2011 NCN@Purdue-Intel Summer School Notes on Photovoltaic Devices

Lecture 14What is Different about Thin-Film PV

M. A. Alam and S. Dongaonkar

alam@purdue.eduElectrical and Computer Engineering

Purdue UniversityWest Lafayette, IN USA

NCN Summer School: July 2011 Notes on the Fundamental of Solar Cell

2

copyright 2011

This material is copyrighted by M. Alam under the following Creative Commons license:

Conditions for using these materials is described at

http://creativecommons.org/licenses/by-nc-sa/2.5/

Alam 2011

The lecture series on solar cells

Introduction to Solar cells Physics of Crystalline Solar cells Simulating Solar Cells What is different about thin film solar cell Organic photovoltaics

3Alam 2011

superposition and recombination

4

Outline of the lecture

4

1) Background information about thin film solar cells

2) Photo current from the transmission perspective

3) Dark current, shunt conduction, and weak diodes

4) Variability, reliability, and lifetime of solar cells

5) Conclusions

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Different types of solar cells

5

Crystalline Silicon

*Google Images

p-n

Amorphous silicon

p-i-n

Flexible organic

m-i-m

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Si too thick and expensive …

Economics of solar cells

6

C-Si CdTe a-Si CIGS OPV

Material/m2 207 50-60 64 100-125 37

Process/m2 123 86 73 130 23-37

Total/m2 350 130 138 230 50-80

Cost/W 1.75 0.94 -1.2 0.9-1.4 1.63 1-1.36

• All costs are approximate (J. Kalowekamo/E. Baker, Solar Energy, 2009. Goodrich, PVSC Tutorial, 2011.

c-Si installation, labor, etc. $3.75/WOthers … $1.00-1.50/W

…. but thin film solar cell has their own problems !

Features of thin film solar cells

7

(2) Thin doped region

Ala-Si:H

TCO

Laser Scribe

Glass

(1) Thin absorption layer

(4) Grain boundaries(3) Contact diffusion

(5) Series connection

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Equivalent circuit of thin film solar cells

Voc

Idark

max ph ocI FVP F= × ×

IT

Iph

Superposition does not hold …

RL

RL

phI

( )0

0

1

1

/( )

ln( )

Bph

ph

qV T

C

k

OV

I

IT q

I I e

k I

= −

⇒ = +

−SHIdIRI

8

IT

9

Outline of the lecture

9

1) Background information about thin film solar cells

2) Photo current from the transmission perspective

3) Dark current, shunt conduction, and weak diodes

4) Variability, reliability, and lifetime of solar cells

5) Conclusions

Alam 2011

10

Basics of current flow

10

Wrong contact loss +Recombination loss

0 03 2L Ln n pJ J J q qυ υ= − = −

0

0 0

4

4 2

Ln n

L L L Rn p n n

J J q

J J J J J

q q

υ

υ υ

≠ =

= − = −

= −

0 0

,,

, , , ,

4 26 6

6 6

L pL n

L n R n L p R p

q q

qG qG

υ υ

γγγ γ γ γ

= × − ×

= × − ×+ +

0 0

,,

, , , ,

3 26

6 6

L pL n

L n R n rec L p R p rec

q

qG

υ υ

γγγ γ γ γ γ γ

= × −

= × − + + + +

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11

Basics of transmission over a barrier

11

,B R Bk TR L Aeγ −← = E

LeftContact

RightContact

,B L Bk TL R Aeγ −← = E

LeftContact

RightContact

,B R Bk TR Aeγ −= E

Left Contact Right ContactDevice

,B L Bk TL Aeγ −= E

Alam 2011

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Photocurrent without recombination

,,

0, , , ,

, ,

0, , , ,

Wph L pL n

L n R n L p R p

W L n R n

L n R n L n R n

Jdx

qG

dx

γγγ γ γ γ

γ γγ γ γ γ

= −

+ +

= − + +

∫

∫

( )/0 0

( )/ ( )/00 0 0 0

E W x kTW

E W x kT E W x kT

edx

e e

υ υυ υ υ υ

− −

− − − −

= −

+ + ∫

0Lγ υ=

( )/0

E W x kTR eγ υ − −=

RL2 2

log cosh coth2 2

D D

D D

L LW WW W

W L W L

= × ≅ −

D

kTL

E≡

‘Price length’ and point of no return …. Alam 2011

Properties of ‘Sokel’ photo-current

2log cosh

2

2coth

2

ph D

D

D

D

J L WW

qG W L

L WW

W L

= ×

≅ −

L Rγ γ

L Rγ γ=

L Rγ γ<

L Rγ γ=

VA=0

Vbi=VA>0

VA>Vbi=0

Vbi

VA

nJqG

Voltage dependent photocurrent, different from Si p-n junction …

Vbi

Sokel and Hughes, JAP, 53(11), 1982.

14

Blocking layer and photocurrent

14

~ 0Rγ

VA=0

Vbi=VA>0

Vbi

VA

phJ

qG

Blocking is essential for many types of thin film PV ….

, ,

0, , , ,

Wph L n R n

L n R n L n R n

Jdx

qG

γ γγ γ γ γ

= − + +

∫

With blocking layer …

phJ qGW=

~ 0Rγ

15

Photocurrent with recombination

0n n n nEυ τ µ τ≡ × = × ×

/ /

01 ec c

W x Wcdx e− − = = − ∫

{ }/1 e np Wn

hJ

qGW W − − −= −

Crandall, JAP, 54(12), 19823

,,

0, , , ,

Wph L pL n

L n R n rec L p R p rec

Jdx

qG

γγγ γ γ γ γ γ

= −

+ + + + ∫

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Photo-current in crystalline cells

16

, /, 1 e ~nph L p WL nn n n n n

J JJD W

qG qG qGτ− = − = − ≡

Voltage independent photocurrent is unaffected by electron mirrors

Electron blocking layer does suppress dark current, increases Voc.

with electron mirror without electron mirror

D Wτ D Wτ

With blocking Without blocking

Numerical validation: Effect of blocking layer

For low quality Si PV, blocking is not essential 17

18

Photocurrent with field/recombination

18

/ ( )/ ( )/0 0

( )/00 0

/ ( )/ ( )/

( )/0 1

n k n

k

n k n

k

x W x W xWph

W x

x W x W xW

W x

J e e edx

qG e

e e edx

e

υ υυ υ

− − − − −

− −

− − − − −

− −

−=

+ −

= +

∫

∫

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2coth

2Dh

D

pJ WW

qGL

LW

≅ −

pD

h

E

n kTL ≡

Matches with numerical simulation very well …

19

Outline of the lecture

19

1) Background information about thin film solar

2) Photo current from transmission perspective

3) Dark current, shunt conduction, and weak diodes

4) Variability, reliability, and lifetime of solar cells

5) Conclusions

Alam 2011

Dark current without recombination

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0 0L R

nL R

RR

LL

nJ qn qγ γ

υ υγ γ γ γ

= −+ +

, ,00 ,0,0 RL R Ln n γ γ=

,0

,0 ,0 0

,

,

0

1

B B bi B

bi B

E k T qV k T

qV kL

T

L

R R

Ae Ae

A eγ

γ

γ γ

− −

−

= =

= × ⇒ =

02

, iR Ann N=

,0,0bi B

Rk T

LqVenn +=

0

2

,bi BqV k Ti

AL en

nN

−=

ND NA

2,0R i An n N=

biV

,0L Dn N=

0R L bi

L R

qV←

←

==

EEAlam 2011

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Calculating dark current without recombination

21

,, 0 000L R

nL R

L RL R

nJ q qnγ γ

υ υγ γ γ γ

= −+ +

( )

1

bi Bq V V k TL

R

eAAγ

γ

− −

= ×=

2

,0bi Bi

A

qV k TL

nn e

N+=

( )

( )/

0

2( )/

,0 ,0

0 11bi B

bi B bi B

n L RR

q

RL

qV k V V

L

q V V k TT k Ti

A

qJ

q ne

N

n n

ee

υγ γ

γυ

γ

−− −

−

= −+

= − +

( )/

2 2

0

( ) /1 1

1bi

B B

Bq V V k TqV k T qV k Tn bii i

d n pA D

V V dn nJ J J q e I e

N N e

µ+ −

− = + = + − ≡ − +

( )

2

/

( ) /1

1bi B

b B

q V V k TqV k Tn bii

A

V V dnq e

N e

µ+ −

− = − +

biV

Theory and practice of thin film dark IV

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A real solar cell IV seldom looks like textbook IV!These wings helped create many complicated models.

0 1BqV k TdI I e ≡ −

Theory Experiment

Butterfly wings!

Alam 2011

Contact diffusion and shunt conduction

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(2) Thin doped region(1) Thin absorption layer

(4) Grain boundaries

(3) Contact diffusion

A real solar cell IV seldom looks like textbook IV

Parasitic shunt leakage

24

shunt

@ low voltage

@ high voltageintrinsic

Dongaonkar and Alam JAP, 2010

Interpretation of ‘shunt’ leakage

1

2 1shunt

VI A

L

δ

δµ+

+= = AE

kTγ

* G. Paasch et al., JAP 106, 084502 (2009)

0

n nJ qnd qndx

µ

κε

=

=

EE

( )

32

32

2 23

98

an

na a

JV L

J V VL

εµεµ

=

=

*

25

Features of shunt leakageSymmetry Exponents

1

2 1shunt

VI A

L

δ

δµ+

+=

Dongaonkar and Alam, EDL, 2010 26

L-3

27

Outline of the lecture

27

1) Background information about thin film solar

2) Photo current from transmission perspective

3) Dark current, shunt conduction, and weak diodes

4) Variability, reliability, and lifetime of solar cells

5) Conclusions

Alam 2011

Contact diffusion and shunt conduction

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(2) Thin doped region(1) Thin absorption layer

(3) Grain boundaries

(4) Contact diffusion

Alam 2011

Variability and weak diodes

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/0

/0

0

(e 1) ( )

(e 1) ( )

ln(1 )

oc

qV kTph

qV kTph oc

oc B ph

I I I V

I I V

V k T I I

= − −

− =

= × +

L I

( )

/0

/

( )e

( )(e 1)

oc

oc

qV kTph oc

q V V kTph oc

I I V

I I V

−

−

≈

= −

( )

( )2

1 /

/2

12 ( )(e 1

(

)

)(e 1)

o

c

c

o

q V V kTph oc

q V V kTph oc

l Vn

I V

I−

−× −

= −

×

Like an impact crater, a single um-sized weak diode can drain away 1-10 mm region !!

( )1 2 /2 2 e !oc ocq V V kTn L l −= ≈

(karpov, PRB 69, 045325, 2004.)

(5) Series connection, shadow degradation, and a very weak diode

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Being in shadow stresses the device

Initial panel operating point

Load lineShaded panel operating point

Initial cell operating point

Shaded cell operating point

Shaded cells can get reverse biased!31

32

Shadow degradation

32

timeNormal operation

Shadow degradation

Dark current

Operating voltage

Dongaonkar et al.IRPS 2011

Light induced degradation

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D. L. Staebler, et al., APL, 1977.

Y. Nakata, et al., JJAP, 1992

T. Shimizu, JJAP, 2004

Slope – t1/3

M. Stutzmann, et al., PRB, 1985

Ongoing discussion about exponent n~1/3

34

Reaction-Diffusion Model for LID

G

RDB

F 0 DB H= GdN

k N - k N Ndt

2H DBH H

dN dNk N

dt dt= -

Free H Generation:

Reaction:

~ 0

( )2 3

1 3 0 1 33 fDB H

r

k N GN k t

k

∝

// /

H2

10-4 10-2 100 102108

109

1010

1011

Dens

ity o

f DBs

[cm

-3]

LS Time [sec]

Slope n ~0.3

Boron-doped Czochralski (Cz) crystalline PV equally susceptible to LID. Float-zone and/or Ga doping better.

Light induced degradation in crystalline PV

metal fingern+ emitter ~ 100 nm

p absorber ~ 100 µm

p+ electron mirror ~ 100 nmback reflector

35Alam 2011

Extrinsic and Intrinsic Solar Cell Reliability

Extr

insic

• Electrochemical corrosion

• Moisture Ingress• Glass fracture• Inverters reliability• Delamination• Improper insulation• Bypass diode failure

Intr

insi

c • Light Induced Degradation

• Hot spot breakdown

• Shunt leakage• Shadow degradation• Weak diodes

36Source - www.nrel.gov/pv/performance_reliability/pdfs/failure_references.pdf

37

conclusions

Economic incentive to develop thin film solar cell.

The unique features of thin film PV make photo-current voltage dependent, increases probability of formation of weak diodes and shunts.

In addition to the extrinsic reliability issues, we need to worry about shadow degradation, light induced degradation, etc.

The reliability/variability are key concerns – making modules less efficient than individual cells.

Alam 2011

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Reference for images

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http://www.viraload.com/2008/08/28/avasolar-raises-104m-for-thin-film-solar/

http://www.solarserver.com/solar-magazine/solar-news/current/kw42/nanosolar-to-expand-thin-film-cigs-solar-cell-manufacturing-to-115mw.htmlhttp://www.solarthinfilms.com/active/en/home/photovoltaics/photovoltaics_and_thinfilms/thinfilm_photovoltaics.htmlhttp://gotpowered.com/2011/ge-develops-thin-film-photovoltaic-panels/

http://kypros.physics.uoc.gr/images/web2.gif