Transcript What is the highest photovoltaic efficiency

Křemík a svĕtlo: termodynamika, sluneční
články a nanotechnologie
Tom Markvart
Solar Energy Laboratory, Engineering Sciences,
University of Southampton, UK
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 lightharvesting.
Absorption coefficient, m-1
Optical absorption of silicon
Wavelength, m
1.2
1
0.8 0.7
0.6
0.5
0.4
0.3
102

W

L


X
K
1
10-2
E
EL
L
0.5
1
1.5
2
2.5
3
Photon energy, eV
3.5
EX Eg

<111>
<100>
4
4.5
X
Capturing sunlight
lost
trapped
Light trapping: a statistical energy balance
The étendue
c 3
Electromagnetic
Energy flux ~ n ~ n 2
refractive
indexn
energy
density
E  n2 cos
 A 
~ n.13 n2
1

c
 Electromagnetic
n2
2
energy
density
refractive
index
~ n.2223
d
n2
A
n12
 ph
4 n 2V

 4n 2 d
E
(Planck, c. 1900) (Yablonovitch
et al, 1982)
Light trapping in practice
PERL cell - UNSW
Dye sensitised (Grätzel) cell
Light
Electron
injection
Dye
Electrolyte
(Iodide /
Tri-iodide)
TiO2
nanocrystals
(diameter
20 nm)
Iodide I-
Figure courtesy of Martin Green,
University of New South Wales.
I3- Tri-iodide
Source: Grätzel lab, EPFL
c
W.H. Weber and J. Lambe, Appl. Opt. 1976
J.S. Batchelder, A.H. Zewail et al, Appl. Opt.
1979
PV cell
Managing light in the frequency space:
fluorescent collectors
PV cell
Managing light in the frequency space:
fluorescent collectors
U.Markvart
W.H.
Rau
Weber
et al,and
and
Appl.
J. Danos,
Lambe,
Phys. Lett.
Appl.
2005;
Opt. 1976
T.
L.
25th
PVSEC,
J.C. Batchelder,
J.S.
Goldschmidt
A.H.
et al,
Zewail
SOLMAT
et al,2009.
Appl. Opt.
2010.
1979
Photonic band stop
spectral flux density
Frequency management:
with photonics
luminescence
incident solar
Absorption channel
reflectance
1
E
Photon transport channel
photon energy
semiconductor
bandgap
photonic
bandgap
Ultrathin c-Si photonic bandgap solar cell
 ph  4n2d eh / kBTo
1.2
Quantum Efficiency
1.0
1m c-Si
with BSR
0.8
500m
c-Si
with
BSR
1m c-Si
with geometric
light trapping
0.6
0.4
1m c-Si with
photonic light
trapping
Efficiency
500m cell 30.3%
1 m cell (BSR only) 15.6%
0.2
1m cell (geometric light trap.) 26.0%
1m cell (photonic light trap.) 30.7%
0.0
300
500
700
wavelength, nm
900
1100
T. Markvart et al, RSC Adv. 2012
Energy transfer to silicon
DiO /
porphyrin
LB film (+ SiOx) or
directly attached
Dyes

spacer
c- silicon
Kuhn, 1970, Danos et al, 2008, 2010
Dye – silicon energy transfer
2
Normalised quenching rate
Near field (optical & dipole-dipole)
1.5
Far-field effects (reflection)
1
0.5
ground
0
0
50
100
150
200
250
300
350
400
Dye-silicon separation (nm)
Sommerfeld, 1909; Chance, Prock and Silbey, 1978
The evanescent field
Evanescent field
(wave optics)
+
-
Trapped electromagnetic field
Classical
(geometrical optics)
silicon
Interaction
Photosynthesis: from a nanochemical factory …
to a nanowire solar cell
_
p +
e
-
membrane
n
D.L. Dexter, J. Luminescence 1979;
L. Danos et al, Thin Solid Films 2008;
T. Markvart, Prog. Quant. Electron. 2000
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.
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 !