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The influence of molecular orientation on organic bulk heterojunction solar cells



In bulk heterojunction organic photovoltaics, electron-donating and electron-accepting materials form a distributed network of heterointerfaces in the photoactive layer, where critical photo-physical processes occur. However, little is known about the structural properties of these interfaces due to their complex three-dimensional arrangement and the lack of techniques to measure local order. Here, we report that molecular orientation relative to donor/acceptor heterojunctions is an important parameter in realizing high-performance fullerene-based, bulk heterojunction solar cells. Using resonant soft X-ray scattering, we characterize the degree of molecular orientation, an order parameter that describes face-on (+1) or edge-on (−1) orientations relative to these heterointerfaces. By manipulating the degree of molecular orientation through the choice of molecular chemistry and the characteristics of the processing solvent, we are able to show the importance of this structural parameter on the performance of bulk heterojunction organic photovoltaic devices featuring the electron-donating polymers PNDT–DTBT, PBnDT–DTBT or PBnDT–TAZ.

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Figure 1: Device architecture and molecular orientation with respect to donor/acceptor heterojunctions.
Figure 2: Device performance strongly correlates with DMO.
Figure 3: Polarized soft X-ray scattering anisotropy reveals molecular order.
Figure 4: Face-on molecular orientation correlating to high performance in other systems.


  1. 1

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

    ADS  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).

    ADS  Article  Google Scholar 

  3. 3

    He, Z. et al. Enhanced power-conversion efficiency in polymer solar cells using an inverted device structure. Nature Photon. 6, 591–595 10.1038/nphoton.2012.190(2012).

    ADS  Article  Google Scholar 

  4. 4

    You, J. et al. A polymer tandem solar cell with 10.6% power conversion efficiency. Nature Commun. 4, 1446 (2013).

    ADS  Article  Google Scholar 

  5. 5

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

    Article  Google Scholar 

  6. 6

    Chen, L.-M., Hong, Z., Li, G. & Yang, Y. Recent progress in polymer solar cells: manipulation of polymer:fullerene morphology and the formation of efficient inverted polymer solar cells. Adv. Mater. 21, 1434–1449 (2009).

    Article  Google Scholar 

  7. 7

    Kline, R. J., McGehee, M. D. & Toney, M. F. Highly oriented crystals at the buried interface in polythiophene thin-film transistors. Nature Mater. 5, 222–228 (2006).

    ADS  Article  Google Scholar 

  8. 8

    Salleo, A., Kline, R. J., DeLongchamp, D. M. & Chabinyc, M. L. Microstructural characterization and charge transport in thin films of conjugated polymers. Adv. Mater. 22, 3812–3838 (2010).

    Article  Google Scholar 

  9. 9

    Rivnay, J., Mannsfeld, S. C. B., Miller, C. E., Salleo, A. & Toney, M. F. Quantitative determination of organic semiconductor microstructure from the molecular to device scale. Chem. Rev. 112, 5488–5519 (2012).

    Article  Google Scholar 

  10. 10

    Chabinyc, M. L. X-ray scattering from films of semiconducting polymers. Polym. Rev. 48, 463–492 (2008).

    Article  Google Scholar 

  11. 11

    Graetzel, M., Janssen, R. A. J., Mitzi, D. B. & Sargent, E. H. Materials interface engineering for solution-processed photovoltaics. Nature 488, 304–312 (2012).

    ADS  Article  Google Scholar 

  12. 12

    Collins, B. A. et al. Polarized X-ray scattering reveals non-crystalline orientational ordering in organic films. Nature Mater. 11, 536–543 (2012).

    ADS  Article  Google Scholar 

  13. 13

    Brabec, C. J., Heeney, M., McCulloch, I. & Nelson, J. Influence of blend microstructure on bulk heterojunction organic photovoltaic performance. Chem. Soc. Rev. 40, 1185–1199 (2011).

    Article  Google Scholar 

  14. 14

    Ma, W. et al. Domain purity, miscibility, and molecular orientation at donor/acceptor interfaces in high performance organic solar cells: paths to further improvement. Adv. Energy Mater. 3, 864–872 (2013).

    ADS  Article  Google Scholar 

  15. 15

    Verlaak, S. et al. Electronic structure and geminate pair energetics at organic–organic interfaces: the case of pentacene/C60 heterojunctions. Adv. Funct. Mater. 19, 3809–3814 (2009).

    Article  Google Scholar 

  16. 16

    Rand, B. P. et al. The impact of molecular orientation on the photovoltaic properties of a phthalocyanine/fullerene heterojunction. Adv. Funct. Mater. 22, 2987–2995 (2012).

    Article  Google Scholar 

  17. 17

    Ojala, A. et al. Merocyanine/C60 planar heterojunction solar cells: effect of dye orientation on exciton dissociation and solar cell performance. Adv. Funct. Mater. 22, 86–96 (2012).

    Article  Google Scholar 

  18. 18

    Yang, L., Tumbleston, J. R., Zhou, H., Ade, H. & You, W. Disentangling the impact of side chains and fluorine substituents of conjugated donor polymers on the performance of photovoltaic blends. Energy Environ. Sci. 6, 316–326 (2013).

    Article  Google Scholar 

  19. 19

    Price, S. C., Stuart, A. C., Yang, L., Zhou, H. & You, W. Fluorine substituted conjugated polymer of medium band gap yields 7% efficiency in polymer–fullerene solar cells. J. Am. Chem. Soc. 133, 4625–4631 (2011).

    Article  Google Scholar 

  20. 20

    Zhou, H. et al. Development of fluorinated nenzothiadiazole as a structural unit for a polymer solar cell of 7% efficiency. Angew. Chem. Int. Ed. 50, 2995–2998 (2011).

    Article  Google Scholar 

  21. 21

    Albrecht, S. et al. Fluorinated copolymer PCPDTBT with enhanced open-circuit voltage and reduced recombination for highly efficient polymer solar cells. J. Am. Chem. Soc. 134, 14932–14944 (2012).

    Article  Google Scholar 

  22. 22

    Gann, E. et al. Soft X-ray scattering facility at the advanced light source with real-time data processing and analysis. Rev. Sci. Instrum. 83, 045110 (2012).

    ADS  Article  Google Scholar 

  23. 23

    Collins, B. A. et al. Absolute measurement of domain composition and nanoscale size distribution explains performance in PTB7:PC71BM solar cells. Adv. Energy Mater. 3, 65–74 (2013).

    ADS  Article  Google Scholar 

  24. 24

    Szarko, J. M. et al. When function follows form: effects of donor copolymer side chains on film morphology and BHJ solar cell performance. Adv. Mater. 22, 5468–5472 (2010).

    Article  Google Scholar 

  25. 25

    Lyons, B. P., Clarke, N. & Groves, C. The relative importance of domain size, domain purity and domain interfaces to the performance of bulk-heterojunction organic photovoltaics. Energy Environ. Sci. 5, 7657–7663 (2012).

    Article  Google Scholar 

  26. 26

    Stribeck, N. X-Ray Scattering of Soft Matter (Springer, 2007).

    Google Scholar 

  27. 27

    Shuttle, C. G., Hamilton, R., O'Regan, B. C., Nelson, J. & Durrant, J. R. Charge-density-based analysis of the current–voltage response of polythiophene/fullerene photovoltaic devices. Proc. Natl Acad. Sci. USA 107, 16448–16452 (2010).

    ADS  Article  Google Scholar 

  28. 28

    Chen, W. et al. Molecular orientation dependent energy level alignment at organic−organic heterojunction interfaces. J. Phys. Chem. C 113, 12832–12839 (2009).

    ADS  Article  Google Scholar 

  29. 29

    Beljonne, D. et al. Electronic processes at organic–organic interfaces: insight from modeling and implications for opto-electronic devices. Chem. Mater. 23, 591–609 (2010).

    Article  Google Scholar 

  30. 30

    Vandewal, K. et al. Efficient charge generation by relaxed charge-transfer states at organic interfaces. Nature Mater. 13, 63–68 (2014).

    ADS  Article  Google Scholar 

  31. 31

    Albrecht, S. et al. On the efficiency of charge transfer state splitting in polymer:fullerene solar cells. Adv. Mater. (2014).

  32. 32

    Gélinas, S. et al. Ultrafast long-range charge separation in organic semiconductor photovoltaic diodes. Science 343, 512–516 (2014).

    ADS  Article  Google Scholar 

  33. 33

    Jailaubekov, A. E. et al. Hot charge-transfer excitons set the time limit for charge separation at donor/acceptor interfaces in organic photovoltaics. Nature Mater. 12, 66–73 (2013).

    ADS  Article  Google Scholar 

  34. 34

    Hexemer, A. et al. A SAXS/WAXS/GISAXS beamline with multilayer monochromator. J. Phys. Conf. Ser. 247, 012007 (2010).

    Article  Google Scholar 

  35. 35

    Kilcoyne, A. L. D. et al. Interferometer-controlled scanning transmission X-ray microscopes at the Advanced Light Source. J. Synchrotron Radiat. 10, 125–136 (2003).

    Article  Google Scholar 

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The authors acknowledge the following support for this collaborative research. Characterization and analysis by J.R.T., B.A.C., E.G., W.M. and H.A. was supported by the US Department of Energy (DOE), Office of Science, Basic Energy Science, Division of Materials Science and Engineering under contract DE-FG02-98ER45737. X-ray data were acquired at the ALS, which is supported by the DOE (DE-AC02-05CH1123). W.Y., L.Y. and A.C.S. are supported by the Office of Naval Research (N000141110235) and an NSF CAREER award (DMR-0954280). W.Y. is a Camille Dreyfus Teacher–Scholar. The authors thank D. Kilcoyne at ALS beamline, A. Hexemer and S. Alvarez at beamline 7.3.3 and C. Wang and A. Young at beamline for assistance with data acquisition and helpful discussions. F. Liu is thanked for TEM measurements and useful discussions. Insightful discussions with M. Toney (SSRL) are also acknowledged.

Author information




J.R.T. and H.A. conceived and designed the experiments. J.R.T. acquired and processed the X-ray data with assistance from B.A.C., E.G., W.M. and H.A. J.R.T., B.A.C. and E.G. conducted X-ray modelling. L.Y. and A.C.S. fabricated and tested devices and prepared all samples. J.R.T., B.A.C. and H.A. wrote the manuscript, with comments and input from all authors. W.Y. and H.A. directed the project.

Corresponding authors

Correspondence to Wei You or Harald Ade.

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The authors declare no competing financial interests.

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Tumbleston, J., Collins, B., Yang, L. et al. The influence of molecular orientation on organic bulk heterojunction solar cells. Nature Photon 8, 385–391 (2014).

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