A. Feltrin

In situ silicon oxide based intermediate reflector for thin-film silicon micromorph solar cells

We show that SiO-based intermediate reflectors (SOIRs) can be fabricated in the same reactor and with the same process gases as used for thin-film silicon solar cells. By varying input gas ratios, SOIR layers with a wide range of optical and electrical properties are obtained. The influence of the SOIR thickness in the micromorph cell is studied and current gain and losses are discussed. Initial micromorph cell efficiency of 12.2% (Voc=1.40V, fill factor=71.9%, and Jsc=12.1mA∕cm2) is achieved with top cell, SOIR, and bottom cell thicknesses of 270, 95, and 1800nm, respectively.

Resistive interlayer for improved performance of thin film silicon solar cells on highly textured substrate

The deposition of thin-film silicon solar cells on highly textured substrates results in improved light trapping in the cell. However, the growth of silicon layers on rough substrates can often lead to undesired current drains, degrading performance and reliability of the cells. We show that the use of a silicon oxide interlayer between the active area and the back contact of the cell permits in such cases to improve the electrical properties. Relative increases of up to 7.5% of fill factor and of 6.8% of conversion efficiency are shown for amorphous silicon cells deposited on highly textured substrates, together with improved yield and low-illumination performance.

Efficient light management scheme for thin film silicon solar cells via transparent random nanostructures fabricated by nanoimprinting

We propose the use of transparent replicated random nanostructures fabricated via nanoimprinting on glass as next-generation superstrates for thin film silicon solar cells. We validate our approach by demonstrating short-circuit current densities for p-i-n hydrogenated microcrystalline silicon solar cells as high as for state-of-the-art nanotextured ZnO front electrodes. Our methodology opens exciting possibilities to integrate a large variety of nanostructures into p-i-n solar cells and allows to systematically investigate the influence of interface morphology on the optical and electronic properties of the device in order to further improve device performance.

Infrared laser-based monitoring of the silane dissociation during deposition of silicon thin films

The silane dissociation efficiency, or depletion fraction, is an important plasma parameter by means of which the film growth rate and the amorphous-to-microcrystalline silicon transition regime can be monitored in situ. In this letter we implement a homebuilt quantum cascade laser-based absorption spectrometer to measure the silane dissociation efficiency in an industrial plasma-enhanced chemical vapor deposition system. This infrared laser-based diagnostic technique is compact, sensitive, and nonintrusive. Its resolution is good enough to resolve Doppler-broadened rotovibrational absorption lines of silane. The latter feature various absorption strengths, thereby enabling depletion measurements over a wide range of process conditions.

Influence of pressure and silane depletion on microcrystalline silicon material quality and solar cell performance

Hydrogenated microcrystalline silicon growth by very high frequency plasma-enhanced chemical vapor deposition is investigated in an industrial-type parallel plate R&D KAI™ reactor to study the influence of pressure and silane depletion on material quality. Single junction solar cells with intrinsic layers prepared at high pressures and in high silane depletion conditions exhibit remarkable improvements, reaching 8.2% efficiency. Further analyses show that better cell performances are linked to a significant reduction of the bulk defect density in intrinsic layers. These results can be partly attributed to lower ion bombardment energies due to higher pressures and silane depletion conditions, improving the microcrystalline material quality. Layer amorphization with increasing power density is observed at low pressure and in low silane depletion conditions. A simple model for the average ion energy shows that ion energy estimates are consistent with the amorphization process observed experimentally. Finally...

Limiting factors in the fabrication of microcrystalline silicon solar cells and microcrystalline/amorphous (‘micromorph’) tandems

This contribution presents the status of amorphous and microcrystalline silicon solar cells on glass, and discusses some material/fabrication factors that presently limit their conversion efficiency. Particular attention is focused on recent results and developments at the Institute of Microtechnology (IMT) in Neuchâtel. The performances and stability of microcrystalline silicon single-junction and amorphous/microcrystalline (‘micromorph’) tandem solar cells are discussed, as a function of material properties. Recent results on the electrical effect of cracks in microcrystalline silicon material are presented. Degradation under the effect of illumination is a well-known limiting factor for amorphous silicon solar cells. As a comparison, studies on the stability of microcrystalline silicon with respect to light-induced degradation are commented upon. The importance of transparent contacts and anti-reflection layers for achieving low electrical and optical losses is discussed. Finally, efforts towards industrialization of micromorph tandem solar cells are highlighted, specifically (i) the development and implementation of an in situ intermediate reflector in a large-area industrial deposition system, and (ii) recent achievements in increasing the growth rate of microcrystalline silicon.

ZnO Transparent conductive oxide for thin film silicon solar cells

There is general agreement that the future production of electric energy has to be renewable and sustainable in the long term. Photovoltaic (PV) is booming with more than 7GW produced in 2008 and will therefore play an important role in the future electricity supply mix. Currently, crystalline silicon (c-Si) dominates the market with a share of about 90%. Reducing the cost per watt peak and energy pay back time of PV was the major concern of the last decade and remains the main challenge today. For that, thin film silicon solar cells has a strong potential because it allies the strength of c-Si (i.e. durability, abundancy, non toxicity) together with reduced material usage, lower temperature processes and monolithic interconnection. One of the technological key points is the transparent conductive oxide (TCO) used for front contact, barrier layer or intermediate reflector. In this paper, we report on the versatility of ZnO grown by low pressure chemical vapor deposition (ZnO LP-CVD) and its application in thin film silicon solar cells. In particular, we focus on the transparency, the morphology of the textured surface and its effects on the light in-coupling for micromorph tandem cells in both the substrate (n-i-p) and superstrate (p-i-n) configurations. The stabilized efficiencies achieved in Neuchâtel are 11.2% and 9.8% for p-i-n (without ARC) and n-i-p (plastic substrate), respectively.

Impact of secondary gas-phase reactions on microcrystalline silicon solar cells deposited at high rate

The role of secondary gas-phase reactions during plasma-enhanced chemical vapor deposition of microcrystalline silicon is a controversial subject. In this paper, we show that the enhancement of such reactions is associated with the improvement of material properties of absorber layers deposited at high constant rate. We detect powder, a product of secondary gas-phase reactions, via infrared laser absorption spectroscopy, laser light scattering, and optical emission spectroscopy. As the powder formation is increased, we measure a systematic improvement of device performance. This demonstrates that secondary gas-phase reactions are not detrimental to the material quality of microcrystalline silicon deposited at high rate.

Research and developments in thin-film silicon photovoltaics

The increasing demand for photovoltaic devices and the associated crystalline silicon feedstock demand scenario have led in the past years to the fast growth of the thin film silicon industry. The high potential for cost reduction and the suitability for building integration have initiated both industrial and research laboratories dynamisms for amorphous silicon and micro-crystalline silicon based photovoltaic technologies. The recent progress towards higher efficiencies thin film silicon solar cells obtained at the IMT-EPFL in Neuchatel in small-area laboratory and semi-large-area industrial Plasma Enhanced Chemical Vapor Deposition (PE-CVD) systems are reviewed. Advanced light trapping schemes are fundamental to reach high conversion efficiency and the potential of advanced Transparent Conductive Oxides (TCO) is presented, together with issues associated to the impact of the substrate morphology onto the growth of the silicon films. The recent improvements realized in amorphous-microcrystalline tandem solar cells on glass substrate are then presented, and the latest results on 1 cm2 cells are reported with up to 13.3 % initial efficiency for small-area reactors and up to 12.3 % initial for large-area industrial reactors. Finally, the different strategies to reach an improved light confinement in a thin film solar cell deposited on a flexible substrate are discussed, with the incorporation of asymmetric intermediate reflectors. Results of micromorph solar cells in the n-i-p configuration with up to 9.8 % stabilized efficiency are reported.

Optical emission spectroscopy to diagnose powder formation in SiH4-H2 discharges

Silane and hydrogen discharges are widely used for the deposition of silicon thin film solar cells in large area plasma-enhanced chemical vapor deposition reactors. In the case of microcrystalline silicon thin film solar cells, it is of crucial importance to increase the deposition rate in order to reduce the manufacturing costs. This can be performed by using high silane concentration, and usually high RF power and high pressure, all favorable to powder formation in the discharge that generally reduces the deposition rate as well as the deposited material quality. This work presents a study of powder formation using time-resolved optical emission spectroscopy. It is shown that this technique is suitable to detect different regimes in powder formation ranging from powder free discharge to discharge producing large dust particles. Intermediate powder formation regimes include the formation of small silicon clusters at plasma ignition as well as cycle of powder growth and ejection out of the discharge, and both are observable by this low-cost and experimentally simple technique.