Solution-Processed Small-Molecule Solar Cells with 7% Efficiency

Alan J. HeegerProfessor of Physics & Materials, UC Santa Barbara

Acknowledgements

Gui BazanGreg Welch

Yanming Sun and Weilin Leong (device fabrication and analysis)

Chris Takacs (High resolution phase contrast TEM)

Loren Kaake (Ultrafast spectroscopy)

This is a CEEM project with partial support from AFOSR

Why Organic Photovoltaics (OPV)?

Thin, flexible, light weight and rugged products

High throughput, roll-to-roll manufacturing

Potential for low cost and low carbon footprint

Unique semi-transparent modules with tunable colors

Remarkably interesting and challenging science --- nanoscale photovoltaics!

Roll-to-Roll Manufacturing

Bulk Heterojunction Solar Cells Fabricated from Small Molecules (Not Polymers)

New Direction

With polymers --- batch-to-batch variations in solubility, molecular weight, polydisperity and purity provide materials with considerably different processing properties.

Difficult, therefore, to evaluate the fundamental structure-performance relationships.

New Direction:Solution-Processed Small-Molecule Solar Cells

with 6.7% Efficiency

eVITO/MoOx/TTPSiBT:PC70BM/Al

-5.4 eV

-4.7 eV

-3.6 eV

-5.2 eV-4.3 eV

-6.1 eV

-4.3 eV

-3.0

-4.0

-5.0

-6.0

-7.0

DTS(PTTh2)2 PC70BM

a

b c

OMe

O

NS

N

NS N

SS

Si

SS S

N SN

300 400 500 600 700 800 900 10000.0

0.2

0.4

0.6

0.8

1.0

1.2

Nor

mal

ized

Abs

orpt

ion

Wavelength (nm)

Solution Film

Design and synthesis:G. Bazan and G. Welch

Dynamics and Time Scales

Mobile carrier sweep out by the internal

voltage (built-in electric field)

Ground state

Interfacial traps and

Interfacial excitons

Excited state and

charge transfer

hν

Recombination

t < 100 fs

t < 100 fs

Energy outns - s

HOMO

LUMO

HOMO

LUMO

Electron Acceptor

Electron Donor

The Initial Discovery (1992):Ultrafast Photo-induced electron transfer

*

*n

hn

e-

+

Ultrafast charge separation with quantum efficiency approaching Unity !

50 fs

Brabec et al Chem Phys Lett 2001

-60 -50 -40 -30 -20 -10 0 1010-1110-1010-910-810-710-610-510-4

Ids1/

2 mA

1/2 )

Ids

Vgs (V)

14.0

0.02.04.06.08.010.0

12.0

0 -10 -20 -30 -40 -50 -60

I ds (

A)

Vds (V)

-20 V-30 V

-40 V

-50 V

-60 V

0.0

-20.0

-40.0

-60.0

-80.0

-100.0 a b

OFET data obtained from pristine DTS(PTTh2)2. a) Output curves b) transfer curves (Vds=-60 V).

Hole mobility (saturation regime) 0.12 cm2/Vs

on/off ratio of 107.

-0.2 0.0 0.2 0.4 0.6 0.8 1.0-14-12-10-8-6-4-202

Cur

rent

den

sity

(mA

/cm

2 )

Voltage (V)

80:20 70:30 60:40 50:50

Relatively small amount of fullerene is required --- Optimum Donor-to-fullerene ratio is 7:3

Device science:Yanming Sun and Wei Lin Leong

Summary of device parameters of SM-BHJ solar cells with active layers cast from solutions with different blend ratios .

DTS(PTTh2)2/PC70BM

(weight ratios)a

Voc (V) Jsc (mA/cm2) FF (%) PCE (%)

80:2070:3060:4050:50

0.800.800.830.84

11.412.57.24.4

39.445.235.428.6

3.604.522.121.06

aActive layer thin films cast from 4% w/v CB solutions with varying weight ratio of the DTS(PTTh2)2:PC70BM components

7:3 G24:PC70BM (η=6.7%)

Top View

SideView

100 nm

7:3 G24:PC70BM (η=6.7%)

James Rogers, Ed Kramer and Gui Bazan

300 400 500 600 700 800 9000

10

20

30

40

50

60

IPC

E (%

)

wavelength (nm)

80:20 70:30 60:40 50:50

IPCE spectra of SM-BHJ solar cells based on DTS(PTTh2)2/PC70BM active layer with different blend ratios.

-0.2 0.0 0.2 0.4 0.6 0.8-14-12-10-8-6-4-202

Cur

rent

den

sity

(m

A/c

m2 )

Voltage (V)300 400 500 600 700 800 9000

10

20

30

40

50

60

70

IPC

E (%

)Wavelength (nm)

0% 0.2% 0.6% 1.0%

Performance of solar cells with a DTS(PTTh2)2:PC70BM active layer as a function of DIO content (as processing additive).

(a) Current-voltage curves (b) IPCE spectra.

Processing with 0.25% DIO (as additive) optimum: PCE = 6.7% FF = 0.59 Voc= 0.78V

Jsc= 14.4 mA/cm2

Each of these values sets a new record for small molecule OPV!

-60 -40 -20 0 20 40 6010-10

10-9

10-8

10-7

10-6

10-5

10-10

10-9

10-8

10-7

10-6

10-5

n-type mode

with 0.25% DIO w/o DIO

I ds (V

)

Vgs (V)

p-type mode

Transfer characteristics of bipolar field-effect transistors based on DTS(PTTh2)2/PC70BM blended films processed with and without DIO additives.

Hole mobility 6×10-3 cm2/Vs Electron mobility 2×10-3 cm2/Vs

Nearly balanced.

1 2 3 4 5 640

45

50

55

60

FF (%

)

Process run1 2 3 4 5 6

4.0

4.5

5.0

5.5

6.0

6.5

7.0

PCE

(%)

Process run

1 2 3 4 5 610

11

12

13

14

15

J sc (m

A/c

m2 )

Process run1 2 3 4 5 6

0.70

0.72

0.74

0.76

0.78

0.80

V oc (V

)

Process run

Performance distribution of SM-BHJ solar cells based on DTS(PTTh2)2/PC70BM active layers with 0.25% DIO (v/v).

Independent 6 batches of device fabrication were made and for each batch at least 2 cells were tested (totally 14 cells).

The average PCE is 6.2%.

DTS(PTTh2)2 BHJ on MoOx (no DIO )(163 nm per side)

(50 nm scale bar)

False Color Raw Image

TEM:Chris Takacs

In-plane stacking of DTS(PTTh2)2 at 0.31 Å-1. (approx. 2 nm d-spacing)

Regions of solid color indicate the spatial extent and direction of the crystal lattice planes.

Defocused --- Phase Contrast TEM

DZ = defocus lengthu = spatial frequency

= e-wavelength (2.5 pm)

)sin(2)()()( 2uZuEuAuCTF D

Phase Contrast TEM E-beam undergoes many small

angle deflections proportional to the density of carbon nuclei

PCBM has higher density of carbon nuclei

E-beam velocity is slower in PCBM than in Polymer

Result: Phase Contrast

Morphology of the BHJ Materials

Defocused (Phase Contrast) TEM imaging of the lattice planes

0 0.1 0.2 0.3 0.4 0.5 0.6 0.7 0.80

0.2

0.4

0.6

0.8

1

1.2x 10

-9

Pow

er S

pect

rum

(Arb

.)

q (Å-1)

Azimuthally integrated power spectrum from BHJ raw images:Blue is from no DIO device (pristine), Red is 0.25% DIO device (optimal) device, Green is 0.60% DIO device. The dashed line represents the theoretical |CTF|2 for a defocus of -1000 nm.

DTS(PTTh2)2 BHJ on MoOx (no DIO )(163 nm per side)

(50 nm scale bar)

False Color Raw Image

TEM:Chris Takacs

In-plane stacking of DTS(PTTh2)2 at 0.31 Å-1. (approx. 2 nm d-spacing)

Regions of solid color indicate the spatial extent and direction of the crystal lattice fringes.

DTS(PTTh2)2 BHJ on MoOx (0.25% DIO)(163 nm per side)

False Color Raw Image

TEM:Chris Takacs

In-plane stacking of DTS(PTTh2)2 at 0.31 Å-1. (approx. 2 nm d-spacing)

Regions of solid color indicate the spatial extent and direction of the crystal lattice fringes.

50 nm scale bar

Caution: Generation of impurity during synthesis

- Remove impurity via extraction with hexanes and column chromatography- Can avoid methyl transfer by lowering reaction temperature

Typically less than 1% impurity

Device Voc (V) Jsc (mA/cm2) FF (%) PCE (%)

G24:PC70BM 0.78 14.4 59.3 6.70

G24 (impurity):PC70BM 0.74 10.2 39.6 3.00

300 400 500 600 700 8000

10203040506070

IPCE

(%)

Wavelength (nm)

G24 G24 with impurity

0.0 0.2 0.4 0.6 0.8-16-14-12-10-8-6-4-202

G24 G24 with impurity

I(mA/

cm̂2)

Voltage(v)

PC84BM Traps in PCDTBT:PC84BM • Introduce first-order (monomolecular) recombination and decrease VOC

• Reduce mobility

• Reduce jSC and prevent fast sweep out

Result: All parameters adversely affected:VOC jSC FF

-0.5 0.0 0.5 1.0-8

-6

-4

-2

0

2

m84:m60 = 1:100

m84:m60 = 0:1

Cur

rent

den

sity

(mA

/cm

2 )

Voltage (V)

m84:m60 = 1:1,000

a.

0.0 10-4 10-3 10-2 10-1 1000.5

0.6

0.7

0.8

0.9

m84

:m60

= 0:1m

84:m

60 = 1:10,000

m84:m60 = 1:1,000m84:m60 = 1:100m84:m60 = 1:0

(estimated)

Ope

n ci

rcui

t vol

tage

(V)

PC84

BM Concentration

d.

SS

N

C8H17 C8H17

n

N NSa OMe

O

Dynamics and Time Scales

Mobile carrier sweep out by the internal

voltage (built-in electric field)

Ground state

Interfacial traps and

Interfacial excitons

Excited state and

charge transfer

hν

Recombination

t < 100 fs

t < 100 fs

Energy outns - s

80

60

40

20

0

CT (

fs)

0.60.40.20.0DIO (% v/v)

-1.0

-0.5

0.0

0.5

1.0

A

(au)

1.51.00.50.0-0.5time (ps)

DIO (% v/v) 0.25 0.16 0.08 0.00

( a ) ( b )

Ultrafast charge separation is enhanced by DiO additive

Continuum of interface states can enable the ultrafast charge transfer

e- center of mass

donor acceptore/h pair

exciton

CT states

CT continuum

CT exciton

CS stateE

Looking for a model General properties of an attractive

potential in a continuum Bound states Scattering states

Loss of carriers at early times due to bimolecular recombination at very high carrier densities

Still lower pump power ------ approx. 2.7x1017 electrons per cm

COMPETITION between SWEEP–OUT and RECOMBINATION

Mobile carrier sweep out by the internal

voltage (built-in electric field)

Ground state

Interfacial traps and

Interfacial excitons

RecombinationTime scale ???

Energy outns - s

0 5 10 15 20

10-3

10-2

10-1

100

Time (s)

J / V

int (m

-1

cm

-2)

0.7V0.6V0.5V0.4V0.2V0.0V-0.5V-1.0V

PCDTBT:PC70BMT = 300K

OMe

O

PCDTBT

PC70BM

Vint = VBI – V

Photoconductance =

Current / Internal voltage

Transient photocurrent in operating solar cell:Competition between Sweep-out and Recombination

When sweep-out is faster than recombination --- high efficiency.

Device Jsc (mA cm-2) Voc (V) FF (%) PCE (%)

70:30 13.39 0.78 62.7 6.50 80:20 9.81 0.78 54.1 4.12 85:15 5.51 0.77 39.6 1.68 90:10 3.55 0.76 37.2 1.0 95:05 0.20 0.73 26.1 0.04 100:0 / 0.50 26.3 /

-0.2 0.0 0.2 0.4 0.6 0.8-14-12-10-8-6-4-202

Cur

rent

den

sity

(m

A/c

m2 )

Voltage (V)

70:30 80:20 85:15 90:10 95:05

300 400 500 600 700 800

10

20

30

40

50

60

70

IPC

E (%

)

Wavelength (nm)

70:30 80:20 85:15 90:10 95:05

Solution-Processed Small-Molecule Solar Cells with 6.7% Efficiency

A Promising New Direction !

Nature Materials (submitted)

G. Bazan, G. Welch, Y. Sun, W-L Leong, C.J. Takacs and AJH

“Plastic” Solar Cells

Effic

ienc

y (%

)

20001995

NREL

NREL

NREL

NREL

United SolarUnited Solar12

8

4

0

16

20

2005UCSB

Cambridge

NREL

U. Linz SiemensKonarka

Konarka

Siemens

2010

12

8

4

0

16

20

OPV single junction

Year

Konarka

PlextronicsUCSB

Solarmer

Efficiency

Konarka

Time evolution of efficiency of “plastic”

solar cells9.3 % Japan;9.2% SCUT (inverted

structure)

“Plastic” Solar Cells

Low $ cost manufacturing

Low energy cost manufacturing

Low carbon “footprint” manufacturing

TechnologyEnergy for production

(MJ.Wp-1)

CO2 footprint

(gr.CO2-eq.Wp-1)

Energy payback

time(years)

mc-Si 24.9 1293 1.95CdTe 9.5 542 0.75CIS 34.6 2231 2.71

Flex OPV 2.4 132 0.19A. L. Roes et al, Progress in Photovoltaics 17, 372 (2009)

Semi-transparent BIPV based on OPV

BIPV for new constructions BAPV for retro-fit existing buildings

Power Plastic: Blending Functionality with Aesthetics

Questions ???