Added value technologies for solar cells

Transcription

1 Added value technologies for solar cells Ingeborg Kaus and Ragnar Fagerberg SINTEF Materials and Chemistry 1

2 Why do we want to add value? Commercial Si based solar cells have an efficiency of ~15% Plenty of room for improvement BUT maximum theoretical efficiency is only ~41%, realistically ~30% 20% of the photons come from this range 2

3 How can we improve this? Primarily by capturing larger portions of the spectrum Up/down-conversion coatings Down conversion: Split a high energy photon into two photons of lower energy or a lower energy photon and thermal energy Up conversion: Combine two low energy photons to create a photon with sufficient energy Tandem structures Placing one or more higher band gap cells on top of a Si based cell Building alternative thin film structures such as CIGS (Cu-In-Ga- Se) 3

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5 1,2 1 0,8 0,6 0,4 100my 200my 300my 50my 10my 0, <700nm <900nm <1100nm % Energy % Photons

6 1,6 1,4 1,2 1 0,8 0,6 0,4 0, Blue = solar spectrum Red = ideal spectral use of the solar spectrum in a Si solar cell (100 micron) when thermal losses and absorption effects are included Approx 50% of the photons are at 800nm +. Potential if these can be taken to nm, where the overall efficiency is high 6

7 Up-conversion Two main mechanisms: Exited State Absorption (ESA): An already excited ion absorbs another photon Energy Transfer Upconversion (ETU): The energy from one excited ion is transferred to another excited ion to create a doubly excited ion 7

8 Solar cell with up-conversion layer Rear contacts Front contacts Light with E f >E g Light with E g >E f Up-converted light Anti reflection coating Solar cell Reflector Up-conversion film 8

9 The chemical composition of an upconversion film Sample no. Er content (%) A host lattice with suitable properties: Low absorption of visible light Commonly used are NaYF 4, and Y Al Garnet (YAG) Y 2 O 3 and YAG containing activators and sensibilizers Activators: Ions that may be doubly exited Activators often have a low absorbance for low energy photons Erbium Sensibilizers: Ions with a larger optical absorption window Can transfer energy to an activator ion through an ETU process Ytterbium Yb content (%)

10 Absorption and emission mechanisms The nature of the emission mechanism is assumed to be independent of the absorption mechanism 10

11 Deposition methods Pulsed laser deposition Y 2 O 3 based materials Chemical solution deposition Y 2 O 3 based materials (SINTEF) YAG based materials (NTNU) Dip coating Spray coating Photo: Melinda Gaal 11

12 Chemical solution deposition Y(NO 3 ) 3 xh 2 O Stirring Stirring Transparent solution Al(NO 3 ) 3 9H 2 O 0.2 M CH 3 COOH 1,2-Ethanediol Calcination Calcination/Annealing Sol Dip-coating Dip-coating Thin films Aim of work: Synthesis of thin films by sol-gel and dipcoating/spin coating methods for PV solar cells applications Substrates: Si and glass Work performed in collaboration with Prof. Tor Grande and Post. Doc. Edita Garskaite, NTNU 12

13 Film characterization Up conversion effect Microstructure Scratch resistance Micrographs of cross section (70 nm thick) and surface (agglomerated primary particles (10 nm) ) 13

14 Pulsed laser deposition Lab established in collaboration with Prof. Thomas Tybell, NTNU Incoming laser beam Focusing lens Holder for source material Plasma cloud Aperture Window for laser Antechamber Substrate Heating element Vacuum chamber 14

15 Film structure Growth in the (111) direction 15

16 Optical characterization Sample holder, x and z adjustable IR filter 50 mm lens 75 mm lens Sample 11mm iris Upconverted light 980 nm laser diode Optical fiber to detector Upconverted light Optical characterization is performed in collaboration with Prof. Mikael Lindgren, NTNU 16

17 Conversion results 10%Er/0%Yb 20%Er/1%Yb 10%Er/1%Yb 1%Er/1%Yb 0.5%Er/1%Yb Conversion to green light (563nm) in all samples Conversion to red (660nm) and traces of blue (410nm) for high concentrations of Er Presence of Yb does not alter the spectrum significantly Normalized data 20% Er gives a significantly lower response than lower Er content 17

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19 Effect of microstructure Edge area (red dot) gives a hundredfold increased emissions Film cracking Better results with multicrystalline films? lots of room for optimization! 19

20 Thin films for tandem cell structures Have recently decided to invest in a PE-CVD unit Oxford Instruments Plasmalab System 100 Have a project for development of thin films for tandem cell structures Si quantum dots in SiO 2 matrix Nanometer size dots of Si dispersed in a SiO 2 matrix Doped thin films of Si:H Amorphous Microcrystalline Investment in collaboration with several groups at NTNU 20

21 How much value did we add? Up-conversion: fractions of a percent but a lot of room for improvement and optimization. Worth a note the up-conversion effect is more efficient at higher incoming intensity, and the effect should therefore be more significant for concentrated solar systems. Theoretical calculations show that the realistic efficiency limit increases from ~30% to ~40%. Tandem cell structures have shown 40% efficiency, 87% theoretical. 21

22 Alternative technologies 22

23 Grätzel cells Have activities building the ceramic structure Titania ZnO Strong activities on TCO films Expanding research into other parts of the cell 23

24 Electrochemical deposition of ZnO Electrochemical reaction involving oxygen (as percursor) Oxygen reduction addition of O 2 Total reaction: Zn 2+ + ½ O 2 + 2e - ZnO High Zn, gold,-1 V, 80ºC Electrolyte: ZnCl 2 + KCl, 50-90ºC, O 2 Cathode: tin oxide glass, gold covered glass Counterelectrode: Pt Reference: Mercurous sulfat electrode (MSE) Potentiostatic: -1 to -1.4 V vs MSE Growth rate varies temperature, chemistry and voltage 1 µm/h at 85ºC, -1.3 V, M ZnCl 2 Low Zn, tin oxide,-1.3 V, 80ºC Ref.: Peulon and Lincot 24

25 Electrochemical deposition of ZnO - 1,20 V Current density (ma/cm 2 ) Cathodic current vs time Electrochemical deposition of ZnO on Al-substrate Time (s) -1 V -1,05 V -1,1 V -1,2 V - 1,05 V - 1,0 V 25

26 ZnO nanostructures by PLD Recently started NANOMAT project Project manager, Prof. Helge Weman, NTNU Collaboration with Colorado School of Mines, Prof. Reuben Collins 26

27 Morphology of titania tubules Low concentration of titanium precursor, tubules with open end High concentration of titanium precursor, tubules with closed end Titania nanotubules Pore size 200nm Substrate film 27

28 Indium tin oxide (ITO) by Chemical Solution Deposition (CSD) Before: Porous and uneven thickness 100 nm Dense microstructures are promoted by: Suitable choice of precursor Suitable temperature treatment procedure Collaboration with Prof. Henk Verweij, Ohio State University Dense Now Dense, columnar grains Micrographs by Ton van Helvoort, NTNU 20 nm 50 nm M.L. Mottern, F. Tyholdt, A. Ulyashin, A.T.J. van Helvoort, H. Verweij and R. Bredesen, Thin solid films 515, 3918 (2007) 28

29 Summary 29

30 Techniques available PLD Upconversion layers ZnO nanostructures Chemical solution deposition Upconversion layers TCO coatings Spin coating Dip coating Spray coating Deposition of complex structures Titania structures PE-CVD/MO-CVD Si based thins films Photo: Lena Gill Sputtering α-sinx:h and α-si:h Close collaboration with NTNU Nanolab and MiNaLab in Oslo to give access to state-of-the-art equipment 30

31 Contact people PLD: Ragnar Fagerberg Chemical solution deposition: Frode Tyholdt, Tommy Mokkelbost, Ingeborg Kaus, Anita Fossdal PE-CVD: Ragnar Fagerberg, Ingeborg Kaus MO-CVD: Frode Tyholdt Titania structures: Juan Yang NTNU Nanolab: Ingeborg Kaus, Anita Fossdal MiNaLab: Frode Tyholdt 31