Photonic strategies for light trapping
in nanostructured solar cells
Otto L. Muskens
School of Physics and Astronomy, University of
Southampton, UK
Acknowledgements
• AMOLF
Ad Lagendijk
Jaime Gómez Rivas
• Philips Research
George Immink
Erik Bakkers (Eindhoven)
Integrated Nanophotonics Group
Started in 2009
Group leader
Otto Muskens
Postdoc
Fedor Tikhonenko
PhD students
Martina Abb
Natasha Fairbairn
Tom Strudley
Integrated Nanophotonics Group
Current projects
Integrated Nanophotonics
Plasmonic
Nanoantennas
- M. Abb
- F. Tikhonenko
Bio-applications
- N. Fairbairn
with A. Kanaras
579 nm
691 nm
819 nm
Light trapping
- T. Strudley
Outline
Omnidirectional AR-coatings
Light trapping by multiple scattering
Plasmonic solar cells
Outline
Photonics challenges in PV
-
Minimize reflective losses
Reduce absorption length (in parts of the spectrum)
Do not compromise electrical performance
Cost-effective
Different systems require different approaches
- Crystalline Si (Generation 1)
- Planar thin-film: a-Si, CdTe, CIGS (Generation 2)
- Nanostructured thin films, Dye-sensitized TiO2, polymer,...
- Nanomaterials: nanowires, quantum dots, ...
Graded-index AR-coating
‘Moth-eye’ coating
- Gradual increase of refractive
index from 1.0 (air) to 3.5 (Si)
- Tapered nanostructures d << λ
(n1 − n2 ) 2
RFresn =
(n1 + n2 ) 2
Diedenhofen et al., Adv. Mater. 2009
Clapham & Hutley, Nature 1973
Huang et al., Nature Nanotech. 2007
Low-index effective medium
Ultrablack CNT film
- Material volume fraction < 10%
- Effective refractive index ~1.01
- Long absorption length (10-100µm)
Ayajan et al., Nano Lett. 2008
Photonic crystals
1D, 2D, or 3D periodicity
- On scale of wavelength (λ/2)
- In 3D using spheres: opals
- Effect of Bragg reflections:
slow light
Stop band
Dye absorption:
Opal
Normal TiO2
Nishimura et al., JACS 2003
Photonic crystals
Our work: 3D opals
- Combination of band structure
& light scattering
Muskens et al., Arxiv 2011 (PRB in press)
Random media
Vapor-Liquid-Solid growth of nanowire photonic materials
VLS: 1050 s
Lateral: 0 s
<d> = 24 nm
φ ~ 5%
L = 1.3 µm
420 oC
Muskens et al, Appl. Phys. Lett. (2006)
630 oC
VLS: 1050 s
Lateral: 1400 s
<d> = 109 nm
φ ~ 55%
L = 1.8 µm
Random media
Light trapping in nanowire layers
- Multiple scattering random walk
- Folding of absorption length
Muskens et al., Nano Lett. 2008
Random media
Light trapping in random media
- Multiple scattering random walk
light diffusion
- Folding of absorption length
Absorption length
Labs = cTabs
Diffusive light transport during Tabs
∆xdiff = DTabs
Definition of diffusion constant D
D = 13 clmfp
∆xdiff =
1
3
Labs lmfp
e.g. if Labs=10 µm, lmfp=0.5µm
then ∆xdiff= 1.3 µm
Random media
Light trapping in random media
- Light scattering leads to diffuse reflection
- Trade-off: diffuse absorption vs. reflective losses
- Possible to reduce device size by factor 3 with R<10%
∆xdiff/Labs
1
0.1
0.01
1
10
Labs/lmfp
100
Labs / lmfp
Muskens et al., Nano Lett. 2008
Random media
Light trapping in random media
- Example: nanowires of GaP, InP, silicon
Light trapping by nanowires
Resonant optical modes in nanowires
Resonant modes of a single Ge
nanowire for perpendicular
illumination.
Cao et al. Nature Mater. (2009)
Light trapping by nanowires
• Resonant extinction of GaP NW,
increasing wire diameters
<d>=51 ± 9 nm
<d>=85 ± 12 nm
<d>=118 ± 19 nm
Weak
scattering
(Rayleigh)
Resonant
scattering (Mie)
Muskens et al., Nano Lett. 2009
Dynamic transport
Question: how much time does the light stay trapped?
- Dynamical transport measurements
- Look at the frequency distribution of scattered light
Light trapping modes
∆ν = T
−1
diff
D
= 2
Lslab
Dynamic transport measurements
Fit of speckle correlation to C1 correlation
(Genack EPL 1990; Van Albada PRL 1991; De Boer PRB 1992)
Experimental parameters:
L = 6.0 ± 0.5µm
l = 0.3 ± 0.04 µm
Fitting result:
D = 21 ± 4 m2/s
Muskens, Lagendijk, Opt. Lett. 2009
Plasmonic solar cells
Using strong light scattering and field enhancement by
metal nanoparticles
Atwater, Polman, Nature Mater. 2010
Plasmonic solar cells
Enhanced photocurrent by silver particle scattering
Stuart, Hall, Appl. Phys. Lett. 1998
Plasmonic solar cells
Our work: nanoantennas for fluorescence enhancement
Muskens et al., Nano Lett. 2007
Conclusions
Photonic strategies
- Minimize R
- Maximize absorption length: slow light, folding,
trapping of light
Some examples of our work
- Characterize light trapping in nanomaterials
- Broadband techniques covering VIS & NIR 500-2000nm
- New directions: ultrafast, microscopic mode mapping