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Exploring the origin of high optical absorption in conjugated polymers


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.

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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.


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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




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.

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Correspondence to Mariano Campoy-Quiles or Jenny Nelson.

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The authors declare no competing financial interests.

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

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