Didier Dominé

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.

Modeling of light scattering from micro- and nanotextured surfaces

We present a calculation routine for the angular and spectral dependence of scattered light after transmission through textured interfaces. Based on a modified Rayleigh–Sommerfeld integral, the treatment requires only measured surface profiles, and the refractive indices of the two materials adjacent to the textured interface but no fitting parameter. For typical surface morphologies used in solar cell fabrication, the calculations correctly reproduce the angle resolved scattering at 543 nm and the total scattered light intensity in the spectral range from 400 to 2000 nm. The model is then applied to predict the behavior of the interface between ZnO and silicon in a thin film solar cell which is not experimentally accessible.

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.

Light trapping effects in thin film silicon solar cells

We present advanced light trapping concepts for thin film silicon solar cells. When an amorphous and a microcrystalline absorber layers are combined into a micromorph tandem cell, light trapping becomes a challenge because it should combine the spectral region from 600 to 750 nm for the amorphous top cell and from 800 to 1100 for the microcrystalline bottom cell. Because light trapping is typically achieved by growing on textured substrates, the effect of interface textures on the material and electric properties has to be taken into account, and importantly, how the surface textures evolve with the thickness of the overgrowing layers. We present different scenarios for the n-i-p configuration on flexible polymer substrates and p-i-n cells on glass substrate, and we present our latest stabilized efficiencies of 9.8% and 11.1%, respectively.

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.

Micromorph Solar Cell Optimization using a ZnO Layer as Intermediate Reflector

The insertion of a zinc oxide (ZnO) intermediate reflector (ZIR) between the top and bottom cell of a superstrate (p-i-n/p-i-n) micromorph tandem solar cell is analyzed, experimentally and by numerical simulation. Solar cells are deposited onto glass plates coated by surface-textured ZnO layers deposited by low-pressure chemical vapour deposition (LP-CVD). The gain in the top cell short-circuit current density (Jsc) obtained by ZIR insertion as well as the corresponding loss for the bottom cell are experimentally observed, for different values of ZIR thickness d. The gain and the loss depend nearly linearly on ZIR thickness for d < 100 nm, the maximum gain is almost 3 mA/cm2. Experimental results are compared with an optical simulation. In the latter a three-layer effective media approximation is used for modeling of thin ZIR layers. Micromorph tandem solar cells were deposited on 2 different types of front LP-CVD ZnO layers: (a) a layer optimized for a-Si:H single-junction solar cells; (b) ZnO layers specially developed for muc-Si:H cells and having undergone a novel surface treatment. In case (a) Jsc=12.1 mA/cm2 and initial conversion efficiency is 11.6 %; in case (b) Jsc=12.8 mA/cm2 and initial conversion efficiency is 11.8 %. The open-circuit voltage (Voc) value could be improved from 1.32 V to 1.41 V with an increased surface treatment time

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.

Self-tracking solar concentrator with an acceptance angle of 32°

Solar concentration has the potential to decrease the cost associated with solar cells by replacing the receiving surface aperture with cheaper optics that concentrate light onto a smaller cell aperture. However a mechanical tracker has to be added to the system to keep the concentrated light on the size reduced solar cell at all times. The tracking device itself uses energy to follow the suns position during the day. We have previously shown a mechanism for self-tracking that works by making use of the infrared energy of the solar spectrum, to activate a phase change material. In this paper, we show an implementation of a working 53 x 53 mm(2) self-tracking system with an acceptance angle of 32° ( ± 16°). This paper describes the design optimizations and upscaling process to extend the proof-of-principle self-tracking mechanism to a working demonstration device including the incorporation of custom photodiodes for system characterization. The current version demonstrates an effective concentration of 3.5x (compared to 8x theoretical) over 80% of the desired acceptance angle. Further improvements are expected to increase the efficiency of the system and open the possibility to expand the device to concentrations as high as 200x (C(geo) = 400x, η = 50%, for a solar cell matched spectrum).

Ultra-Light Amorphous Silicon Cell for Space Applications

For space applications, solar cells should be optimized for highest power density rather than for highest efficiency. In this context, relatively low efficiency thin-film solar cell may well surpass multi-junction III-V based solar cells if they can be made thin enough. In thin-film solar cells the power density is mostly limited by the substrate. The introduction of ultra-thin polymeric substrates is the key for decreasing the cell mass. In this work, a very thin polyimide film LaRCtrade-CP1 was used as substrate or superstrate for amorphous silicon solar cell fabrication. CP1 films were either fixed on a glass carrier or spin coated onto a glass carrier coated with a release agent. By depositing amorphous silicon cells on 6 mum thick CP1 films, a power density of 2.9 W/g under AM1.5g and of 3.9 W/g (estimated) under AM0 illumination spectra was achieved, in substrate (n-i-p) configuration (for a cell area of ca. 0.25 cm2). A similar cell deposited in superstate (p-i-n) configuration exhibits a record power density of 3.2 W/g under AM1.5g and an estimated value of 4.3 W/g under AM0 illumination spectra. Release of the finished solar cells from the glass carrier was also tested

Light scattering at nano-textured surfaces in thin film silicon solar cells

State-of-the-art solar cells based on thin film silicon make use of random textures for absorption enhancement; in the past, the development of these textures was carried out empirically, and in most applications this is still the case. Attempts to understand light scattering at the internal interfaces rely on quantities like haze and angle resolved scattering that are only measurable in air and need to be extrapolated, normally by means of scalar scattering theory. In this context it is unfortunate that the description of scattering into air requires modifications with empiric parameters whose scaling is unknown. We present an alternate approach for predicting angular properties and intensities of scattered light which is based only on measurable quantities like the surface morphology and the refractive index dispersion, no adjustable parameters are needed.