| 1

Plastic solar cells

M. A. Loi

Zernike Institute for Advanced Materials

University of Groningen,

The Netherlands

e-mail M.A.Loi@rug.nl

| 2

Overview

1st hourSolar cells in general Solar Radiation p-n junction The organic version

2nd hour Improving plastic solar cells Low band-gap polymers Charge transfer states is detrimental?

| 3

Solar Cells I

› Long duration power supply

• Satellites

• Space vehicles

• Remote locations on earth

› Valid alternative to fossil fuels

› Pollution free

| 4

› Photovoltaic effect

• Becquerel (1839)

• Fritts {Selenium} (1883)

• Ohl {semiconductor junction solar cell}(1946)

• Chapin, Fuller, Person {Silicon p-n junction solar cells} (1954)

Solar Cells II

| 5

Motivations

› ENERGY Increasing energy need

Exhaustion of fossil fuels

Diversification of energy sources

Energy for all (2 billion people without electricity)

› ECOLOGY Pollution of environment

CO2 Responsible Climate

change

› ECONOMY Energetically independent

| 6

Solar Radiation› Every second in the sun

6 x 1011 kg H2 → He + 4 x 1020 J ☼

› At the average distance of the earth the solar radiation is 1353 W/m2

› The atmosphere attenuates the solar radiation

• Absorption water - IR

• Absorption Ozone – UV

• Scattering

Air Mass

| 7

Air Mass

› Air mass = the path length of the light from a celestial source relative to that at the zenith at sea level.

› increases as the angle between the source and the zenith increases (AM38 at the horizon).

› Out of the atmosphere AM0

› On earth surface with sun at the zenith AM1

› Average for terrestrial applications - 45˚ from the zenith AM1.5

AM= sec zenith angle

| 8

Solar spectrum

| 9

| 10

Solar cells – inorganic case

› Single bandgap material

• Photons with h<Eg lost energy

• Photons with h=Eg used energy

• Photons with h>Eg (h-Eg) lost

energy

Illuminated p-n junction

| 11

P-n junction solar cells

| 12

Ideal solar cell

IL current produced by solar radiation

Is diode saturation current

RL load resistance

LkTqV

s IeII )1( /

Shockley diode equation

kTE

n

n

Ap

p

DVCs

geD

N

D

NNAqNI /11

A device area

| 13

IV characteristics

oc

Lsc

V

II Short circuit current

Open circuit voltage

LkTqV

s IeI oc )1(0 /

s

L

s

Loc I

I

q

kT

I

I

q

kTV 1ln

| 14

Ideal solar cell

VIeVIIVP LkTqV

s )1( /

mmm VIP

i

m

P

P

| 15

IV characteristics-realistic

Shunt resistance – leakage current

SH

SSsL

SHDL

R

IRV

kT

IRVqIII

IIII

1)(

exp

Series resistanceJunction, impurity concentration

| 16

IV characteristics-realistic

SH

SSsL

DL

R

IRV

kT

IRVqIII

III

1)(

exp

The effect RSH is negligible

Rs in Si solar cells 0.7-0.4

| 17

Conversion efficiency

› FF; IL; Voc should be maximized for efficient solar cells!

ocL

mm

VI

VIFF Fill factor

Conversion efficiency

in

ocL

in

mm

P

VIFF

P

VI

EQE or IQE, quantum efficiency-percentage of photon converted in carriers (ISC)

| 18

Ideal efficiencies

| 19

Real efficiency

| 20

Plastic Solar cells

2000

2008

| 21

Pro & conAdvantages

› tailoring of opto-electronic properties

› large areas

› low temperatures (RT)

› processing from solution

› roll to roll manufacturing

› light weight

› transparent

› low cost…….maybe…

Power paint?

| 22

Problems

› low ambient stability

› strongly bound excitons (Frenkel like)

› Exciton diffusion length rather short 5-20 nm.

› low mobility of charge carriers

•μn (c-Si) > 1000 cm2/Vs

•μh (polymer) ≈ 0.1 cm2/Vs

› difficult to obtain low band-gap materials

Pro & con

| 23

Nevertheless

Disposable low-end applications!

http://www.konarka.com

| 24

-

+

-

+

Frenkel exciton Stronghly bound (0.4 eV in PPV); radius 5 Å

Molecola-

+

Charge Transfer exciton

Polarons

Molecular semiconductorsMolecular semiconductors• coulomb interaction • elettron-phonon coupling

To start - photoexcitations

| 25

Triplet excitons

Frenkel excitons

Ground state

Flu

ores

cenc

e

Intermolecular excitons

non radiative states

Non

rad

iativ

e-em

issi

on

Phos

phor

esce

nce

| 26

The first examples

› Early works inspired by nature (photosynthesis)

› Porphyrins, phthalocyanines, perylenes (xerography), merocyanines

› Organic heterojunction devices: p-type / n-type organic semiconductors

› – 1970’s until 1995: organic heterojunction bilayers

› – 1985 Tang cell: PTCBI (45 nm) and CuPc (25 nm)

› 1% efficiency

| 27

The Kodak approach

Tang et al., APL 2005

| 28

The polymer approach!

› Active layer:

bulk heterojunction - hole conducting material - electron conducting material

› Operation principle:

• Exciton photoexcitation

• Diffusion of the excitons towards the organic-organic interface

• Charge separation/electron transfer

• Transport of charge carriers towards the electrodes

| 29

Photoinduced Charge Generation

MDMO PPV 3,7 - dimethyloctyloxy methyloxy

PPV

PCBM1-(3-methoxycarbonyl) propyl-1-

phenyl [6,6]C61

O

O n

DONOR ACCEPTOR

N. S. Sariciftci et al., Science 258, 1474 (1992)

An ultra-fast e- transfer occurs between Conjugated Polymer / Fullerene composites upon illumination. The transition time is less than 40 fs.

exciton

Back transfer very slow! s - ms

| 30

The driving force!

› Electron affinity fullerene derivatives!

OMe

O

PolymerPCBM

-6 eV-5.2 eV

-4.2 eV

-3.5 eV

| 31

Bulk Heterojunctions

h MDMO-PPVPCBMPCBM

e-

ITO on Glass / Plastic

e-

P+

e-

e-

e-

e-

e-

P+

Al Electrode

Al Electrode

e-

| 32

P-Solar Cells - FILM PREPARATION

Spin Casting is a easy coating technique for small areas. Material loss is very high. Doctor Blade Technique

was developed for large area coating

Doctor Bladehas no material loss

| 33

Production - Large Area

a)

b)

Large Area Thin Film Production using Doctor/Wire Blading

| 34

Plastic Solar Cells - CONTACTING

The cathode electrode is

applied by evaporation.

Different electrodes are used

for different applications.

Sealing is absolutely necessary for anincreased life time of plastic solar cells.

| 35

Characterization under A.M. 1.5

| 36

Bulk Heterojunctions

h MDMO-PPVPCBMPCBM

e-

ITO on Glass / Plastic

e-

P+

e-

e-

e-

e-

e-

P+

Al Electrode

Al Electrode

e-

| 37

The morphology issue…

S. E. Shaheen, Appl. Phys. Lett., 78, 841–843 (2001)

2,5%< 1%

| 38

Now..

Organic solar cells performances depend on the material properties and microscopic structure of the bulk heterojunction!

P3HT

4,5-5.0 %

> 60 polymers checked last 5 years!

| 39

Optimization

eff = Isc * Voc * FF / Iinc

Isc Tuning of the Transport Properties - Mobility

Voc Tuning of the Electronic Levels of the Donor Acceptor

Systems

FF Tuning of the Contacts and Morphology

Iinc Tuning of the Spectral Absorbance/Absorbing more

light (low bandgap)

| 40

The future?

| 41

Intermezzo!

| 42

Organic Solar cells

Polymer(donor)

PCBM(acceptor)

Power conversion efficiency ~ 5 - 6%

bulk heterojunction

bulk heterojunction3D heterostructure

hole conducting material

+electron conducting material

| 43

Remember-Organic Solar Cells› Working mechanism-steps

• Excitons photoexcitation

• Diffusion of the excitons towards the interface

• Charge separation/electron transfer

• Transport of charge carriers towards the electrodes

› Organic solar cells performances depend on

• the material properties

• the microscopic structure of the bulk hetero-junction

| 44

The driving force!

› Donor and acceptor LUMO energy offset!

OMe

O

PolymerPCBM

-6 eV-5.2 eV

-4.2 eV

-3.5 eV

Ultrafast phenomena!

| 45

Enhancing devices efficiency

› Optimize the materials properties• Matching solar spectrum! NIR materials

• Relative position of the energy levels of the donor and acceptor

optimal offset between LUMO (D) – LUMO(A)

for electron transfer at least 0.3 – 0.5 eV

P3HT:PCBM: LUMO (D) – LUMO(A) ~1.1 eV

› Optimize the morpholog• microscopic phase separation

( exciton diffusion length ~ 5 – 7 nm )

• presence of a percolation pathway

| 46

Remember-Solar cells parameters

› JSC – short-circuit current

› Jph – photocurrent

› FF – fill factor:

› VOC – open circuit voltage

SCOC JV

JVFF

maxmax

light

SCOC

lightin

out

P

JVFF

P

P

P

P max

› power conversion efficiency

LUMO (A)3

4

5

6

Don

or

Acc

epto

r

En

ergy

(eV

)

Voc

HOMO (D)

LUMO (D)

| 47

En

ergy

(eV

)

P3HT:PCBM

The reduction of the LUMO offset

• power conversion efficiency ~ 3.8 %• LUMO offset ~ 1.1 eV• Voc~ 0.59 V

Voc

3

4

5

6

Don

or

Acc

epto

r

bisPCBM

• LUMO offset ~ 1.0 eV• Voc~ 0.73 V

Power conversion efficiency ~ 4.5 % !!!

M. Lenes et al, Adv. Mater. 2008, 20, 2116

| 48

PL of thermally annealed films

The devices performance:

P3HT:PCBM – 3% P3HT:bisPCBM – 3.6%

electron transfer is more efficient for P3HT:PCBM

P3HT:bisPCBM – PL ≈ 60 ps P3HT:PCBM – PL ≈ 41 ps

| 49

PL of solvent annealed films

The devices performance:

P3HT:PCBM – 3.8% P3HT:bisPCBM – 4.6%

electron transfer is more efficient for P3HT:PCBM

P3HT:bisPCBM – PL ≈ 38 ps P3HT:PCBM – PL ≈ 31 ps

| 50

AFM measurementsP3HT:PCBM P3HT:bisPCBM

spin coated

slow dried

3.9 nm

12.4 nm10.7 nm

4.6 nm

• surfaces is smoother for

samples prepared by

thermal annealing

• difference in RMS

roughness between

P3HT:PCBM and

P3HT:bisPCBM

10x10m

| 51

The blend in solution

P3HT:PCBM & P3HT:bisPCBM

PL ≈ 156 ps

→ the efficiency of the electron transfer is the same in both blends

| 52

Conclusion 1› Increasing power conversion efficiency by tailoring

the energy levels

• P3HT:bisPCBM – higher Voc

• P3HT:bisPCBM – higher power conversion efficiency

• P3HT:PCBM – faster PL decay in the thin film

• In solution – the same PL decay

› A small reduction of the LUMO offset does not have a significant influence on the electron transfer

› The P3HT:bisPCBM blend is limited by diffusion -the morphology can be still optimize

| 53

-

+

-

+

Frenkel exciton Stronghly bound (0.4 eV in PPV); radius 5 Å

Molecule-

+

Charge Transfer exciton

Polarons

Molecular semiconductorsMolecular semiconductors• coulombic interaction • elettron-phonon coupling

Remember-Photoexcitations

| 54

Intermediate state?

M. A. Loi et al., Adv. Funct. Mat. 17, 2111 (2007)

CT like intermediate states

are considered for

modelling IV of solar cells

V.D. Mihailetchi et al., PRL (2004)

Recent reports consider the energy transfer from the polymer to the PCBM as the first step of the charge separation

OMe

ONN

S

R

R

*

*

n

*RR

S NSN

S*

n

| 55

OMe

O

PCBM

NN

S

R

R

*

*

n

F8BT

10-4

10-3

10-2

10-1

100

101

102

-0,4 -0,2 0 0,2 0,4 0,6 0,8 1

Ph

oto

curr

ent

(mA

/cm

2 )

Voltage (V)Very poor PV performances!!

Energy transfer?

| 56

450 500 550 600 650 700 750 800 850

PCBM PLcorr

P2+66%PCBM PLcorr

P2+33%PCBM PLcorr

P2+20%PCBM PLcorr

P2+0.5%PCBM PLcorr c

P2 PLcorr c

Pho

tolu

min

esc

en

ce

Wavelength (nm)

F8BT

0,5% PCBM

20% PCBMPCBM

33%PCBM

66%PCBM

M. A. Loi et al., Adv. Funct. Mat. 17, 2111 (2007)

OMe

O

PCBM

NN

S

R

R

*

*

n

F8BT

Energy transfer?

400 500 600 700 800

Pho

tolu

min

esce

nce

Wavelength (nm)

F8BT

0,5% PCBM

20% PCBM

PCBM

33%PCBM 66%PCBM

| 57

0 500 1000 1500 2000

F8BT @ 530 nm

66%PCBM @ 520nm

PCBM @ 720nm

66%PCBM @ 720nm

Pho

tolu

min

esce

nce

Time (ps)

Energy Transfer?

The polymer PL

decay becomes very

fast upon PCBM

blending

Energy transfer?

| 58

-10 0 10 20 30 40 50

PCBM-TR1 Nlaser N66%F8BTP

hoto

lum

ines

cenc

eTime (ps)

No clear evidence!

Long rise-time also in pristine PCBM

M. A. Loi et al., Adv. Funct. Mat. 17, 2111 (2007)

Energy transfer to the PCBM singlet state then transferred to the triplet state.

S. Cook et al., APL (2006)

| 59

Electron transfer

*RR

S NSN

S*

n

OMe

O

F8DTBTPCBM

~4%

Polyfluorene copolymers

promising low-band gap

materials for PV

applications 10-4

10-3

10-2

10-1

100

101

102

-0,4 -0,2 0 0,2 0,4 0,6 0,8 1

Ph

otoc

urr

ent

(m

A/c

m2 )

Voltage (V)

M. Svensson et al., Adv. Mat. 15, 988 (2003);Q. Zhou et al., Appl. Phys. Lett. 84, 1653 (2004);F. Zhang et al., Adv. Funct. Mater. 16, 667 (2006)

| 60

OMe

O

PCBM 550 600 650 700 750 800 850

Pho

tolu

min

esce

nce

Wavelegth (nm)

F8DTBT

0.5% PCBM

10%PCBM20% PCBM

66% PCBM

*RR

S NSN

S*

nF8DTBT

| 61

Charge transfer excitons

550 600 650 700 750 800 850

Pho

tolu

min

esce

nce

Wavelegth (nm)

F8DTBT

5% PCBM

20% PCBM

PCBM 33%PCBM 66%PCBM

Red-shift with the increasing of the average dielectric constant

ε(PCBM) ~ 3.9;ε(Polymer) ~ 2.5-3.0

222

4

2 n

eEE gn

Rydberg-like transitions

| 62

PCBM1 = 320 ps; 2 = 3.1 ns

PCBM1 = 350ps; 2 = 1.0 ns

M. A. Loi et al., Adv. Funct. Mat. 17, 2111 (2007)

0 500 1000 1500 2000

F8DTBT @ 630nm

66%PCBM @ 630nm

F8DTBT @ 730nm

66%PCBM @ 730nm

PCBM @ 820nm

66%PCBM @ 820nm

Pho

tolu

min

esce

nce

Time (ps)

| 63

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

300 400 500 600 700 800

PCBM

F8DTBT

66%F8DTBT+33%PCBM

33%F8DTBT+66%PCBM

66%F8DTBT+33%PCBM CALC

33%F8DTBT+66%PCBM CALC

Abs

orpt

ion

Coe

ffici

ent (

105 c

m-1

)

Wavelength (nm)

OMe

O

PCBM

*RR

S NSN

S*

n

F8DTBT

Ground state interaction?

| 64

0.0

0.5

1.0

1.5

2.0

2.5

3.0

3.5

300 350 400 450 500 550 600

PCBMF8BT66%F8BT+33%PCBM33%F8BT+66%PCBM66%F8BT+33%PCBM CALC33%F8BT+66%PCBM CALC

Wavelength (nm)

Abso

rptio

n C

oeffic

ien

t (1

05 c

m-1

)OMe

O

PCBM

NN

S

R

R

*

*

n

F8BT

| 65

Charge separation

There is an intermediate state between the Frenkel exciton and the free charge!

| 66

Is it general?Are more systems showing this phenomena?Typical of narrow band gap polymers?

D. Muehlbacher et al. Adv. Mater. 2006

Solar cells efficiency: =3.2 %

FIRST narrow band-gap O-semiconductor

PCPDTBT

nSS

NS

N

*

*

| 67

Narrow band-gap polymer

PCPDTBTblend PCBM 1:1

| 68

Concentration dependence

0 PCBM

1/11 PCBM1/3 PCBM

1/2 PCBM

Charge Transfer Exciton

(1,0)

(10,1) (2,1)(1,1)

| 69

New excited state in the blend with long decay time!

0 PCBM= 100ps

1/11 PCBM1 = 50ps2 = 1190ps

1/3 PCBM = 510ps

1/2 PCBM = 478ps

1/2 PCBM= 5ps

(ns)

0 PCBM= 150ps

Exciton CT Exciton

| 70

CTE detrimental for PV?

PCPDTBT/PCBM 1:1

| 71

Conclusions II

Evidences of an excited state intermediate between

the exciton and the free carriers in heterojunctions

containing narrow band gap polymers – present also in

working devices!

Additives can reduce the CTE component acting on

the microstructure of the blend.

The suppression of the intermediate states in bulk

hetero-junctions is extremely important for the

optimization of organic solar cells. PCPDTBT/PCBM 3.8%

→5.5%