Enhancing Light Capture for Silicon Solar Cells: From Thermodynamics to Nanotechnology

1. Křemík a svĕtlo: termodynamika, sluneční články a nanotechnologie Solar Energy Laboratory, Engineering Sciences, University of Southampton, UK Tom Markvart

2. Outline • Introduction: how to enhance light capture by crystalline silicon solar cells • The thermodynamics of light trapping: from Planck to the photonic bandgap solar cell • Sub-wavelength and nanoscale structures: about molecules, waveguides and photosynthetic light-harvesting.

3. Optical absorption of silicon

4. Capturing sunlight

5. Energy flux Electromagnetic energy density refractive index . c Electromagnetic energy density refractive index . d Light trapping: a statistical energy balance (Yablonovitch et al, 1982) (Planck, c. 1900)

6. I- 3 Light trapping in practice Dye sensitised (Grätzel) cell PERL cell - UNSW Light Electron injection Dye Electrolyte (Iodide / Tri-iodide) TiO2 nanocrystals (diameter 20 nm) Iodide I- Tri-iodide Figure courtesy of Martin Green, University of New South Wales. Source: Grätzel lab, EPFL

7. PV cell Managing light in the frequency space: fluorescent collectors W.H. Weber and J. Lambe, Appl. Opt. 1976 J.S. Batchelder, A.H. Zewail et al, Appl. Opt. 1979

8. PV cell PV cell Managing light in the frequency space: fluorescent collectors U. Rau et al, Appl. Phys. Lett. 2005; J.C. Goldschmidt et al, SOLMAT 2009. W.H. Weber and J. Lambe, Appl. Opt. 1976 J.S. Batchelder, A.H. Zewail et al, Appl. Opt. 1979 T. Markvart and L. Danos, 25th PVSEC, 2010.

9. 1 DE reflectance Photonic band stop photon energy semiconductor bandgap photonic bandgap Frequency management: with photonics luminescence incident solar spectral flux density Absorption channel Photon transport channel

10. 1mm c-Si with geometric light trapping 1mm c-Si with BSR 1mm c-Si with photonic light trapping Ultrathin c-Si photonic bandgap solar cell 500mm c-Si with BSR Efficiency 500mm cell 30.3% 1 mm cell (BSR only) 15.6% 1mm cell (geometric light trap.) 26.0% 1mm cell (photonic light trap.) 30.7% T. Markvart et al, RSC Adv. 2012

11.                   Time-resolved fluorescence DiO / porphyrin LB film (+ SiOx) or directly attached spacer Energy transfer to silicon Dyes c- silicon Kuhn, 1970, Danos et al, 2008, 2010

12. Dye – silicon energy transfer Near field (optical & dipole-dipole) Far-field effects (reflection) Sommerfeld, 1909; Chance, Prock and Silbey, 1978

13. Interaction Evanescent field (wave optics) Trapped electro-magnetic field Classical (geometrical optics) Classical (geometrical optics) The evanescent field silicon

14. to a nanowire solar cell e- membrane Photosynthesis: from a nanochemical factory … D.L. Dexter, J. Luminescence 1979; L. Danos et al, Thin Solid Films 2008; T. Markvart, Prog. Quant. Electron. 2000

15. Conclusion • We have reviewed the techniques currently available for enhancing the light capture by poorly absorbing materials such as silicon. Aside from geometric concentration these include: • Light trapping, closely linked with the directional equilibrium of the photon gas. Extension to the full thermodynamic equilibrium leads to the concepts of fluorescent concentrators / collectors and the photonic bandgap solar cell. • Sub-wavelength and nanoscale techniques, including photon tunnelling directly into the trapped (waveguiding) modes via the evanescent field, and the excitation of electron-hole pairs mediated by the dipole near field, similar to the light harvesting energy collection in photosynthesis.

16. Thanks to:  Current and previous staff and students at Solar Energy Laboratory, University of Southampton Funding sources: PV21 Supergen, Engineering and Physical Sciences Research Council, UK; The Carbon Trust, UK  … and to you for your attention !