1. Ch.4: Polymer Solar Cells: Photoinduced Charge Transfer ▪ Photoinduced Electron Transfer between Conjugated Polymer and Fullerene N. S. Sariciftci & A. J. Heeger et al., Science 258 (1992) C.B. V.B. C 60 LUMO Level  50fs ~ ms Conjugated Polymer C 60 C 60 HOMO Level High Quantum Efficiency ~ 100% -Very efficient process faster than any other competing processes such as polaron, bipolaron, and exciton formation ( <50 fs ). -Back transfer is slowed down by forming polarons in C60. (>1 ms) 1

2. ▪ Photoinduced Electron & Hole Transfer Photoinduced Charge Transfer Process (c.f. Photoinduced Energy Transfer) ▪ Experimental Evidence: LESR 2

3. ▪ Experimental Evidence: Luminescence Quenching 1.MEH-PPV:PCBM (95:5) 2.MEH-PPV:PCBM (90:10) 3.MEH-PPV:PCBM (80:20) 4.MEH-PPV:PCBM (50:50) HOMO LUMO MEH-PPV PCBM Photoinduced Charge Transfer 3

4. ▪ Experimental Evidence: Photoconductivity Measurements Transient photoconductivity measurement C.H.Lee et al., Phys. Rev. B. 48, 15425 (1993). ▪ Experimental Evidence: Photoinduced Infrared Spectroscopy Infrared photoexcitation spectroscopy K.Lee, S. N. Saricifci, A. J. Heeger Phys. Rev. B. 49, 5781 (1994). C 60 - peak at 1.15 eV and 1.25 eV between t 1u and t 1g 4

5. Bicontinuous donor/acceptor bulk heterojunction -+ Al PEDOT:PSS ITO Glass Light e e-e- - - + + + + e - PCBM P3HT G. Yu & A. J. Heeger et al., Science 270 (1995) Interpenetrating Network in Polymer/C60 Composites Advantages Bulk donor-acceptor heterojunction Achievement of large interfacial area Requirements Formation of percolation path Morphology control Photovoltaic Cells using Polymer/C 60 Composites 5

6. Metal/Semiconductor/Metal structure Photovoltaic Effect in Polymer Solar Cells HOMO LUMO MEH-PPV PCBM Al ITO Typical PV effect in MEH-PPV/PCBM mixture 6