1. “Plastic” Electronics and Optoelectronics
Low Cost “Plastic” Solar Cells
Light emitting Field Effect Transistor (LEFET)
CLEO
Baltimore
May 7, 2007

2. Low Cost “Plastic” Solar Cells:
A Dream or Reality???
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

3. Printing of Plastic Electronics
“inks” ---- with
electronic functionality!
Plastic Substrate
Solar Cells
We should remember to explain that “Plastic electronics is based on conducting and semiconducting polymers
Much lower capital and materials cost. 
The Dream 
Functional
Ink

4. “Plastic” Solar Cells
Ultrafast charge separation with quantum efficiency approaching Unity ! e - ħ R O
1992
50 femtoseconds!!

5. Ultrafast electron transfer is important ---
Charge transfer is 1000 times faster than any competing process,
and
Back charge transfer is inhibited
Therefore,
Quantum efficiency for charge separation approaches unity!
Every photon absorbed yields one pair of separated charges!
(Route to photovoltaics and photodetectors)

6. How do we create a material with
charge-separating junctions everywhere?? e - ħ R O
All must be accomplished at nanometer length scale!

7. Self-assembled nanoscale material with
PCBM
P3HT
Bulk Heterojunction Material
Bicontinuous interpenetrating network
Self-assembled nanoscale material with
charge-separating junctions everywhere!

8. “Bulk” D-A Heterojunction Material A self-assembled nanomaterial
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

9. “Bulk” Heterojunction Material
Before 150oC
anneal
150oC anneal
30 min
150oC anneal
2 hours
P3HT
PCBM
TEM images of the P3HT/PCBM
interpenetrating network

10. P3HT/PCBM (1:0.8) prior to annealing

11. After annealing for 10 minutes at 150oC

12. Spatial Fourier Transform
Relevant length scale --- peak corresponds to structure
with “periodicity” of nm
Large scale segregation or slow variation in thickness
for distances > 100 nm
III. Noise at very short length scales

13. Spatial Fourier Transform
High temperature annealing o C
1. Stable for long times at High-T
2. 10 minutes is sufficient to lock in the structure

14. ”Period” Å
Distance from any point in the material to a
charge separating interface  Å
Less than the mean exciton diffusion length ---
High charge separation efficiency.

15. Series resistance decreased from 113  to 7.9 
Solar Cell Performance
Device structure: ITO/PEDOT/P3HT:PCBM/Al.
Eff = 5.0%
VOC = V
FF = 68%
w/o anneal
70oC, 30 min
150oC, 30 min
Series resistance decreased from 113  to 7.9 

16. New Device Architecture: Optical Spacer
ITO
Glass
PEDOT Al
Active Layer
Conventional Device
Device with Optical Spacer
Light intensity in
Dead zone
Optical spacer

17. Device architecture Charge separation layer TiOX Ti OR OH + H2O O
Device architecture Ti OR OH
+ H2O
TiOX O
Glass
ITO
PEDOT:PSS
P3HT:PCBM
TiOX
Aluminum
Charge separation layer

18. Power Conversion Efficiency
Green light (532 nm)
Efficiency
e = 8.1% → 12.6%
AM1.5 (solar spectrum)
Efficiency
e = 2.3% → 5.0%
Optical spacer: 50% improvement !

19. Polymer Solar cells: Present status in our labs --- 5% - 6%
What can we expect to achieve?? --- Eff  15%
New Architecture --- optical spacer % improvement
(already included)
Devices with eff.  5 - 6% fabricated with P3HT
Band gap too large --- missing half the solar spectrum
Opportunity: Potential for 50% improvement using
polymer with smaller band gap
Increase open circuit voltage ---
Opportunity: Potential for 50% improvement
Tandem Cell ---
Opportunity: Potential for > 50% improvement

20. Semiconducting polymers with smaller band gap?( ) n O C H 3 a b c
Zhengguo Zhu
(ZZ50)
Absorption 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

21. Best performance for a single cell architecture( ) n O C H 3 a b c
ZZ50
5.5%
Best performance for a single cell architecture

22. Tandem Cell
(Multilayer architecture equivalent to two solar cells in series)
Open circuit voltage doubled
Efficiency 6.5%

23. We can do even better ----
Increased performance of
each of the two subcells ---
Already demonstrated.
Goal for coming months: 10%

24. Air-stable polymer electronic devices
Thin layer of amorphous TiOx (x<2) improves performance
and
Enhances Lifetime of Polymer Based Solar Cells

25. Bulk Heterojunction Solar Cells
Single TiOx passivation layer signficantly enhances the lifetime
Factor of 100 !
Goal: Simple inexpensive barrier materials will be
sufficient for achieving long lifetime.

26. Lifetime: Light soak of flex devices
Flex OPV with low cost packages pass °C, 1 sun
Fully flexible devices under 1 sun, 65°C
Flexible devices packaged with commercial, low cost films
(Konarka)

27. Lifetime: Damp heat stability
Flex OPV with low cost packages pass 500 hrs in damp heat.
Cells are fairly stable, though slow degradation observed
% Efficiency change under 65C/85%RH
-100
-80
-60
-40
-20 20 40
200
400
600
800
1000
1200
Time (hours)
% Change of Efficiency
Structure 1
Structure 4
Flexible devices packaged with commercial, low cost films

28. Environmental lifetime (Konarka)
Little (no!) degradation observed after ½ year outdoor testing.
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.

29. Polymer Solar cells: Present status in our labs --- 5 - 6%
What can we expect to achieve?? --- Eff  15%
New Architecture --- optical spacer % improvement
(already included)
Devices with eff.  5 - 6% fabricated with P3HT
Band gap too large --- missing half the solar spectrum
Opportunity: Potential for 50% improvement using
polymer with smaller band gap (ongoing ---)
Increase open circuit voltage ---
Opportunity: Potential for 50% improvement
Tandem Cell ---
Opportunity: Potential for > 50% improvement

30. Active development program at Konarka Technologies
Can we achieve Eff > 10% with the polymer
“bulk heterojunction” nano-material?
Active development program at Konarka Technologies
in USA and in Europe.

31. Goal: Roll-to-roll manufacturing by printing and
coating technologies
Joint Venture: Konarka - KURZ (Germany)

32. Low Cost “Plastic” Solar Cells:
A Dream Becoming a Reality

33. Light Emitting FETs Gate-Induced Insulator-to-Metal Transition ---
Field induced metallic state in organic FETs
Light Emitting FETs
and --- the relationship between the two

34. Field Effect Transistor
Drain
Source
Semiconductor Layer
++++
Gate Insulator
Gate Electrode
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 the
insulator-to-metal boundary???

35. 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 approaching metallic densities

36. Gated four-probe measurement (P3HT)
Material deposition by spin casting
Device annealed at 235C
Carrier mobilities  0.1 cm2/Vs
source
L = 16 m
drain
W = 1000 m
SiO2 dielectric, 200 nm
gate, Si (n++)
We measure the voltage drop inside the transistor channel
Typically Rcontact < channel resistance

37. Field-induced M-I Transition in poly(3-hexyl thiophene) --- PRL (2006)
P3HT
VG = 150 V
Metallic
VG = 50V
Critical line
VG = 40 V
Insulator
Power law indicates critical line
M-I Transition in 2d --- (Zero-T Quantum Phase Transition)

38. 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)

39. Polymer Light Emitting Field-Effect Transistorhv
Drain
(Ag)
Conjugated Polymer
Source
(Ca)
Gate Dielectric
Gate
Qualitative description of device operation:
Gate controlled “p-n junction”
Light emission from the overlap recombination zone

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

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

42. Device physics analyzed in detail by Smith and Ruden
experiment
theory
D. L. Smith and P. P. Ruden, Appl Phys Lett 2006, 89,

43. |Ids|1/2 vs gate voltage,
(where Ids is the total current between the Au and Ca electodes).
µh = 2.1 x10-5 cm2/V/s
µe = 1.6 x10-5 cm2/V/s
Vth(holes) = 150V -103V = 47 V
Large threshold voltages imply disorder
and a high density of traps at the
semiconductor-dielectric interface.
Vth(electrons) = 65 V

44. and holes in the -band only for a narrow range of gate voltages
|Ids|1/2 vs gate voltage,
(where Ids is the total current between the Au and Ca electodes).
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.

45. Calculated curves (Smith and Ruden):
(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,

46. Definitive demonstration of LEFET
Location of emission zone controlled by gate voltage
Good agreement with
Smith and Ruden
VG = 87 V
VG = 93 V
VG = 99 V
Definitive demonstration of LEFET

47. Spatial Resolution of the Emission Zone (EZ)
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)

48. Emission at the extremes --- VG  0 and VG  150 V
Occurs 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

49. Conclusions
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 density
of deep traps within the band gap.

50. The LEFET as the route to injection lasers fabricated
from luminescent semiconducting polymers
The data imply inverted populations:
High density of electrons in *-band
High 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

51. 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

52. Acknowledgement Professor Kwanghee Lee ---UCSB and GIST (Korea)
Dr. Jin Young Kim
Christoph Brabec and colleagues at Konarka
James Swensen (LEFET)