Skip to main content

Thank you for visiting nature.com. You are using a browser version with limited support for CSS. To obtain the best experience, we recommend you use a more up to date browser (or turn off compatibility mode in Internet Explorer). In the meantime, to ensure continued support, we are displaying the site without styles and JavaScript.

Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends

Abstract

Control of blend morphology at the microscopic scale is critical for optimizing the power conversion efficiency of plastic solar cells based on blends of conjugated polymer with fullerene derivatives. In the case of bulk heterojunctions of regioregular poly(3-hexylthiophene) (P3HT) and a soluble fullerene derivative ([6,6]-phenyl C61-butyric acid methyl ester, PCBM), both blend morphology and photovoltaic device performance are influenced by various treatments, including choice of solvent, rate of drying, thermal annealing and vapour annealing. Although the protocols differ significantly, the maximum power conversion efficiency values reported for the various techniques are comparable (4–5%). In this paper, we demonstrate that these techniques all lead to a common arrangement of the components, which consists of a vertically and laterally phase-separated blend of crystalline P3HT and PCBM. We propose a morphology evolution that consists of an initial crystallization of P3HT chains, followed by diffusion of PCBM molecules to nucleation sites, at which aggregates of PCBM then grow.

This is a preview of subscription content

Access options

Rent or Buy article

Get time limited or full article access on ReadCube.

from$8.99

All prices are NET prices.

Figure 1: Absorption spectra for P3HT:PCBM films and EQE spectra for P3HT:PCBM devices.
Figure 2: Lateral segregation in P3HT:PCBM films as observed by optical microscopy, Raman spectroscopy and atomic force microscopy.
Figure 3: Vertical composition profiles in P3HT:PCBM films as deduced using ellipsometry.
Figure 4: Real-time evolution of P3HT:PCBM blend morphology.

References

  1. 1

    Halls, J. J. M. et al. Efficient photodiodes from interpenetrating networks. Nature 376, 498–500 (1995).

    CAS  Article  Google Scholar 

  2. 2

    Yu, G., Gao, J., Hummelen, J. C., Wudl, F. & Heeger, A. J. Polymer photovoltaic cells: Enhanced efficiencies via a network of internal donor–acceptor heterojunctions. Science 270, 1789–1791 (1995).

    CAS  Article  Google Scholar 

  3. 3

    Hoppe, H. & Sariciftci, N. S. Morphology of polymer/fullerene bulk heterojunction solar cells. J. Mater. Chem. 16, 45–61 (2006).

    CAS  Article  Google Scholar 

  4. 4

    Shaheen, S. E. et al. 2.5% efficient organic plastic solar cells. Appl. Phys. Lett. 78, 841–843 (2001).

    CAS  Article  Google Scholar 

  5. 5

    Ma, W., Yang, C., Gong, X., Lee, K. & Heeger, A. J. Thermally stable, efficient polymer solar cells with nanoscale control of the interpenetrating network morphology. Adv. Funct. Mater. 15, 1617–1622 (2005).

    CAS  Article  Google Scholar 

  6. 6

    Reyes-Reyes, M., Kim, K. & Carroll, D. L. High-efficiency photovoltaic devices based on annealed poly(3-hexylthiophene and 1-(3-methoxycarbonyl)-propyl-1-phenyl-(6,6) C61 blends. Appl. Phys. Lett. 87, 083506 (2005).

    Article  Google Scholar 

  7. 7

    Kim, Y. et al. A strong regioregularity effect in self-organizing conjugated polymer films and high-efficiency polythiophene:fullerene solar cells. Nature Mater. 5, 197–203 (2006).

    CAS  Article  Google Scholar 

  8. 8

    Li, G. et al. High-efficiency solution processable polymer photovoltaic cells by self-organization of polymer blends. Nature Mater. 4, 864–868 (2005).

    CAS  Article  Google Scholar 

  9. 9

    Kim, Y. et al. Device annealing effect in organic solar cells with blends of regioregular poly(3-hexylthiophene) and soluble fullerene. Appl. Phys. Lett. 86, 063502 (2005).

    Article  Google Scholar 

  10. 10

    Vanlaeke, P. et al. Polythiophene based bulk heterojunction solar cells: Morphology and its implications. Thin Solid Films 511–512, 358–361 (2006).

    Article  Google Scholar 

  11. 11

    Mihailetchi, V. D. et al. Origin of the enhanced performance in poly(3-hexylthiophene):[6,6]-phenyl C61-butyric acid methyl ester solar cells upon slow drying of the active layer. Appl. Phys. Lett. 89, 012107 (2006).

    Article  Google Scholar 

  12. 12

    Kim, K., Liu, J. & Carroll, D. L. Thermal diffusion processes in bulk heterojunction formation from poly-3-hexylthiophene/C60 single heterojunction photovoltaics. Appl. Phys. Lett. 88, 181911 (2006).

    Article  Google Scholar 

  13. 13

    Chirvase, D., Parisi, J., Hummelen, J. C. & Dyakonov, V. Influence of nanomorphology on the photovoltaic action of polymer–fullerene composites. Nanotechnology 15, 1317–1323 (2004).

    CAS  Article  Google Scholar 

  14. 14

    Zhao, Y., Xie, Z., Qu, Y., Geng, Y. & Wang, L. Solvent-vapor treatment induced performance enhancement of poly(3-hexylthiophene):methanofullerene bulk-heterojunction photovoltaic cells. Appl. Phys. Lett. 90, 043504 (2007).

    Article  Google Scholar 

  15. 15

    Osterbacka, R., An, C. P., Jiang, X. M. & Vardeny, Z. V. Two-dimensional electronic excitations in self-assembled conjugated polymer nanocrystals. Science 287, 839–842 (2000).

    CAS  Article  Google Scholar 

  16. 16

    Zhokhavets, V., Erb, T., Gobsch, G., Al-Ibrahim, M. & Ambacher, O. Relation between absorption and crystallinity of poly(3-hexylthiophene)/fullerene films for plastic solar cells. Chem. Phys. Lett. 418, 347–350 (2006).

    CAS  Article  Google Scholar 

  17. 17

    Vanlaeke, P. et al. P3HT/PCBM bulk heterojunction solar cells: Relation between morphology and electro-optical characteristics. Sol. Energy Mater. Sol. Cells 90, 2150–2158 (2006).

    CAS  Article  Google Scholar 

  18. 18

    Klimov, E., Li, W., Yang, X., Hoffmann, G. G. & Loos, J. Scanning near-field and confocal Raman microscopic investigation of P3HT-PCBM systems for solar cell applications. Macromolecules 39, 4493–4496 (2006).

    CAS  Article  Google Scholar 

  19. 19

    Yang, X. et al. Nanoscale morphology of high-performance polymer solar cell. Nano Lett. 5, 579–583 (2005).

    CAS  Article  Google Scholar 

  20. 20

    Zhokhavets, V., Erb, T., Hoppe, H., Gobsch, G. & Sariciftci, N. S. Effect of annealing of poly(3-hexylthiophene)/fullerene bulk heterojunction composites on structural and optical properties. Thin Solid Films 496, 679–682 (2006).

    CAS  Article  Google Scholar 

  21. 21

    Savenije, T. J., Kroeze, J. E., Yang, X. & Loos, J. The effect of thermal treatment on the morphology and change carrier dynamics in a polythiophene-fullerene bulk heterojunction. Adv. Funct. Mater. 15, 1260–1266 (2005).

    CAS  Article  Google Scholar 

  22. 22

    Swinnen, A. et al. Tuning the dimensions of C60-based needlelike crystals in blended thin films. Adv. Funct. Mater. 16, 760–765 (2006).

    CAS  Article  Google Scholar 

  23. 23

    Yang, X., Alexeev, A., Michels, A. J. & Loos, J. Effect of spatial confinement on the morphology evolution of thin poly(p-phenylenevinylene)/methanofullerene composite films. Macromolecules 38, 42890–4295 (2005).

    Google Scholar 

  24. 24

    Tompkins, H. G. & Irene, E. A. (eds) Handbook of Ellipsometry (William Andrew Publishing, Norwich, New York, 2005).

  25. 25

    Fried, M. et al. Dose-dependence of ion implantation-caused damage in silicon measured by ellipsometry and backscattering spectrometry. Thin Solid Films 455, 404–409 (2004).

    Article  Google Scholar 

  26. 26

    Zaumseil, P., Krüger, D., Kurps, R., Fursenko, O. & Formanek, P. Precise measurement of Ge depth profiles in SiGe HBT’s—a comparison of different methods. Solid State Phenom. 95–96, 473–482 (2004).

    Google Scholar 

  27. 27

    Reyes-Reyes, M. et al. Meso-structure formation for enhanced organic photovoltaic cells. Org. Lett. 7, 5749–5752 (2005).

    CAS  Article  Google Scholar 

  28. 28

    Waldauf, C. et al. Highly efficient inverted organic photovoltaics using solution based titanium oxide as electron selective contact. Appl. Phys. Lett. 89, 233517 (2006).

    Article  Google Scholar 

  29. 29

    Goffri, S. et al. Multicomponent semiconducting polymer systems with low crystallization-induced percolation threshold. Nature Mater. 5, 950–956 (2006).

    CAS  Article  Google Scholar 

  30. 30

    Arias, A. C. et al. Vertically segregated polymer-blend photovoltaic thin-film structures through surface-mediated solution processing. Appl. Phys. Lett. 80, 1694–1697 (2002).

    Article  Google Scholar 

  31. 31

    Björnström, C. M. et al. Multilayer formation in spin-coated thin films of low-bandgap polyfluorene:PCBM blends. J. Phys. Condens. Matter 17, L529–L534 (2005).

    Article  Google Scholar 

  32. 32

    Björnström, C. M. et al. Vertical phase separation in spin-coated films of a low bandgap polyfluorene/PCBM blend—effects of specific substrate interaction. Appl. Surf. Sci. 253, 3906–3912 (2007).

    Article  Google Scholar 

  33. 33

    Heriot, S. Y. & Jones, R. A. L. An interfacial instability in a transient wetting layer leads to lateral phase separation in thin spin-cast polymer-blend films. Nature Mater. 4, 782–786 (2005).

    CAS  Article  Google Scholar 

  34. 34

    Snaith, H. J., Greenham, N. C. & Friend, R. H. The origin of collected charge and open-circuit voltage in blended polyfluorene photovoltaic devices. Adv. Mater. 16, 1640–1645 (2004).

    CAS  Article  Google Scholar 

  35. 35

    Xue, J., Rand, B. P., Uchida, S. & Forrest, S. R. Mixed donor–acceptor molecular heterojunctions for photovoltaic applications. II. Devices performance. J. Appl. Phys. 98, 124903 (2005).

    Article  Google Scholar 

  36. 36

    Heutz, S., Sullivan, P., Sanderson, B. M., Schultes, S. M. & Jones, T. S. Influence of molecular architecture and intermixing on the photovoltaic, morphological and spectroscopic properties of CuPc–C60 heterojunctions. Sol. Energy Mater. Sol. Cells 83, 229–245 (2004).

    CAS  Article  Google Scholar 

  37. 37

    Sims, M. et al. On the use of optical probes to monitor the thermal transitions in spin-coated poly(9,9-dioctylfluorene) films. J. Phys. Condens. Matter 17, 1–12 (2005).

    Article  Google Scholar 

  38. 38

    Kim, H., So, W.-W. & Moon, S.-J. Effect of thermal annealing on the performance of P3HT/PCBM polymer photovoltaic cells. J. Kor. Phys. Soc. 48, 441–445 (2006).

    CAS  Google Scholar 

  39. 39

    Si, L., Massa, M. V., Dalnoki-Veress, K., Brown, H. R. & Jones, R. A. L. Chain entanglement in thin freestanding polymer films. Phys. Rev. Lett. 94, 127801 (2005).

    Article  Google Scholar 

  40. 40

    Ellison, C. J. & Torkelson, J. M. The distribution of glass-transition temperatures in nanoscopically confined glass formers. Nature Mater. 2, 695–700 (2003).

    CAS  Article  Google Scholar 

  41. 41

    Sharp, J. S. & Forrest, J. A. Free surfaces cause reductions in the glass transition temperature of thin polystyrene films. Phys. Rev. Lett. 91, 235701 (2003).

    CAS  Article  Google Scholar 

  42. 42

    Campoy-Quiles, M., Sims, M., Etchegoin, P. G. & Bradley, D. D. C. Thickness-dependent thermal transition temperatures in thin conjugated polymer films. Macromolecules 39, 7673–7680 (2006).

    CAS  Article  Google Scholar 

  43. 43

    van Zanten, J. H., Wallace, W. E. & Wu, W.-L. Effect of strongly favourable substrate interaction on the thermal properties of ultrathin polymer films. Phys. Rev. E 53, R2053–R2056 (1996).

    CAS  Article  Google Scholar 

  44. 44

    Hummelen, J. C. et al. Preparation and characterization of fulleroid and methanofullerene derivatives. J. Org. Chem. 60, 532–538 (1995).

    CAS  Article  Google Scholar 

  45. 45

    Campoy-Quiles, M. et al. Ellipsometric characterization of the optical constants of polyfluorene gain media. Adv. Funct. Mater. 15, 925–933 (2005).

    CAS  Article  Google Scholar 

Download references

Acknowledgements

The authors thank the UK Engineering and Physical Sciences Research Council (project no. EP4/C5403361) and BP Solar (OSCER project) for financial support. We also thank I. McCulloch, M. Heeney and M. Giles, from Merck Chemicals, for supplying the P3HT polymer. M.C.-Q. thanks S. Choulis, S. Sidat and S. Sohel for useful discussions at the early stages of these investigations, J. Dane for help with the evaporation mask designs and Rod Bottom and Phil Williams (Mettler Toledo Ltd) for their help with the real-time microscopy measurements.

Author information

Affiliations

Authors

Corresponding authors

Correspondence to Mariano Campoy-Quiles, Donal D. C. Bradley or Jenny Nelson.

Supplementary information

Supplementary Information

Supplementary figures S1-S4 and supplementary table S1 (PDF 341 kb)

Rights and permissions

Reprints and Permissions

About this article

Cite this article

Campoy-Quiles, M., Ferenczi, T., Agostinelli, T. et al. Morphology evolution via self-organization and lateral and vertical diffusion in polymer:fullerene solar cell blends. Nature Mater 7, 158–164 (2008). https://doi.org/10.1038/nmat2102

Download citation

Further reading

Search

Quick links