Post on 22-May-2020
2132-1
Winter College on Optics and Energy
J. Nelson
8 - 19 February 2010
Imperial CollegeLondon
U.K.
Third generation photovoltaics
Optics & Solar Energy : Nelson : Lec. 3
ICTP Winter College on Optics and Energy8 – 19 February 2010
Jenny NelsonDepartment of PhysicsImperial College London
(jenny.nelson@imperial.ac.uk)
Third generation photovoltaics
Optics & Solar Energy : Nelson : Lec. 3
THE FUTURE FOR PV
Optics & Solar Energy : Nelson : Lec. 3
Strategies to cost reduction
Use lessphotovoltaic
material?
Use cheaperphotovoltaic
material?
More workper photon?
1995 2000 2005 2010 2015 2020 2025 2030 2035
1
10 ~ 30 c / kWh
system module
Cos
t / U
SD
Wp-1
Year
~ 3 c / kWh
Barrier to implementation is the capital cost of the module
Dominated by the cost of silicon
Rep
ort I
EA
-PVP
S T
1-16
:200
7
Optics & Solar Energy : Nelson : Lec. 3
Outline
1. Future directions for PV
2. Using cheaper photovoltaic materials
3. Limiting efficiency of solar cells
4. Routes to more work per photon
5. More efficient light harvesting
Optics & Solar Energy : Nelson : Lec. 3
Cheaper photovoltaic materialsCheaper photovoltaic materials
To be treated in detail in future lectures
Optics & Solar Energy : Nelson : Lec. 3
Device structures
• Donor‐acceptor bulk heterojunction devices
Al cathode
Donor-Acceptor blend: e.g. polymer / fullerene, polymer / nanocrystal, polymer / polymer
ITO anode
Glass substrate
h+e-
• Both components deposited from same solution
Optics & Solar Energy : Nelson : Lec. 3
contacts active layer
Current and voltage output
flexible substratebarrier coating
Light
Generic device structure
• Active layer ~100 nm thick
• Low temperature processing enables use of flexible substrates
Optics & Solar Energy : Nelson : Lec. 3
Outline
1. Future directions for PV
2. Using cheaper photovoltaic materials
3. Limiting efficiency of solar cells
4. Routes to more work per photon
5. More efficient light harvesting
Optics & Solar Energy : Nelson : Lec. 3
National Renewable Energy Laboratory
Optics & Solar Energy : Nelson : Lec. 3
Detailed balance limit
(i) One electron hole pair per photon with hν > Eg,
(ii) Carriers relax to form separate Fermi distributions at lattice temperature Tambient with quasi Fermi levels separated by Δμ.
(iii) All electrons extracted with same electrochemical potential Δμ = eV
(iv) Only loss process is spontaneous emission
Band gap
Optics & Solar Energy : Nelson : Lec. 3
Limiting power conversion efficiency
Ta
Ta, μ
Ts
Ta
θs
Work
A fraction Xβ of the “sky” emits solar radiation at a black body temperature of Tsun
A fraction (1 ‐ Xβ) of the “sky”emits ambient radiation at a black body temperature of
Tambient
The photovoltaic energy converter emits ambient radiation at a black body
temperature of Tambient and a chemical potential of Δμ
The remaining charge pairs provide a current of electrons with chemical potential of Δμ
Optics & Solar Energy : Nelson : Lec. 3
Calculation of limiting efficiency
( ) ∫ −=Δ Δ−
max
min1
2,,, /)(
2
23maxmin
E
ETkE dE
eE
chTEEN
Bμ
πμ
( ) ∫ −=Δ Δ−
max
min1
2,,, /)(
3
23maxmin
E
ETkE dE
eE
chTEEL
Bμ
πμ
( ) ( ) ( ) ( ){ }qVTENTENXfTENXfqVJ agagssgs ,,,0,,,10,,,)( ∞−∞−+∞=
)0,,,0( sss TLXfP ∞=sP
VVJ )(=η
Photon flux density from sun (T = Tsun) or ambient (T = Tambient)
Irradiance from sun (T = Tsun)
Balance fluxes to obtain current
Power conversion efficiency
Optics & Solar Energy : Nelson : Lec. 3
Ts, ΔμTs, 0
Δμ N
Tsun,0
Ts, 0
Xβ
1 - Xβ
N
Particle flux
( ) ( ) ( ) ( )dEEbedEEbXdEEbXeJ
ggg Eambient
kT
Eambient
Esun ∫∫∫
∞Δ
∞∞
−−+= /1 μββ
( )1/0 −−= kTeV
sc eJJJ
Optics & Solar Energy : Nelson : Lec. 3
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Pho
ton
flux
dens
ity /
s-1 m
-2 e
V-1
Photon Energy / eV
sun ambient
Eg
Optics & Solar Energy : Nelson : Lec. 3
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Pho
ton
flux
dens
ity /
s-1 m
-2 e
V-1
Photon Energy / eV
sun ambient
Eg
Jsc
J0
Optics & Solar Energy : Nelson : Lec. 3
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Pho
ton
flux
dens
ity /
s-1 m
-2 e
V-1
Photon Energy / eV
sun ambient
Eg
Optics & Solar Energy : Nelson : Lec. 3
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
Pho
ton
flux
dens
ity /
s-1 m
-2 e
V-1
Photon Energy / eV
sun ambient
Eg
Jsc
J0
Optics & Solar Energy : Nelson : Lec. 3
0.0 0.5 1.0 1.5 2.0
0
200
400
600
800
Eg = 2.0 eV
Eg = 0.7 eV
Cur
rent
den
sity
/ A
m-2
Voltage / V
0.0 0.5 1.0 1.5 2.0 2.5 3.00
100
200
300
400
500
600
700
800
0
2
4
Voc
Jsc
Phot
ocur
rent
den
sity
/ A
m-2
Band gap / eV
Ope
n ci
rcui
t Vol
tage
/ V
0.0 0.5 1.0 1.5 2.0 2.5 3.00.0
0.1
0.2
0.3
0.4
0.5
0.6
X = 1
Effic
ienc
y
Band gap / eV
Optics & Solar Energy : Nelson : Lec. 3
Limiting efficiency of single band gap cell
0
200
400
600
800
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00
Photon Energy (eV)
Irrad
ianc
e (W
m-2
eV-1
)
Optimum cell converts 31% of power
Power spectrum from black body sun at 5760K
Lost by thermalisation
Lost by transmission
Band gap
Optics & Solar Energy : Nelson : Lec. 3
Actual versus ideal PV performance
0.00
0.10
0.20
0.30
0.40
0.50 1.00 1.50 2.00 2.50
Band Gap / eV
Effic
ienc
y
Molecular
amorphous SiCdTe
GaAsc-Si
poly-SiCIGS
Optics & Solar Energy : Nelson : Lec. 3
Actual versus ideal PV performance
0
10
20
30
40
50
60
70
80
0.5 0.6 0.7 0.8 0.9 1 1.1 1.2
Voc /V
Jsc
/ mA
cm
-2
GaAs
a-Si
CdTe
poly-Si
c-Si
CIGS
theoretical limit
Optics & Solar Energy : Nelson : Lec. 3
Limiting efficiency under full concentration
( ) ∫ −=Δ Δ−
max
min1
2,,, /)(
2
23maxmin
E
ETkE dE
eE
chTEEN
Bμ
πμ
( ) ∫ −=Δ Δ−
max
min1
2,,, /)(
3
23maxmin
E
ETkE dE
eE
chTEEL
Bμ
πμ
)0,,,0( sss TLXfP ∞=sP
VVJ )(=η
( ) ( ){ }qVTENTENqVJ agsg ,,,0,,,)( ∞−∞=
Photon flux density from sun (T = Tsun) or ambient (T = Tambient)
Irradiance from sun (T = Tsun)
Balance fluxes to obtain current
At full concentration
Power conversion efficiency
Optics & Solar Energy : Nelson : Lec. 3
Limiting efficiency under full concentration
Ta
Ta, μ
Ts
Ta
θs
Work
Ta
Ta, μ
Ts
θs
Work
0
0.1
0.2
0.3
0.4
0.5
0 0.5 1 1.5 2 2.5 3
Band gap / eVEf
ficie
ncy
1 sun
full concentration
Si GaAs
Optics & Solar Energy : Nelson : Lec. 3
Outline
1. Future directions for PV
2. Using cheaper photovoltaic materials
3. Limiting efficiency of solar cells
4. Routes to more work per photon
5. More efficient light harvesting
Optics & Solar Energy : Nelson : Lec. 3
How to get more ...
• More work per photon with:
– more band gaps
• tandems and intermediate bands
– more work per photon
• prevent carrier thermalisation in “hot carrier” solar cells
– more electrons per photon
• Impact ionisation and up‐or down‐conversion of light
Optics & Solar Energy : Nelson : Lec. 3
Route 1: Multiple band gaps
IR IR
red
red blue blue
Single band gap Two band gaps
0
200
400
600
800
0.00 0.50 1.00 1.50 2.00 2.50 3.00 3.50 4.00
Photon Energy (eV)
Irrad
ianc
e (W
m-2
eV-1
) Black body sunOne band gapTwo band gapsThree band gaps
• Approaches using tandem cells, spectral splitting and quantum semiconductor structures
Optics & Solar Energy : Nelson : Lec. 3
Triple junction cell
GaInP GaAs Ge
AlInP GaInP
GaInP
AlGaInP
GaInP
GaAs
GaAs
GaAs
GaInP
GRID
n+
n
n
n
p
p
n
p
p
Ge substrate p doped
TOP CELL
GaAs GaAs n++
p++ TUNNEL JUNCTION
MIDDLE CELL
CONTACTING LAYER
GaAs GaAs n++
p++ TUNNEL JUNCTION
(Al)GaAs n
Ge n BOTTOM CELL
Highest efficiency for a 3 junction tandem: 40.7 % under concentration(Spectrolab, Dec 2006)
Optics & Solar Energy : Nelson : Lec. 3
Quantum well structures in solar cells
• Additional low energy photons absorbed in QWs in active region
• Charge carriers escape from QWs ‐ less energy lost by thermalisation than in homojunction
• Voc intermediate between values expected for QW and barrier material. Higher efficiency possible.
Optics & Solar Energy : Nelson : Lec. 3
Single Junction Quantum Well Solar Cell
June 2009 world record:28.3% at 535 suns
1µm
>1µm
Optics & Solar Energy : Nelson : Lec. 3
Route 2: Slowed cooling
generation equilibriation cooling recombination
3/2 kT h 3/2 kT a
• Use quantum nanostructures or molecular systems to harvest photogenerated charges before cooling?
Optics & Solar Energy : Nelson : Lec. 3
Hot carrier solar cells
Photogenerated electrons scatter with each other but not with lattice⇒ Fermi Dirac distribution at temperature TH (>Ta) and μH
Electrons extracted isoentropicallythrough narrow bandwidth contact
EoutEgμout = qV
0
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.5 1 1.5 2
Band gap / eVEf
ficie
ncy
1 sun
full concentration
Efficiency of hot carrier solar cell:
Maximum efficiency same as thermodynamic limit
Optics & Solar Energy : Nelson : Lec. 3
Route 3: Utilising high and low energy photons by up‐ and down‐conversion
• Up‐conversion: low energy photons converted into high energy photon (~ Eg)and absorbed by device. E.g. rare earth doped ceramics
• Down‐conversion: high energy photons generate multiple lower energy photons (~ Eg)
Solar cellInsulator
up-converter
mirrorE~Eg
Optics & Solar Energy : Nelson : Lec. 3
Multiple electrons per photon
c.b.
v.b.
k k
E c0
0
E
E g
k + k 0
• Generate multiple electrons per photon by impact ionisation (or Auger generation)
• Observed experimentally in Si and Ge, and now in PbSequantum dots
• Analysis equivalent to hot electron solar cell with μH = 00
0.1
0.2
0.3
0.4
0.5
0.6
0.7
0.8
0.9
1
0 0.5 1 1.5 2
Band gap / eV
Effic
ienc
y
Auger full
Auger 1 sun
Ellingson et al, SPIE, 2006
Optics & Solar Energy : Nelson : Lec. 3
Outline
1. Future directions for PV
2. Using cheaper photovoltaic materials
3. Limiting efficiency of solar cells
4. Routes to more work per photon
5. More efficient light harvesting
Optics & Solar Energy : Nelson : Lec. 3
More More photogenerationphotogeneration per unit volumeper unit volumeb s
b s
Anti-reflection coatings
X b s
Concentration
b s
Light trapping
b s
Reabsorbing luminescence
Optics & Solar Energy : Nelson : Lec. 3
Concentration
• Parabolic concentrators, Fresnel lenses
• Need for direct sunlight
• Use with high efficiency (III‐V) photovoltaic materials
• Use with optical fibre technology?
0.00
0.10
0.20
0.30
0.40
0.00 0.50 1.00 1.50 2.00 2.50
Band Gap (eV)
Effic
ienc
y
1 sun
1000 suns
Parabolic concentrator
Fresnel lens
SolarSystems
FISE, Ioffe Inst.
Optics & Solar Energy : Nelson : Lec. 3
Light trapping
• Light trapping structures: increase optical path length, reduce reflection
• Aim to reduce silicon cell thickness from ~300 μm to ~10 μm
More in Prof Bagnall’s lecture …
Optics & Solar Energy : Nelson : Lec. 3
Better light harvesting with optical design: planar structures
• Device structures ~ wavelength of light
• Interference critically important
• Design device structures to maximise absorption in active layer and at donor‐acceptor interface
• Can afford to lose absorption at short λ to gain at long λ
e.g., Pettersson et al, J.Appl.Phys. 86, 487 (1999)
Optics & Solar Energy : Nelson : Lec. 3
Type 1 Type 2 Type 3
Light trapping in grating structures
• Larger absorption enhancements available with 1D or 2D grating structures.
• Rigorous Coupled Wave Analysis to predict absorptance of patterned device
• Soft embossing to imprint grating on polymer
Felix Braun
Optics & Solar Energy : Nelson : Lec. 3
Luminescent concentrator
Optics & Solar Energy : Nelson : Lec. 3
Luminescent concentrator
• Luminescent dyes or quantum dots dispersed in refractive material
• Cheaper than imaging concentrators
• Harvests diffuse light
• Able to concentrate photon energies near band gap of cell
+-
Optics & Solar Energy : Nelson : Lec. 3
Stacked luminescent concentrators
• Stacked luminescent concentrators to split the solar spectrum
• Selective band gap cells for higher efficiency (more work per photon)
• Research at Imperial on use of II‐VI quantum dots as luminescent species, (A. J. Chatten et al.)
Small band gap cell
Intermediate band gap cell
Large band gap cell
Optics & Solar Energy : Nelson : Lec. 3
Summary
• The cost per Watt of PV electricity can be increased by:
• Reducing the cost of PV material
– By use of inorganic thin film materials
– E.g. by use of molecular PV materials
• Increasing the amount of work per photon
– with multijunction “tandem” structures or other (quantum) heterostructures
– Exploiting hot carrier effects or multiple electrons per photon
• Reducing the amount of PV material per photon harvested
– Concentration of light (including luminescent concentrators)
– Light trapping
Optics & Solar Energy : Nelson : Lec. 3
Key challenges
VoltageC
urre
nt d
ensi
ty J
Silicon
Voc limited by:interfacial recombinationinterfacial energy levels
Jsc limited by: absorption of red light,interface morphologycharge transport
Fill factor limited by: low mobilityseries resistanceshort-circuits
Best η 25%
Organic solar cellBest η ~6%
400 500 6 00 70 0 80 0 900 10 00 110 0 120 0 1 3000
1
Irrad
ianc
e /W
m-2nm
-1
W a v e le n g th / nm
Optics & Solar Energy : Nelson : Lec. 3
Exploiting Plasmonic Effects in Solar Cells
Internal Scattering Field EnhancementSurface Plasmon
Polariton Propagation
Optics & Solar Energy : Nelson : Lec. 3
0.0 0.5 1.0 1.5 2.0
0
200
400
600
800
Eg = 2.0 eV
Eg = 0.7 eV
Cur
rent
den
sity
/ A
m-2
Voltage / V
0.0 0.5 1.0 1.5 2.0 2.5 3.00
100
200
300
400
500
600
700
800
0
2
4
Voc
Jsc
Phot
ocur
rent
den
sity
/ A
m-2
Band gap / eV
Ope
n ci
rcui
t Vol
tage
/ V
0.0 0.5 1.0 1.5 2.0 2.5 3.00.0
0.1
0.2
0.3
0.4
0.5
0.6
X = 1
Full concentration
Effic
ienc
y
Band gap / eV