1. Nanostructured Polymer Solar Cells
2008 IEEE INTERNATIONAL RELIABILITY PHYSICS SYMPOSIUM
April 29, 2008
Nanostructured Polymer Solar Cells
D. Xi, C. Shi, Y. Yao, Y. Yang, Q. Pei
Materials Science and Engineering
California NanoSystems Institute
University of California, Los Angeles

2. Polymer Solar Cell Efficiency of inorganic solar cells: ~10–20%
Konarka
Efficiency of inorganic solar cells: ~10–20%
Current Polymer Solar Cell: ~5%
Max Inorganic: ~40%
No clean room or high T steps needed (large-area, low cost)
Flexible panels (form factor)
Versatility of polymer structure and property via synthesis
Nanostructural tailoring

3. How Does It Work?
Yu, Heeger, et al, Science, 270, 1789(1995)

4. Mechanism and Efficiency
ηIQE = ηA × ηED × ηCT × ηCC
Stephen R. Forrest,
MRS Bulletin, 30 (2005) p.28
VOC = D ( ELUMO – EHOMO – EExiton binding )

5. Mechanism and Efficiency
RSH = Shunt resistance (quality of diode)
RS = Series resistance (quality of contacts & transport in bulk of film)
Stephen R. Forrest, MRS Bulletin, 30 (2005) p.28-32

6. Main Factors Limiting the Efficiency: Low absorption
H. Hoppe & NS Sariciftci, J. Mater. Res., Vol. 19, 1926 (2004)

7. Main Factors Limiting the Efficiency: Short Exiton Lifetime
Exciton diffusion length in ordered polymers is 5-14 nm
A. Haugeneder, et al, Phys. Rev. B, 59(23), 15346: 1999
T. Stubinger and W. Brutting, J. APPL. PHYS., 90(7), 3632: 2001

8. Bulk Heterojunction in Polymer Blend
ITO
Donor/Acceptor
Blend (100+ nm) Al
N.S. Sariciftci, Heeger, et al.
S. Forrest, et al.

9. Bulk Heterojunction
Inganäs, et al, Adv. Mater.,
13, 1871: 2001
Stalmach U. et al, J. Am. Chem. Soc.,122, 5464 (2000)

10. Alkoxythiophene polymers?
FETs
Solar cells ??
P3HT+PCBM (200nm) A l k o x y P V ( M E H - ) O *
Alkoxy to alkyl:
Larger bandgap
Lower mobility
Less stable

11. Synthesis of regioregular polymers and copolymers
Shi, et al., J. AM. CHEM. SOC. 2006, 128,

12. UV-Vis-NIR of spin-coated filmsX O O O X O X O X X O O X O O X O X X O O X

13. Energy Levels of Semiconductors
P3OOT
1.91eV Ca
P3DOT
PF-co-DTB Al
1.64eV
1.60eV
1.92eV
1.78eV
ITO
POT
-co-
DOT
P3HT
PCBM

14. Solar Cell Structure A Al (80 nm) LiF (1 nm) Polymer/PCBM (80-100 nm)
PEDOT:PSS (25 nm)
ITO/Glass

15. Characteristics of Bulk Heterojunction Cells
(AM 1.5G irradiation at 100 mW/cm2).
Polymer Polymer:PCBM Jsc Voc (V) FF (%) PCE (%) (w/w ratio) (mA/cm2)
P3DOT 1:
POT-co-DOT 1:
PF-co-DTB 2:
PF-co-DTB 1:
PF-co-DTB 1:
PF-co-DTB 1:

16. IPCE plot of PF-co-DTB/PCBM (1:4) BH cells
Shi, et al., J. AM. CHEM. SOC. 2006, 128,

17. C60 PCBM vs C70 PCBM
Y. Yan, et al., APL 89, (2006)

18. Absorption of PF-co-DTB/[70]PCBM blends
Y. Yan, et al., APL 89, (2006)

19. AFM of PF-co-DTB/PCBM blends
Tapping mode Phase mode
PF-co-DTB: 1
[60]PCBM: 4
[70]PCBM: 4

20. Cell performance vs. PCBM concentration

21. IV Characteristics of polymer/PCBM BH cells

22. EQE of polymer/PCBM BH cells
Y. Yan, et al. APL 89, (2006)

23. Other Small Eg Polymers

24. Bulk Heterojunction in Nanorod/Polymer Blend
7nmx7nm
7nmx60nm
(Huynh W.U., Science 295,2425, 2002)

25. Bulk Heterojunction in Porous TiO2 / Polymer
Sintering TiO2 nanocrystals + P3HT
Quantum efficiency only 6%
Due to incomplete filling and random distributed inferface
Well ordered 8nm pore TiO2 film + P3HT
Incomplete PL quench due to twist of polymer into 8nm pores;
optimized infiltration depth 20nm, QE, 10%, power efficiency 0.45%
(Kevin M. Coakley, Adv. Funct. Mat 13, 301, 2003)

26. Bulk Heterojunction Based on CuPc Nanowires
PCE FF
Voc
CuPc nanowires by CVD.
Scale bar: 500 nm
ITO / CuPc / PTCBI / BCP / Ag
Power efficiency 2.7%
(Fan Y., Nature Materials 4, 37, 2005)

27. Interdigitated p-n Nanohybrid
transparent electrode
top electrode Au
p-Conjugated polymer
n-semiconductor
ITO/PEDOT
Diameter ~20nm,
Height ~200nm
Space between rods ~20nm
Two bicontinuous phases, effectively split exciton before recombination
Carriers have straight pathway to electrodes
Prevent holes from reaching the negative electrode and electrons from positive electrodes

28. Interdigitated p-n Nanohybrid: Polymer nanotube array
500 nm b b c
1 mm d e
100 nm
Xi et al. Nanotechnology 18 (2007)

29. Interdigitated p-n Nanohybrid: CdS Nanorod array
500 nm
CdS + PT by electropolymn
CdS + P3HT by infiltration

30. Summary
Alkoxythiophene is a useful building block for highly- conjugated, low bandgap (co)polymers.
BH solar cells based on PF-co-DTB and [70]PCBM:
Jsc: 6.34mA/cm2
Voc: 0.76V
FF: 50.5%
PCE: 2.4%
More work is needed to improve mobility and band edge matching (PF-co-DTB: mh = 2x10-5cm2/Vs)
Interdigitated p-n nanohybrid is a good architecture but challenging to fabricate perfect nanostructure/material