Arun Madan

Evidence for graphitic-type bonding in glow discharge hydrogenated amorphous silicon carbon alloys

Amorphous silicon carbon (a‐SiC:H) films have been deposited by the glow discharge technique using SiH4 and CH4 gas mixtures. At high discharge powers and low deposition chamber pressures, evidence for graphitic‐type bonding in C‐deficient a‐SiC:H is found and correlations are made between the appearance of this bonding with significant changes in the electronic and structural properties. This graphitic‐type bonding can be minimized by significant H attachment to C via CHn (n=2,u20093) bonding. This results in a‐SiC:H films with low gap state densities and sharp Urbach tails.

A new modular multichamber plasma enhanced chemical vapor deposition system

Abstract The present work reports on a new modular UHV multichamber PECVD system with characteristics which prevent both the incorporation of residual impurities and cross contamination between different layers. A wide range of intrinsic and doped hydrogenated amorphous silicon (a-Si:H) materials have been produced and single junction pin solar cells with an efficiency greater than 10% have been readily obtained with little optimization. The system contains three UHV modular process zones (MPZs); the MPZs and a load lock chamber are located around a central isolation and transfer zone which contains the transport mechanism consisting of an arm with radial and linear movement. This configuration allows for introduction of the substrate into the MPZs in any sequence so that any type of multilayer device can be produced. The interelectrode distance in the MPZs can be adjusted between 1 and 5 cm. This has been found to be an important parameter in the optimisation of the deposition rate and of the uniformity. The multichamber concept also allows individually optimized deposition temperatures and interelectrode distances for the various layers. The system installed in Utrecht will be employed for further optimization of single junction solar cells and for research and development of stable a-Si:H tandem cells.

High deposition rate amorphous and polycrystalline silicon materials using the pulsed plasma and “Hot-Wire” CVD techniques

Abstract The cost of amorphous silicon solar panels are dictated by the deposition rate, the utilization rate of the silane gas and stability issues. In this context, we present data of amorphous silicon materials and solar cells using pulsed plasma PECVD (plasma enhanced chemical vapor deposition) technique with the i-layer fabricated with high deposition rates. “Hot-Wire” CVD deposition technique has attracted a considerable amount of interest because of the ability to produce amorphous silicon at high deposition rates and with low hydrogen concentration of H which could minimize the stability phenomena. Further, under suitable conditions, low-temperature polycrystalline silicon can be produced. We present data of high deposition rates of polycrystalline Si (∼10xa0A/s) and discuss its potential usefulness in a hybrid tandem (combination of amorphous and polycrystalline) junctions.

Progress in high deposition rate amorphous and polycrystalline silicon materials using the pulsed plasma and hot wire CVD deposition techniques

Abstract 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 15xa0A/sec, using a modulation frequency in the range of 1–100xa0kHz 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 (>15xa0A/s) and polycrystalline Si and discuss their usefulness to photovoltaic applications.

High Deposition Rate Amorphous Silicon Solar Cells and Thin Film Transistors Using the Pulsed Plasma Pecvd Technique

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 and solar cells. In this paper, we report the deposition of a-Si at rates of up to 15 A/sec a using a modulation frequency in the range of 1-100kHz and the impact it has on solar cell conversion efficiency and thin film transistor performance.

Material properties of glow-discharge a-SiSn:H alloys

Abstract In this study, a series of a-SiSn:H alloys is investigated. A transition from n- to p-type in the conduction mechanism is found with Sn incorporation, while the μτ products of electrons and holes decreased drastically at the same time. We attribute this to the creation of additional states in the lower half of the gap. Similar trends can be observed in a-SiGe:H and a-SiC:H. Phosphorous doping recovers the μτ products of the photo-carriers in a-SiSn:H.

Deposition of amorphous and microcrystalline silicon using a graphite filament in the hot wire chemical vapor deposition technique

The use of a graphite filament in the “hot wire” chemical vapor deposition technique is demonstrated to produce “state-of-the-art” intrinsic and doped (p- and n-) amorphous silicon (a-Si:H) material and microcrystalline silicon (μc-Si) materials. Preliminary p-i-n type solar cells have led to a conversion efficiency of >8.5%. The filament is found to be rugged and remains intact even after deposition of ∼500u200aμm in thickness. This is in contrast to the use of conventional filament materials, such as W or Ta, whose longevity is limited to less than a few microns of deposition. Unlike the case of a Ta filament, the deposition rate remains constant with the use of a graphite filament.

An Assessment of a-SiGe:H Alloys with a Band GaP of 1.5Ev as to their Suitability for Solar Cell Applications

Hydrogenated amorphous silicon germanium alloys a-Si 1-x Ge x :H are being actively investigated for their application as a low band gap material in cascade solar cells [1,2]. To date, such alloys produce material of reasonable electronic quality only if the Ge-content is kept low ( 16%, which are ultimately hoped to be obtained using the cascade approach [2]. Other low band gap alloys such as a-Si 1-x Sn x :H have been shown to be even less suitable with regard to their electronic properties [3]. The cause of the degradation in electronic properties with increased alloying is not yet understood. Factors such as preferential attachment of H to Si rather than Ge [4] or microstructure observed in alloys have been suggested as a cause for the electronic degradation, [5,6] but no unique correlations have been established between such findings and the electronic properties.

Microcrystalline and wide band gap p+ window layers for a-Si p-i-n solar cells

Abstract Amorphous silicon p-i-n solar cells have been made with amorphous silicon, a-Si:B:H, amorphous silicon carbide, a-SiC:B:H, and microcrystalline silicon, μc-Si:B:H p+ layers. The open circuit voltage increases with the introduction of a-SiC or μc-Si p+ layers due to an increase in the built-in field of the p-i-n junction. Bias dependent quantum efficiency and temperature dependent current voltage measurements indicate that the open circuit voltage is limited by a combination of interface and bulk recombination.

Influence of Transparent Conducting Oxide on Amorphous Silicon Solar Cell Performance

The use of transparent conducting oxides (TCO) as electrical contacts in a-Si:H solar cells has stimulated interest in the multitude of effects that these layers have on a-Si:H solar cell performance. The study of a-Si:H p-i-n junctions using a TCO contact involves many factors such as, interdiffusion, transmission, reflection, and resistivity. In this paper, we attempt to distinguish between these factors through the role they play in determining the solar cell device performance. Devices were characterized via dark and illuminated current vs. voltage (I-V) measurements, and spectral response. It was found that the properties of the TCO have an important role in influencing FF and Jsc in the devices.