Post on 02-Aug-2020
2132-2
Winter College on Optics and Energy
J. Nelson
8 - 19 February 2010
Imperial CollegeLondon
U.K.
Physics of Solar Cells (I)
Optics & Solar Energy : Nelson : Lec. 1
ICTP Winter College on Optics and Energy8 – 19 February 2010
Jenny NelsonDepartment of PhysicsImperial College London
(jenny.nelson@imperial.ac.uk)
Physics of Solar Cells (I)
Optics & Solar Energy : Nelson : Lec. 1
Outline
1. Photovoltaic applications
2. The solar resource
3. Photovoltaic energy conversion
4. The p‐n junction solar cell
5. Solar cell performance characteristics
Optics & Solar Energy : Nelson : Lec. 1
• Radiant power at Earth’s surface ~ 100000 TW
• Electricity consumption ~ 2 TW
• 80% from fossil fuels & nuclear, 0.1% from PV
The solar energy resource
Optics & Solar Energy : Nelson : Lec. 1
Some concepts in PV
Solar cellModule
System
Optics & Solar Energy : Nelson : Lec. 1
1.1. PHOTOVOLTAIC APPLICATIONS
Optics & Solar Energy : Nelson : Lec. 1
Photovoltaic applications
158
249
1347
54Domestic off-grid
Non-domestic off-grid
Grid-connecteddistributedGrid-connectedcentralised
Cumulative PV capacity 2003 (MW): 1.8 GWp
Optics & Solar Energy : Nelson : Lec. 1
Photovoltaic applications
311 430
8220
4464 Domestic off-grid
Non-domestic off-grid
Grid-connecteddistributedGrid-connectedcentralised
Cumulative PV capacity 2008 (MW): 13.4 GWp
Optics & Solar Energy : Nelson : Lec. 1
Optics & Solar Energy : Nelson : Lec. 1
Optics & Solar Energy : Nelson : Lec. 1
Optics & Solar Energy : Nelson : Lec. 1
Optics & Solar Energy : Nelson : Lec. 1
Optics & Solar Energy : Nelson : Lec. 1
1.2. THE SOLAR RESOURCE
Optics & Solar Energy : Nelson : Lec. 1
U V IR red
yello
w
gree
n
blue
purp
le
3 00 4 00 50 0 6 00 7 00 90 08 00 11 00 12 00 13 0 0
4.1 4 3 .10 2 .4 8 2 .0 7 1 .77 1 .3 81 .55 1 .2 4 1 .03 0 .9 6
λ [n m ]
E [eV ]
200
400
600
800
1000
1200
1400
1600
1800
2000
0 .0
0.5
1.0
1.5
2.0
Irrad
ianc
e / W
m-2 n
m-1
W ave leng th / nm
A M 0
Solar power spectrum
Optics & Solar Energy : Nelson : Lec. 1
U V IR red
yello
w
gree
n
blue
purp
le
3 00 4 00 50 0 6 00 7 00 90 08 00 11 00 12 00 13 0 0
4.1 4 3 .10 2 .4 8 2 .0 7 1 .77 1 .3 81 .55 1 .2 4 1 .03 0 .9 6
λ [n m ]
E [eV ]
200
400
600
800
1000
1200
1400
1600
1800
2000
0 .0
0.5
1.0
1.5
2.0
Irrad
ianc
e / W
m-2 n
m-1
W ave leng th / nm
A M 0 B B 5780K
Solar power spectrum
Power density 1353 W m-2 outside earth atmosphere
Optics & Solar Energy : Nelson : Lec. 1
U V IR red
yello
w
gree
n
blue
purp
le
3 00 4 00 50 0 6 00 7 00 90 08 00 11 00 12 00 13 0 0
4.1 4 3 .10 2 .4 8 2 .0 7 1 .77 1 .3 81 .55 1 .2 4 1 .03 0 .9 6
λ [n m ]
E [eV ]
200
400
600
800
1000
1200
1400
1600
1800
2000
0 .0
0.5
1.0
1.5
2.0
Irrad
ianc
e / W
m-2 n
m-1
W ave leng th / nm
A M 0 B B 5780K A M 1.5
Solar power spectrum
Atmosphere absorbs and scatters light
~15% of solar radiation is ‘diffuse’
datm
γ d atm
x nAir
Mass
s
Air Mass = Cosec γs
Standard spectrum is Air Mass 1.5, 1000 Wm-2
Optics & Solar Energy : Nelson : Lec. 1
200
400
600
800
1000
1200
1400
1600
1800
2000
2200
2400
2600
2800
3000
0 .0
0.5
1.0
1.5
2.0
Irrad
ianc
e / W
m-2 n
m-1
W ave leng th / nm
A M 1.5
Vibrational-rotational modes of H2O
Vibrational-rotational modes of CO2
Optics & Solar Energy : Nelson : Lec. 1
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0
2 00
4 00
6 00
8 00Irr
adia
nce
/ W m
-2 e
V-1
P h o ton E n e rg y / e V
A M O B B 5 7 8 0 K B B 3 0 0 K A M 1 .5
Solar power spectrum
visible
( )nm//1240eV/ λ=E
Optics & Solar Energy : Nelson : Lec. 1
0.0 0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.0
0
2 00
4 00
6 00
8 00Ph
oton
flux
den
sity
/ s-
1 m-2 e
V-1
P h o to n E n e rg y / e V
A M O B B 5 7 8 0 K B B 3 0 0 K A M 1 .5
Solar photon flux spectrum
Important quantity for PV is solar photon fluxdensity (not power density)
Optics & Solar Energy : Nelson : Lec. 1
Average ~ 200 Wm-2 (5 peak sun hours)
Southern Britain ~ 125 Wm-2 (~3 peak sun hours)
100 W m-2
100
150
150
200
200
250
Optics & Solar Energy : Nelson : Lec. 1
JAN
FEB
MAR
APR
MAY
JUN
JUL
AUG
SEP
OCT
NOV
DEC
0
1
2
3
4
5
6
7
Peak
Sun
Hou
rs
Month
Paris Reykjavik
JAN
FEB
MAR
APR
MAY
JUN
JUL
AUG
SEP
OCT
NOV
DEC
0
1
2
3
4
5
6
7
Peak
Sun
Hou
rs
Month
Tokyo
JAN
FEB
MAR
APR
MAY
JUN
JUL
AUG
SEP
OCT
NOV
DEC
0
1
2
3
4
5
6
7
Peak
Sun
Hou
rs
Month
Bulawayo Penang Guantanamo
JAN
FEB
MAR
APR
MAY
JUN
JUL
AUG
SEP
OCT
NOV
DEC
0
1
2
3
4
5
6
7
Peak
Sun
Hou
rs
Month
Sydney
Optics & Solar Energy : Nelson : Lec. 1
-80 -60 -40 -20 0 20 40 60 800
1
2
3
4
5
6
7
0
40
80
120
160
200
240
280
Ave
rage
irra
dian
ce /
W m
-2
5.5*cos(Latitude)
Aver
age
Peak
Sun
Hou
rs
Latitude / Degrees N
What will be the consequences of latitude on off-grid power generation?
Optics & Solar Energy : Nelson : Lec. 1
Energy consumption per capita in Western Europe ~ 5 kW
Mean solar irradiance in Southern Britain ~ 125 W m-2
What land area is needed to supply the energy needs of a city of 10 million people using solar irradiation:
(a) with conversion efficiency of 50%?
(b) with conversion efficiency of 5%
Optics & Solar Energy : Nelson : Lec. 1
Optics & Solar Energy : Nelson : Lec. 1
Optics & Solar Energy : Nelson : Lec. 1
Optics & Solar Energy : Nelson : Lec. 1
1.3. PHOTOVOLTAIC ENERGY CONVERSION
Optics & Solar Energy : Nelson : Lec. 1
Light energy conversion
• Packets of light energy (photons) defined by the wavelength of light
• May be absorbed in matter to promote electrons to higher energy
What happens next depends on the system …
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00
100
200
300
400
500
600
Irrad
ianc
e / W
m-2 e
V-1
Photon energy / eV
Optics & Solar Energy : Nelson : Lec. 1
0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00
100
200
300
400
500
600
Irrad
ianc
e / W
m-2 e
V-1
Photon energy / eV
Ene
rgy
Heat
Light
Solar thermal
Light ΔμChemical potential energy
Solar chemical
Light Δμ Electricwork
Solar photovoltaic
Optics & Solar Energy : Nelson : Lec. 1
Energy
Heat, light
Light
Semiconductor - open system
μ
μ+Δμ
Δμ electric workΔμ dN
Optics & Solar Energy : Nelson : Lec. 1
Photons in, electrons out
eV
• Photovoltaic energy conversion requires:
– photon absorption across an energy gap
– separation of photogenerated charges
– asymmetric contacts to an external circuit
Optics & Solar Energy : Nelson : Lec. 1
Photons in, electrons out
movie
Optics & Solar Energy : Nelson : Lec. 1
Photons in, electrons out
p type silicon
n ty
pe
silic
on
movie
Optics & Solar Energy : Nelson : Lec. 1
p type silicon
n ty
pe efficiency ~ 14%
power rating ~ 80 Wp
Applications
CIS Tower, Manchester0.4 MWp
(Solar Century)Solar powered refrigeration
~100 Wp
~1 mWp
Optics & Solar Energy : Nelson : Lec. 1
1.4. THE P-N JUNCTION
p type
0 V 0 V
n type
EgLight
Optics & Solar Energy : Nelson : Lec. 1
0 V 0 V
No driving force to direct photocurrent
How do the electrons know which way to go??
EgLight
Optics & Solar Energy : Nelson : Lec. 1
0 V+V0
Applied voltage drives photocurrent, but power consumed
J
EgLight
Optics & Solar Energy : Nelson : Lec. 1
p type
0 V 0 V
Compositional change drives photocurrent
n type
EgLight
Optics & Solar Energy : Nelson : Lec. 1
p type
Compositional change drives photocurrent
Jn type
EgLight
Optics & Solar Energy : Nelson : Lec. 1
• To generate electric power from solar radiation we need
– An energy gap (to keep photo‐generated electrons at high Δμ)– A preferred direction for electron extraction
• A semiconductor provides the energy gap
• Asymmetric contacts (for directed charge extraction) can be provided by a p‐n or n‐p junction
Optics & Solar Energy : Nelson : Lec. 1
Energy
μ
intrinsic
μ
n-type
μ
p-type
Optics & Solar Energy : Nelson : Lec. 1
Energy
μ
n-type
μ
p-type
Optics & Solar Energy : Nelson : Lec. 1
Energy
μ
n-type
μ
p-type
Optics & Solar Energy : Nelson : Lec. 1
Energy
μμ
n-typep-type
++++
----
Electric field
Optics & Solar Energy : Nelson : Lec. 1
Semiconductor p‐n junction
• Band gap enables photogeneration
• Compositional change drives photocurrent
EgLight
p type
0 V 0 V
n type
Optics & Solar Energy : Nelson : Lec. 1
1.5. SOLAR CELL PERFORMANCE CHARACTERISTICS
-JSC
VOC
Power density
-Jm
Cur
rent
den
sity
P
ower
den
sity
Voltage / V Vm
Current density
Optics & Solar Energy : Nelson : Lec. 1+
Voltage
Cur
r ent
In the dark
Optics & Solar Energy : Nelson : Lec. 1+
Voltage
Cur
r ent
- +
Voc
Open circuit voltage Voc
Open circuit
Optics & Solar Energy : Nelson : Lec. 1+
Voltage
Cur
r ent
Short circuit current density -Jsc
Short circuit
Optics & Solar Energy : Nelson : Lec. 1+
Voltage
Cur
r ent
Load Operating point
Operating: photovoltage x photocurrent = electric power
Optics & Solar Energy : Nelson : Lec. 1
Solar cell characteristics
• Quantum efficiency
• Short circuit current density
-Jsc
0
1
2
400 800 1200
Wavelength / nm
Exte
rnal
qua
ntum
ef
ficie
ncy
QE
solar spectrum
)()(
in photonsout electrons
EqbEJQE
sun
sc==
∫ ×= dEEEqJ sc )(QE )(densityflux photon Voltage
Cur
r ent
Optics & Solar Energy : Nelson : Lec. 1
Solar cell characteristics
• Short circuit current density Jsc,
• Open circuit voltage Voc
• Fill factor FF
• Power conversion efficiency
• Measured under Standard Test Conditions (AM1.5, 1000 Wm-2, 25°C)
in
ocsc
PFFVJPCE =
∫= dEEEbP inin )(
ocsc
mm
VJVJFF =
-JSC
VOC
Power density
-Jm
Cur
rent
den
sity
P
ower
den
sity
Voltage / V Vm
Current density
Optics & Solar Energy : Nelson : Lec. 1
Cell Type Area (cm2)
Voc (V) Jsc (mA /cm2) FF (%) Efficiency (%)
c-Si
UNSW PERL
4.0 0.696 42.0 83.6 24.9
c-GaAs Kopin 3.91 1.022 28.2 87.1 25.1
poly-Si
UNSW/ Eurosolare
1.0
0.628
36.2
78.5 19.8
a-Si
Sanyo 1.0 0.887 19.4 74.1 12.7
CuInGaSe2 NREL 1.04
0.669
35.7
77.0 18.4
Cd Te
NREL 1.131 0.848 25.9 74.5 16.4
polymer / fullerene
various 0.1 ~0.6-0.8 ~12-15 ~60 5 - 6
Solar cell performance
Optics & Solar Energy : Nelson : Lec. 1
More on this in next Monday’s lectures
Optics & Solar Energy : Nelson : Lec. 1
Summary
• Installed photovoltaic capacity reached 13 GWp in 2008, mainly in grid connected applications
• Overhead sun provides ~ 1 kW m-2 of radiant power at the Earth’s surface, annually averaged irradiance ~ 100 – 200 W m-2 depending on latitude
• In photovoltaic energy conversion, – absorbed photons promote electrons to higher energy levels within
a semiconductor– opposite charges are driven towards opposite contact by electrode
selectivity – charges are extracted ( current) with some electrochemical
potential energy ( voltage)
• Power conversion efficiency is about 25% for the best single junction solar cells.
Optics & Solar Energy : Nelson : Lec. 1
Energy consumption per capita in Western Europe ~ 5 kWMean solar irradiance in Southern Britain ~ 125 W m-2What land area is needed to supply the energy needs of a city of 10 million people using solar irradiation:
(a) with conversion efficiency of 50%?
107 x 5000/(125 x 0.5) m2 = 8 x 108 = 800 km2
Optics & Solar Energy : Nelson : Lec. 1
Energy consumption per capita in Western Europe ~ 5 kWMean solar irradiance in Southern Britain ~ 125 W m-2What land area is needed to supply the energy needs of a city of 10 million people using solar irradiation:
(b) with conversion efficiency of 5%?
107 x 5000/(125 x 0.05) m2 = 8 x 108 = 8000 km2
Optics & Solar Energy : Nelson : Lec. 1
Device physics of solar cells
• Physics governed by charge continuity
• Generation G = light absorption rate
• Recombination R usually linear
• Current density J Dominated by minority carrier diffusion.
• Differential equation for carrier density
• Boundary conditions
01 =−+ RGdxdJ
e
xsunebG
αα −=
τnBnpR ≈=
μεqndxdnqDJ +=
Gtn
dxndD
nn −=−
2
( ) kTVenn /00 = ( ) 0=dJn
Optics & Solar Energy : Nelson : Lec. 1
Gen
erat
ion
rate
np
n-p junction
Car
rier d
ensi
ty
n p
Cur
rent
den
sity
JnJp
Optics & Solar Energy : Nelson : Lec. 1
Device physics of solar cells
• Result: diode equation for J‐V ( )1/0 −+−= mkTqVsc eJJJ
• Important parameters:
– absorption coefficient α, optical depth αd, reflectivity
– charge diffusion length L = (Dτ)1/2
– type of recombination (e.g. linear, bimolecular, via defect states)
– parasitic resistances: series R and shunt R (leakage)
Optics & Solar Energy : Nelson : Lec. 1
24.4% efficient PERL cell
Solar cell design
• good optical absorption:
– Increase thickness
– Anti‐reflection coating, texturing
– Fine contacts
• efficient charge separation:
– Increase junction built‐in bias, junction area
– Suppress recombination
• efficient transport
– Improve material quality
• low electrical losses
– reduce series resistance
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