7_LossesOptimization_2010

7/28/2019 7_LossesOptimization_2010

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7LOSSES and OPTIMIZATION


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Solar cell operating principles

1.Absorption of photons

generation of electron-hole pairs

2. Separation of carriers in the internal electric field created by p-njunction and collection at the electrodes

potential difference and current in the external circuit

3. Potential difference at the electrodes of a p-njunction injection and recombination of carriers losses

The resulting current in the external circuit: I = IL - ID (V)

photocurrent IL dark (diode) current ID


3/29

- 56% spectral mismatch

- 9% reflection & transmission

- 13% fundamental recombination

- 7% excess recombination,

resistance, etc

15%

Typical commercial c-Si solar cellsunlight

solar cell

electricity

waste

heat

Solar cell performance

Single-junction solar cell:


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Non-absorptionThermalization

Reflection

Transmission

Area loss

Recombination

- bulk

- surface

( ) 0 ( )R1

g opt QE

t

f

A

A

elQE


Optical and collection losses:


5/29

( )

=0

0

I d

hcP

( )0 Photon flux density: number of photons per

unit area per unit time and unit wavelength

Non-absorption Eph


6/29

( )

( ) d

ch

dE

0

0

0

0g

g

g

EC

EV

EphEG

Thermalization

Eph

>EG

g

Thermalization

g ph >


Optical losses: Thermalization

( )

=0

0

I d

hcP

( )0 Photon flux density: number of photons per

unit area per unit time and unit wavelength


7/29

EC

EV

Eph EG

Thermalization

Eph

>EG

EC

EV

EphEG

Non-absorption Eph


8/29

( ) 0 ( )R1

elQE

Optical losses: Reflection and transmission

Reflection:

Different refractive indices

Transmission:

finite thickness of a cell

absorption coefficient

Area loss:

metal electrode coverage

t

f

A

A

g opt QE



9/29

( ) 0 ( )R1 Recombination:

bulk recombination

(minority carrier lifetime)

surface recombination

(surface recombinationvelocity)

g opt QE

t

f

A

A

elQE

( )t

felgoptmaxsc

A

AQEQER1JJ =

( )

=

g

0

0

max dqJ

Collection losses: Recombination



10/29

I

ocsc

P

ffVJ =

( )0I0

hcP d

=

( ) ( )g

0fsc opt g el

t 0

AJ 1 R QE QE q d

A=

( )

( )( )

g

0

0 f

g opt el oc0 t

0

q dA

= 1-R QE QE V ff Ahc d

( )

( )

( )

( )

( )

g g

g

0 0

G

0 0 ocfg opt el

0 t G0

0 0

h c

E d d q VA= 1-R QE QE ff

A Ehc h c d d


Efficiency:

Overstraeten, Mertens: Physics, technology and Use of Photovoltaics, Adam Hilger 1986


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8. Fill factor

7. Voltage factor

6. Loss due to recombination5. Loss by incomplete absorption due to the finite thickness

4. Loss by reflection

3. Loss by metal electrode coverage2. Loss by excess energy of photons

1. Loss by long wavelengths

( )

( )

( )

( )

( ) ffE

VqQEQER1

A

A

d

ch

dE

d

ch

d

ch

g

oceloptg

t

f

0

0

0

0

g

0

0

0

0

g

gg

=


Overstraeten, Mertens: Physics, technology and Use of Photovoltaics, Adam Hilger 1986


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Optical losses:

Non-absorption

Thermalization

ReflectionTransmission

Area loss

Optical gap

Optical gap

Refractive indicesAbsorption coefficient

Metal grid design

Properties:



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Collection losses:

Recombination

- surface

- bulk

Surface recombination velocity

Minority carriers lifetimeDiffusion coefficient


Properties:


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Total current: LkTqV

0T I1eII =

Short circuit current (V=0):

LSC II =

High Isc :

Minimize front surface reflection

-

antireflection coatings

Minimize transmission losses

-

thick absorber

Minimize surface recombination

-

passivation

layers

Minimize bulk recombination

-

large diffusion lengths

-

high electronic quality material


Optimal design:


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Total current: LkTqV

0T I1eII =

Open circuit voltage (I=0):

+= 1lnV

0

OCI

I

q

kT L

+=

Dp

ip

An

in

NL

nDq

NL

nDqAI

22

0

Low I0: High doping densities

Low surface recombination

velocities

Large diffusion lengths


Optimal design:


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p++ p++

Al

Al Al

SiO2 n+

p-typec-Si

Absorption versus collection:

Thickness of the absorber layer

Minority carrier diffusion length

Al


Optimal thickness of absorber layer:


17/29

p++ p++

Al

Al Al

SiO2 n+

p-typec-Si

Al

Le







18/29

p++ p++

Al

Al Al

SiO2 n+

p-typec-Si

Le

Le







19/29

p++ p++

Al

Al Al

SiO2 n+

p-typec-Si

Le

p++ p++

Al







20/29

p++ p++

Al

Al Al

SiO2 n+

p-type

c-Si







21/29

Increase absorption:

-

Surface texture

-

Antireflection coating

Avoid surface recombination:-

Surface passivation

SiO2

n+

p++ p++

Al

AlAl




22/29

IL1 2 V

+

-

I

current source IL diode diffusion current

diode recombination current

1

2

I-V characteristics

Voltage

Curr

ent

IL

ID

IT

ISC

VOC


Equivalent circuit:


23/29

RS

RshIL

1 2 V+

-

I

series resistorRS

parallel resistorRsh

( )ph

p

sasa JR

ARJV

Tkn

ARJVqJJ

+

= 1exp0

Jph

= 400 A/m2

Voc

= 0.6904 V

= 16 mm2


Equivalent circuit:


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Series resistance (RS)

Bulk resistance of semiconductor

Bulk resistance of metal electrodes

Contact resistance between semiconductor and metal

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-400

-300

-200

-100

0

100

200

Voltage [V]

C

urrentDensity[A/m2]

Rp = 1e4 Ohm

Voc

Rs = 0 Ohm

Rs = 2.5 Ohm

Rs = 5 Ohm

Rs = 7.5 Ohm

Rs = 10 Ohm

Rs



25/29

Shunt (parallel) resistance (RP)

Leakage across the p-n junction around the edge

Crystal defects, pinholes, impurity precipitates

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.8-400

-300

-200

-100

0

100

200

Voltage [V]

C

urrentDensity[A/m2]

Rs = 0 Ohm

Voc

Rp = 0.001 Ohm

Rp = 0.005 Ohm

Rp = 0.01 Ohm

Rp = 0.03 Ohm

Rp = 1e4 Ohm

Rp



26/29

n-type Si

p-type Si

(+) (+)

(-)

Fabricated in 1954 wrap-around structure

p-n junction formed by B

dopant diffusion high resistive losses in the p-

layer

efficiency 6%

Crystalline Si solar cells

First c-Si solar cell:


27/29

University of New

South Wales (Australia)


Efficiency improvement:

C lli Si l ll


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Passivated

Emitter and

Rear Locally diffused

External parameters:

Jsc

=42.7 mA/cm2

Voc

=0.705 V

ff = 0.828

= 25.0 %

Record c-Si solar cell: PERL structure (UNSW)


C lli Si l ll


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Surface texture (inverted pyramids for light trapping)

Selective emitter(n+-layer for contact, n-layer for active part ofsurface)

Passivation of surface (SiO2 on both sides of solar cell)

Thin metal fingers on the front side

Back side metalization with small contact area to the basematerial

Locally diffused regions under contact points at the back(BSF field)

Minority diffusion lengths well in excess of device thickness


Key attributes for high efficiency solar cells:

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