Innovation: Thin Si Film Based Hybrid Solar Cells On Low-Cost Substrates

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Keywords: 
energy storage, microelectronics, energy saving, conservation, energy transport, renewable sources of energy, energy conversion, solar energy, energy efficiency, electronics
The ThinSi project will develop a solar cell processing chain for high throughput, cost-effective manufacturing of thin film silicon based solar cells on low-cost silicon substrates. The substrates will be made on the basis of an innovative powder-to-substrate concept. In line with the Workprogramme topic addressed, it will reduce the cost of solar cell modules compared to those made by the conventional wafer based approach. A set of innovative processes will be developed to realise the new low-cost concept and transfer the results into production. The new silicon based substrates will be made from low-cost material using state-of-the-art ceramics technologies. Cost effective processes for the formation of the thin film silicon base and the complete solar cell structure will be developed.
New methods for optical confinement will be investigated. The electronic properties of individual solar cell materials and their interfaces as well as the relationship between the deposition parameters and the device properties will be analysed using advanced characterisation and modelling. It will also develop a better understanding of relevant materials issues. Manufacturing procedures suitable for pilot scale production will be developed based on an innovative process chain. The produced solar cells will be assembled into complete modules. The project will develop innovative technologies and equipment prototypes that can easily be scaled up and transferred to production lines by the end of the project. New market opportunities for the SME and industrial partners will be created, both as production tool suppliers and as end-users of the technology.

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Project context and objectives:

THINSI is an ambitious project which aims to develop a solar cell processing chain for high-throughput, cost-effective manufacturing of thin film silicon based solar cells on low-cost silicon substrates. The substrates will be made on the basis of an innovative powder-to-substrate concept. In line with the work programme topic addressed, it will reduce the cost of solar cell modules compared to those made by the conventional wafer based approach. In general, the THINSI solar cell structure based on a low-cost silicon (Si) substrate is very similar to a conventional bulk crystalline Si solar cell, but the low-cost substrate substitutes the Si wafer.

A set of innovative processes will be developed to realise the new low-cost concept and transfer the results into production. The new Si based substrates will be made from low-cost material using state-of-the-art ceramics technologies. Cost-effective processes for the formation of the thin film silicon base and the complete solar cell structure will be developed. New methods for optical confinement will be investigated. The electronic properties of individual solar cell materials and their interfaces as well as the relationship between the deposition parameters and the device properties will be analysed using advanced characterisation and modelling. Manufacturing procedures suitable for pilot scale production will be developed based on an innovative process chain. The produced solar cells will be assembled into complete modules.

The project will also develop innovative technologies and equipment prototypes that can easily be scaled up and transferred to production lines by the end of the project.

The project is supposed also to reduce the thermal budget for deposition of high quality poly-Si thin layers, since the highly doped Si based substrates contain high concentrations of impurities. Normal deposition is usually done by epitaxial high temperature growth at 1000-1200 degrees Celsius and the risk to contaminate the growing/crystallised high quality Si active layer on such a substrate is rather high. THINSI will apply several approaches to address this issue:
(i) lower temperatures for the deposition of thin Si layers;
(ii) alternative to conventional chemical vapour deposition (CVD) methods for Si layers based on thermal spray and magnetron sputtering;
(iii) gettering.

THINSI also adopts advanced methods for optical confinement to increase the chances of reaching photovoltaic conversion efficiencies similar to single crystal wafer based solar cells:
(i) back-side reflector;
(ii) texturing;
(iii) surface plasmon effects.

The THINSI solar cell structure, being wafer and Si based, is very similar to traditional cells. This will ensure a low acceptance threshold in the solar cell industry, which presently is 95 % based on crystalline Si. In spite of these similarities, several essential bottlenecks and developments in low-cost process methods, and therefore in the whole process flow for the processing of poly-Si on low-quality Si substrates solar cell, must be addressed:
(i) reduction of cost and simplification of processing of low-cost Si based substrates using a powder-to-substrate concept;
(ii) methodology for cost-effective deposition of high quality active layers on low cost substrates, including Si base, emitter and TCO antireflection coating retaining a high cell efficiency;
(iii) thorough understanding of the electronic properties of the deposited solar cell thin films and their interfaces as a function of deposition parameters;
(iv) development of advanced methods for optical confinement;
(v) implementation of powder-to-substrate concept based processes flow for the solar cell fabrication to existing industrial lines currently operating in frame of single Si wafer based approach.

Project results:

WP1: The main aim of this part of the project is to identify and to develop processes, which will be used as a background for an innovative wafer equivalent technology based on powder-to substrate concept. Therefore, essential part of the activity in the first 18 months was concentrated on fabrication of Si powder with the desired purity, crystallinity and particle size. Experience from the partners using the Si powder for thermal spraying and tape casting followed by a spark plasma sintering (SPS), showed that a special refinement and adjustment of the Si powder quality was needed for each Si wafer sintering method, both with regard to conductivity and particle size distribution. ELKEM Solar has therefore started work on optimising the powder production. ELKEM Solar has produced and delivered nine different Si powder qualities for use in low cost substrates based on feedback from the consortium. Several routes were investigated to sintering tape casting silicon tapes at
temperatures below the melting point of Si. It was established that a SPS of silicon powder with and without doping elements is a promising method to produce Si wafers with the desirable properties. By use of this method, substrates which satisfied the requirements of the project have been produced. For the substrates both Al and B doped Si powder has been used. B and Al doping has been done by mechanically mixing the Si powder with B/Al at SINT, while B-doped Si powder also has been produced at ELKS. As a result of an optimisation of the SPS process, dense and highly conductive Si substrates have been fabricated.

From the investigations carried out, it is concluded that the SPS process provides partial melting and recrystallisation of Si at certain process conditions, providing formation of Si material similar to a conventional multi-Si. Porous Si monolayer has been processed on such material. Given the fact that the resistivity sample is about 0.006 Ohm-cm, the thickness can be considered as normal. Porous Si stack in the top part of the multi-Si substrate made from the ingot sintered using Si powder was etched. Total reflectance measurements shows a clear reflection band around 800-900 nm, which is a very good result for a first attempt of etching a porous Si Bragg reflector in an unknown substrate.

Free standing approximately 300-500 µm thick 50x65 mm2, as well as 156x156 mm2 Si wafers have been produced from a low-cost and solar grade Si powders by the thermal spray method. These wafers exhibit all necessary properties, although have a reduced density.

It is concluded therefore that formation of a por-Si reflector directly on such wafers is not possible and as a possible solution of a problem bonding of an exfoliated por-Si layer, prepared separately from a high-quality crystalline Si substrate has to be used. Such porous Si layers including those, which consist of multi-layer structure (Bragg reflector), have been fabricated by IMEC.

WP2: In the first 18 months the main activity in this WP has been concentrated on the following:
(i) design and building of an advanced Plasma-enhanced CVD (PECVD) and magnetron sputtering equipment, which allows deposition of Si layers at elevated temperatures, including some tests of such equipment;
(ii) development of a thermal spray process for the deposition of thin Si layers using Si powder;
(iii) to investigate possibility to exfoliate high quality thin Si layers from the crystalline Si substrate and to bond them onto low-quality Si powder based substrate;
(iv) to verify possibility to deposit thin Si layers with less than 105 defects per cm2 at temperatures less than 1050 degrees of Celsius.

OIPT has developed PECVD and magnetron systems which can be used for the deposition of Si layers at elevated temperatures.

Moreover, an upgraded additional tool OIPT System 400 has been developed. This tool currently has following characteristics and differs from the System 100 tool by:
(i) 300 degrees of Celsius table temperature;
(ii) base vacuum less than 1x10-7 Torr;
(iii) four target capability; (iv) four wafers of up to 150 mm;
(v) fixed target to substrate separation.

Using the developed magnetron sputtering equipment OIPT has demonstrated the following:
(i) high rate a-Si deposition up to 162 nm/min (H2 free);
(ii) time to grow 10 µm 1hr;
(iii) magnetron process exceeds PECVD capability;
(iv) 10µm a-Si deposited on prime silicon wafers and low cost THINSI wafers at RT, 200 and 300 degrees of Celsius;
(v) excellent film adhesion - other technologies have tensile stress;
(vi) compressive film stress reduces with RF bias and temperature.

Pyrogenesis has demonstrated that thin Si powder based layers can be deposited on any supporting substrate using thermal spray. Such layers have been deposited even on a glass substrate. Moreover, similar layers have been deposited on low-cost Si powder substrates using electronically grade Si powder. Properties of such layers are under the investigation currently. It can be concluded that the substrate temperature during the thermal spray deposition does not exceed approximately 600 degrees of Celsius.

IMEC has prepared a set of exfoliated thin Si layers, formed on Cz Si substrates. These samples are then send to SINTEF where detach and bonding tests are under the development currently. It is supposed that such layers will be bonded on top of any type of highly doped Si powder based supporting substrate.

Epitaxial processes for the deposition of thin Si layers at lower temperatures have been done on monocrystalline Si substrates in an atmospheric pressure CVD system at ISE. Defect densities of the epitaxial layers have been measured. The goal of less than 105 defects per cm2 was achieved for the temperature of 1050 degrees of Celsius.

WP3: Since the beginning of the project properties of Si powder based substrates have been analysed by X-ray diffraction (XRD), scanning electron microscope (SEM), energy-dispersive X-ray spectroscopy (EDX), electron backscatter diffraction (EBSD), Raman, conductivity measurements. Properties of indium tin oxide (ITO), AZO and fluorine-doped tin oxide (FTO) films have been investigated by AFM, XRD, transmittance ellipsometry and conductivity measurements. Properties of solar cell structures and interfaces have been analysed by SEM, AFM, SSRM and IV measurements.

Several porous silicon gettering layers have been fabricated in a monolithic approach (porous silicon gettering layers etched in low quality Si substrates) and in standalone / exfoliated form. Such layers were systematically characterised in regard to their optical, mechanical and morphological properties. Porous Si multilayer stacks (porous Si reflector) on top of highly doped Si powder based supporting substrates were fabricated both for analysis and device fabrication. Metal contamination tests for Si powder low-cost substrates made of recrystallised A+ powder, have been performed. The SPS samples both highly and lowly doped were analysed again their dissolution reaction at various chemical etching solutions. Their morphology was examined via SEM and optical microscopy. It is found that dense sintered Si regions and inclusions of not fully sintered agglomerated Si could be identified by cross section SEM. Inclusions present in the material seen by optical microscopy were also
visible with the EBSD maps, which revealed many black structures where no indexation was possible. The SEM-EDX study reveals that the sintered silicon contains structures with a different composition. The grains with weak EBSD quality rise difficulty in detecting the crystal structure. The orientation of the EBSD pattern remains the same within the same large structure.

WP4: Reference solar cells on highly doped low-cost UMG-Si substrates were successfully processed with standard technologies and reached a maximum conversion efficiency of 13.6 %. Reference solar cells on highly doped multicrystalline and monocrystalline Si substrates reached 13.3 % and 15.3 %, respectively. First solar cells on Si powder based substrates prepared via wafering of an ingot made of remelted B-doped low purity silicon powder have been processed. The conversion efficiency for the best solar cell reached 11.9 %.

WP5: A cost model for a standard technology of monocrystalline silicon cells has been developed to be used as reference for the THINSI process chain. The breakdown of the model consists of wafer, solar cell and module steps. The wafer step implies polysilicon, ingot growing and wafering; the results of solar cell step show the following details: equipments (energy and depreciation), labour, material (materials and consumables), yield losses and fixed costs. Estimations, made on the basis of the model, which has been developed, show that the cost of the THINSI solar cell based modules will reach approximately 1 EUR/Wp in case if such solar cells have efficiency approximately 15 %.

List of websites: http://www.sintef.no/Projectweb/ThinSi/

Collaboration sought: N/A

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This innovation is the result of the project

Title: Thin Si Film Based Hybrid Solar Cells On Low-Cost Substrates

Acronym: 
THINSI

Runtime: 
01.01.2010 to 31.12.2012

Status: 
completed project

Organisations and people involved in this eco-innovation.

Please click on an entry to view all contact details.

STIFTELSEN SINTEF

(Norway)

Role in project: Project Coordination

Contact person: Ms. HØNSTAD Tove Lillian

Website: http://www.sintef.no

Contact

AGENZIA NAZIONALE PER LE NUOVE TECNOLOGIE,L'ENERGIA E LO SVILUPPO ECONOMICO SOSTENIBILE

(Italy)

Contact person: Dr. SYTCHKOVA Anna

Website: http://www.enea.it

Phone: +39-0630484441

Contact

ELKEM SOLAR AS

(Norway)

Contact person: Dr. GLØCKNER Ronny

Website: http://www.elkem.com

Phone: +47-98215512

Contact

FRAUNHOFER-GESELLSCHAFT ZUR FOERDERUNG DER ANGEWANDTEN FORSCHUNG E.V

(Germany)

Contact person: Mr. STEIERT Maximilian

Website: http://www.fraunhofer.de

Phone: +49-76145885308

Contact

INNOVATIVE MATERIALS PROCESSING TECHNOLOGIES LTD

(United Kingdom)

Contact person: HILL Jean

Phone: +44-1159225932

Contact

INTERUNIVERSITAIR MICRO-ELECTRONICA CENTRUM VZW

(Belgium)

Contact person: Ms. VAN HOUTVEN Christine

Website: http://www.imec.be

Phone: +3216281613

Contact

ISOFOTON S.A.

(Spain)

Contact person: Dr. CABALLERO Luis

Website: http://www.isofoton.com

Phone: +34-951233500

Contact

NT-MDT EUROPE BV

(Netherlands)

Contact person: Dr. KOZODAEV Dmitry

Website: http://www.ntmdt.com

Phone: +31-402631200

Contact

OXFORD INSTRUMENTS PLASMA TECHNOLOGY LTD

(United Kingdom)

Contact person: Mr. BEEKMAN Knut

Phone: +44-1934837000

Contact

PYROGENESIS SA

(Greece)

Contact person: Dr. VARDAVOULIAS Michalis

Website: http://www.pyrogenesis-sa.gr

Phone: +30-2292023477

Contact

THE UNIVERSITY OF NOTTINGHAM

(United Kingdom)

Contact person: Mr. CARTLEDGE Paul

Phone: +44-1159515627

Contact