“Plastic” Electronics and Optoelectronics

CLEOBaltimoreMay 7, 2007

Low Cost “Plastic” Solar Cells

Light emitting Field Effect Transistor (LEFET)

We have an energy problem.

Nuclear, hydroelectric, wind etc must all contribute to the solution.

Solar cells --- Power from the Sun must be --- and will be a significant contribution.

Two problems that must be solved with solar:1. Cost 2. Area

Low Cost “Plastic” Solar Cells:

A Dream or Reality???

PrintingPrinting of Plastic Electronics of Plastic Electronics

Functional Functional InkInk

Plastic SubstratePlastic Substrate

Solar CellsSolar Cells

“inks” ---- with electronic functionality!

The Dream

Ultrafast charge separation with quantum efficiency approaching

Unity !

e-

ħ

RO OR

RO OR

RO OR

RO OR

1992

50 femtoseconds!!

“Plastic” Solar Cells

Ultrafast electron transfer is important ---

Charge transfer is 1000 times faster than any competing process,

and

Back charge transfer is inhibited

Every photon absorbed yields one pair of separated charges!

(Route to photovoltaics and photodetectors)

Therefore,

Quantum efficiency for charge separation approaches unity!

How do we create a material with charge-separating junctions everywhere??

All must be accomplished at nanometer length scale!

e-

ħ

RO OR

RO OR

RO OR

RO OR

Self-assembled nanoscale material with charge-separating junctions everywhere!

Bulk Heterojunction MaterialBicontinuous interpenetrating network

P3HT

S n

PCBM

OMe

O

“Bulk” D-A Heterojunction Material

A self-assembled nanomaterial

Electrical Contact

Electrical Contact

Must break the symmetry --- use two different metals with

different work functions.Electrons will automatically go toward lower work function contact and holes toward higher work function contact

“Bulk” Heterojunction Material

TEM images of the P3HT/PCBM interpenetrating network

Before 150oC anneal

150oC anneal 2 hours

150oC anneal 30 min

P3HT

S n

PCBM

OMe

O

P3HT/PCBM (1:0.8) prior to annealing

After annealing for 10 minutes at 150oC

0.01 0.1 1

P.S

.D.(

a.u

.)

Spatial Frequency(1/nm)

10min annealing Room temperature

Spatial Fourier Transform

II. Relevant length scale --- peak corresponds to structurewith “periodicity” of 16-20 nm

I. Large scale segregation or slow variation in thicknessfor distances > 100 nm

III. Noise at very short length scales

Spatial Fourier Transform

High temperature annealing --- 150o C1. Stable for long times at High-T2. 10 minutes is sufficient to lock in the structure

0.01 0.1 1

P

.S.D

.(a

.u.)

Spatial Frequency(1/nm)

10min annealing 30min annealing 2hour annealing

”Period” 160 -200 Å

Distance from any point in the material to a charge separating interface 40-50 Å

Less than the mean exciton diffusion length --- High charge separation efficiency.

Solar Cell Performance

Device structure: ITO/PEDOT/P3HT:PCBM/Al.

w/o anneal

70oC, 30 min

150oC, 30 min

Eff = 5.0%VOC = 0.625 VFF = 68%

Series resistance decreased from 113 to 7.9

ITOGlass PEDOT AlActive LayerGlass ITO PEDOT AlActive Layer

Light intensity in

Conventional Device Device with Optical Spacer

New Device Architecture: Optical Spacer

Dead zone Optical spacer

Gla

ss

ITO

PE

DO

T:P

SS

P3H

T:P

CB

M

TiO

X

Al

um

inu

m

Device architecture

Charge separation layer

 

Ti

OR

OR

 

OR

Ti

OH

OH

OH

OH

  + H2O

TiOX

TiO

Ti

Ti

TiO

O

O

O

O

Power Conversion Efficiency

AM1.5 (solar spectrum)

Efficiency

e = 2.3% → 5.0%

Green light (532 nm)Efficiency

e = 8.1% → 12.6%

Optical spacer: 50% improvement !

Polymer Solar cells: Present status in our labs --- 5% - 6%

What can we expect to achieve?? --- Eff 15%

Devices with eff. 5 - 6% fabricated with P3HTBand gap too large --- missing half the solar spectrum

Opportunity: Potential for 50% improvement using polymer with smaller band gap

New Architecture --- optical spacer --- 50% improvement(already included)

Increase open circuit voltage --- Opportunity: Potential for 50% improvement

Tandem Cell --- Opportunity: Potential for > 50% improvement

Semiconducting polymers with smaller band gap?

300 400 500 600 700 800 900 10001E-8

1E-7

1E-6

1E-5

1E-4

1E-8

1E-7

1E-6

1E-5

1E-4

P3HT/P3HT:PCBM

Pho

tocu

rren

t (ar

b.u.

)

Energy (eV)

Pho

tocu

rren

t (ar

b.u.

)

ZZ50/ZZ50:PCBMAbsorption and photoresponse

out to 900 nm in the IR.

Should be capable of 7% in single cell and >10% in a Tandem Cell with P3HT

Zhengguo Zhu

(ZZ50)S S

N

S

N

( )n

S n

OCH3O

(a)

(b)

(c)

S S

N

S

N

( )n

S n

OCH3O

(a)

(b)

(c)

S S

N

S

N

( )n

S n

OCH3O

(a)

(b)

(c)

S S

N

S

N

( )n

S n

OCH3O

(a)

(b)

(c)

Best performance for a single cell architecture

5.5%

ZZ50

0.0 0.2 0.4 0.6 0.8 1.0 1.2-12

-10

-8

-6

-4

-2

0

PCPDTBT single cell P3HT single cell Tandem cell

Cu

rren

t D

ensi

ty (

mA

/cm

2 )

Bias (V)

Open circuit voltage doubled

Efficiency 6.5%

Tandem Cell(Multilayer architecture equivalent to two solar cells in

series)

We can do even better ----

Increased performance of each of the two subcells ---

Already demonstrated.

Goal for coming months: 10%

Air-stable polymer electronic

devices

Thin layer of amorphous TiOx (x<2) improves performance

and

Enhances Lifetime of Polymer Based Solar Cells

Bulk Heterojunction Solar CellsSingle TiOx passivation layer signficantly enhances the lifetime

Factor of 100 !

Goal: Simple inexpensive barrier materials will be sufficient for achieving long lifetime.

0 50 100 150

0.01

0.1

1 with TiOx

Conventional

Eff

icie

ncy (

%)

Time (Hours)

Lifetime: Light soak of flex devices

Flex OPV with low cost packages pass 1000 hrs @ 65°C, 1 sun

Flexible devices packaged with commercial, low cost films(Konarka)

0 200 400 600 800 1000 12000,0

0,2

0,4

0,6

0,8

1,0

1,2

Effi

cien

cy [a

.u.]

Time [hours]

Fully flexible devices under 1 sun, 65°C

Lifetime: Damp heat stability

% Efficiency change under 65C/85%RH

-100

-80

-60

-40

-20

0

20

40

0 200 400 600 800 1000 1200

Time (hours)

% C

han

ge

of

Eff

icie

ncy

Structure 1

Structure 4

Flexible devices packaged with commercial, low cost films

Flex OPV with low cost packages pass 500 hrs in damp heat. Cells are fairly stable, though slow degradation observed

Environmental lifetime (Konarka)

Little (no!) degradation observed after ½ year outdoor testing.

OPV BL2 Module Aging on Rooftop Test Station - under Load

-80%

-60%

-40%

-20%

0%

20%

40%

60%

80%

9/20/06 10/4/06 10/18/06 11/1/06 11/15/06 11/29/06 12/13/06 12/27/06 1/10/07 1/24/07 2/7/07 2/21/07 3/7/07

Date

Cha

nge

in P

ower

(%

) x

x

-20

-10

0

10

20

30

40

50

60

70

80

Air

Tem

pera

ture

(°C

)

G4

G6

S5

S6

(Encapsulated, with UV Protection)

Data at 1 Sun (1,000 W/m2)

Proprietary Information of Konarka Technologies Inc.

t0 Efficiencies: G4: 1.35% G6: 1.21% S5: 1.88% S6: 1.41%

Air Temperature

OPV modules loaded on roof top test in Lowell, MA

Reduced efficiency observed in the winter period is not degradation --- efficiency increases with increasing temperature.

Polymer Solar cells: Present status in our labs --- 5 - 6%

What can we expect to achieve?? --- Eff 15%

Devices with eff. 5 - 6% fabricated with P3HTBand gap too large --- missing half the solar spectrum

Opportunity: Potential for 50% improvement using polymer with smaller band gap (ongoing ---)

New Architecture --- optical spacer --- 50% improvement(already included)

Increase open circuit voltage --- Opportunity: Potential for 50% improvement

Tandem Cell --- Opportunity: Potential for > 50% improvement

Can we achieve Eff > 10% with the polymer “bulk heterojunction” nano-material?

Active development program at Konarka Technologies

in USA and in Europe.

Goal: Roll-to-roll manufacturing by printing andcoating technologies

Joint Venture: Konarka - KURZ (Germany)

Low Cost “Plastic” Solar Cells:

A Dream Becoming a Reality

Gate-Induced Insulator-to-Metal Transition ---Field induced metallic state in organic FETs

Light Emitting FETs

and --- the relationship between the two

Field Effect Transistor

Gate Electrode

Gate Insulator

Semiconductor Layer

Drain Source

When voltage is applied between gate and source-drain,mobile electronic charge carriers (positive or negative) are induced into the semiconductor.

++++

Can we increase the field induced carrier density to levels sufficient to cross theinsulator-to-metal boundary???

Can we increase the field induced carrier density to high enough levels sufficient to cross the

insulator-to-metal boundary???

Q = CV = (A/d)V

N = A(/e)(V/d)

nA = (/e)(V/d) electrons (or holes) per unit area

Vmax/d = EBreakdown

These charges are confined to a thin layer (approx 10 nm) in the semiconductor adjacent to the interface with the gate dielectric.

Conclusion: n 1020 cm-3 --- approaching metallic densities

source drain

SiO2 dielectric, 200 nm

W = 1000 m

L = 16 m

gate, Si (n++)

Gated four-probe measurement (P3HT)

• We measure the voltage drop inside the transistor channel

• Typically Rcontact < channel resistance

• Material deposition by spin casting

• Device annealed at 235C

• Carrier mobilities 0.1 cm2/Vs

Sn

P3HT

Field-induced M-I Transition in poly(3-hexyl thiophene) --- PRL (2006)

10-5

10-4

10-3

10-2

10-1

100

101

0.1 0.2 0.3 0.4 0.5

(

-1cm

-1)

1/T1/2 (K-1/2) -12

-10

-8

-6

-4

-2

0

2 3 4 5ln

lnT

VG = 150 V Metallic

VG = 50VCritical line VG = 40 V

Insulator

Sn

P3HT

Power law indicates critical line

M-I Transition in 2d --- (Zero-T Quantum Phase Transition)

Conclusion:

We can reach metallic densities in organic FETs (gate–induced through field effect)

n 1020 cm-3

Question:

Can we reach metallic densities (field-induced)for both electrons and holes in the same device?

Answer: Yes!Light-emitting Field Effect Transistor (LEFET)

Polymer Light Emitting Field-Effect Transistor

hv

Gate

Drain(Ag)

Gate Dielectric

Source(Ca)

Conjugated Polymer

+ + + + + + + + + + + + + +

- - - - - - - - - - - - - - - - - -

Qualitative description of device operation:

Gate controlled “p-n junction”

Light emission from the overlap recombination zone

Au Ca150 V ground

Vg < Vsd/2h+ transport dominates

Ambipolar transport dominates

hhhh

Vg Vsd/2e- transport dominates Vg > Vsd/2

0 30 60 90 120 1501E-9

1E-8

1E-7

Vgs [V]

|Ids|

[A

]

0

2

4

6

8

10

12

14

PM

T C

urre

nt |

a.u.

|

Gate voltage controls the electron current,the hole current, and the brightness!

Ambipolar transport dominates

hhhh

Vg Vsd/2

0 30 60 90 120 1501E-9

1E-8

1E-7

Vgs [V]

|Ids|

[A

]

0

2

4

6

8

10

12

14

PM

T C

urre

nt |

a.u.

|

Maximum efficiency at crossover point where electron and hole currents are equal.

0 50 100 150 2001E-8

1E-7

1E-6

Vgs [V]

Ids

[A

]

D. L. Smith and P. P. Ruden, Appl Phys Lett 2006, 89, 233519

experiment theory

Device physics analyzed in detail by Smith and Ruden

0 25 50 75 100 125 1500

10

20

30

40

|Ids|1

/2

A1/2

Vgs [V]

|Ids|1/2 vs gate voltage, (where Ids is the total current between the Au and Ca electodes).

22 thgid VVL

WCI

Vth(holes) = 150V -103V = 47 V

Vth(electrons) = 65 V

Large threshold voltages imply disorder and a high density of traps at the semiconductor-dielectric interface.

µh = 2.1 x10-5 cm2/V/s

µe = 1.6 x10-5 cm2/V/s

0 25 50 75 100 125 1500

10

20

30

40

|Ids|1

/2

A1/2

Vgs [V]

|Ids|1/2 vs gate voltage, (where Ids is the total current between the Au and Ca electodes).

22 thgid VVL

WCI

The LEFET operates in the ambipolar regime with electrons in the *-band and holes in the -band only for a narrow range of gate voltages

65 V < VG < 103 V.

(a) (b)

Calculated curves (Smith and Ruden): Gate-to-Channel potential and Carrier Densities (electrons and holes) vs x

Gate to channel potential 0 between the hole and electron accumulation regions.

This location defines the center of the light emission zone (and the center of the charge recombination zone).

D. L. Smith and P. P. Ruden, Appl Phys Lett 2006, 89, 233519

Location of emission zone controlled by gate voltage

Definitive demonstration of LEFET

VG = 87 V

VG = 93 V

VG = 99 V

Good agreement withSmith and Ruden

Diagram of a confocal microscrope(Prof. S. Burrato, UCSB)

Images of the emission zone collected as EZ is moved across the channel

Cross section plots

of emission intensity vs. lateral position

Full width at half maximum approx 2 m(determines recombination rate, Smith & Ruden)

Spatial Resolution of the Emission Zone (EZ)

Au Ca150 V ground

Vg < Vsd/2h+ transport dominates

e- transport dominates Vg > Vsd/2

0 30 60 90 120 1501E-9

1E-8

1E-7

Vgs [V]

|Ids|

[A

]

0

2

4

6

8

10

12

14

PM

T C

urre

nt |

a.u.

|

Emission at the extremes --- VG 0 and VG 150 VOccurs adjacent to the Ca and the Au electrodes:

PLEDs with electron (hole) accumulation layer extending

all the way across the channel as electrode with hole(electron) injection by tunneling

Hole accumulation layer acts as low resisitance cotnact across the channel

Electron accumulation layer acts as low resistance contact across the channel

At the voltage extremes, the electron (hole) density extends all the way across the 16 m channel length such that the accumulation layer functions as the electrode for an LED with

opposite carrier injection by tunneling.

In these two limits the carrier densities (electrons in the *- band or holes in the - band) are sufficiently high that the accumulation layer functions as a low resistance contact,

implying near metallic transport.

These very long distances for electron and hole accumulation, respectively, more than 10,000 repeat units on the semiconducting polymer chain and more than 10,000 interchain

spacings imply the existence of well-defined - and *- bands with a very small densityof deep traps within the band gap.

Conclusions

The LEFET as the route to injection lasers fabricated from luminescent semiconducting polymers

1. The data imply inverted populations: High density of electrons in *-bandHigh density of holes in - band

2. Minimal loss from the source and drain electrodes

3. Minimal loss from ITO gate electrode

4. Electron & hole densities confined to narrow

region near the gate dielectric

• Reduced ‘electrode absorption losses’ by gate-induced injection through the source and drain

• Minimized ‘charge induced absorption losses’ because chargetransport is perpendicular to the waveguide direction

Light Emitting FET (LEFET) as theRoute to the Polymer Injection Laser

Light emitting FETs

The high gate induced carrier densities imply inverted population in the LEFET

High density of electrons in the *–band and

High density of holes in the –band.

LEFET Injection Laser: Important Opportunity

Professor Kwanghee Lee ---UCSB and GIST (Korea)Dr. Jin Young KimChristoph Brabec and colleagues at KonarkaJames Swensen (LEFET)

Acknowledgement