2132-2 Winter College on Optics and...

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

([email protected])

Physics of Solar Cells (I)


Outline

1. Photovoltaic applications

2. The solar resource

3. Photovoltaic energy conversion

4. The p‐n junction solar cell

5. Solar cell performance characteristics


• 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


Some concepts in PV

Solar cellModule

System


1.1. PHOTOVOLTAIC APPLICATIONS


Photovoltaic applications

158

249

1347

54Domestic off-grid

Non-domestic off-grid

Grid-connecteddistributedGrid-connectedcentralised

Cumulative PV capacity 2003 (MW): 1.8 GWp


Photovoltaic applications

311 430

8220

4464 Domestic off-grid

Non-domestic off-grid

Grid-connecteddistributedGrid-connectedcentralised

Cumulative PV capacity 2008 (MW): 13.4 GWp


1.2. THE SOLAR RESOURCE


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


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


Power density 1353 W m-2 outside earth atmosphere


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


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


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


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


visible

( )nm//1240eV/ λ=E


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)


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


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


-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?


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%


1.3. PHOTOVOLTAIC ENERGY CONVERSION


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


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


Energy

Heat, light

Light

Semiconductor - open system

μ

μ+Δμ

Δμ electric workΔμ dN


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



movie



p type silicon

n ty

pe

silic

on

movie


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


1.4. THE P-N JUNCTION

p type

0 V 0 V

n type

EgLight


0 V 0 V

No driving force to direct photocurrent

How do the electrons know which way to go??

EgLight


0 V+V0

Applied voltage drives photocurrent, but power consumed

J

EgLight


p type

0 V 0 V

Compositional change drives photocurrent

n type

EgLight


p type

Compositional change drives photocurrent

Jn type

EgLight


• 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


Energy

μ

intrinsic

μ

n-type

μ

p-type


Energy

μ

n-type

μ

p-type


Energy

μμ

n-typep-type

++++

----

Electric field


Semiconductor p‐n junction

• Band gap enables photogeneration

• Compositional change drives photocurrent

EgLight

p type

0 V 0 V

n type


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


Voltage

Cur

r ent

- +

Voc

Open circuit voltage Voc

Open circuit


Voltage

Cur

r ent

Short circuit current density -Jsc

Short circuit


Voltage

Cur

r ent

Load Operating point

Operating: photovoltage x photocurrent = electric power


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


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


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


More on this in next Monday’s lectures


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.


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


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


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


Gen

erat

ion

rate

np

n-p junction

Car

rier d

ensi

ty

n p

Cur

rent

den

sity

JnJp


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)


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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