d. cahen, weizmann inst. 02/’12 how good can solar cells be? assessing possibilities for solar...

D. Cahen, Weizmann Inst. 02/’12 www.weizmann.ac.il/AERI/presentations/html

How good can Solar Cells be?Assessing Possibilities for Solar Cells

by Identifying Basic Limitations to PV Performance

THANKS to many COLLEAGUES, esp.

Pabitra Nayak Juan Bisquert, Un. Jaume, Antoine Kahn, Princeton Un.

Lee Barnea, Ron Milo + other Weizmann Inst. colleagues Bolko von Roedern, David Ginley, Keith Emery, Rommel Noufi

NREL, US National Renewable Energy Lab

Bernard Kippelen, Georgia Tech, Arie Zaban, Bar Ilan U.;K.L. Narasimhan, TIFR, 3G Solar & many others!


on

e e

lect

ron

en

erg

yon

e e

lect

ron e

nerg

y

spacspacee

Generalized picture•Metastable high and low energy states

•Absorber transfers charges into high and low energy state

•Driving force brings charges to contacts

•Selective contacts

(1) cf. e.g., Green, M.A., (1) cf. e.g., Green, M.A., Photovoltaic principles.Photovoltaic principles. Physica E, 14 (2002) Physica E, 14 (2002) 11-1711-17

The Photovoltaic (PV) effect:

High High energyenergystatestate

Low Low energyenergystatestate

AbsorberAbsorber

e-

p+

conta

ct

conta

ct


Types of PV Cells

Elemental Semiconductors Single or multi-crystal Polycrystalline Amorphous thin film

Inorganic Compound Semiconductors Single crystal Polycrystalline thin film

Organic, Excitonic (molecules, polymer) Polycrystalline Interpenetrating network Nanocrystalline;

dye-sensitized

Primarily based on solid-state electronic material systems

Si,Ge

(Ga,In)(As,P)Cu(In,Ga)Se2

CdTe

Phenylene-vinylidene, PCBM++

Ru-dye+TiO2

(non)concentrator;single-& multi-

junction

homo- &hetero-junction;photo-electro-

chemical;MIS


A schematic of a Solar Cell


Solar Cell (r)evolutions

1st generation

Si

cm cheaper? simpler?

2nd generation

CdTe, CIGS

poly-crystalline

mmicro-crystalline & amorphous

nano-crystalline

~ 20 nm

3d generation

TiO2

organic (polymer/ small molecules

nano-crystalline

~ 20 nm

3d generation

TiO2

nano-crystalline

~ 20 nm

3d generation

TiO2

organic (polymer/ small molecules

single- & multi-crystalline

Quantum dot Cells

4 4.5 6 15 nm

next generations

work horse

stabilization self-healing

self-assembly

&nano !!

great science


OUTLINE• PV cell performances today

• Limits of PV solar energy conversion

• Empirical guides to limits or possibilities

• Losses in “excitonic” cells

• Summary & Future


IN

OUT

PowerRadiativeSolar

PowerElectrical %100

Definition of efficiency:

Lowest Loss Laboratory PV cells (1-4 cm2 except for tandems):

Data from Solar Cell Eff #39, Progr in PV 2012


12.3%ETA cells

/ WIS


Efficiency(%) Manufacturer Technology (area, if < 600 cm2) BESTrated/minimum commercial

module /cell19.6 /?? SunPower Single-crystal Si non-standard jnctn

78% 17.1 / 16.3 Sanyo Single-crystal Si HIP jnctn 74% 15.2 / 14.6 Kyocera Multi crystal Si standard junction 75% 13.4 / 12.7 Evergreen EFG(ribbon) Si standard junction 77%

12.2 / 10.8 First Solar CdTe 71 % 13.1 / 11.2 Miasole CIGS 67 % 12.6 /11.4 Q-cells CIGS 64 %

6.3 / ?? Kaneka a-Si single junction * 66 %6.7 / 5.7 Uni-Solar a-Si, triple junction * 54 %

* stabilized values

5#,** 3GSolar dye (225) ~40 %3.9# Solarmer Organic polymer/ molecule (225)

~40 % # Pilot modules; few yrs stability; **not yet commercially available

Possibilities for Technological Progress

Nayak et al. Adv Mat. 2011 Module data from B v Roedern, NREL, 11/2011


Infra- visible ultra- -Red -violet

(IR) (UV)Solar Energy

Spectrum

Photovoltaic Conversion is a Quantum (threshold) Conversion

Process

WHY ?In Solar Cells Most Solar Energy

is “Lost” as Heat


e-

-voltage (qV)

Energy

e-

n-typep-type

h

h+

e-

useable photo-voltage (qV)

Energy

e-

n-typep-type

h

h+

Single p-n junction solar cell

O. Niitsoo

space

in Solar Cells Most Solar Energy is “Lost” as Heat


>Eg thermalized

< Eg not absorbed

Etendu; Photon entropy –TD~0.3eV @RT, lack of concentration

Carnot factor –TD

Emission loss- (current)

Electrical power out

Current – Voltage Characteristics

After Hirst & Ekins-DaukesProg.Photovolt:Res:Appl. (2010)

Losses in PV cell

0 1 2 3 40

10

20

30

40

50

60

70

80

Cur

rent

(m

A/c

m2)

Energy (eV)

Eg

Nayak et al. Energy Environ. Sci., 2012 (In Press)


Prince, JAP 26 (1955) 534Loferski, JAP 27 (1956) 777Shockley & Queisser JAP (1961)

Shockley-Queisser (SQ) Limit

photosynthesis

0.5 1.0 1.5 2.0 2.55

10

15

20

25

30

OPV

CIGS

c-Si

Eff

icie

ncy

(%

)

Band Gap (eV)

GaAs

InP

CdTe

DSCa-Si

SQ Limit


What can we do?

tandem (stacked)spectral splitting

(spatial)

Make better use of sunlight: “Photon management”

e.g., multi-junction photovoltaics

Bandgap (eV)

5 6 7 8 9 1 25 6 7 8 9 1 2

Four-junction device with bandgaps1.8 eV/1.4 eV/1.0 eV/0.7 eV

Theoretical efficiency > 52%


SPECTRAL SPLITTING

A two junction, four terminal photovoltaic device forenhanced light to electric power conversion usinga low-cost dichroic mirrorSven Ruhle, Akiba Segal, Ayelet Vilan, Sarah R. Kurtz, Larissa Grinis, Arie Zaban,Igor Lubomirsky, and David CahenJOURNAL OF RENEWABLE AND SUSTAINABLE ENERGY 1, 013106 2009



Electrical Performance with Dichroic Mirror



Electrical Performance with Dichroic Mirror

Needed:

cheap, h

igh E G +

high

V max

PV Cells


Improve performance using concentrated sunlight

but … diffuse (scattered)

radiation lost upon concentration


“Etendue” VOC loss (“photon entropy”) ΔF = ΔH – TΔS

ΔqVOC = EG – kT ln W = EG – kTln 46,200 = EG – 10.7 kT =

280 meV @ 300K

Experiments on III-V alloys

yield

350 -550 mV for Eg-Voc

Hirst & Ekins-DaukesProc. 24th Eur. PV Solar En. Conf. Hamburg


0.0

0.5

1.0

1.5

2.0

0.6 1 1.4 1.8 2.2 2.6 3Bandgap Eg (eV)

Eg

/q,

Vo

c, a

nd

(E

g/q

) -

Vo

c (

V)

0

100

200

300

400

500

600

700

800

Inte

ns

ity

pe

r U

nit

Ph

oto

n E

ne

rgy

(W

/(m

2 . e

V))

VocEg from EQE(Eg/q) - Vocradiative limitAM1.5D, low-AOD

Voc of solar cells with wide range of bandgaps and comparison to radiative limit

d-A

lGa

InP

Ga

As

1.4

- e

V G

aIn

As

o-G

aIn

P

AlG

aIn

As

d-A

lGa

InP

d-G

aIn

P

d-A

lGa

InP

0.9

7-e

V G

aIn

As

Ga

InN

As

1.1

0-e

V G

aIn

As

1.2

4-e

V G

aIn

As

1.3

0-e

V G

aIn

As

Ge

(i

nd

ire

ct g

ap

)

AlG

aIn

As

Richard King

“Etendue” VOC loss (“photon entropy”)


Empirical Guide 1Current


0.5 1.0 1.5 2.0 2.5 3.0 3.5 4.00

20

40

60

80

100Maximum Current Density Available

in 1 Sun @ AM 1.5G

J

(m

A/c

m2 ),

EQ

E (

%)

Energy (eV)

J, 100%

J0-, EQE()

0

1

2

3

4

5

Sp

ect

ral P

ho

ton

Flu

x D

en

sity

( 1

014 p

ho

ton

s/se

c-cm

2 )

from B. Kippelen, Georgia Tech

Current Limitation

Harvest more photons


JMP q d

(JMP JSCmax )

a From EQE * for best performing cell of given type

:Limit or Opportunity ?

Nayak et al. Adv. Mater., May 2011


Maximal possible vs. experimental photocurrents

Natural PS<~10-2

mA/cm2

Thanx 2 Lee B

1.0 1.5 2.0

10

20

30

40

50

DSSC(c)DSSC

(b)

OPV (a)

(Jsc

Max)

JMP

JSC

Curr

ent

dens

ity(

mA/c

m2 )

Absorption Edge (eV)

Si

CI GS

I nP GaAs

CdTe

DSSC(a)

OPV (b)

a-Si

DSSC

(a) = black dye, (b) = N719

& (c) = YD2-O-C8+Y123

OPV (a) = Mitsubishi, (b) = Konarka

PKN, JB, DC, 2011, AM


Empirical Guide 2Voltage


:Limit or Opportunity ?

JSC q d

(JSC JSCmax )




Voc / EG : Limit or Opportunity ?




1000 800 600 400

1 1.5 2 2.5 30.3

0.4

0.5

0.61000 800 600 400

1 1.5 2 2.5 30.3

0.4

0.5

0.6

CO

ST:

qV

hν –

qV

OC

[eV

]

Absorption Edge Energy [eV]

Shockley-Queisser or detailed balance limit COST as function of minimal

excitation energy Wavelength [nm]

WRONG

S-Q based on R.Milo,WIS Thanx 2 Lee B


0.0 0.5 1.0 1.5 2.0 2.5 3.00

10

20

30

40

50

60

70

Curr

ent

dens

ity(

mA/c

m2 )


Geometrical illustration of solar spectrum loss

due to “over-potential”Consider ~ 1 eV or 2 eV absorption edge “Assume 1 eV “overpotential” shifts reference energy 1 eV to right small purple rectangle gives new optimal energy value.

\

After Ron Milo, WIS PKN, JB, DC, 2011, AM


0.0 0.5 1.0 1.5 2.0 2.5 3.00

10

20

30

40

50

60

70

Curr

ent

dens

ity(

mA/c

m2 )


Geometrical illustration of solar spectrum loss

due to “over-potential”Consider ~ 1 eV or 2 eV absorption edge “Assume 1 eV “overpotential” shifts reference energy 1 eV to right small purple rectangle gives new optimal energy value.

After Ron Milo, WIS PKN, JB, DC, 2011, AM


VMP / EG : Limit or Opportunity?

a From EQE * for best performing cell of given type Nayak et al. Adv. Mater., May 2011


Absorption Edge Energy [eV]

Shockley-Queisser or detailed balance limit LOSS as function of minimal

excitation energy

S-Q based on R.Milo,WISPKN, JB, DC, 2011, AM Thanx 2 Lee B

1000 800 600 400

1 1.5 2 2.5 3

0.3

0.4

0.5

0.6

qV

hν –

qV

op

era

tion

(=M

P) [

eV

]1000 800 600 400

1 1.5 2 2.5 3

0.3

0.4

0.5

0.6


...............

Includes basic add’l loss of amorphous/disordered material

1.0 1.2 1.4 1.6 1.8 2.0

0.2

0.4

0.6

0.8

1.0 DSC-latestOPV Mitsubishi

GaAs

OPV Konarka

a-Si PS

CuGaSe2

Absorption Edge [eV]

qVhv

- qV

oper

atio

nal (

= M

P) [e

V]

SQ- Limit Loss

(GaI n)P

DSC-N719

I nPCI GS

C -Si

DSC-Black CdTe

PS: natural photsynthesis

Shockley-Queisser or detailed balance limit LOSS as function of minimal

excitation energy

S-Q from R.Milo,WIS PKN, JB, DC, 2011, AM

qV

hν –

qV

op

era

tion

(=M

P) [

eV

]


Why / how do we loose

with the excitonic cells?


D

-

Cathode

Anode

SubstrateLight

DA

Bulk heterojunction cell

Paul W. M. Blom et al.

Static Disorder


D+A-

D*A

DA

Nuclear co-ordinate

En

erg

y (

eV

)

Eg

λ

ΔG*

ΔG0

Vibronic relaxation after electron transfer

A-

λrel (1)

A

Nuclear co-ordinate

Ener

gy (e

V)

λrel (2)

ΔG0rec

Loss = λrel (hole) + λrel

(electron)

λrel (hole) = ~150 meV (UPS)

λrel (electron) = ~150 meV (DFT)

λ = λrel (1) + λrel (2)

Consider electron transfer & Vibronic relaxation

Nayak et al., EES, in press


Static and Dynamic disorder

Tail states

After Kera, Yamane and Ueno Progress in Surface Science, 2009



En

erg

y (

eV

)

CBM

VBM

Exciton binding energy < kT

→ dissociation by space charge region E-field

p n

EFp

Inorganic

Wannier exciton

LUMO (D)

HOMO (D)

LUMO (A)

HOMO (A)

12 3

3

ΔD/AEn

erg

y (

eV

)

Donor Acceptor

Exciton binding energy >> kT

→ requires donor/acceptor, (D/A) type structure

Organic

Frenkel exciton

p/n vs. excitonic solar cells


Inorganic semiconductor

Exciton binding energy

< kT

→ dissociation by space charge

region E-field

Electron-hole pair:Organic vs. Inorganic PV

cells

from A. Kahn, Princeton U

Organic semiconductor

MOLECULAR PICTURE

Exciton binding

energy >> kT

→ requires donor/

acceptor, (D/A) type structure


p/n vs. excitonic solar cellsINORGANICINORGANIC

• high dielectric constant

• minority carrier device

ORGANIORGANICC

• low dielectric

constant

• exciton splitting

• includes jiggling & wigglingfrom B. Kippelen, Georgia

Tech

Exciton binding energy ~ 10 meV ~ 0.1-0.3 eV

EF

n

* 4

2 2 20(4 ) 2B

m eE

dielectric constant


{LUMO(A) – HOMO(D) gap} Voc correlation

Rand et al., Phys. Rev. B 75,

(2007)Δ= IE(D) – EA(A) – EBA/2 (?)

IE EA

Evac

Afer A. Kahn, Princeton U

EF

HOMO

LUMO

EFDAΔ

VOC = IE(D) – EA(A) –(≥0.3) ?

Origin of VVOCOC


G0 Gibbs free energy reorganization energy, relaxation due to vibronic modes Vif electronic coupling

0 00 exp BDA

B

J J e Nk T

k

k :rate of charge transfer (s-1) e: electron charge (C) NDA: surface density of DA complexes[cm-2]

212

02( )

exp4if

B

GV

kk

T

0

lnOCSCB JT

eV

kn

J

Origin of VVOCOC

after B. Kippelen, Georgia Tech

Voc as J0 & J00



SummaryThere are limits, beyond Shockley-Queisser for photo-conversion with organics (OPV, DSSC, PS, AP)

• static disorder ~ 0.2-0.3 eV

• dynamic disorder

vibronic coupling ~ 0.2-0.3 eV

• low dielectric constant~ 0.1 -0.3 eV Σ = 0.5 - 0.9 eV; cf. 1.05 eV !

Nayak et al. Adv. Mater., May 2011 PKN, JB, DC -TBP


1.0 1.5 2.0 2.5 3.00

5

10

15

20

25

30

35

Cal

cula

ted

Eff

icie

ncy

(%)

Band gap (eV)

S-Q Limit

Shockley-Queisser (SQ) Limit

0.5 1.0 1.5 2.0 2.55

10

15

20

25

30

OPV

CIGS

c-Si

Effi

cien

cy (

%)

Band Gap (eV)

GaAs

InP

CdTe

DSC a-Si

SQ Limit



1.0 1.5 2.0 2.5 3.00

5

10

15

20

25

30

35

Cal

cula

ted

Eff

icie

ncy

(%)

Band gap (eV)

+ 80% EQE + Fill factor loss (n=2) + Tail state loss = 0.2eV + Vibronic loss = 0.25eV + Dielectric loss = 0.2eV + (Dielectric + vibronic) = 0.3eV

S-Q Limit

Extra Losses in Molecular Cells

0.5 1.0 1.5 2.0 2.55

10

15

20

25

30

OPV

CIGS

c-Si

Effi

cien

cy (

%)

Band Gap (eV)

GaAs

InP

CdTe

DSC a-Si

SQ Limit



SummaryThere are limits, beyond Shockley-Queisser for photo-conversion with organics (OPV, DSSC, PS, AP)

• static disorder ~ 0.2-0.3 eV

• dynamic disordervibronic coupling ~ 0.2-0.3 eV

• low dielectric constant~ 0.1 -0.3 eV Σ = 0.5 - 0.9 eV; cf. 1.05 eV !

High(er) optical absorption edge systems

Ways to beat those limits in artificial systems * composite materials? *smarter photon management• ………………………………….

Nayak et al. Adv. Mater., May 2011, EES, in press


0.2 GWp ( ~40 MWc) plant at Golmud, PRC

World’s Largest Solar-Electric Plant (2009)

30 TWp (~ 6 TWC)requires 1 such plant, every HOUR, for the next~ 20 years (+ a bit of

storage…)

Solar Cell Power Stations TODAY

01/’12 Global installed PV power

~0.067 TWp

PRC goal >2011≥ 0.002 TWp/yr


Israel’s electricity generation capacity ~11 GW ~ 0.011 TW (1.6 kW/capita)

China’s electricity growth plan: 0.1 TW/year ……

One such plant, every day,

for the next … 11 years

10 TW electricity from COAL ?

אם תרצואין זו אגדה


Thin Film Solar Cells: Present Status

Data from B v Roedern, NREL, 2011


Energy Pay-back Time for PV Cells

d. cahen, weizmann inst. 02/’12 how good can solar cells be? assessing possibilities for solar...

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