NSLS-II Beamline Development Proposals Workshops, Soft X-ray Spectromicroscopy, May 20, 2011

Photovoltaic Materials and Devices:

Organic Bulk Heterojunction Solar Cells

Chang-Yong Nam

Electronic Nanomaterials Group

Center for Functional Nanomaterials

• Abundance of solar energy, in the form of light

• Conversion of light into electricity: High cost, low efficiency

• Better understanding and control over material/device parameters:

Can soft x-ray techniques be useful?

Ultimate source of renewable energy: Sun

Light Photovolatics

Sun

Issues of fossil fuel

2

• Inorganic solar cells: High materials and manufacturing costs

• Organic solar cells: Low material & fabrication costs

• Efficiency issue due to its distinctive PV conversion processes

Konarka’s Power Plastic®

Organic solar cells

Konarka’s roll-to-roll processing

Conventional Si solar cells

3

• PV conversion process is “excitonic” : Low dielectric constant

• Exciton diffusion & dissociation at D-A interface

• Current output ~ absorber band-gap

• Voltage (potential energy) output ~ LUMOacceptor – HOMOdonor

• Bilayer device: Low efficiency due to insufficient light absorption

Organic solar cells

LD

Photon

LD: Exciton diffusion length

L~ 5-20 nm

p-type

(donor)

n-type

(acceptor)

Full absorption length:

>200 nm

Energy level diagram under short circuit

4

• Bulk heterojunction: Blend of p- & n-type materials, using solution

• Active layer can be much thicker than exciton diffusion length (LD)

• Best efficiency ~4-5% (P3HT-PCBM) & ~7% (PBDTTT-PCBM)

Organic bulk heterojunction

~LD

Bulk heterojunction

Photon

P3HT (poly-3-

hexylthiophene)

p-type

PCBM

(Derivatized C60)

n-type

L >> LD

Nanoscale phase-separated

morphology

5

US DOE target efficiency

• Multi year plan for

Solar Energy Technology Program

6

DOE target efficiency

• Multi year plan for

Solar Energy Technology Program

7

• Large optical band gap: Not enough light absorption

• Low charge mobility (especially hole): Carrier recombination loss

• Why inefficient charge transport in these materials?

Solar spectrum

Efficiency limiting factors

400 600 800

0.1

0.2

0.3

0.4

(nm)

Ab

so

rba

nce

Optical absorbance

of P3HT:PCBM

Thickness

~135 nm

P3HT: 650 nm

(1.9 eV)

P3HT: 22% coverage

~10% efficiency?

P3HT

8

Charge transport in blend layer

• Complex blend morphology, low crystallinity, hopping, charge traps

• Very low charge mobility : ~10-5-10-3 cm2/V-s

• Significant charge recombination loss

• How to measure charge mobility?

bieff Vμ

L=

2

,ansit timeCarrier tr

? ?

“Dead ends”

9

100

200

300

020

Glazing incidence

x-ray scattering of P3HT

Limited crystallinity in polymer Intermolecular hopping

& charge traps

e.g., ~ 10 s for = 10-5 cm2/V-s

How to measure charge mobility?

• Field effect transistor: “Wrong” orientation, influence of interface

• Time of flight: Thick sample requires, dispersive transport

• Space charge limited current method

10

S D

G

Oxide

Organic semiconductor

VDS

VGS

IDS

A

Back-gated field effect transistor Time of flight measurement

Light pulse

Glass/ITO

Oscilloscope V

P-type, accumulation mode

0

(before saturation)

(after saturation)

or

IDS

VGS

DSOX

VGS

D VL

WC

dV

dI

DS

2

2TGOXDS VV

L

WCI

@VDS

IPhoto

tTOF

Vt

L

TOF

2

t

Photocurrent transient

Space charge limited current

• Bulk limited transport: Built-up charge in organics due to low mobility

• Mott-Gurney law: J ~ V2 with in the proportional constant

• Modification: Effects of traps, electric field

• Example: Mobility in air-processed P3HT:PCBM blend device

11

3

2

8

9

d

VμεεJ so

Metal-insulator-metal,

Low mobility, trap-free

Deep traps, field-dependent detrapping Shallow traps

d

Vμ

d

VJ

d

VJ

ofield

fieldro

effro

891.0exp where

8

9 dependent, Field

8

9 trap,Shallow

3

2

3

2

Space charge limited current

Air-processed P3HT:PCBM blend solar cells

• Spin-coating/contact deposition, ambient air processing

• Post-fabrication vacuum anneal for reduction of oxygen charge trap

ITO/PEDOT:PSS

coated glass

P3HT:PCBM

1-2% Solution Thickness~100-140 nm

1. Spin coating of active layer

Al ~70-100 nm

2. Top metal contact deposition

Nam, Su, et. al., Adv. Func. Mater. 19 (2009)

Device cross-section viewed by TEM

12

Air-processed P3HT:PCBM blend solar cells

• Spin-coating/contact deposition, ambient air processing

• Post-fabrication vacuum anneal for reduction of oxygen charge trap

• Charge mobility through blend network? Nam, Su, et. al., Adv. Func. Mater. 19 (2009)

0.0 0.3 0.6-12

-8

-4

0

4 P3HT:PCBM blend

Curr

ent

density (

mA

/cm

2)

Voltage (V)

Dark

1 Sun, AM1.5G

Efficiency ~4%

Jsc ~9-10 mA/cm2

Voc ~0.65 V

Jsc

Voc

Solar cell current density vs voltage characteristics Device cross-section viewed by TEM

13

P3HT hole mobility in the blend

• Hole mobility: Two conducting channels & selective contacts

• Mobility in 10-5 cm2/V-s range, ~2 orders lower than reported

• Vacuum anneal improves mobility a bit (~20%)

• Poole transport at high bias: NT ~1020 cm-3, oxygen trap at ~0.4 eV

• Nonetheless, decent PV performance

14

Low bias: SCLC High bias: Poole conduction

ITO/PEDOT:PSS

Au

0 1 20

10

20

30

0 2 4 6 8

102

103

104

As-cast

Blend-anneal

JR

1/2 (

A1/2/m

)

Va-V

bi (V)

520

518

517

516

8.0x10-91.0x10

-8

1/F

ln (

J/F

2)

~8.5 V

JR (

A/m

2)

Va-V

bi (V)

(a) (b)

0.1 1 1010

1

102

103

104

2000 4000 600010

1

102

103

104

JF

-R (

A/m

2)

Va-V

bi (V)

JF

-R (

A/m

2)

F0.5

(V0.5

/m0.5

)

~1 V(c) (d)

0 1 20

10

20

30

0 2 4 6 8

102

103

104

As-cast

Blend-anneal

JR

1/2 (

A1/2/m

)

Va-V

bi (V)

520

518

517

516

8.0x10-91.0x10

-8

1/F

ln (

J/F

2)

~8.5 V

JR (

A/m

2)

Va-V

bi (V)

(a) (b)

0.1 1 1010

1

102

103

104

2000 4000 600010

1

102

103

104

JF

-R (

A/m

2)

Va-V

bi (V)

JF

-R (

A/m

2)

F0.5

(V0.5

/m0.5

)

~1 V(c) (d)(c) (d)

Vd

μεεJ so 3

1

8

9d

V

kT

esJ

2exp

Nam, Su, et. al., Adv. Func. Mater. 19 (2009)

Selective electrical contact

P3HT

PCBM

?

Open circuit voltage

• New polymer, PBDTTT: Eg ~1.77 eV (~700 nm), Jsc ~13 mA/cm2

• Improved Voc (~0.76 V) leads to PCE ~ 6.77%

Chen et. al., Nat. Photon. 3 (2009), Solarmer & Yu’s group

15

–3.70

What is open circuit voltage?

• Circuit model vs. energy band model: No driving electric field at Voc

• Voc,max should be 1.4 V or 0.9 V (contact) for P3HT:PCBM

• In reality, Voc ~0.6 V, why?

• Charge recombination should be considered

16

R

I

Light

Solar cell V

V

I

0

Isc

Voc

Simplified circuit model Energy band model: Flat band @ Voc

-3.2 eV

-3.7 eV

-5.1 eV

-6.1 eV

-4.3 eV

-5.2 eV

Al

P3HT

PCBM

PEDOT:

PSS

Quasi Fermi level, Voc, and recombination

• Max potential energy ~ quasi Fermi level difference: qVoc =EFN – EFP

• Steady state charge density n determine Voc

– At steady state: G = R, where G and R are generation and recombination rates

• When light is turned off, Voc decreases due to R: dn/dt = –R = –n/

• Voc transient provides carrier lifetime

17

-3.2 eV

-3.7 eV

-5.1 eV

-6.1 eV

P3HT

PCBM

EV

EC

EFN

EFP

states) ofdensity (eff. ~ and where

exp

exp

VC

FPVV

CFNC

NNpn

kT

EENp

kT

EENn

n

p

qVoc E

q

E

N

n

q

kTV

C

oc ln2

1212

q

kT

dt

dn

nq

kT

dt

dVoc

Preliminary transient Voc data

• Measure voltage required to force zero current under light

• Preliminary vs Voc data, valid only Voc < ~0.3 V

• Linear in semi-log plot: Bimolecular recombination at low n

• If extrapolate, ~10 s at Voc = 0.575 V (full illumination)

18

-0.2 0.0 0.2 0.4 0.6-8

-6

-4

-2

0

J (

mA

/cm

2)

V (V)

1SUN, AM1.5G

PCE = 2.0%

Jsc

= 6.76 mA/cm2

Voc

= 0.575 V

FF = 0.52

2 month old P3HT:PCBM device

0.0 0.5 1.0 1.5 2.0

0.0

0.1

0.2

0.3

0.4

0.5

0.6

0.7

Off

Vo

c (

V)

t (s)

Light on

0.05 0.10 0.15 0.20

0.01

0.1

Lifetim

e (

s)

Voc

(V)

Voc transient PV characteristics

Lifetime vs Voc

.2

ln 1

ion)recombinatr bimolecula(for and ln2 2

constVkT

qn

nn

Rq

E

N

n

q

kTV

oc

C

oc

n

qVoc E

Bimolecuar

recombination

Important material properties

• Structural & energy characteristics: Crystallinity, molecular

orientation, interfaces, amorphous region, energy levels, electronic

defects

• How to spatially map these parameters and correlate them with

device photovoltaic and electrical performance?

van Bavel, et. al., Adv. Func. Mater. 20 (2009)

Transmission electron microscopy (TEM) images of P3HT:PCBM blend

before and after thermal anneal

Summary

• Organic bulk heterojunction solar cells

– Blend of donor (p-type) and acceptor (n-type) organic semiconductor

• Charge transport and mobility measurement

– Field effect transistor, time of flight, Space charge limited current

– Oxygen effects on P3HT:PCBM blend

• Origin of open circuit voltage and transient measurement

– Carrier lifetime

– Bimolecular recombination

• Acknowledgement

– Electronic Nanomaterials, CFN: Chuck Black, Jon Allen, Danvers Johnston

– Electron Microscopy, CFN: Dong Su

– Condensed Matter Physics: Ben Ocko, Htay Hliang

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