1. P3HT:PCBM Possible way to home-use solar cell “foliage” Ge, Weihao

2. Major steps of the working process of all solar cells Efficiency - Stability - Synthesis Absorb photon Create charge Collect charge Sunlight Acceptor Donor Anode Cathode Schematic of solar cell working process

3. Efficiency - Stability - Synthesis Step1: Absorption Factors: Intensity at the active layer concentrating devices decreasing surface reflection Band structure of the material

4. Absorption Spectrum: Photoelectric effect for λ > λ max, photon passes through for λ < λ max, Excessive energy is wasted in the form of heat. Absorption spectrum of P3HT:PCBM blends: Efficiency - Stability - Synthesis Step1: Absorption Cook et al. J. Phys. Chem. C, Vol. 113, No. 6, 2009

5. Efficiency - Stability - Synthesis Step1: Absorption Spectrum: Difference from inorganic solar cell materials Band gap width: Si: organics: higher absorption at UV Adjustable Chapin et al. J.Appl.Phys.25 (1954) pp. 676 Cook et al. J. Phys. Chem. C, Vol. 113, No. 6, 2009

6. Efficiency - Stability - Synthesis Step2: Charge generation Exciton creation: Electron and hole are paired via (screened) Coulomb interaction Binding energy of excitons A very high separation rate: which is not good. Ashcroft et al. “Solid State Physics” ISBN: 7-5062-6631-8/O482 pp.626-628 Cook et al. J. Phys. Chem. C, Vol. 113, No. 6, 2009

7. Efficiency - Stability - Synthesis Step2: Charge generation Exciton separation: Diffusion, -> recombination / separation Difference from inorganic ones higher binding energy lower diffusion range

8. Efficiency - Stability - Synthesis Step2: Charge generation Exciton separation: Internal field at junctions how is an internal field built up? – Fermi energy must be matched when in equilibrium Some properties of heterojunction Window effect superinjection Bulk heterojunction in organic materials Enlarge D-A interface Excitons meet field within diffusion range Alferov, Nobel Lecture, Dec. 8, (2000) Hoppe, et, al. J.Mater.Res., Vol.19, No.7, Jul (2004)

9. Efficiency - Stability - Synthesis Step 3: Charge Collection Challenges: poor charge mobility High surface resistivity Diffused metal particles from the cathode impairs acceptor’s strength Bulk heterojunction Hoppe, et, al. J.Mater.Res., Vol.19, No.7, Jul (2004) Christoph, et, al. Adv. Funct. Mater. 2001, 11, No. 5, October Mayer, et,al. Materials today, Vol.10, No.11, Nov. (2007) pp.28-33

10. Efficiency - Stability - Synthesis Methods to improve efficiency Additional layers: Optical spacer Buffer layer P.D. Andersen et al., Opt. Mater. (2008), doi:10.1016/j.optmat.2008.11.014 Y.Zhao,etal.,Sol.EnergyMater.Sol.Cells(2009),doi:10.1016/j.solmat.2008.12.007

11. Efficiency - Stability - Synthesis Methods to improve efficiency Morphology: Experiment results Enhanced absorption Enhanced charge mobility P. Vanlaeke et al. Solar Energy Materials & Solar Cells 90 (2006) 2150–2158 F. Padinger, et al. Adv. Func. Mat. 13 (2003) 85.

12. Efficiency - Stability - Synthesis Methods to improve efficiency Thermal annealing Nanocrystal of PCBM P3HT rod-like crystalline X.Yang, et al. Nano Lett., Vol. 5, No. 4, 2005 P.Vanlaeke et al. Solar Energy Materials & Solar Cells 90 (2006) 2150–2158

13. Efficiency - Stability - Synthesis Methods to improve efficiency Thermal annealing In blends, crystallization inhibited by annealing, resumed P.Vanlaeke et al. Solar Energy Materials & Solar Cells 90 (2006) 2150–2158 J.Zhao, et al. J. Phys. Chem. B 2009, 113, 1587–1591

14. Efficiency - Stability - Synthesis Methods to improve efficiency Supplements Dye Enhance IR absorption Lower exciton separation percentage E.Johansson, et.al. J. Phys. Chem. C, 2009, 113 (7), 3014-3020

15. Efficiency - Stability - Synthesis Methods to improve efficiency Supplements Hole-extraction layer, increasing surface conductivity ITO anode PEDOT:PSS, as modifier and as anode Carbon nanotube J.Kang, et.al; Electrochemical and Solid-State Letters, 12 3 H64-H66 2009 A. Colsmann et al. Thin Solid Films 517 (2009) 1750–1752 R.A. Hatton et al., Org. Electron. (2009), doi:10.1016/j.orgel.2008.12.013

16. Efficiency - Stability - Synthesis Stability test, experimental results P3HT:PCBM 1 year lifetime out-door J.A. Hauch et al. Solar Energy Materials & Solar Cells 92 (2008) 727–731

17. Efficiency - Stability - Synthesis Methods to enhance stability Protection from electrodes: slow down phase transition Selection of Cathod material An outlook: self-repair and defect-tolerating material J.Zhao, et al. J. Phys. Chem. B 2009, 113, 1587–1591 De Bettingnies, et.al. Synthetic Metals, 156 (2006) pp.510-513 DOE office of Science. “Basic Research Needs for Solar Energy Utilization”, Apr. (2005)

18. Efficiency - Stability - Synthesis Synthesis methods Wet processing spin coating repeatable Screening printing easy to define pattern Good choice of solvent also increases efficiency www.brewerscience.com F.Krebs, Solar Energy Materials & Solar Cells 93 (2009) 465–475 S.E. Shaheen, et.al. Appl. Phys. Lett. 79, 2996 (2001) Maher Al-Ibrahim,et.al. Appl. Phys. Lett. 86,201120 (2005)

19. Summary Main challenge to organic solar cells: efficiency Morphology plays an important role. Possible for low-cost mass-production Additionally, there’re other advantages.

20. References : [1] Cook et al. J. Phys. Chem. C, Vol. 113, No. 6, 2009 [2] Chapin et al. J.Appl.Phys.25 (1954) pp. 676 [3] Ashcroft et al. “Solid State Physics” ISBN: 7-5062-6631-8/O482 pp.626-628 [4] Alferov, Nobel Lecture, Dec. 8, (2000) [5] Hoppe, et, al. J.Mater.Res., Vol.19, No.7, Jul (2004) [6] Christoph, et, al. Adv. Funct. Mater. 2001, 11, No. 5, October [7] Mayer, et,al. Materials today, Vol.10, No.11, Nov. (2007) pp.28-33 [8] P.D. Andersen et al., Opt. Mater. (2008), doi:10.1016/j.optmat.2008.11.014 [9] Y.Zhao,etal.,Sol.EnergyMater.Sol.Cells(2009),doi:10.1016/j.solmat.2008.12.007 [10] P. Vanlaeke et al. Solar Energy Materials & Solar Cells 90 (2006) 2150–2158 [11] F. Padinger, et al. Adv. Func. Mat. 13 (2003) 85. [12] X.Yang, et al. Nano Lett., Vol. 5, No. 4, 2005 [13] J.Zhao, et al. J. Phys. Chem. B 2009, 113, 1587–1591 [14] E.Johansson, et.al. J. Phys. Chem. C, 2009, 113 (7), 3014-3020 [15] J.Kang, et.al; Electrochemical and Solid-State Letters, 12 3 H64-H66 2009 [16] A. Colsmann et al. Thin Solid Films 517 (2009) 1750–1752 [17] R.A. Hatton et al., Org. Electron. (2009), doi:10.1016/j.orgel.2008.12.013 [18] J.A. Hauch et al. Solar Energy Materials & Solar Cells 92 (2008) 727–731 [19] De Bettingnies, et.al. Synthetic Metals, 156 (2006) pp.510-513 [20] DOE office of Science. “Basic Research Needs for Solar Energy Utilization”, Apr. (2005) [21] www.brewerscience.com [22] F.Krebs, Solar Energy Materials & Solar Cells 93 (2009) 465–475 [23] S.E. Shaheen, et.al. Appl. Phys. Lett. 79, 2996 (2001) [24] Maher Al-Ibrahim,et.al. Appl. Phys. Lett. 86,201120 (2005)