Uwe Rau Institut für Energieforschung 5 Photovoltaik- Forschungszentrum Jülich GmbH

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1 Mitglied der Helmholtz-Gemeinschaft Materialforschung für f r DünnschichtphotovoltaikD nnschichtphotovoltaik- Status und neue Entwicklungen Uwe Rau Institut für Energieforschung 5 Photovoltaik- Forschungszentrum Jülich GmbH

2 Inhalt 1. Marktentwicklung 2. Solarzellentechnologien und allgemeine Prinzipien 3. Dünnschichttechnologien (CIGS,CdTe, a/μc-si ) - Forschungsbeispiele

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4 Top 10 producers in 2007 JA Solar 29,5 132,4 300 Yingli ,5 280 Sanyo SolarWorld Motech First Solar , CdTe Production 2007 Production Kyocera Suntech Sharp Q-Cells , ,2 434, MW source: Institut PHOTON für Energieforschung International 5 Photovoltaik 3/2008

5 Thin-film module manufacturers ranking thin-film manufactures 2007 Bangkok Solar Fuji Electric Systems 11,1 6,8 12 0,5 all others a-si (/µc-si) Mitsubishi Heavy Industries Sinonar Würth Solar CIS Production 2007 Production 2006 Sharp 8,2 21 Trony Kaneka United Solar First Solar 5, CdTe MW source: PHOTON International 3/2008

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7 Photovoltaic technologies (and their working principles)

8 Different types of solar cells Wafer cell Thin-film cell Electrochemical cell Organic solar cell cryst. Si amorph. Si, Cu(In,Ga)Se 2, CdTe nanopor. TiO 2 PPV/C 60 Efficiency 25 % 19 % 10 % 5 %

9 Excitonic and bipolar (classical) solar cells 3. charge separation at interface Excitonic Bipolar 3. charge separation by built-in field J n _ + J x J p 2. diffusion J n J p 2. diffusion _ + x light 1. generation light 1. generation Gregg and Hanna, J. Appl. Phys. 93, 3605 (2003).

10 General charge separation scheme illumination _ excitation excitonic transport _ + first charge separation p-contact _ final charge separation bipolar transport _ n-contact U. Rau et al., J. Phys. Chem B (2003) K. Schwarzburg et al., Coord. Chem. (2005).

11 pin-type and pn-type devices pin-type n-type contact light _ absorber 0 J n x _ J x + p-type contact x + d J p E C E V interface dissociation bulk dissociation x n-type contact _ light J n T. Kirchartz, J. Mattheis, U. Rau, Phys. Rev. B 78 (2008) absorber 0 x _ J x p-type contact x + + d J p E C E V x pn-type

12 Dominant currents at junction (x=+0) J n J x _ + J n J p _ + x J p x = 0 d short circuit current density J sc [macm -2 ] J n bipolar Bipolar J x J x J p J p T. Kirchartz, J. Mattheis, U. Rau, Phys. Rev. B 78 (2008) J n excitonic Excitonic dissociation lifetime τ D [s] τ D = 10-3 s cf. Fig. 3 Criterion: Currents at interface

13 Thin-film photovoltaic technologies

14 Thin film PV technologies CIGS-solar cells: CuIn 1-x Ga x Se 1-y S y Light CdTe-solar cells: CdTe Light a-si technology Example: a-si/µc-si tandem cell ( Micromorph ) Light n ZnO:Al i-zno (1 µm) (0.05 µm) Glass (1-4mm) Glass (1-4mm) p CdS CIGS Mo (0.05 µm) 4µm (2 µm) (0.5 µm) TCO CdS (0.1 µm) CdTe (3-8 µm) p i n p i TCO a-si:h µc-si:h 3µm Glass (3 mm) Metal n ZnO Ag

15 c-si 15 % CdTe 11 % a-si 6 % Cu(In,Ga)Se 2 CIGS 12 % Crystalline silicon Thin-films

16 Solar cell efficiencies (Labscale) 30 III-V multijunct. Efficiency η[%] CuInSe c-si 2 CdTe stacked a-si:h single Year

17 Efficiency limits Silicon Cu(In,Ga)Se 2 33 % SQ CdTe a-si/μc-si 25 % Best Lab 19 % Mod. 20 % 17 % 13 % 14 % 12 % 11 % 9 % 9 % 7 % 7 %

18 Schematic representation of a CIGS module fabrication process. Mo sputtering laser patterning absorber deposition patterning chemical deposition of CdS ZnO deposition patterning

19 Absorber deposition CIGS solar cells Selenization (two-stage) Co-evaporation I. Deposition II.Selenisation

20 Cu-poor and Cu-rich CuInGaSe 2 E g = 1.5 ev CuInS 2 Cu-arme Präparation Cu-rich (etched) = stoichiometric Cu-reiche Präparation Cu-poor CuInSe 2 CuGaSe 2 E g = 1.04 ev E g = 1.65 ev

21 Photoluminescence of CuInGaSe 2 E g = 1.5 ev CuInS 2 Cu-arme Präparation Cu-reiche Präparation Cu-rich (etched) Cu-poor CuInSe 2 CuGaSe 2 E g = 1.04 ev E g = 1.65 ev Cu-rich Cu-poor S. Zott, K. Leo, M. Ruckh, H.W. Schock, J. Appl. Phys. 82 (1997)

22 Band diagram (CuInGaSe 2 ) buffer window absorber Mo CdS CIGS back contact ZnO

23 Recombination mechanisms E nkt ln a 00 VOC = q q jsc j buffer window absorber (A): Interface recombination E a = Φ b (B-D): Volume recombination back contact E a = E g

24 Recombination mechanisms CuInS 2 E g =1.50 ev y CuInSe 2 E g =1.04 ev Cu-rich preparation Cu-poor preparation E x nkt ln a 00 VOC = q q jsc CuGaSe 2 E g =1.68 ev j Open Circuit Voltage V oc (V) E g =1.49 ev E g =1.22 ev 0.6 E g =1.43 ev 0.4 E g =1.15 ev CuIn(Se,S) 2 Cu-poor Cu-rich Temperature T (K) M.Turcu, O. Pakma, U. Rau, Appl. Phys. Lett. 80 (2002)

25 Recombination mechanisms Cu(In 1-x Ga x )(Se 1-y S y ) 2 Activation Energy E a (ev) x=0 1.6 x~0.25 Cu-poor y= x~ Cu-rich x= Band Gap Energy E g (ev) CdS Φ b p ΔΦ b Cu(In,Ga)(Se,S) 2 Cu-poor surface layer M.Turcu, O. Pakma, U. Rau, Appl. Phys. Lett. 80 (2002)

26 Cu(In,Ga)(Se,S) 2 research issues buffer absorber window back contact Faster absorber growth Metastability Na from glass reproducibility Improved buffer window/combinations (Cd-free) stability

27 Process sequence for CdS/CdTe solar cells Front contact deposition CdS deposition CdS deposition back contact final dev ice structure

28 CdS/CdTe interface electronic structure not activated activated E CB E F 0,3-0,08 0,55 1,49 E CB E F 0,1-0,08 0,35 2,42 1,49 1,01 2,42 E VB 0,17 1,01 CdS CdTe Interface defects E VB CdS interdiffusion CdTe W. Jaegermann, A. Klein, T. Mayer, Adv. Mat.. 21 (2009)

29 CdS/CdTe research issues buffer absorber window doping grain boundaries back contact contact material stability resistance improved understanding of CdS/CdTe interface

30 a-si/μc-si thin-film tandem solar cell Light 1.2 VIS IR 1.2 glass (1-4mm) ZnO a-si:h µc-si:h 3µm externe Quanteneffizienz a-si:h E G =1.7 ev µc-si:h E G =1.1 ev spektr. Teilchenstrom (rel.) λ (nm) 0.0

31 a:si:h/µc-si:h phase transition microcrystalline amorphous L. Houben, Dissertation, FZJ (IFF/IPV), Uni Düsseldorf O. Vetterl et al., Sol. Energ. Mat. Sol. Cells 62 (2000)

32 Multi-junction solar cells μc-si:h

33 Optimized ZnO for light trapping ZnO glass HCl 0.5 % quantum efficiency, absorbance qe 1 µm µc-si optimised short dip smooth wavelength (nm)

34 Simulations: Tandem cells on textured ZnO:Al K. Ding, Master Thesis, RWTH Aachen

35 Absorptance distribution Front p-layer Glass Back contact Doped layers Front TCO Reflexion a-si:h absorber µc-si:h absorber

36 Loss analysis (0V)

37 III: Thickness dependence Matching loss Recombination loss Transmission loss Matching loss

38 Tandem Solar Cell with Intermediate Reflector Requirements for the Intermediate Reflector: glass TCO a-si:h intermediate reflector 50 nm -150 nm µc-si:h sufficient conductance low absorption low refractive index to achieve high refractive index difference between Si and SiO x back reflector

39 SiO x intermediate reflector external quantum efficiency EQE ma/cm² No IL 9.9 ma/cm² 11.5 ma/cm² Δ EQE Top ma/cm² Δ EQE Top ma/cm² 22.2 ma/cm² 12.4 ma/cm² 10.7 ma/cm² Δ EQE Top ma/cm² with IL wavelength λ [nm] C. Das, et al., Appl. Phys. Lett. 92 (2008)

40 Scanning near-field optical microscopy Effect of micro-/nano-structures of textured ZnO on local optical properties Analyzed by Scanning Near field Optical Microscopy topography SNOM measured SNOM calculated Topography enters into first principle calculations K. Bittkau, et al., Phys. Rev. B 76 (2007) SNOM

41 asi/μcsi research issues Improved ligth scattering μc-si:h intermediate reflector back side reflector p-front contact faster absorber growth

42 Conclusions Photovoltaics has become a billion business.. on a partly (but then heavily) subsidized market Political goals can be met (on the technological level) Cost reduction is still (and more than ever) a major issue Challenges for thin-film technologies: Close the gap between lab and production scale efficiencies Faster and more reliable production methods Improved scientific understanding of optics, materials and interfaces