Light management for photovoltaics. Ando Kuypers, TNO Program manager Solar

Transcription

1 Light management for photovoltaics Ando Kuypers, TNO Program manager Solar

2 Global energy consumption: 500 ExaJoule/Year Solar irradiation on earth sphere: ExaJoule/year 2

3 Capturing 0,01% covers total energy need Requires km 2 present day solar cells (Surface area of Spain = km 2 ) Present world wide installation about 200 km 2 3

4 Present world wide installation about 200 km 2 Installed in 2010: >15 GWp or >100 km : 100 GW p, meaning 1000 km 2 /year For comparison: 2010 LCD market 300 km 2 /year Optical media market: 400 km 2 /year Architectural glass production 2000 km 2 /year 4

5 Contribution of PV and CSP will be substantial EJ/a geothermal other renewables solar thermal (heat only) solar power (photovoltaics (PV) & solar thermal generation (CSP) wind energy biomass (advanced) biomass (traditional) hydroelectricity nuclear power gas coal oil year 2100 Source: German Advisory Council on Global Change, 2003, adapted by Wim Sinke, ECN 5

6 Still 50% cost reduction required for grid parity Light management in solar cells important for $/W p cost reduction Photonics for photovoltaics? Only if technology is feasible on km 2 scale! And brings higher efficiency at lower cost (total module < 100/m 2 ) 6

7 The solar cell primary process: converting light into free charge carriers 7

8 Light in, charge carriers out Challenges: Maximize light incoupling Maximize effective surface area Minimize reflection Optimize angle of incidence Interaction probability with active layer Maximize interaction length Minimize chance of recombination Material properties, defect control Minimize travelling distance Electrodes with minimum resistivity loss Low contact resistance at boundaries Maximize conductivity, minimize electrode width These are competing properties! 8

9 Light capture in silicon wafer based solar modules R=4% n air = 1 Front cover glass A wavelength =5-20% n glass =1,5 Polymer Si wafer cell n = 3,4 Back cover glass T = 1 A R R = ((n 1 -n 2 )/(n 1 +n 2 )) 2 = 0,04 for air-glass 9

10 Light capture Si wafer solar cell 10 Source: Wikipedia Incident angle: texture Antireflection: SiN Interaction length: Al reflector Recombination: doping profile and AlOx passivation Resistivity loss: Ag

11 Electrodes: current out or light in? A compromise Reduced effective surface area Shadowing Limitations of screen printing 11

12 Contact formation in silver screenprinting proces not easily replaced Silverpaste with lead oxide and glass frit on SiN antireflection layer Firing >600 o C: liquid glass frit etches SiN Lead oxide etches Si via redox reaction resulting in liquid lead Liquid lead makes silver particles melt, and liquid silverlead selectively etches specific Si crystal orientation Lead positively influences silver-si contacting upon cooling 12

13 Maintain screenprinting proces Back contacts and optimalised line pattern Solland Sunweb cells (developed i.c.w. ECN) Larger effective surface area and less open space between cells Ease of integration 13

14 Combined screenprinting and electrochemistry: Decoupling contacting and conduction Meco, Metalor (CH), Photovoltech (B) Thin screen printed traces Electrochemical silver Higher conductivity Larger effective surface area by reduced shadowing Cell efficiency from 15,7% (ref) to 15,9% 14

15 Reduced shadowing effect by light management Texturing of electrode surface to compensate shadowing by reflection (1366 Technologies, US) 15

16 Case 2 Light in, current out: Thin film PV Glass or encapsulating layer Transparent front contact (TCO) Active layer: a-si, CIGS, CdTe, organic Back contact Encapsulating carrier or layer 16

17 Longer interaction length of light in active layer by light trapping Source: pvcdrom.pveducation.org 17

18 Longer interaction length of light in a-si by light trapping 18

19 Use of transparent conductor (TCO) for light trapping Transparent conductors: low charge carrier concentration with high mobility Doped oxides: ITO, F:SnO, Al:ZnO, B:ZnO, etc Maximized transmission for useful wavelength domain Texture by morphology control (crystal size and shape) 19

20 Cheapest TCO: atmospheric CVD tin oxide in on-line float glass production process CVD-positions Loading raw material 1050 C 600 C ~1600 C Melt oven Tin float bath Cooling and anneal 40 C Cutting Storage and distribution Source picture: Micro structures for PV light management at a few $/m 2! 20

21 Thin film on foil without cover glass Flexcell Helianthos Two a-si thin film modules: pink and black 21

22 Roll to roll light management Transparent protecting foil 1-2 µm Transparent conducting oxide Amorphous silicon solar cell Back contact (Flexible) substrate SnO 2 :F p i n metal Roll-to-roll atmospheric pressure chemical vapour deposition 22

23 Applied Materials 23

24 Applied Materials SunFab (thin film a-si) Turn key >20 MW p /yr/line Basis: TCO coated glass (typically 0,5 km 2 /yr) Present total world wide solar glass requirement >100 km 2 /yr (3-4% of total glass consumption) Source: Applied Materials 24

25 Thin film organic solar cells Organic transparent conductors by printing State of the art: indium-tin oxide TCO Novelty (2009): printable organic conductor (PEDOT) with thin ( invisible ) metallisation (Agfa and Holst Centre) 25

26 Photonics for PV: From micro to nano on square kilometers Quantum dot applications Up and down conversion Light concentrators Quantum dot production Kg-scale production fold price reduction required Quantum dot processing Prevent additional process steps: incorporation in existing layers Photonic crystals Light trapping Replication techniques Self assembly 26

27 Plasmonic photovoltaics Albert Polman et al Integration of metal nanostructures in thin film (a-si) solar cells Resonant light scattering from metal nanoparticles and light trapping Full solar spectrum absorption by coupling light into surface plasmon polaritons and photonic modes See also: Review article Plasmonics for improved photovoltaic devices, H.A. Atwater and A. Polman, Nature Materials, Feb

28 For a future with durable energy: Hard work on Light management ando.kuypers@tno.nl 28