Processing options for CdTe thin film solar cells

Processing options for addressing critical issues associated with the fabrication of thin film CdTe solar cells are presented, including window and buffer layer processing, post-deposition treatment, and formation of stable low resistance contacts. The paper contains fundamental data, engineering relationships and device results. Chemical surface deposited CdS and Cd_1−xZn_xS films are employed as the n-type heteropartner window layers. Maintaining junction quality with ultra-thin window layers is facilitated by use of a high resistance oxide buffer layer, such as SnO₂, In₂O₃ or Ga₂O₃, between the heteropartner and the transparent conductive oxide. Thermal annealing of the CdTe/CdS heterostructure in the presence of CdCl₂ and O₂ shifts the chemical equilibrium on the surface of the absorber layer, which influences the bulk electrical properties. Aspects of back contacting CdTe/CdS devices, including etching, Cu application, contact annealing, back contact chemistry and secondary contacts, are discussed. Two commonly employed etches used to produce a Te-rich layer, nitric acid/phosphoric acid mixtures and Br₂/methanol are compared, including the nature and stability of the final treated CdTe surface. The diagnostic abilities of the surface sensitive VASE and GIXRD techniques are highlighted. Various methods of Cu delivery are discussed with consideration to; reaction with Te, processing simplicity, processing time and possible industrial scale-up. Some aspects of back contact stability are presented, including discussion of apparent robust back contacts, which contain a thick Te component.     

Cadmium flows and emissions from CdTe PV: future expectations

Cadmium telluride photovoltaic (CdTe PV) technology is growing rapidly, and already represents the largest contributor to non-silicon based photovoltaics worldwide. We assessed the extent to which CdTe PV will play a notable role in the Cd use and emission flows in the future, and whether it will be environmentally beneficial or detrimental. Our results show that while CdTe PV may account for a large percentage of future global Cd demand, its role in terms of Cd sequestration may be beneficial. We calculated that its potential contribution to yearly global Cd emissions to air and water may well be orders-of-magnitude lower than the respective current Cd emissions rates in Europe.     

pin double-heterojunction thin-film solar cell p-layer assessment

The simplest realization of a pin double-heterojunction thin-film solar cell would consist of a lightly doped, moderate-bandgap absorber i-layer; a heavily doped, wide-bandgap n-layer window (cathode); and a heavily doped, wide-bandgap p-layer window (anode) in which the anode and cathode are electrically contacted by at least one transparent conductor. The focus herein is on p-layer interfacial assessment, which is accomplished using modern Schottky barrier and heterojunction theory and is directed to the analysis of p-windows for copper indium gallium diselenide (CIGS) and cadmium telluride (CdTe) thin-film solar cells. A p-type window layer serves as an electron reflector and also aids in the formation of an ohmic anode contact. Ohmic anode contacts are particularly difficult to form in CIGS and CdTe thin-film solar cells since these materials have very large ionization potentials, i.e., IP_S=5.65 (CIGS) and 5.78 V (CdTe) and significant interfacial screening, characterized by extremely small Schottky barrier interface parameters, i.e., S=0.14 (CIGS) and 0.21 (CdTe). An ideal p-type window material would be heavily doped, p-type, and would have a wide bandgap, a large ionization potential, and a smaller charge neutrality level energy than that of the absorber layer.     

Regulatory policy governing cadmium-telluride photovoltaics: A case study contrasting life cycle management with the precautionary principle

Market projections for cadmium-telluride (CdTe) thin-film photovoltaics (PV) are tempered by global environmental policies based on the precautionary principle which restrict electronic products containing cadmium, a known human carcinogen. An alternative to the precautionary principle is life cycle management, which involves manufacturers assuming product stewardship from beginning to end of product life. Both approaches have the aim of minimizing environmental contamination, but attempt to do so in different ways. Restrictions on electronic products containing cadmium by the precautionary principle-based restriction of hazardous substances (RoHS) directive in the European Union and a similar policy in China are presented, relative to their potential impact on CdTe PV. Life cycle environmental risks with respect to potential release of cadmium to the environment are also presented for routine operation of CdTe PV panels, potential catastrophic release of cadmium from a residential fire, and at the end of the product life. There is negligible risk of environmental cadmium contamination during routine operation and insignificant risk during catastrophic exposure events such as fire. At the end of the product life, risks of contamination are minimized by take-back programs that may be paid for by insurance premiums incorporated into the cost of the product. Therefore, policies based on the precautionary principle that could potentially ban the product based on its cadmium content may not be warranted.     

Electro-optical characterization and modeling of thin film CdS–CdTe heterojunction solar cells

Optoelectronic characteristics of thin film CdTe–CdS solar cells fabricated at four different laboratories were measured and analyzed. Current versus voltage measurements revealed that, under one sun illumination, tunneling was the dominant current flow mechanism in all cells. Tunneling was also the dominant current flow mechanism in the dark for all types except P3 which exhibited a generation-recombination type current flow process in the dark. A theoretical model involving bulk traps in CdTe and a charged thin layer (T-layer) near the junction under forward bias and/or illumination was developed. The model is able to explain all significant features in the experimental results obtained from current versus voltage, and capacitance.     

HIGH EFFICIENCY THIN FILM CdS/CdTe SOLAR CELLS†

Cadmium telluride is a most promising thin film photovoltaic material as shown by the higher than 10% efficient CdS/CdTe heterojunction solar cells. In this work, thin film CdS/CdTe solar cells have been prepared using CdS films grown from an aqueous solution and p-CdTe films deposited by metalorganic chemical vapor deposition (MOCVD) and close-spaced sublimation (CSS). The solution-grown CdS films, with high photoconductivity ratios and good optical transmission, are well-suited for optoelectronic devices. The CdTe films deposited by CSS show considerably better microstructure than those by MOCVD because of the higher substrate temperature used in the CSS process. The properties of CdS/CdTe heterojunctions have been studied. Solar cells of 1 cm2 with an AM 1.5 efficiency of 14.6%, verified at the National Renewable Energy Laboratory, have been CdTe films     

Electrical and optical properties of CdTe films prepared by vacuum evaporation with close spacing between source and substrate

CdTe films for solar cell application were prepared at various growth temperatures by a vacuum evaporation system having close spacing between the source and substrate. The CdTe thin films had a cubic structure highly oriented with the (1 1 1) direction perpendicular to the substrate surface, regardless of the growth temperature. The crystallites are of random shape and reach up to about 2 μm in size with increasing growth temperature. The higher growth temperature contributed to the reduction of dark resistivity from 6×10⁷ Ω cm at room temperature to 5.4×10⁶ Ω cm at 300°C. The photovoltaic properties of the CdS/CdTe solar cell were considerably improved with the increase in the growth temperature, which was caused by the increase of (1 1 1) texture and grain size in CdTe films.     

LASER-DRIVEN PHYSICAL VAPOR DEPOSITION FOR THIN-FILM CdTe SOLAR CELLS

We review the use of laser-driven physical vapor deposition from solid targets for the growth of CdTe-related photovoltaic materials on glass substrates. The physical conditions in the laser-driven plume from 15 nsec pulses at λ = 308 nm from an XeCI laser have been studied by optical multichannel spectroscopy and time-of-flight methods. Film growth has been optimized in terms of substrate temperature, laser pulse energy density, and overall growth rate through the use of optical absorption, Raman scattering, x-ray diffraction, electron microscopy, energy dispersive x-ray spectroscopy, and electrical measurements on the films. Early results of device fabrication have yielded an all-LDPVD CdS/CdTe cell of 8.7% AM 1.5 efficiency.     

Electrodeposition of fluorine-doped cadmium telluride for application in photovoltaic device fabrication

Electrodeposition (ED) technique has been used to deposit thin-film cadmium telluride (CdTe) on glass-/fluorine-doped tin oxide substrates using a two-electrode system from acidic, aqueous medium containing Cd(NO₃)₂ 4H₂O, TeO₂ and CdF₂ as dopants. Photoelectrochemical cell measurement, DC conductivity measurement, X-ray diffraction, optical absorption, thickness measurement and atomic force microscopy were carried out to study the electrical, structural, optical, compositional and morphological properties of the as-grown, heat-treated and CdCl₂-treated ED-CdTe layers, respectively. The characterization data obtained signified 1370 mV as the best growth potential and CdCl₂ treatment at 400 °C for 15 min as the best post-growth treatment to achieve an optimal result.     

Boosting performance of CdTe photocathode with pulse-reverse electrodeposited nickel telluride-nickel phosphide dual co-catalysts

Herein, noble-metal free Ni–Te/Ni–P dual co-catalysts are investigated as attractive candidates for improving the photoelectrochemical (PEC) performance of CdTe photocathodes. The Ni–Te/Ni–P dual co-catalysts are deposited on CdTe photocathodes via pulse-reverse electrodeposition and post annealing. The Ni–Te/Ni–P catalyzed CdTe photocathodes show a remarkable PEC performance, especially in terms of the photocurrent density, which is approximately 11.06 times higher than that of the pristine CdTe photocathode. The Ni–Te/Ni–P dual co-catalysts boost the catalytic activity via enhanced charge transfer and surface reaction kinetics. The photogenerated electrons and holes are effectively separated by the Schottky contact between the Ni–Te/Ni–P dual co-catalysts and the CdTe absorbing layer. Thus, the rational design of the CdTe photocathode with the Ni–Te/Ni–P dual co-catalysts can dramatically enhance the PEC performance of the CdTe photocathodes.     

Zone melting recrystallization of silicon films for crystalline silicon thin-film solar cells

Coarse-grained silicon films for crystalline silicon thin-film solar cells have been prepared by zone melting recrystallization. A zone melting heater was modified to obtain better temperature homogeneity of the sample and higher reproducibility of the melt process. Various substrate materials of different purity and surface roughness have been tested concerning their suitability for, silicon deposition, zone melting and solar cell process. Solar cell efficiencies up to 10.5% could be achieved on silicon sheets from powder, capped by an intermediate layer. Silicon films on SiAlON ceramics were successfully processed to solar cells by a completely dry solar cell process.     

Study of the electrical and junction properties of ITO thin films deposited on CdTe and InP substrates

Thin films of indium tin oxide (ITO) have been deposited on fused quartz substrates by the spray deposition and rf-magnetron sputtering methods and their optical and electrical properties investigated. The junction properties of fabricated ITO/pCdTe and ITO/pInP structures have also been studied and their electrical properties presented. It is established that the deposited ITO films have a lattice constant of 10.14 Å and grow in cubic modifications along the (400) direction. Their transmittance value rises up to between 85% and 90% and extends well beyond the visible range of the spectrum. In the forward direction of the ITO/pCdTe structure, it is suggested that tunnelling dominated processes determine the current flow mechanism. Recombination currents at the interface region and thermionic-emission currents, however, dominate in the ITO/pInP structure at low bias and high bias respectively. The two structures can best be described as heterojunctions.     

Photovoltaic characteristics of silicon nanowire arrays synthesized by vapor-liquid-solid process

Silicon nanowire (SiNW) arrays were grown directly on a P type Si substrate, pre-deposited with gold catalyst, and then made into solar cell for photovoltaic characteristic measurement. Different growth conditions of SiNWs, including variation of the flow rate ratio of SiH₄ versus N₂, and the thickness of Au film, which can be sputtered into different size of nanoparticles, will be made in order to obtain an optimum photovoltaic conversion efficiency. The morphologies and crystalline structure of the nanowires are studied by SEM, TEM and XRD. The SiNW array surface is shown to have good antireflection property, and is expected to raise light absorption and short circuit current. The photovoltaic performance of the solar cells with SiNWs grown at different conditions is measured and discussed. More effort is still needed to raise the performance of SiNW solar cells.     

Spray deposition of CdTe–Te thin films using ethylene-diamine-tetra-acetic acid as a complexing agent in the precursor solution

CdTe thin films have been deposited by spray pyrolysis technique using varying concentration of Ethylene-diamine-tetra-acetic acid (EDTA) as a complexing agent (CA) in the precursor solution. Scanning electron micrographs show the films deposited with EDTA have a more homogeneous distribution of fine grains. X-ray diffraction patterns show that the films consist of a mixture of CdTe and Te, the Te peaks intensity increasing with EDTA concentration. X-ray photoelectron spectroscopy study of the surface reveals that there is a change-over in chemical environment of Te from −2 to +4 state after using CA. A broad photoluminescence peak at 1.40 eV is observed in the films without CA, which can be resolved into two peaks 1.43 and 1.40 eV. The intensities of both peaks are increased with EDTA due to formation of elemental Te and Cd vacancies. Optical study indicates that an increment in optical reflection due to Te metallic cluster formation in the film. Thus, the stoichiometry of the CdTe films can be controlled by varying EDTA concentration.     

Photovoltaic properties of CdTe-Cu2Te

The photovoltaic output of two types of CdTe - Cu₂Te solar cells has been investigated. Type A devices are fabricated on a CdTe single crystal substrate etched by 2% bromine in methanol in the presence of an excimer laser with an intensity of 30 mW/cm² as an illumination source during the etching process, while type B devices are fabricated on CdTe single crystal substitute etched by 2% bromine in methanol only. The current - voltage characteristics and capacitance-voltage characteristics of these two groups of devices are demonstrated to be strongly influenced by the process of etching. The electrical properties such as open circuit voltage, short circuit current, fill factor, efficiency, diffusion voltage, carrier concentration, barrier height and ideality factor of the CdTe - Cu₂Te heterojunction are calculated for both types of devices.     

Surface textured MF-sputtered ZnO films for microcrystalline silicon-based thin-film solar cells

Highly conductive and transparent aluminum-doped zinc oxide (ZnO:Al) films were prepared by reactive mid-frequency (MF) magnetron sputtering at high growth rates. By varying the deposition pressure, pronounced differences with respect to film structure and wet chemical etching behavior were obtained. Optimized films develop good light-scattering properties upon etching leading to high efficiencies when applied to amorphous (a-Si:H) and microcrystalline (μc-Si:H) silicon-based thin-film solar cells and modules. Initial efficiencies of 7.5% for a μc-Si:H single junction and 9.7% for an a-Si:H/μc-Si:H tandem module were achieved on an aperture area of 64 cm².     

Photovoltaic cells based on cadmium sulphide–phthalocyanine heterojunction

Hybrid photovoltaic (PV) cells based on cadmium sulphide (CdS) single crystal and phthalocyanine (Pc) films have been developed and their PV performance was measured. Five different Pcs have been selected as candidates for the PV cell, PcCu, PcMn, PcZn, PcMg, and PcVO. It was found that all the chosen Pcs are capable of forming a hybrid heterojunction with the CdS surface, and that illumination results in charge separation at the interface. However, the performance of the In/CdS/Pc/Au device was dependent on the Pc used. PV cells with PcMg and PcZn showed the best results. An unoptimized cell with the PcZn film showed an open-circuit voltage V_oc=0.595 V, a short-circuit current density J_sc=1.88 μA/cm², a fill factor FF=0.265, and a power conversion efficiency PCE=3.0×10⁻⁴% under the AM1.5 conditions.     

Light and voltage dependence of the junction transport properties of CdTe/CdS photovoltaics

The J–V curve of CdTe/CdS photovoltaics does not consist of a simple superposition of a loss current and a light generated current with a considerable loss in conversion efficiency. This paper uses capacitance/voltage measurements and J–V measurements at a variety of temperatures and light levels to develop a model for this non-superposition. It was found that a light dependent tunneling mechanism dominates at low voltages. Moreover, the tunneling takes place from a trap level within the CdTe.     

A new technique for large-area thin film CdS/CdTe solar cells

The atmospheric pressure CSS method has been developed as a reproducible and efficient process. Thin film CdTe grown under atmospheric pressure has a rough surface morphology. The density of carbon black powder in the graphite carbon paste for screen printing is a key factor in reducing the series resistance of the device with rough surface CdTe. Using graphite carbon paste with 7 wt% carbon black powder has resulted in cells with a relatively low back contact resistance. A highly efficient large-area CdS/CdTe solar cell (11.0%, 5327 cm²) sub-module has been fabricated using the new technique.