Ultrafast photogeneration of inter-chain charge pairs in polythiophene films

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Abstract

Photoexcitation dynamics in films of polythiophenes with different side groups were studied by transient absorption spectroscopy using 80 fs pulses. We show that inter-chain charge pairs (CP) are generated with ∼20% efficiency in a polymer with short inter-chain distance (∼4 Å) within 100 fs after photoexcitation. We discuss two mechanisms for inter-chain charge separation: (i) electron or hole transfer from an initially excited intra-chain singlet state or (ii) optical excitation of mixed exciton–charge transfer states, which quickly evolve to inter-chain CPs.

Introduction

Conjugated polymers, including polythiophenes, show attractive performance in optoelectronic devices, e.g. light-emitting diodes 1, 2, plastic lasers 3, 4and photovoltaic cells 5, 6. Their photophysics has been extensively studied during the last decade, and great progress has been achieved in understanding the nature of excited states [7]. Light emission in conjugated polymers arises mainly from the intra-chain singlet excitations, while inter-chain interactions were found to reduce the photoluminescence (PL) quantum efficiency in solid films unless the conjugated backbones are well separated 8, 9, 10, 11, 12. The origin of PL quenching is, however, not well understood and the interpretations are controversial. Early studies of photoinduced absorption (PA) suggested that inter-chain charge pairs (CP), referred to also as polaron pairs, are formed with high efficiency (80–90%) in films of poly(p-phenylene vinylene) (PPV) and methoxy-substituted PPV (MEH–PPV) 11, 12. The polaron concept refers to the charge carrier (electron or hole) with a polarised electron and nuclear distribution in its neighbourhood. CPs have been suggested to account for magnetic field effects on photoconductivity and luminescence [13]and for the increase of microwave photoconductivity upon chain aggregation [14]in PPV and derivatives. Recent progress in polymer synthesis has made it possible to increase the PL quantum efficiency in films of non-substituted PPV to 27% [15], which argues against a high quantum yield of inter-chain CPs even in polymers with dense chain packing. Spectral signatures of polarons are observed by spin-selective steady state and picosecond PA 16, 17, but their yield and formation dynamics remain obscure. Inter-chain physical dimers 8, 18, 19, 20or excimers (excited-state dimers) 10, 21, 22were also suggested to contribute to PL quenching in some polymers. A detailed understanding of excitation dynamics and its dependence on chain packing has not yet been achieved.

In this Letter, we report on the excitation dynamics in two polythiophenes, containing different side groups (Fig. 1) that define the distance between the conjugated backbones. Poly[3-(2,5-dioctyl-phenyl)thiophene] (PDOPT) with bulky side groups is partially crystalline in the solid state and a shortest inter-chain distance of ∼10 Å has been estimated from X-ray diffraction [23]. The other polymer, poly[3-(4-octyl-phenyl)-2,2′-bithiophene] (PTOPT) is amorphous and the inter-chain distance in the sites of face-to-face packing is expected, on average, to be 3.8–4 Å, as determined for other polythiophenes with chemical structures similar to that of PTOPT 8, 23, 24. This estimate is consistent with the 2.5 times larger optical density of the PTOPT film than that of the PDOPT for the same thickness.

Section snippets

Experimental

Synthesis and characterisation of the substituted polythiophenes is described elsewhere 25, 26. The iron content was about 0.02 wt% in PTOPT and 0.002 wt% in PDOPT as determined by flame absorption. Solid films were prepared by spin-casting from concentrated chloroform solutions in open air and quickly sandwiched between two glass plates. This simple procedure allowed the suppress of the photo-oxidation of films and we have not observed any changes of the transient absorption spectra or

Results and discussion

Fig. 2 shows the transient absorbance spectra of polymer films recorded at various time delays. The decrease of sample absorbance (negative ΔA) in the visible spectral range is due to photoinduced bleaching (BL) of the ground-state absorption (above 2 eV) and amplification of the probe light due to stimulated emission (SE). The spectral shape of the latter closely resembles the shape of the PL spectrum. The PA, with a maximum at 1 eV (PA1), is observed in both polymers at an early time delay

Acknowledgements

Financial support from the Swedish Natural Science Research Council, the Royal Swedish Academy of Science, the Swedish Institute and Knut and Alice Wallenberg Foundation is gratefully acknowledged.

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