EELS microanalysis of polycrystalline silicon thin films for solar cells grown at low temperatures

The aim of this work is the quantitative chemical analysis of polycrystalline silicon thin films grown on glass substrates at temperatures <600°C by means of transmission electron microscopy (TEM) and electron energy-loss spectrometry (EELS). Specimens produced with two different methods were investigated. We found significant differences in grain size and morphology, as well as in the distribution of oxygen. A surprisingly high amount of Ba diffusion from the subtrate was detected.     

Low temperature polycrystalline silicon: a review on deposition, physical properties and solar cell applications

This review article gives a comprehensive compilation of recent developments in low temperature deposited poly Si films, also known as microcrystalline silicon. Important aspects such as the effect of ions and the frequency of the plasma ignition are discussed in relation to a high deposition rate and the desired crystallinity and structure. The development of various ion energy suppression techniques for plasma enhanced chemical vapour deposition and ion-less depositions such as HWCVD and expanding thermal plasma, and their effect on the material and solar cell efficiencies are described. The recent understanding of several important physical properties, such as the type of electronic defects, structural effects on enhanced optical absorption, electronic transport and impurity incorporation are discussed. For optimum solar cell efficiency, structural considerations and predictions using computer modelling are analysed. A correlation between efficiency and the two most important process parameters, i.e., growth rate and process temperature is carried out. Finally, the application of these poly Si cells in multijunction cell structures and the best efficiencies worldwide by various deposition techniques are discussed.     

Fluorinated carboxylic membranes deposited by plasma enhanced chemical vapour deposition for fuel cell applications

Among the fuel cell technologies, the polymer electrolyte membrane fuel cells (PEMFCs) are particularly promising because they are energy-efficient, clean, and fuel-flexible (i.e., can use hydrogen or methanol). The great majority of PEM fuel cells rely on a polymer electrolyte from the family of perfluorosulfonic acid membranes, nevertheless alternative materials are currently being developed, mainly to offer the alternative workout techniques which are required for the portable energy sources. Plasma polymerization represents a good solution, as it offers the possibility to deposit thin layer with an accurate and homogeneous thickness, even on 3D surfaces. In this paper, we present the results for the growth of proton conductive fluoro carboxylic membranes elaborated by plasma enhanced chemical vapour deposition. These membranes present conductivity values of the same order than the one of Nafion^®. The properties of the membrane, such as the chemical composition, the ionic conductivity, the swelling behaviour and the permeability were correlated to the plasma process parameters. The membranes were integrated in fuel cells on porous substrates and we present here the results regarding the barrier effect and the power output. Barrier effect similar to those of 40 μm Nafion^® layers was reached for 10 μm thick carboxylic membranes. Power outputs around 3 mW cm⁻² were measured. We discuss the results regarding the gas barrier effect and the power outputs.     

Nanocrystalline silicon solar cells from excimer laser crystallization of amorphous silicon

Excimer laser crystallized nanocrystalline silicon layers were used to fabricate solar cells. The laser crystallized layers are characterized with Raman spectroscopy for structural investigations and with atomic force microscopy to study surface modifications upon crystallization. The current–voltage characteristics of the devices completed with aluminium back contacts were investigated with air mass 1.5 G solar simulations. The resulting nanocrystalline solar cells show inferior performance compared with amorphous silicon devices. The degradation of open circuit voltages and short circuit currents with the increase of crystallization energy density is explained to be due to thermal modification of the p-type/intrinsic layer interface and band gap enhancement of fine-grained nanocrystalline layers of the devices along with increased surface and grain boundary recombination of carriers.     

Structural defects caused by a rough substrate and their influence on the performance of hydrogenated nano-crystalline silicon n-i-p solar cells

We present a cross-sectional transmission electron microscopy study of a set of hydrogenated nano-crystalline silicon n-i-p solar cells deposited by hot-wire chemical vapour deposition on Corning glass substrates coated with ZnO-covered Ag layers with various surface roughnesses. Strip-like structural defects (voids and low-density areas) are observed in the silicon layers originating from micro-valleys of Ag grains. A correlation between the opening angles of the textured surface and the appearance of these strips was found. We propose that in order to grow high-quality hydrogenated nano-crystalline silicon absorber layers for solar cell applications, the morphology of the Ag surface is a critical property, and the micro-valleys at the ZnO surface with an opening angle smaller than around 110° should be avoided.     

Germanium-doped Czochralski silicon for photovoltaic applications

Germanium (Ge)-doped Czochralski (GCZ) silicon has been grown for photovoltaic (PV) applications. It is found that Ge doping improves the mechanical strength of CZ silicon, resulting in the reduction of breakage during wafer cutting, cell fabrication and module assembly. Boron-oxygen (B-O) defects that lead to the light-induced degradation (LID) of carrier lifetime are effectively suppressed by Ge doping. The decrease in the maximum concentration of B-O defects increases with an increase of Ge concentration. The efficiency of GCZ silicon solar cells and the power output of corresponding PV modules both exhibit smaller loss under sunlight illumination. The current work suggests that GCZ silicon should be potentially a novel substrate for thin solar cells with low LID effect.     

Microcrystalline silicon films and solar cells deposited by PECVD and HWCVD

The application of microcrystalline silicon (μc-Si:H) in thin-film solar cells is addressed in the present paper. Results of different technologies for the preparation of μc-Si:H are presented, including plasma enhanced chemical vapour deposition (PECVD) using 13.56 MHz (radio frequency, rf) and 94.7 MHz (very high frequency, vhf) and hot-wire chemical vapour deposition (HWCVD). The influence of the silane concentration (SC) on the material and solar cell parameters is studied for the different techniques as the variation of SC allows to optimise the solar cell performance in each deposition regime. The best performance of μc-Si:H solar cells is always observed near the transition to amorphous growth. The highest efficiency obtained so far at a deposition rate of 5 Å/s is 9.4%, achieved with rf-PECVD in a deposition regime of using high pressure and high discharge power. High deposition rates and solar cell efficiencies could be also achieved by vhf-PECVD. An alternative approach represents the HWCVD which also demonstrated high deposition rates for μc-Si:H. However, good material quality and solar cell performance could only be achieved at low substrate temperatures and, consequently, low deposition rates. The μc-Si:H solar cells prepared by HWCVD exhibit comparably high efficiencies up to 9.4% and exceptionally high open circuit voltages up to 600 mV but at lower deposition rates (≈1 Å/s). The properties of PECVD and HWCVD solar cells are carefully compared.     

Conformal thin film silicon photovoltaic modules

A polymer composite material system and a process for encapsulation of thin film solar cells were developed for profiled roofing elements, in view of building-integrated photovoltaic (PV) applications. Amorphous silicon cells were deposited on a polyethylene naphthalate film and encapsulated with additional polymer layers in the form of a flat laminate using industrial processes. The process technology developed in this work included a pre-forming step of the PV laminate and a moulding step of a glass fibre-reinforced polyester composite. These two steps were optimised using thermo-elastic analyses, with attention paid to laminate designs and process windows compatible with the thermo-mechanical limits of the fragile active PV layers. A demonstrator with standard 1.0 m×1.8 m corrugated roofing profile, a weight of 6.3 kg and 60 W of output power was produced. The long-term endurance of the conformal modules was also validated using humidity-freeze, damp-heat and water immersion tests.     

Nanocrystalline Ti-oxide-based solar cells made by sputter deposition and dye sensitization: Efficiency versus film thickness

Nanocrystalline TiO₂-based films with a special penniform microstructure were prepared by reactive DC magnetron sputtering followed by dye sensitization. The films were integrated in a solar cell configuration and were able to yield a higher photocurrent than sol–gel-produced Ti-oxide-based films of similar thickness. The photoelectric conversion efficiency reached 4% in 10-μm-thick sputter deposited films.     

Modification in the chemical bath deposition apparatus, growth and characterization of CdS semiconducting thin films for photovoltaic applications

In this paper, growth and characterization of CdS thin films by Chemical Bath Deposition (CBD) technique using the reaction between CdCl₂, (NH₂)₂CS and NH₃ in an aqueous solution has been reported. The parameters actively involved in the process of deposition have been identified. A commonly available CBD system has been sucessfully modified to obtain the precious control over the pH of the solution at 90°C during the deposition and studies have been made to understand the fundamental parameters like concentrations of the solution, pH and temperature of the solution involved in the chemical bath deposition of CdS. It is confirmed that the pH of the solution plays a vital role in the quality of the CBD–CdS films. Structural, optical and electrical properties have been analysed for the as-deposited and annealed films. XRD studies on the CBD–CdS films reveal that the change in Cadmium ion concentration in the bath results in the change in crystallization from cubic phase with (1 1 1) predominant orientation to a hexagonal phase with (0 0 2) predominant orientation. The structural changes due to varying cadmium ion concentration in the bath affects the optical and electrical properties. Optimum electrical resistivity, band gap and refractive index value are observed for the annealed films deposited from 0.8 M cadmium ion concentration. The films are suitable for solar cell fabrication. Further on, annealing the samples at 350°C in H₂ for 30 min resulted in an increased diffraction intensity as well as shifts in the peak towards lower scattering angles due to enlarged CdS unit cell. This in turn brought about an increase in the lattice parameters and narrowing in the band-gap values. The results are compared with the analysis of previous work.     

ZnO-based nanocrystalline powders with applications in hybrid photovoltaic cells

In recent years there has been a growing interest in the development of hybrid photovoltaic cells consisting of new materials, such as devices based on the combination of a wide gap semiconductor and an organic dye (dye-sensitized solar cells, DSSC). In this paper we obtain nano-zinc oxide particles whose optical and electrical properties have been modified by the presence of small amounts of Al or In acting as dopants. The aim of this study is to improve the compatibility of each of the compounds present in the photovoltaic solar cell. The knowledge gained will provide input to guide the processes in the manufacture of hybrid solar cells.     

Chemical vapour deposited SnO2:Sb heat mirror coatings for cylindrical solar collectors

A chemical vapour deposition technique for growth of SnO₂:Sb heat mirror coatings on the inner walls of long cover glass tubes for cylidrical solar collectors is reported. The best performance of the tin-oxide film is obtained for those films grown from a source of SnCl₂ + 1 mol% Sb on Corning glass tubes at 520°C. These films, supported on 2 mm glass substrates, have a solar transmittance of 0.85 and an infrared reflectance of 0.8. The heat mirror coatings are observed to increase the stagnation temperature of the absorber in anevacuated tubular collector from 142 to 161°C under incident optical flux of 1150 W/m²     

Amorphous silicon alloy photovoltaic technology and applications

The last year has seen enormous advances in the science and technology of amorphous silicon alloy photovoltaic. A new world record in cell efficiency has been achieved; a large volume manufacturing plant is also operational. Innovative building-integrated products have been developed which are aesthetically pleasing and easy to install on the roof. In this paper, we give a brief summary of these developments.     

Mechanical effects of chemical etchings on monocrystalline silicon for photovoltaic use

The mechanical effects of two etching treatments commonly applied on silicon wafers for the PV industry, are considered. The failure characteristics of this material under concentrated load are shown . In both cases, the maximum elongation and sustainable load of the etched wafers were measured to be higher than those of the original sample. The employed experimental procedure and results are presented here and a statistical data analysis substantiates the results observed. An attempt of explanation for this effect is offered based on the removal of a shallow highly defective layer induced by the etching of the material.     

Influence of hydrogen dilution on structural, electrical and optical properties of hydrogenated nanocrystalline silicon (nc-Si:H) thin films prepared by plasma enhanced chemical vapour deposition (PE-CVD)

Hydrogenated nanocrystalline silicon (nc-Si:H) thin films were deposited from pure silane (SiH₄) and hydrogen (H₂) gas mixture by conventional plasma enhanced chemical vapour deposition (PE-CVD) method at low temperature (200 °C) using high rf power. The structural, optical and electrical properties of these films are carefully and systematically investigated as a function of hydrogen dilution of silane (R). Characterization of these films with low angle X-ray diffraction and Raman spectroscopy revealed that the crystallite size in the films tends to decrease and at same time the volume fraction of crystallites increases with increase in R. The Fourier transform infrared (FTIR) spectroscopic analysis showed at low values of R, the hydrogen is predominantly incorporated in the nc-Si:H films in the mono-hydrogen (SiH) bonding configuration. However, with increasing R the hydrogen bonding in nc-Si:H films shifts from mono-hydrogen (SiH) to di-hydrogen (SiH₂) and (SiH₂)_n complexes. The hydrogen content in the nc-Si:H films decreases with increase in R and was found less than 10 at% over the entire studied range of R. On the other hand, the Tauc's optical band gap remains as high as 2 eV or much higher. The quantum size effect may responsible for higher band gap in nc-Si:H films. A correlation between electrical and structural properties has been found. For optimized deposition conditions, nc-Si:H films with crystallite size 7.67 nm having good degree of crystallinity (84% ) and high band gap (2.25 eV) were obtained with a low hydrogen content (6.5 at%). However, for these optimized conditions, the deposition rate was quite small (1.6 Å/s).     

Boron-doped zinc oxide layers grown by metal-organic CVD for silicon heterojunction solar cells applications

In this work, we focus on ZnO:B layers as an alternative TCO for application on a-Si:H/c-Si heterojunction solar cells. First, the optimization of the material has been done in terms of optical and electrical properties. We have also studied the behaviour of ZnO:B against ageing. Finally, complete heterojunction solar cells have been fabricated with different back-side TCO configurations to understand the ageing mechanisms and to deeply study the influence of degradation on the solar cells parameters. Stable efficiencies up to 16.6% on polished n-type c-Si were obtained on 25 cm² heterojunction solar cells fabricated at low temperatures.     

Electrical and optical properties of F-doped textured SnO2 films deposited by APCVD

This paper presents the structural, electrical and optical properties of transparent conducting F-doped textured SnO₂ films prepared by atmosphere pressure chemical vapour deposition (APCVD). Polycrystalline SnO₂:F films having a variable preferred orientation have been obtained with resistivity as low as 5 × 10⁻⁴ Ωcm, with carrier concentrations between 3.5 × 10²⁰ and 7 × 10²⁰ cm⁻³, and Hall mobilities from 15.7 to 20.1 cm²/V/s. The average transmittance (including diffusion transmittance) is as high as 94% in the wavelength range of the visible spectrum and the maximum infrared reflectance reaches 92% for a film 655 nm thick. The figure of merit ƒ_TC = T10/_sh, (7.12 × 10⁻² S) of these films is the highest amongst the results reported on doped SnO₂ films.     

Fabrication of thin film nanocrystalline silicon solar cell with low light-induced degradation

Nanocrystalline silicon thin films have been deposited at different total gas flow rates and plasma excitation frequencies and samples with similar crystalline volume fraction have been compared. In hydrogenated nanocrystalline silicon solar cells, amorphous component is not necessarily the only determining factor for light-induced degradation. Smaller grain size less than 3 nm diameter and intermediate range order provide a better stability in the i-layer near the p/i interface, thus improving the overall stability of the solar cell. Light-induced degradation (LID) of efficiency of the cell mainly depends on the light-induced degradation of short-circuit current density and light-induced degradation of fill factor (FF). Degradation of open-circuit voltage is less than 1%. Minimum degradation of efficiency obtained in this work is 2%.     

All ambient environment‐based perovskite film fabrication for photovoltaic applications

Low‐temperature solution process‐able perovskite solar cells are highly desirable for future photovoltaics. Chemical root was utilized to synthesize and optimize mixed halide‐based MAPbIBr₂ light absorber perovskites on electron transport layer of TiO₂ nanoparticles in ambient atmosphere. For the first time all synthesis work was performed in an ambient environment and observe material behavioral characteristics. To accurately control the film morphology, one‐step deposition technique was applied to investigate material's optoelectronic behavior. The role of the perovskite structure, physical, and optical properties in planner device architecture was studied through ultraviolet visible, X‐ray diffraction, X‐ray photoelectron spectroscopy, and scanning electron microscope characterization techniques to confirm a band gap of 1.76 eV with cubic crystalline structure having a particle size of 12.5–13.0 nm, which is highly suitable for perovskite solar cells.