PROMISING THIN FILMS MATERIALS FOR PHOTOVOLTAICS Emmanuelle ROUVIERE CEA Grenoble (France) emmanuelle.rouviere@cea.fr
Outline Introduction Photovoltaic technologies and market Applications Promising Thin film technologies Stacked layers Lab performances Nano based solar cell Nanowires Nanocrystals Conclusion
The four missions of the CEA 5 700 people R & D for nuclear Total Budget : 3 300 M 15 000 people 1400 Patents energy Fundamental Research 2 700 People Technological Research For industry Defense programs 4 600 people New technologies for energy and nanomaterials Budget : 220 M 2000 People Micro-nanotechnologies for information and communication Software based systems
Key numbers for CEA-Liten CHAMBERY Solar&accomodation with low energy consumption 150p. GRENOBLE Automotive application Portable component Nanomaterials 400p Staffing Staffing 2008 2008 550 550 scientists scientists and and technicians technicians Patent Patent 320 320 acting acting 85 85 filled filled in in 2008 2008 Budget Budget for for 2008:80M 2008:80M 55 55 M M external external funding funding 25 25 M M internal internal funding funding
The CEA Liten research areas Nano Materials Technologies Division ( DTNM ) PV and Energy management for building CEA LITEN develops solar & accommodation with low consumption Solar Cells Bulk Silicon Organic PV modules Systems Energy storage Hydrogen and Fuel Cells for automotive applications CEA LITEN develops the hydrogen pathway : -Production -Transport and storage -Conversion Hydrogen production by high-temperature electrolysis Fuel Cells PEMFC SOFC Systems architecture Nanomaterials in high technology industrial applications CEA LITEN develops nanomaterials :Synthesis, Handling, safety, integration Micro-sources of energy containing nano-objects : micro-fuel cells Micro-batteries Thermoelectric generator Thin films Photovoltaics CNT, SiNW & Nanocomponents Nanostructured surfaces : surface energy,nanocatalyzers Flexible electronics
Principles of PV energy conversion 1. Light absorption 2. Electron-holes pair formation 3. Charge transport and separation 4. Charge extraction I generation: crystalline silicon technology II generation: Thin-film technologies(cigse, CdTe, silicon (amorphous & polycrystalline) III generation: Innovative high efficiency and low-cost technologies (multijunctions, nano structures) Movie from Mathieu ROY
pecific products for specific applications Standard roofs Stand alone system Street light Mobile phone Wireless sensors Building Integration Flat roofs Facades Parking Solar plants
Best Research Cell Efficiencies Multi Junction Silicon Thin films Organic
Cost of electricity from photovoltaics [Abengoa Solar website] Need to develop low cost and high efficiency technologies Grid parity in 2020 for sunny regions [EPIA] DNI : Direct Normal Irradiance
Thin film market share and module cost by technology Source: Harry Zervos, IDTechEx in rrenewable Energy World, April 2008
Photovoltaics at CEA: technologies 1 st generation 2 nd generation 3 rd generation Cristalline Si Polycristalline Si Metallurgical Si HIT technology Thin films CIGS a-sih mc-si CdTe Nanostructured PV Nanowires (Si, Ge) Quantum dot (Si, Ge) 10µm
Outline Introduction Photovoltaic technologies and market Applications Promising Thin film technologies Stacked layers Lab performances Nano based solar cell Nanowires Nanocrystals Conclusion
CdTe & CIGS thin film solar cells Low cost technology: Glass or metallic substrates, Large area process, with low material consumption. Potential for high efficiency: Commercial devices: 10% - 12%. Best laboratory devices: 16.5% for CdTe & 19.5% for CIGS (NREL) Superstrate structure Substrate structure Powalla and Bonnet, Advances in OptoElectronics (2007)
Wide bandgap CIGS absorber Standard Cu(In,Ga)Se 2 solar cells: Ga ~ 30% bandgap ~ 1.2 ev open circuit voltage ~ 600 mv Objective: Wide bandgap Cu(In,Ga)Se 2 solar cells with high open circuit voltage (useful for mobile applications). Results: ZnO:Al (0,35 µm) i-zno (0,08 µm) CdS (0,05 µm) CIGS (1,8 µm) Mo (0,4 µm) Glass (1 mm) Elaboration of wide bandgap CIGS thin film (Ga~50%, bandgap~1.35 ev), using an optimized evaporation process V oc (mv) 900 800 700 600 500 400 H. Marko et al. to be submitted x (-) 0,0 0,2 0,4 0,6 0,8 1,0 Droite d'idéalité X Our result X Cette étude Colorado State University (2005) ASC (2003) IEC (1996) 1,0 1,1 1,2 1,3 1,4 1,5 1,6 1,7 Eg (ev) η (%) 17 16 15 14 13 12 11 10 9 8 7 x (-) 0,0 0,2 0,4 0,6 0,8 1,0 Our result X Cette étude Colorado State University (2005) ASC (2003) IEC (1996) X 1,0 1,1 1,2 1,3 1,4 1,5 1,6 1,7 Eg (ev) Efficiency η = 14.7% Open circuit voltage V oc = 797 mv Future works: Replacing the CdS buffer layer by a Cd-free buffer layer, such as Zn(O,S).
CdTe solar cell Standard CdTe solar cells: Bandgap 1.45 ev open circuit voltage ~ 800 mv Objectives: Better improvement on film micro morphologies and interfaces Results: SER 121.3 (AM 1.5) (Close space sublimation) CSS CBD I (A) 0,07 0,06 0,05 0,04 0,03 0,02 0,01 0-0,3-0,2-0,1 0 0,1 0,2 0,3 0,4 0,5 0,6 0,7 0,8 0,9 1 1,1 1,2 1,3-0,01-0,02-0,03 Isc : 23mA/cm² Voc:810mV FF:69% Efficieny:13% U (V) Future works: CdTe absorber thickness < 2 µm with comparable efficiency > 13%
Outline Introduction Photovoltaic technologies and market Applications Promising Thin film technologies Stacked layers Lab performances Nano based solar cell Nanowires Nanocrystals Conclusion
Silicon nanowire solar cells CVD-grown silicon nanowires Radial pn junction light 30 µm Kayes, J. Appl. Phys. (2005) Kelzenberg, IEEE PV Specialist Conf. (2008) Low-cost substrate CVD growth of silicon nanowires on glass or metallic substrates low cost (similar to thin film technology). Radial pn junction architecture high efficiency ( 15%, similar to silicon wafer technology), even for low-quality silicon material. Challenges: Silicon nanowire growth on low-cost substrates, with control of nanowire size and density. Radial pn junction formation. Front electrical contact formation Base Emitter
Integration of Si NW arrays into PV cells Results: For 100 mw/cm² U OC = 250 mv J SC = 17 ma/cm² η = 1.9% R S =5 Ω.cm² Perraud et al., Solar Energy Materials and Solar Cells (2009) The series resistance (5 Ω.cm²) is much lower than that obtained for CVD-grown silicon nanowire array solar cells reported in the recent literature (several 10 Ω.cm²). [Tsakalakos, Appl. Phys. Lett. (2007); Stelzner, Nanotechnology (2008)] This clear improvement is ascribed to the front surface planarization used in our process. Future works: Formation of radial junction silicon nanowires.
Silicon nanocrystal solar cells Tandem structure : very high efficiency (as high as 30%, similar to III-V compound technology). All-silicon thin-film technology : potential for cost reduction (compared to expensive technology based on epitaxial III-V compounds)
Conclusion CdTe semiconductor constitutes a first industrial demonstration for high efficiency thin film technologies. CIGS technology is promising in terms of efficiencies due to the adjustable bandgap (1eV-1,6eV) and processes (PVD, printing). 3 rd generation (NW,QD, multi-junctions) is still at a R&D level. Thin films and nano technologies will be competitives when their conversion efficiencies exceed ~15% on large area Cost-effective, High throughput. Materials and device innovations are required for further improvements Encapsulation materials (flexible substrate). Wide range of applications : building & solar plant, wireless sensors networks or mobile phone.
Acknowledgements : Jean Pierre JOLY _ CEA/LITEN/INES Simon PERRAUD _ CEA/LITEN Sergio BERNARDI _ CEA/LETI THANK YOU FOR YOUR ATTENTION
Photovoltaic energy harvesting MAIN CHARACTERISTIC ADVANTAGES DISADVANTAGES Si mono and poly Bandgap 1.1eV Best efficiency lab. 20-25% Voc 700mV:Jsc 40mA/cm² Abundant material and non toxic Technologies from Microelectronic Non optimal bandgap (indirect bandgap) a-si:h Bandgap 1.7eV Best efficiency lab. 12% Voc 880mV:Jsc 19.4mA/cm² Low cost and low material consumption process Staebler-Wronsky instability (degradation of properties under illumination) Low efficiency CIGS Bandgap 1.2eV Best efficiency lab. 20% Voc 640mV:Jsc 35mA/cm² Ajustable Bandgap Good performance with polycristalline material quality Production tools not available In ressource CdTe Bandgap 1.5eV Best efficiency lab. 15.8% Voc 843mV:Jsc 25.1mA/cm² Ideal Bandgap Te ressource Cd not environmental friendly AsGa Bandgap 1.4eV Best efficiency lab. 25% Voc 1022mV:Jsc 28mA/cm² Ideal Bandgap High efficiency Expensive process Multijunction GaInP/GaAs Best efficiency lab. 33% Voc 2488mV:Jsc 14.2mA/cm² High efficiency Expensive process