Amorphous silicon thin film solar cells deposited entirely by hot-wire chemical vapour deposition at low temperature (< 150 °C)

Amorphous silicon n-i-p solar cells have been fabricated entirely by Hot-Wire Chemical Vapour Deposition (HW-CVD) at low process temperature < 150 °C. A textured-Ag/ZnO back reflector deposited on Corning 1737F by rf magnetron sputtering was used as the substrate. Doped layers with very good conductivity and a very less defective intrinsic a-Si:H layer were used for the cell fabrication. A double n-layer (µc-Si:H/a-Si:H) and µc-Si:H p-layer were used for the cell. In this paper, we report the characterization of these layers and the integration of these layers in a solar cell fabricated at low temperature. An initial efficiency of 4.62% has been achieved for the n-i-p cell deposited at temperatures below 150 °C over glass/Ag/ZnO textured back reflector.     

Optical properties of bismuth niobate thin films studied by spectroscopic ellipsometry

We performed optical analysis of bismuth niobate thin films using spectroscopic ellipsometry (SE). The films were grown on Pt/Ti/SiO₂/Si substrates with pulsed laser deposition. Six films were prepared using various deposition temperatures and thermal-annealing times. The room-temperature SE spectra of these films were measured by a rotating-analyzer ellipsometer from 1.12 to 6.52 eV at incidence angles of 50, 55, 60, 65, and 70°. The resulting refractive indices and extinction coefficients show significant changes with deposition temperature and thermal annealing.     

Defect analysis of thin film Si-based alloys deposited by hot-wire CVD using junction capacitance methods

We evaluate and compare the electronic properties of hot-wire CVD deposited a-Si:H and a-Si,Ge:H films with those produced by the glow discharge (PECVD) method. A good indicator of film quality with respect to solar cell applications is the narrowness of the band tail widths determined by transient photocapacitance (TPC) spectroscopy. We focus on the excellent electronic properties of hot-wire CVD a-Si,Ge:H alloys that have recently been produced by a 1800  °C filament temperature process. These alloy samples were compared to a-Si,Ge:H films of the same optical gaps deposited by PECVD. Light-induced degradation was examined in a few samples and compared to the behavior PECVD a-Si,Ge:H alloys of similar optical gap. The effects of intentional oxygen contamination were also studied on a series of HWCVD a-Si,Ge:H samples containing 29at.% Ge.     

Studying early time HWCVD growth of a-Si:H by real time spectroscopic ellipsometry

We have applied real time spectroscopic ellipsometry and secondary ion mass spectrometry to study the growth of amorphous silicon by hot-wire chemical vapor deposition. Differences in temperature and hydrogen content affect the optical properties of the film. These effects provide valuable insight into the growth process. We have compared a-Si:H films grown at two different temperatures to better understand these effects. Our studies reveal the presence of a distinct 100–200-thick layer at the top of the growing film. The properties of this layer are primarily determined by the ambient conditions in the growth chamber and appear relatively independent of substrate temperature. In contrast, the properties of the bulk of the film are strongly influenced by substrate temperature. These results imply that differences in film properties associated with substrate temperature are the result of subsurface reconstruction and diffusion processes.     

Small-angle neutron scattering studies of hot-wire CVD a-Si:H

Several a-Si:H and a-Si:D films prepared by hot-wire chemical vapor deposition have been examined by small-angle neutron scattering (SANS) to search for H non-uniformity in this material. The SANS measurements were supplemented by small-angle X-ray scattering measurements. The differences in H/D detection sensitivity of these two techniques allow distinction of the scattering mechanisms. Two- or three-phase models are used to interpret the results quantitatively. Significant H non-uniformity, as well as a small fraction of microvoids, was found in the best-quality material. Samples grown with higher deposition rates or lower substrate temperatures have much larger void fractions. The size scale of the heterogeneity spans a range from 2 nm to more than 50 nm, with the largest features assigned to surface roughness.     

Hot-wire deposition of amorphous and microcrystalline silicon using different gas excitations by a coiled filament

Microcrystalline silicon (μc-Si:H) and amorphous silicon (a-Si:H) films were deposited using a hot-wire CVD (HWCVD) system that employs a coiled filament. Process gasses, H₂ and Si₂H₆, could be directed into the deposition chamber via different gas inlets, either through a coiled filament for efficient dissociation or into the chamber away from the filament, but near the substrates. We found that at low deposition pressure (e.g. 20 mTorr) the structure of the films depends on the way gases are introduced into the hot-wire chamber. However, at higher pressure (e.g. 50 mTorr), Raman measurement shows similar results for films deposited with different gas inlets.     

Comparison between newly developed and classical determinations of index and thickness of thin films on substrates by ellipsometry

In the well-known classical determination of the index and thickness of thin transparent films formed on known substrates from the ellipsometric data, the film thickness is computed from its index value which is found first, independently of the former. However, these parameters of very thin films well under 100 Å may not be accurately thus determined in the presence of experimental angular errors particularly that in the angle of incidence. In this paper, not only the film parameters but also the angle of incidence are found using a new method from the ellipsometric data measured on two film-absorbing substrate reflecting system formed with the same transparent film but up to different thicknesses. The parameters obtained and their uncertainties estimated from the two methods are compared and consequently the limits of application of the methods are defined. The new method shows resistance to the propagation of angular errors to the parameters sought. Furthermore, a correlation detectedn between the error in the substrate real index and the uncertainty in the angle of incidence computed allows one to correct one of these by making use of the angle of incidence measured. These results are illustrated by applying the method presented to recently published experimental data.     

ZnO:Al films deposited by in-line reactive AC magnetron sputtering for a-Si:H thin film solar cells

Throughout the last years strong efforts have been made to use aluminium doped zinc oxide (ZnO:Al) films on glass as substrates for amorphous or amorphous/microcrystalline silicon solar cells. The material promises better performance at low cost especially because ZnO:Al can be roughened in order to enhance the light scattering into the cell. Best optical and electrical properties are usually achieved by RF sputtering of ceramic targets. For this process deposition rates are low and the costs are comparatively high. Reactive sputtering from metallic Zn/Al compound targets offers higher rates and a comparable high film quality in respect to transmission and conductivity. In the presented work the process has been optimised to lead to high quality films as shown by reproducible cell efficiencies of around 9% initial for single junction amorphous silicon solar cells on commercial glass substrates. The crucial point for achieving high efficiencies is to know the dependency of the surface structure after the roughening step, which is usually performed in a wet etch, on the deposition parameters like oxygen partial pressure, aluminium content of the targets and temperature. The most important insights are discussed and the process of optimisation is presented.     

Piezoresistive properties of nanocrystalline silicon thin films deposited on plastic substrates by hot-wire chemical vapor deposition

The piezoresistive property of n-type and p-type nanocrystalline silicon thin films deposited on plastic (PEN) at a substrate temperature of 150 °C by hot-wire chemical vapor deposition, is studied. The crystalline fraction decreased from 80% to 65% in p-type and from 84% to 62% in n-type films, as the dopant gas-to-silane flow rate ratio was increased from 0.18% to 3-3.5%. N-type films have negative gauge factor (− 11 to − 16) and p-type films have positive gauge factor (9 to 25). In n-type films the higher gauge factors (in absolute value) were obtained by increasing the doping level whereas in p-type films higher gauge factors were obtained by increasing the crystalline fraction.     

Film stoichiometry and gas dissociation kinetics in hot-wire chemical vapor deposition of a-SiGe:H

The gas phase dissociation rates of silane and germane are measured for HWCVD on a tantalum filament and compared to the a-SiGe:H film composition. The Ge from dissociated germane is converted entirely into film on the substrate and chamber walls. Approximately 85% of the Si from the dissociated silane is converted into film, with the rest incorporated into the filament. The dissociation rate per unit partial pressure of germane is 2-3 times that of silane. The pressure dependence of feed gas depletion rates suggests that the dissociation on the filament is rate limited by filament reactive site availability.     

Some physical properties of Sn-doped CdO thin films prepared by chemical bath deposition

Undoped and Sn-doped CdO thin films were prepared by the chemical bath deposition method by means of a procedure that improves the deposition efficiency. All as-grown films were crystallized in the cubic structure of cadmium peroxide (CdO₂) and transformed into CdO with a cubic structure after an annealing process. The as-grown films have a high resistivity (> 10⁶ Ω cm) and an optical bandgap around 3.6 eV. Undoped CdO displays an optical bandgap around 2.32–2.54 eV and has an electrical conductivity of 8 × 10^− 4 Ω cm. The Sn incorporation into CdO produces a blue shift in the optical bandgap (from 2.55 to 2.84 eV) and a decrease in the electrical conductivity.The deposition procedure described here gives colloid-free surface thin films as indicated by the surface morphology analysis.     

Deposition and structural characterization of poly-Si thin films on Al-coated glass substrates using hot-wire chemical vapor deposition

The growth of polycrystalline Si films onto Al-coated Corning 7059 glass substrates using hot-wire chemical vapor deposition (HW-CVD) was investigated. The crystalline fraction, grain structure and average grain size of the films were compared as a function of the growth rate and the Si/Al thickness ratio. Micrometre-size Si grains were achieved with a Si/Al ratio of 2 and Si thickness of 2 μm at a growth rate of 1 μm h⁻¹. It was found that the films had a bimodal grain size distribution, which included nanocrystalline Si, and that the growth of micrometre-size crystallites does not continue as the thickness of Si film increases. At a growth rate of 5 μm h⁻¹, films are similar to those grown on glass with an average grain size less than 60 nm and crystalline fraction of 75%.     

Mechanical properties of a-C:H and a-C:H/SiOx nanocomposite thin films prepared by ion-assisted plasma-enhanced chemical vapor deposition

a-C:H and a-C:H/SiO_x nanocomposite thin films were deposited on silicon, aluminum and polyimide substrates at 25 °C in an asymmetric 13.56 MHz r.f.-driven plasma reactor under heavy ion bombardment. Fourier transform infrared spectra of the films indicate that the nanocomposite filmsappears to consist of an atomic scale random network of a-C:H and SiO_x. Raman spectroscopy revealed that the sp² carbon fraction in the nanocomposite film was reduced compared with the a-C:H film. The intrinsic stress of both films increased with increasing negative bias voltage (−V_dc) at the substrate. However, the nanocomposite films exhibited lower intrinsic stress compared w with a-C:H-only films. Especially, a thin SiO_x-rich interlayer was very effective in reducing the film stress and enhancing the bonding strength at the interface. The interlayer allowed deposition of thick films of up to 5 μm. Also, the nanocomposite films were stable in 0.1 M NaOH solution and showed good microhardness.     

Tungsten filament effect on electronic properties of hot-wire CVD silicon films for heterojunction solar cell application

In this study, we describe the correlation between cell efficiency and wire aging during hot-wire chemical vapor deposition in detail. The new and aged tungsten (W) filaments were used to deposit the n-type microcrystalline silicon (μc-Si) films for heterojunction (HJ) Si solar cell applications. Tungsten silicide (WSi_x) was coated on the W catalyzer surface (center and end regions) after each deposition, and which was investigated and determined by scanning electron microscopy and electron probe microanalysis. The wire age has an effect on the resulting electronic properties of the grown film, thought to be related to differences in dark conductivity with aged versus new wires. It was found that the aging process is related to the formation of a silicide at the surface. A limited amount of silicon was observed in the bulk of catalyzer, suggesting that silicon diffusion into the wire has occurred. The original single-side HJ solar cell with efficiency of 15.3% has been fabricated using the new wires. The quality of n-type μc-Si films and efficiency of HJ solar cells were reduced when the aged W filament was employed. The quality of silicon films and the efficiency of HJ solar cell could be improved after regeneration process.     

Determination of band offsets in a-Si:H/c-Si heterojunctions from capacitance-voltage measurements: Capabilities and limits

The capabilities and limitations of the well-known C-V technique for the determination of the conduction band offsets in (n)a-Si:H/(p)c-Si heterojunctions are presented. In particular, the effects due to the presence of an inversion layer in c-Si and a non-negligible defect density at the a-Si:H/c-Si interface on the reliability of the C-V intercept method are discussed. The influence of the Fermi level positions in (p)c-Si and (n)a-Si:H on the inversion layer formation and the influence of the interface defect density have been analysed using numerical simulations and experimental measurements.     

Silicon surface passivation by hot-wire CVD Si thin films studied by in situ surface spectroscopy

Silicon thin films can provide an excellent surface passivation of crystalline silicon (c-Si) which is of importance for high efficiency heterojunction solar cells or diffused emitter solar cells with well-passivated rear surfaces. Hot-wire chemical vapor deposition (hot-wire CVD) is an attractive method to synthesize Si thin films for these applications as the method is ion-bombardment free yielding good quality films over a wide range of deposition rates. The properties of the interface between hot-wire CVD Si thin films and H-terminated c-Si substrates have been studied during film growth by three complementary in situ techniques. Spectroscopic ellipsometry has been used to determine the optical properties and thickness of the films, whereas information on the H-bonding modes and H-depth profile has been obtained by attenuated total reflection infrared spectroscopy. Second-harmonic generation (SHG), a nonlinear optical technique sensitive to surface and interface states, has been used to probe two-photon resonances related to modified Si-Si bonds at the interface. By correlating the observations with ex situ lifetime spectroscopy experiments the growth and surface passivation mechanism of the Si films are discussed.     

Photoconducting a-Si:H films grown at high deposition rates by pulsing a VHF 100 MHz discharge

In a novel experiment hydrogenated amorphous silicon films were deposited by modulating the very high frequency (VHF) (100 MHz) discharges, at low frequency (2 Hz) with a nonzero low power level, using pure as well as 25% hydrogen and 25% helium diluted silane as the source gases. During these studies deposition rate is found to depend on the dwell time as in the case of RF pulsed plasma CVD reported earlier by the authors. The films were characterised for optical bandgap, dark and photoconductivity, hydrogen content, microstructure factor, Urbach energy and defect density. The results indicate that, unlike the RF pulsed plasma case, there is an order of magnitude improvement in the photoconductivity of the material due to pulsing the VHF discharges. Urbach energy and defect density studies also indicate an improvement in the film quality. The improvements are more pronounced in diluted silane deposited films. Controlled ion bombardment (of high flux and lower energy) and the resulting ion bombardment induced preparation of the growth surface in the VHF discharges are believed to be the main factors contributing to the observed results. Thus, a more favourable sheath characteristics as obtained during pulsed VHF discharge conditions over RF (13.56 MHz). Silane discharges holds the key to obtain high growth rate deposition of a-Si:H films of acceptable opto-electronic quality     

Monte Carlo simulations on large-area deposition of amorphous silicon by hot-wire CVD

Scale-up of hot-wire CVD reactors for commercial production of a-Si:H based solar cells requires understanding of the large-area deposition process. Therefore, the process was simulated using the Direct Simulation Monte Carlo-method (G.A. Bird, Clarendon Press, Oxford (1994)), considering reactions at the filaments, in the gas phase and at the substrate, and in particular large-area deposition by modeling the gas shower and the filament grid, which were found to determine the uniformity and quality of the a-Si:H films (Thin Solid Films 395 (2001) 61; Solar Energy Mater. Solar Cells 73 (2002) 321). The distance between the filament grid and the substrate (d_fil–S) and the distance between the filaments (d_fil) were systematically varied, and the simulation results were compared to experimental results obtained in our large-area deposition system (Thin Solid Films 395 (2001) 61; Solar Energy Mater. Solar Cells 73 (2002) 321). The experimentally obtained optimum filament-to-substrate distance was supported by an optima in the simulated Si₂H₄-concentration. For other species, the existence was confirmed but a definite value for optimum d_fil–S could not be concluded. The simulations also confirmed the influence of the filament geometry on the uniformity as obtained in the experiments.     

Hot-wire chemical vapor deposition and characterization of p-type nanocrystalline Si films for thin film photovoltaic applications

P-type nanocrystalline Si (p-nc-Si) films were deposited by hot-wire chemical vapor deposition (HWCVD) system using SiH₄, B₂H₆, and H₂ as reactants. The effect of H₂ flow rate on the material properties of p-nc-Si films were investigated using Raman spectroscopy, X-ray diffractormeter, ultraviolet-visible-near infrared spectrophotometer, Fourier transform infrared spectroscopy, field emission scanning electron microscopy (FESEM), and transmission electron microscopy (TEM). Moreover, the electrical properties, such as carrier concentration, activation energy, dark conductivity, and Hall mobility, of p-nc-Si films were also measured. It was found that H₂ flow rate played an important role in forming of p-nc-Si, decreasing the deposition rate, and increasing the crystallinity of p-nc-Si films. FESEM and TEM micrographs also showed the enhancement of crystallinity with adding H₂ flow rate. Furthermore, the change of microstructure at various H₂ flow rates was found to affect the electrical properties of p-nc-Si films. Details of the growth mechanism in p-nc-Si films will be discussed also. Moreover, the optimum p-nc-Si film was used as window layer in n-type crystalline Si heterojunction (HJ) solar cell. After the deposition parameters were optimized, the Si HJ solar cell with the open-circuit voltage of 0.58 V, short-circuit current density of 33.46 mA/cm², fill factor of 64.44%, and the conversion efficiency of 12.5% could be obtained.     

Single layer and multilayered films of plasma polymers analyzed by nanoindentation and spectroscopic ellipsometry

Well-defined single layer and multilayered a-SiC:H films, deposited from tetravinylsilane at different powers by plasma-enhanced chemical vapor deposition on silicon, were intensively studied by in situ spectroscopic ellipsometry, nanoindentation, and atomic force microscopy. A realistic model of the sample structure was used to analyze ellipsometric data and distinguish individual layers in the multilayered film, evaluate their thickness and optical constants. Dispersion dependences for the refractive index were well separated for each type of individual layer, if the thickness was decreased from 315 to 25 nm, and corresponded to those of the single layer. A beveled section of the multilayered film revealed the individual layers that were investigated by atomic force microscopy and nanoindentation to confirm that mechanical properties in multilayered and single layer films are similar.