of CdTe layer thickness at an air mass of 1.5 without antireflection coatings. This is important for establishing a strong foundation before developing a new structure (e.g., glass/ITO/CdS/CdTe/ZnTe/Ag configuration) with a back surface field of wide-bandgap material (e.g., ZnTe).

Prospective development in diffusion barrier layers for copper metallization in LSI

The most recent development together with some challenging opportunities on barrier layers for copper metallization has been reviewed. This review study mainly focuses on the technology trends in interconnect metallization, with emphasis on barrier layer materials, mechanism that dominates diffusion in barrier layer materials, and promising candidate barrier layers for copper metallization in LSIs. The applicability of different materials as diffusion barriers in copper-based interconnects has also been assessed for practical usage.

Amorphous Silicon Single-Junction Thin-Film Solar Cell Exceeding 10% Efficiency by Design Optimization

The conversion efficiency of a solar cell can substantially be increased by improved material properties and associated designs. At first, this study has adopted AMPS-1D (analysis of microelectronic and photonic structures) simulation technique to design and optimize the cell parameters prior to fabrication, where the optimum design parameters can be validated. Solar cells of single junction based on hydrogenated amorphous silicon (a-Si:H) have been analyzed by using AMPS-1D simulator. The investigation has been made based on important model parameters such as thickness, doping concentrations, bandgap, and operating temperature and so forth. The efficiency of single junction a-Si:H can be achieved as high as over 19% after parametric optimization in the simulation, which might seem unrealistic with presently available technologies. Therefore, the numerically designed and optimized a-SiC:H/a-SiC:H-buffer/a-Si:H/a-Si:H solar cells have been fabricated by using PECVD (plasma-enhanced chemical vapor deposition), where the best initial conversion efficiency of 10.02% has been achieved ( V,  mA/cm2 and ) for a small area cell (0.086 cm2). The quantum efficiency (QE) characteristic shows the cell’s better spectral response in the wavelength range of 400 nm–650 nm, which proves it to be a potential candidate as the middle cell in a-Si-based multijunction structures.

Prospects of Back Surface Field Effect in Ultra-Thin High-Efficiency CdS/CdTe Solar Cells from Numerical Modeling

Polycrystalline CdTe shows greater promises for the development of cost-effective, efficient, and reliable thin film solar cells. Results of numerical analysis using AMPS-1D simulator in exploring the possibility of ultrathin, high efficiency, and stable CdS/CdTe cells are presented. The conventional baseline case structure of CdS/CdTe cell has been explored with reduced CdTe absorber and CdS window layer thickness, where 1 μm thin CdTe and 50 nm CdS layers showed reasonable efficiencies over 15%. The viability of 1 μm CdTe absorber layer together with possible back surface field (BSF) layers to reduce minority carrier recombination loss at the back contact in ultra thin CdS/CdTe cells was investigated. Higher bandgap material like ZnTe and low bandgap materials like Sb2Te3 and As2Te3 as BSF were inserted to reduce the holes barrier height in the proposed ultra thin CdS/CdTe cells. The proposed structure of SnO2/Zn2SnO4/CdS/CdTe/As2Te3/Cu showed the highest conversion efficiency of 18.6% (Voc = 0.92 V, Jsc = 24.97 mA/cm2, and FF = 0.81). However, other proposed structures such as SnO2/Zn2SnO4/CdS/CdTe/Sb2Te3/Mo and SnO2/Zn2SnO4/CdS/CdTe/ZnTe/Al have also shown better stability at higher operating temperatures with acceptable efficiencies. Moreover, it was found that the cells normalized efficiency linearly decreased with the increased operating temperature with relatively lower gradient, which eventually indicates better stability of the proposed ultra thin CdS/CdTe cells.

Prospects of Cu 2 ZnSnS 4 (CZTS) solar cells from numerical analysis

In the rapid growth of thin film solar cells, Cu2ZnSnS4 (CZTS) poses to be a potential and alternative absorber layer of CIGS based cells. Besides solving the scarcity issue of rare materials like In or Ga in CIGS based solar cells, the CZTS based cells do not contain any toxic material and can lead to produce nontoxic thin film solar cells with excellent optical properties. In this work, absorber layer parameters have been studied by Solar Cell Capacitance Simulator (SCAPS) in terms of CZTS layer thickness and band gap to find out the optimum electrical performance. A promising result has been achieved with an efficiency of 7.55 % (with Voc = 0.5136 V, Jsc = 30.83 mA/cm2 and fill factor = 47.65 %) by using CZTS/CdS structure. It has also been found that the high efficiency of CZTS absorber layer thickness lies between 1 and 2.2 µm. This result can be explained in the practical work as non-stoichiometric composition of CZTS may result in lower efficiency of the solar cells. Quantum efficiency is almost 80% in the region of 350–500 nm, due to less absorption of light in the buffer layer. In addition, it is revealed that the highest efficiency cell can be achieved with the In2S3 buffer layer band gap of 2.74–2.90 eV. The study suggests that the proposed solar cell can be widely exploited in response to the fabrication of high efficiency thin film photovoltaic devices.

Synthesis and Characterization of Silver Nanoparticles and Silver Inks: Review on the Past and Recent Technology Roadmaps

AbstractThe synthesis of silver nanoparticles for silver ink formation has attracted broad interest in the electronic part printing and semiconductor chip industry due to the extraordinary electrical and mechanical properties of these materials. The preparation of silver nanoparticles through a physical or chemical reduction process is the most common methodology applied to obtain nanoparticles with the required size, shape and surface morphology. The chemical solution or solvent carrier applied for silver ink formulation must be applied simultaneously with the direct writing technique to produce the desired adherence, viscosity, and reliable performance. This review paper discusses the details concerning the past and recent advancement of the synthesis and characterization of silver nanoparticles and silver ink formation. A review on the advantages of various sintering techniques, which aim to achieve the electrical and mechanical properties of the required printed structure, is also included. A brief summary concerning the recent challenges and improvement approaches is presented at the end of this review.

Recent Developments of Flexible CdTe Solar Cells on Metallic Substrates: Issues and Prospects

This study investigates the key issues in the fabrication of CdTe solar cells on metallic substrates, their trends, and characteristics as well as effects on solar cell performance. Previous research works are reviewed while the successes, potentials, and problems of such technology are highlighted. Flexible solar cells offer several advantages in terms of production, cost, and application over glass-based types. Of all the metals studied as substrates for CdTe solar cells, molybdenum appears the most favorable candidate, while close spaced sublimation (CSS), electrodeposition (ED), magnetic sputtering (MS), and high vacuum thermal evaporation (HVE) have been found to be most common deposition technologies used for CdTe on metal foils. The advantages of these techniques include large grain size (CSS), ease of constituent control (ED), high material incorporation (MS), and low temperature process (MS, HVE, ED). These invert-structured thin film CdTe solar cells, like their superstrate counterparts, suffer from problems of poor ohmic contact at the back electrode. Thus similar strategies are applied to minimize this problem. Despite the challenges faced by flexible structures, efficiencies of up to 13.8% and 7.8% have been achieved in superstrate and substrate cell, respectively. Based on these analyses, new strategies have been proposed for obtaining cheaper, more efficient, and viable flexible CdTe solar cells of the future.