Symposium PV Suisse 2005, Genève. Recherches en photovoltaïque à l IMT : état de l art et perspectives

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1 Symposium PV Suisse 2005, Genève Recherches en photovoltaïque à l IMT : état de l art et perspectives C. Ballif Laboratoire de photovoltaïque et couches minces électroniques PV-Lab, IMT, Université de Neuchâtel Remerciements A tout le groupe IMT PV-Lab (Prof. A. Shah) A Unaxis et VHF-Technology OFEN, CTI, EU, FN

2 Institute of Microtechnology in Neuchâtel Since 1986: focus on thin film silicon solar cells prepared by plasma processes 1987 VHF plasma deposition 1994 First efficient microcrystalline Si solar cells followed by the micromorph devices (a-si/µc-si) Starting «technology transfer» Worldwide: over 50 research institutes and 15 industries active in this area A strong heritage of Prof. A. Shah and the pionnering work of his research group

3 An area of strong industrial interest Product manufacturers Kaneka Mitsubitshi Uni-solar Schott FEE, Pilot-line / consumer products Sanyo VHF-Technologies ITFT Akzo Nobel Entering the market Fuji Sharp CSG Equipment suppliers Unaxis Solar Applied Films EPV Ishikawa HI Developping a-si/µc-si technology

4 Photovoltaics activities at IMT Thin film silicon on flexible substrate Thin film silicon on glass c-si, special Applications, PV technology

5 Thin film silicon: challenge and opportunities Advantages Low material costs (0.3-2 µm thickness) Availability of base materials (SiH 4 ) and dopants Medium efficiency up to 10-13% in principle possible with stacked cells Favourable temperature coefficient Proven technology Synergy with flat-panel display sector Challenges Development of suitable transparent conductive oxides Master cost-effective large area plasma deposition processes for a-si:h and µc-si:h Demonstrate higher devices and modules efficiency Demonstrate low production prices

6 Light trapping in thin film silicon Back reflector P-i-n TCO Glass n=1.5 Because of the low optical absorption in infra-red need for light trapping Surface texture scatters light Typical dimension λ/n ~ 300 nm Superstrate configuration: Combine electrode with naturally textured transparent conductive oxyde (TCO) Scatter light Be highly conductive Front TCO must Be highly transparent Be cost effective

7 LPCVD ZnO development at IMT S. Fay J. Steinhauser R. Schluechter Fabrication by lowpressure chemical vapor deposition 1 µm SEM Plane view Typical LPCVD ZnO : highly transparent, R = 10 Ohm/square for 2 µm Customized layers : Control of roughness, Light scattering Conductivity Mininum absorption over the whole spectrum Collaboration

8 Solar cells on glass results Record amorphous cell on LPCVD ZnO 9.4% substrate (IMT, 2003) J. Meier et al. J. Bailat, D. Dominé, X. NIquille Record thin (1µm) microcrystalline cells on LPCVD ZnO, 8.4% (2005) and 9.2% on Glass/Ag Microcrystalline cells in industrial Kai-S reactor: 7.4% at 7 Å/s (2005) Meier, Dominé, Bailat Micromorph solar cells with efficiencies ~10-11% ( ) L. Feitknecht, Poster F. Freitas, C. Bucher Collaboration E g top (a-si:h): ~ 1.75 ev E g bottom (µc-si:h) ~ 1.1eV

9 Micromorph solar cell : SEM view Glass light Front ZnO 2 µm LPCVD ZnO p-i-n a-si:h cell Intermediate Reflector p-i-n µc cell 300 nm PECVD a-si 70 nm sputtered ZnO 2 µm PECVD Micro-crystalline Si ZnO Back Reflector 500 nm Photo: G. Buerki, EMPA Thun 2 µm LPCVD ZnO +White paste or Ag (back reflector)

10 Amorphous Si:H: substrate configuration Low-cost PET substrate: how to trap light? Use nanotextured PET, with grating structure «total internal diffraction» V. Terrazoni-Daudrix F. Freitas, J. Bailat With AFM on pet substrate Nanotextured PET Roll

11 A-Si on PET nanotextured PET substrate External quantum efficiency J. Bailat, F. Freitas V. Terrazoni-Daudrix X. Niquille Substrate OVDn4, A, 13mA/cm2 2 flat Flat PEN, substrate 12.5mA/cm2 2 Double Gratingperiod, B 12.9mA/cm2 2 ZnO Grating CVD C replica, 14.4 ma/cm2 2 Morf Grating, D 13.4mA/cm ma/cm 2 20% gain in current! Wavelength [nm] 7.3% a-si stabilised solar cell on PET

12 Solar cells on plastic results All efficiencies stabilised and with «industry compatible substrates» Amorphous cells on PET : 7.3% Microcrystalline cells on PET : 7.3% Micromorph devices on PET : 8.3% (Preliminary tests) J. Bailat, F. Freitas V. Terrazoni-Daudrix X. Niquille Poster V. Terrazoni-Daudrix et al. EU-STREP Flexcellence ( ) Roll-to-roll technology for the production of highefficiency low cost thin film silicon modules IMT coordinator, VHF-Technolgies partner 7 millions CHF/ 5 millions from EU

13 Perspectives for thin film silicon at IMT Short Term (1-2 years) Stabilization and robustness of processes Upgrade of infrastructures / fabrication / measurements Support to our industrial partners time Medium term (3-6 years) Thin film Si solar cells with > 13% on glass > 11% on flexible substrates Innovate with new processes Industry compatible processes Long term Contribute to price < 1 /Wp Thin film silicon modules on every rooftop!

14 New infrastructure and upgrade of systems New sputtering system Partial automation of industrial Kai deposition system R. Tscharner C. Burcher R. Schlüter time Decoupling of two LPCVD systems for ZnO Starting to build a new «twochamber» industrial large area system (40 x 50 cm 2 ) Partial automation of development systems Retro-fitting of older large area sputtering systems

15 Deposition systems 6 small area PECVD reactors ( up to 10x10 cm2) 2 medium size (PECVD reactors) 6 systems for preparation of metallic and TCO layers

16 Upgrade and new measurement systems time FTPS (measurements of electronic defects in µc-si devices) V-I-M (measurement at variable intensity) Infra-Red Lock-in Thermography for shunt detection in solar cells and modules F. Meillaud E. Vallat Faster I-V measurements Faster Spectral Response measurements Faster, better understanding Yield, industry like approach

17 New at IMT : Infra-red lock-in thermography IR-Camera + computer Lock-in demodulation of each pixel ( T < 0.1 mk) Optical microscopy Modulated electrical excitation of solar cell (0-0.3V) Calibrated translation stage

18 Upgrade and new measurement systems J. Steinhauser F. Jeanneret IR - Image IR Lock-in picture: V Scale (0-3mK)

19 Other related PV activities High efficiency on thin silicon wafers a-si:h/c-si heterojunction, wafer sawing (with HCT, EMPA) PV technology expertise for specific projects, e.g. solar-impulse Special application of PV (watches, integrated energy sources) Characterisation systems (Project with Q-Cells)

20 Conclusion Mastering a-si:h, µc -Si:H, a-si/ µc-si on different substrates Several record efficiencies and strong potential for further improvements New activities on processing of «thin crystalline Si wafers» Working together with the indutry: setting the bases to make PV cheaper and more efficient