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Transcript of “Plastic” Electronics and Optoelectronics CLEO Baltimore May 7, 2007 Low Cost “Plastic”...
“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