We have used small-angle x-ray scattering (SAXS) in conjunction with X-ray diffraction (XRD) to study the nanostructure of hydrogenated nanocrystalline silicon (nc-Si:H). The crystallite size in the growth direction, as deduced from XRD data, is 24 nm with a preferred [220] orientation in the growth direction of the film. Fitting the SAXS intensity shows that the scattering derives from electron density fluctuations of both voids in the amorphous phase and H-rich clusters in the film, probably at the crystallite interfaces. The SAXS results indicate ellipsoidal shaped crystallites about 6 nm in size perpendicular to the growth direction. We annealed the samples, stepwise, and then measured the SAXS and ESR. At temperatures below 350◦C, we observe an overall increase in the size of the scattering centers on annealing but only a small change in the spin density, which suggests that bond reconstruction on the crystallite surfaces takes place with high efficacy.

We demonstrate that in nanostructured films of nanocrystalline silicon imbedded in a hydrogenated amorphous silicon matrix, carriers generated in the amorphous region are transported out of this region and therefore do not recombine in the amorphous phase. Electron paramagnetic resonance (EPR) and photoluminescence (PL) measurements show that the EPR and PL from the amorphous phase are rapidly quenched as the volume fraction of Si nanocrystals exceeds about 30 vol. %. We propose the use of similar structures to dramatically increase the open circuit voltages in solar cell devices.

Our efforts focus on developing a method to produce hydrogenated nanocrystalline silicon (nc-Si:H) with larger crystallites to enhance carrier transport properties. A new PECVD methodology, called double pulsed PECVD (DPPECVD), employs alternating low frequency and high frequency discharge sub-cycles to sequentially grow and etch the evolving film, respectively. This confers enhanced process control compared to conventional methods, and provides a pathway to achieve our goal of enhanced carrier mobility. Preliminary results demonstrate nc-Si:H films possessing grains as large as 29 nm, with (220) preferred orientation, which is suitable for solar cell applications. Reactions between plasma species in a SiF4:H2:SiH4 glow discharge, which expectedly contribute to evolution of large grains, are also discussed. Our findings suggest the double pulse strategy is a valuable method for manipulating the microstructural evolution of PECVD grown thin film materials.

A study of the electroluminescence degradation of a-SiC:H based light emitting devices (LED) is presented for the first time. The best initial peak brightness obtained is 4.2 cd/m2. All LEDs reported in this paper emit a red light which, when operated under continuous bias in a not fully darkened room, is visible for several minutes, depending on degradation rate. The time dependence of LED degradation, which is reversible upon annealing, can be explained if self-annealing is taken into account. There is evidence of an improved LED performance for lower temperature operation. Pulsed operation, with respect to dc operation, produces a markedly lower defect production rate, associated to a higher brightness after degradation. The possibility of some optimization of the operation parameters (peak current, duty cycle) is discussed.

a-Si:H / c-Si heterojunction diodes were produced by PECVD with varying amorphous silicon layer thickness and hydrogen dilution of the gas phase. An accurate determination of the growth rate also in the initial stages of the deposition was made possible by an original chemical method based on the dissolution of the films followed by spectroscopical analysis of the obtained solution. The electrical characterization of the diodes confirms the generation - recombination -multitunneling nature of the transport. Although H2 dilution is important, however, beyond a certain level it is detrimental for the junction quality, probably due to the transition to a microcrystalline phase deposition. Solar cells were also produced, the best results being an open circuit voltage of 610 mV and an intrinsic efficiency of 14.2%.

The attraction for amorphous Silicon based alloys (a-Si) stems in large part from its ease in its depositing uniformly over large areas in a cost effective manner. We here at Glasstech Solar, Inc. (GSI) wish to report on the development of a-Si films for electrophotographic applications. For this specific purpose we have developed thick films (˜30–40 μm) of intrinsic a-Si deposited on TCO at deposition rates of up to 14 Å/S We tested the material for its photoconductivity and breakdown voltage, and its suitability for the application at hand with one measured parameter, the 400 volt switching time, TΔ400. Essentially the goal consisted of developing a high voltage photoconductive switch, that would swing 400 volts at the terminals in a few μsec.

Thin film amorphous silicon diodes are being examined as a possible high energy particle detector for use in the superconducting super collider. One of the key requirements for any such detector is the ability to withstand relatively high doses of high energy particles without degradation of performance. We report here results of degradation studies of amorphous silicon p-i-n diodes to protons and neutrons with energies in excess of 100 keV.

A study of charge-carrier recombination in intrinsic hydrogenated amorphous silicon (a-Si:H) is presented using pulsed electrically and pulsed optically detected magnetic-resonance spectroscopies in order to measure the influence of spin-dependent recombination on photoluminescence (PL) and photoconductivity (PC). The experiments show band tail state recombination influencing the PL but not the PC which constitutes geminate recombination of correlated charge carriers that do not contribute to charge transport. In contrast, nongeminate recombination through silicon dangling bonds is observed influencing both PL and PC. The experiments presented constitute a direct and unambiguous observation of geminate and nongeminate recombination in a-Si:H.

Nanocrystalline silicon (nc-Si) based p-i-n solar cells were fabricated onto various substrates using modified pulsed PECVD technique. Dark J-V characteristics of nc-Si p-i-n solar cells were found to depend strongly on the substrates and are studied at different i-layer thickness and varying the p/i interface structures. In this work, we report an almost constant diode quality factor (n = 1.2–1.3) up to the thickness of 3.8 µm for the devices grown on “suitably textured” ZnO substrates. The rather insensitive variation of n with i-layer thickness suggests that the dark J-V characteristics are not dominated by bulk recombination for the devices grown on textured ZnO, which prevents grain collision in the i-layer. In contrast to that, a significant change of n (1.8 − 1.3) was found while changing the p/i interface using various duration of H2 plasma treatment of nc-p surface (ST). The p/i interface structure in nc-Si p-i-n device plays the crucial role either by changing the p/i interface defects or the film structure at p/i interface determines the quality of subsequently grown nc-i layer and hence the whole device performance.

The role of p/i interface in nanocrystalline Si p-i-n type solar cells is very critical due to the fact that the nc-Si grown by CVD technique often starts with an amorphous incubation phase, the extent of which sensitively varies with the film growth conditions. We used the reflectance spectra at UV region, where the additional reflections (at 365 nm and 275 nm) appear due to the presence of any crystalline phase, to get an idea about the structure of the film at p/i interface. In this work, the usual pulsed PECVD technique has been modified to provide for extra degrees of freedom to manipulate the growth surface reactions and hence the control of initial film growth via altering the density and energy of various radicals (like H, SiH/sub 3/ etc.) and ions. The use of modified pulsed PECVD method for a surface treatment on nc-p layer leads to an elimination or a reduction of the incubation layer at the p/i interface and thereby improves the device performances significantly.

Pulsed plasma enhanced chemical vapour deposition (PECVD) involves modulation of standard 13.56 MHz RF plasma in the kilohertz range. This allows an increase in the electron density during the 'ON' cycle, while in the 'OFF' cycle, neutralising the ions responsible for dust formation in the plasma. The authors report the development of state-of-the-art nanocrystalline Si (nc-Si:H) materials using a pulsed PECVD technique with 220 crystallite orientation, grain size of ∼200 Å, low O concentration and a minority carrier diffusion length Ld of ∼1.2 μm. The crucial effects of the p/i interface and the incubation layer have been investigated and an efficiency of ∼7.5% for a single junction nc-Si:H p-i-n device has been achieved for an i-layer thickness of 1.4 μm, using non-optimised textured substrates

The pulsed PECVD technique allows an increase in the electron density during the 'ON' cycle, while in the 'OFF' cycle it neutralizes the ions responsible for dust formation in the plasma. We have developed a modified pulsed PECVD technique, which has the inherent ability to reduce powder formation in the plasma and to grow nc-Si:H p-i-n solar cells. An efficiency of /spl sim/7.5% (FF of 0.69) at an i-layer thickness of /spl sim/1.4 /spl mu/m has been obtained by optimizing the i-layer growth temperature. The 7.5% efficiency device is (220) oriented with I/sub 220//I/sub 111/ value of /spl sim/2.0 and exhibits no significant Si-O bonding in FTIR, which yields a hydrogen content of /spl sim/7 at%.

The pulsed plasma CVD technique has been shown to increase the deposition rate without an increase in the particulate count in the plasma which is an important factor in determining the yield of commercial products such as solar cell modules. The technique is also more easily scaled to larger areas than the VHF-PECVD technique. In this paper, we report on the deposition of microcrystalline silicon (/spl mu/c-Si) films over large area substrates (30 cm /spl times/ 40 cm) as well on the optimization of /spl mu/c-Si solar cell devices. The effects of nucleation and substrate pre-treatment on the p/i interface are discussed.

In this paper, we report the deposition of microcrystalline Si materials and microcrystalline n–i–p and p–i–n devices via the Pulsed PECVD technique. The crystallite orientation of the films changes from a random orientation to (220) orientation near the microcrystalline-to-amorphous transition. The observed change in orientation (220 vs. 111) is correlated with the solar cell performance, with the best efficiency seen for (220) oriented i-layers. The role of ion bombardment and grain boundary interfaces on the VOC of these devices is also investigated.

This paper presents the development of low dark current amorphous silicon (a-Si:H) based heterojunction photodiodes. A series of p-i-n and n-i-p structures have been deposited by plasma-enhanced chemical vapor deposition (PECVD). Junction properties and carrier transport are investigated in terms of dark and light current-voltage characteristics, time dependence of the dark current, and spectral photoresponse measurements. It is demonstrated that a thin (∼4 nm) undoped a-SiC:H buffer layer introduced between the p and i layers reduces the leakage current and improves the diode ideality factor. A dark current density of ∼10 pA/cm 2 at reverse bias of 1 V was achieved for the n-i-p structure. Optimization of device design for further improvement of dark current and photoresponse is discussed.

The pulsed plasma CVD technique has been shown to increase the deposition rate without an increase in the particulate count in the plasma, which is an important factor in determining the yield of commercial products such as solar cell modules. In this paper, we report on the use of this technique in a small area deposition system, and show deposition rates of a-Si:H of up to 15 Angstroms/sec can be acheived using a modulation frequency in the range of 1-100 kHz. Simple solar cells of the p/i/n configuration, deposited using this technique, have shown initial efficiencies of 9% with intrinsic-layer deposition rates of up to 7 A/sec. The application of this technique to a large-area 30cm X 40cm system is also discussed. In particular, we report a deposition rate of >6A/s, thickness uniformity (<5%), gas utilization rate(>25%), and the performance of small area (0.25 cm(2)) devices using this approach.

For the ''Hot Wire" chemical vapor deposition (HWCVD) method to be applicable for photovoltaic applications certain critical technical issues need to be addressed and resolved such as, lifetime of the filaments used reproducibility, large area demonstration of the material and stable devices. We have developed a new approach which addresses some of these problems, specifically longevity of the filaments and reproducibility of the materials produced. This new technique does not seem to introduce contaminants into the materials from the source and can produce high quality amorphous Silicon (intrinsic and doped) and intrinsic microcrystalline silicon films.

The pulsed plasma deposition can increase the deposition rate of amorphous silicon (a-Si) without an increase in the particulate count in the plasma which is an important factor determining the yield of commercial products such as active matrix displays. In this paper, we report the deposition of a-Si at rates of up to 15Å/sec, using a modulation frequency in the range of 1–100kHz and the impact it has on solar cell conversion efficiency. The hot wire CVD deposition technique has attracted a considerable amount of interest because of the ability to produce a-Si at a high deposition rate and with low hydrogen concentration which could minimize the instability phenomena. Further, under suitable conditions, low temperature polycrystalline silicon can be produced. We present data of high deposition rates for a-Si (>15A/s) and polycrystalline Si and discuss their usefulness to photovoltaic applications.

The emitter of amorphous/crystalline silicon heterojunction (HJ) solar cells is normally very thin. Consequently, the metal used as a front contact can produce a partial or even total depletion of this layer. As a result, the diffusion potential of the p-n junction deviates from its maximum value. In this paper, we report the results concerning HJ in which either metal dots (Au, Al), semitransparent metal layers, or indium tin oxide (ITO) dots or layers were used as front contact on the same HJ structure, namely (p)a-Si:H / (i)a-Si:H / (n)c-Si / Al. We show that, for thin p-layers, the dark and light J-V characteristics of HJ solar cells depend on the material used as front contact. In particular, we found that the dark saturation current increases if a low work function material is used. This increase is interpreted in terms of p-layer depletion, and is shown to directly influence the J-V characteristics under illumination, producing a reduction of the open circuit voltage of solar cells.

Amorphous silicon carbide films were deposited by the plasma-enhanced chemical vapour deposition technique in SiH4-CH4-H-2 gas mixtures and the effect of hydrogen dilution on the optoelectronic properties investigated using photothermal deflection spectroscopy, photoconductivity and dark electrical conductivity, photoluminescence and Fourier transform infrared spectroscopy. Large H-2 dilution leads to materials of improved quality whose E(g) is about 2.0 eV. The materials were also incorporated into a solar cell device structure to confirm our conclusions.