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.

Exploring the origin of high optical absorption in conjugated polymers

Abstract

The specific optical absorption of an organic semiconductor is critical to the performance of organic optoelectronic devices. For example, higher light-harvesting efficiency can lead to higher photocurrent in solar cells that are limited by sub-optimal electrical transport. Here, we compare over 40 conjugated polymers, and find that many different chemical structures share an apparent maximum in their extinction coefficients. However, a diketopyrrolopyrrole-thienothiophene copolymer shows remarkably high optical absorption at relatively low photon energies. By investigating its backbone structure and conformation with measurements and quantum chemical calculations, we find that the high optical absorption can be explained by the high persistence length of the polymer. Accordingly, we demonstrate high absorption in other polymers with high theoretical persistence length. Visible light harvesting may be enhanced in other conjugated polymers through judicious design of the structure.

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: Extinction coefficient κ (imaginary part of refractive index) spectra and maximum value of κ for a range of conjugated polymers.
Figure 2: Molecular structures and refractive indices of DPP-TT-T C1 and C3 polymers.
Figure 3: Calculated oscillator strength, normalized by the number of π-electrons Nπ, and corresponding ɛ2 spectra.
Figure 4: Extinction coefficient of polymers with high and low theoretical persistence length (λp), and the effect of absorption coefficient on external quantum efficiency EQE.
Figure 5: Resonance Raman intensity analysis (RRIA) of dilute solutions of high- and low-MW C3 polymer in 1,2-dichlorobenzene.
Figure 6: Calculated and experimental summed oscillator strength per π-system electron.

References

  1. 1

    Pope, M., Swenberg, C. E. & Pope, M. Electronic Processes in Organic Crystals and Polymers 2nd edn (Oxford Univ. Press, 1999).

    Google Scholar 

  2. 2

    Köhler, A. & Bässler, H. Electronic Processes in Organic Semiconductors: An Introduction (John Wiley, 2015).

    Book  Google Scholar 

  3. 3

    Spano, F. C. & Silva, C. H- and J-aggregate behavior in polymeric semiconductors. Annu. Rev. Phys. Chem. 65, 477–500 (2014).

    CAS  Article  Google Scholar 

  4. 4

    Tian, B., Zerbi, G., Schenk, R. & Mullen, K. Optical-spectra and structure of oligomeric models of polyparaphenylenevinylene. J. Chem. Phys. 95, 3191–3197 (1991).

    CAS  Article  Google Scholar 

  5. 5

    Puschnig, P. et al. Electronic, optical, and structural properties of oligophenylene molecular crystals under high pressure: an ab initio investigation. Phys. Rev. B 67, 235321 (2003).

    Article  Google Scholar 

  6. 6

    Prest, W. M. & Luca, D. J. Origin of the optical anisotropy of solvent cast polymeric films. J. Appl. Phys. 50, 6067–6071 (1979).

    CAS  Article  Google Scholar 

  7. 7

    Prest, W. M. & Luca, D. J. The alignment of polymers during the solvent-coating process. J. Appl. Phys. 51, 5170–5174 (1980).

    CAS  Article  Google Scholar 

  8. 8

    Koynov, K. et al. Molecular weight dependence of chain orientation and optical constants of thin films of the conjugated polymer MEH-PPV. Macromolecules 39, 8692–8698 (2006).

    CAS  Article  Google Scholar 

  9. 9

    Clark, J., Chang, J. F., Spano, F. C., Friend, R. H. & Silva, C. Determining exciton bandwidth and film microstructure in polythiophene films using linear absorption spectroscopy. Appl. Phys. Lett. 94, 163306 (2009).

    Article  Google Scholar 

  10. 10

    Hestand, N. J. et al. Confirmation of the origins of panchromatic spectra in squaraine thin films targeted for organic photovoltaic devices. J. Phys. Chem. C 119, 18964–18974 (2015).

    CAS  Article  Google Scholar 

  11. 11

    Yao, K., Xu, Y. X., Li, F., Wang, X. F. & Zhou, L. A simple and universal method to increase light absorption in ternary blend polymer solar cells based on ladder-type polymers. Adv. Opt. Mater. 3, 321–327 (2015).

    CAS  Article  Google Scholar 

  12. 12

    Sjoqvist, J., Linares, M., Lindgren, M. & Norman, P. Molecular dynamics effects on luminescence properties of oligothiophene derivatives: a molecular mechanics-response theory study based on the CHARMM force field and density functional theory. Phys. Chem. Chem. Phys. 13, 17532–17542 (2011).

    Article  Google Scholar 

  13. 13

    Hedstrom, S., Henriksson, P., Wang, E., Andersson, M. R. & Persson, P. Light-harvesting capabilities of low band gap donor–acceptor polymers. Phys. Chem. Chem. Phys. 16, 24853–24865 (2014).

    Article  Google Scholar 

  14. 14

    Grimm, B., Risko, C., Azoulay, J. D., Bredas, J. L. & Bazan, G. C. Structural dependence of the optical properties of narrow bandgap semiconductors with orthogonal donor–acceptor geometries. Chem. Sci. 4, 1807–1819 (2013).

    CAS  Article  Google Scholar 

  15. 15

    Mishra, A. et al. A-D-A-type S, N-heteropentacenes: next-generation molecular donor materials for efficient vacuum-processed organic solar cells. Adv. Mater. 26, 7217–7223 (2014).

    CAS  Article  Google Scholar 

  16. 16

    Xu, Y. X. et al. Improved charge transport and absorption coefficient in indacenodithieno[3,2-b]thiophene-based ladder-type polymer leading to highly efficient polymer solar cells. Adv. Mater. 24, 6356–6361 (2012).

    CAS  Article  Google Scholar 

  17. 17

    Rau, U. Reciprocity relation between photovoltaic quantum efficiency and electroluminescent emission of solar cells. Phys. Rev. B 76, 085303 (2007).

    Article  Google Scholar 

  18. 18

    Campoy-Quiles, M., Alonso, M. I., Bradley, D. D. C. & Richter, L. J. Advanced ellipsometric characterization of conjugated polymer films. Adv. Funct. Mater. 24, 2116–2134 (2014).

    CAS  Article  Google Scholar 

  19. 19

    Bronstein, H. et al. Thieno[3,2-b]thiophene-diketopyrrolopyrrole-containing polymers for high-performance organic field-effect transistors and organic photovoltaic devices. J. Am. Chem. Soc. 133, 3272–3275 (2011).

    CAS  Article  Google Scholar 

  20. 20

    Wood, S. et al. Natures of optical absorption transitions and excitation energy dependent photostability of diketopyrrolopyrrole (DPP)-based photovoltaic copolymers. Energy Env. Sci. 8, 3222–3232 (2015).

    CAS  Article  Google Scholar 

  21. 21

    Meager, I. et al. Photocurrent enhancement from diketopyrrolopyrrole polymer solar cells through alkyl-chain branching point manipulation. J. Am. Chem. Soc. 135, 11537–11540 (2013).

    CAS  Article  Google Scholar 

  22. 22

    Meager, I. et al. Power conversion efficiency enhancement in diketopyrrolopyrrole based solar cells through polymer fractionation. J. Mater. Chem. C 2, 8593–8598 (2014).

    CAS  Article  Google Scholar 

  23. 23

    Kline, R. J. et al. Dependence of regioregular poly(3-hexylthiophene) film morphology and field-effect mobility on molecular weight. Macromolecules 38, 3312–3319 (2005).

    CAS  Article  Google Scholar 

  24. 24

    Chang, J.-F. et al. Molecular-weight dependence of interchain polaron delocalization and exciton bandwidth in high-mobility conjugated polymers. Phys. Rev. B 74, 115318 (2006).

    Article  Google Scholar 

  25. 25

    Fox, M. Optical Properties of Solids (Oxford Univ. Press, 2001).

    Google Scholar 

  26. 26

    Koch, F. P. Synthesis and Physical Chemistry of a ‘Monomer-up Approach’ PhD thesis, ETH Zurich (2013).

  27. 27

    Schumacher, S. et al. Effect of exciton self-trapping and molecular conformation on photophysical properties of oligofluorenes. J. Chem. Phys. 131, 154906 (2009).

    Article  Google Scholar 

  28. 28

    Li, W. et al. One-step synthesis of precursor oligomers for organic photovoltaics—a comparative study between polymers and small molecules. ACS Appl. Mater. Interfaces 7, 27106–27114 (2015).

    CAS  Article  Google Scholar 

  29. 29

    van Faassen, M., de Boeij, P. L., van Leeuwen, R., Berger, J. A. & Snijders, J. G. Ultranonlocality in time-dependent current-density-functional theory: application to conjugated polymers. Phys. Rev. Lett. 88, 186401 (2002).

    CAS  Article  Google Scholar 

  30. 30

    Albuquerque, R. Q., Hofmann, C. C., Kohler, J. & Kohler, A. Diffusion-limited energy transfer in blends of oligofluorenes with an anthracene derivative. J. Phys. Chem. B 115, 8063–8070 (2011).

    CAS  Article  Google Scholar 

  31. 31

    Rossi, G., Chance, R. R. & Silbey, R. Conformational disorder in conjugated polymers. J. Chem. Phys. 90, 7594–7601 (1989).

    CAS  Article  Google Scholar 

  32. 32

    Soos, Z. G. & Schweizer, K. S. Absorption-spectrum of flexible conjugated polymers—the weak-disorder limit. Chem. Phys. Lett. 139, 196–200 (1987).

    CAS  Article  Google Scholar 

  33. 33

    Flory, P. J. Statistical Mechanics of Chain Molecules (Interscience Publishers, 1969).

    Book  Google Scholar 

  34. 34

    Jackson, N. E. et al. Conformational order in aggregates of conjugated polymers. J. Am. Chem. Soc. 137, 6254–6262 (2015).

    CAS  Article  Google Scholar 

  35. 35

    Chung, W. J., Shibaguchi, H., Terao, K., Fujiki, M. & Naito, M. Evaluation of global conformation of polydialkylsilane using correlation between persistence length and excitonic absorption. Macromolecules 44, 6568–6573 (2011).

    CAS  Article  Google Scholar 

  36. 36

    Vanhee, S. et al. Synthesis and characterization of rigid rod poly(p-phenylenes). Macromolecules 29, 5136–5142 (1996).

    CAS  Article  Google Scholar 

  37. 37

    Bronstein, H. et al. Indacenodithiophene-co-benzothiadiazole copolymers for high performance solar cells or transistors via alkyl chain optimization. Macromolecules 44, 6649–6652 (2011).

    CAS  Article  Google Scholar 

  38. 38

    Venkateshvaran, D. et al. Approaching disorder-free transport in high-mobility conjugated polymers. Nature 515, 384–388 (2014).

    CAS  Article  Google Scholar 

  39. 39

    Marcus, M., Tozer, O. R. & Barford, W. Theory of optical transitions in conjugated polymers. II. Real systems. J. Chem. Phys. 141, 164102 (2014).

    Article  Google Scholar 

  40. 40

    Hestand, N. J. & Spano, F. C. The effect of chain bending on the photophysical properties of conjugated polymers. J. Phys. Chem. B 118, 8352–8363 (2014).

    CAS  Article  Google Scholar 

  41. 41

    Hayes, S. C. & Silva, C. Analysis of the excited-state absorption spectral bandshape of oligofluorenes. J. Chem. Phys. 132, 214510 (2010).

    Article  Google Scholar 

  42. 42

    Myers, A. B. & Mathies, R. A. in Biological applications of Raman Spectroscopy: Resonance Raman Spectra of Polyenes and Aromatics Vol. 2 (ed. Spiro, T. G.) 1–58 (1987).

    Google Scholar 

  43. 43

    van Franeker, J. J., Turbiez, M., Li, W. W., Wienk, M. M. & Janssen, R. A. J. A real-time study of the benefits of co-solvents in polymer solar cell processing. Nature Commun. 6, 6229 (2015).

    CAS  Article  Google Scholar 

  44. 44

    Campoy-Quiles, M., Nelson, J., Bradley, D. & Etchegoin, P. Dimensionality of electronic excitations in organic semiconductors: a dielectric function approach. Phys. Rev. B 76, 235206 (2007).

    Article  Google Scholar 

Download references

Acknowledgements

M.S.V. and S.F. are grateful to the Engineering and Physical Sciences Research Council (EPSRC) for a doctoral training award and a CDT studentship (EP/G037515/1), respectively. G.P. and S.C.H. acknowledge the University of Cyprus for funding through the internal grant ‘ORGANIC’. B.D., A.R.G. and M.C.-Q. acknowledge financial support from the Ministerio de Economía y Competitividad of Spain through projects CSD2010–00044 (Consolider NANOTHERM), SEV-2015–0496 and MAT2012–37776 and the European Research Council through project ERC CoG648901. I.Meager., R.S.A. and I.McCulloch acknowledge support from the European Commission FP7 Project ArtESun (604397). J.N. is grateful to the Royal Society for a Wolfson Merit Award, and acknowledges financial support from EPSRC grants EP/K030671/1, EP/K029843/1 and EP/J017361/1. The authors thank I. Alonso for performing supplementary ellipsometric measurements; we thank T. Kirchartz, J. Moore Frost, C. Müller, I. Alonso and A. Myers for helpful discussions.

Author information

Affiliations

Authors

Contributions

M.S.V. coordinated the experimental work, made films, performed solution ultraviolet–visible measurements, and did electrical characterization. S.F. did the quantum chemical calculations. I.M. and H.B. made the DPP-TT-T and IDTBT polymers, respectively, under the supervision of I.McCulloch. G.P. and S.C.H. performed the RR spectroscopy measurements and subsequent analysis. B.D., A.R.G. and M.C.-Q. did the ellipsometry measurements. R.S.A. made the devices. J.N. supervised the work.

Corresponding authors

Correspondence to Mariano Campoy-Quiles or Jenny Nelson.

Ethics declarations

Competing interests

The authors declare no competing financial interests.

Supplementary information

Supplementary Information

Supplementary Information (PDF 6529 kb)

Rights and permissions

Reprints and Permissions

About this article

Verify currency and authenticity via CrossMark

Cite this article

Vezie, M., Few, S., Meager, I. et al. Exploring the origin of high optical absorption in conjugated polymers. Nature Mater 15, 746–753 (2016). https://doi.org/10.1038/nmat4645

Download citation

Further reading

Search

Quick links

Nature Briefing

Sign up for the Nature Briefing newsletter — what matters in science, free to your inbox daily.

Get the most important science stories of the day, free in your inbox. Sign up for Nature Briefing