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 qpei@seas.ucla.edu

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

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

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

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

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

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

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

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

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

Synthesis of regioregular polymers and copolymers Shi, et al., J. AM. CHEM. SOC. 2006, 128, 8980-8986

UV-Vis-NIR of spin-coated films X O O O X O X O X X O O X O O X O X X O O X

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

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

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 1:1 0.14 0.02 26.5 0.0007 POT-co-DOT 1:1 0.60 0.22 41.2 0.054 PF-co-DTB 2:1 0.74 0.83 25.5 0.16 PF-co-DTB 1:1 2.92 0.78 32.8 0.74 PF-co-DTB 1:2 4.00 0.76 44.6 1.27 PF-co-DTB 1:4 4.31 0.76 48.6 1.60

IPCE plot of PF-co-DTB/PCBM (1:4) BH cells Shi, et al., J. AM. CHEM. SOC. 2006, 128, 8980-8986

C60 PCBM vs C70 PCBM Y. Yan, et al., APL 89, 153507 (2006)

Absorption of PF-co-DTB/[70]PCBM blends Y. Yan, et al., APL 89, 153507 (2006)

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

Cell performance vs. PCBM concentration

IV Characteristics of polymer/PCBM BH cells

EQE of polymer/PCBM BH cells Y. Yan, et al. APL 89, 153507 (2006)

Other Small Eg Polymers

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

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)

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)

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

Interdigitated p-n Nanohybrid: Polymer nanotube array 500 nm b b c 1 mm d e 100 nm Xi et al. Nanotechnology 18 (2007) 095602

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

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