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J. Phys. Chem. B 2001, 105, 7624-7631
Intra- and Interchain Luminescence in Amorphous and Semicrystalline Films of Phenyl-Substituted Polythiophene Arvydas Ruseckas,† Ebinazar B. Namdas,† Tapan Ganguly,†,| Mathias Theander,‡ Mattias Svensson,§ Mats R. Andersson,§ Olle Ingana1 s,‡ and Villy Sundstro1 m*,† Department of Chemical Physics, Lund UniVersity, Box 124, SE-22100 Lund, Sweden, Laboratory of Applied Physics, Department of Physics and Measurement Technology (IFM), Linko¨ ping UniVersity, SE-58183 Linko¨ ping, Sweden, and Departments of Organic Chemistry and Polymer Technology, Chalmers UniVersity of Technology, SE-41296 Go¨ teborg, Sweden ReceiVed: February 8, 2001; In Final Form: May 7, 2001
We report time-resolved photoluminescence (PL) measurements of spin-cast amorphous films of a regioregular polythiophene derivative poly[3-(4-octylphenyl)thiophene] and of annealed semicrystalline films of the same polymer with an interchain stacking distance of 5 Å. Red-shifted PL appears at long delay times in both pristine and annealed films, which we assign to interchain aggregates populated by excitation energy migration. Aggregate luminescence in annealed films exhibits a pronounced vibronic structure indicating the coupling to a CdC bond stretch with a Huang-Rhys factor S ) 2. Two types of aggregates are distinguished in annealed films: in a few picoseconds most excitations are trapped by aggregates with a large energy gap (about 1.83 eV) between the lowest excited singlet state and the ground state. Excitation energy is transferred or the aggregates relax structurally with a time constant of about 200 ps to aggregates with a smaller energy gap (about 1.75 eV). The radiative lifetime of aggregates with smaller energy gap is estimated to be about 35 ns, two times longer than that of aggregates with the larger energy gap. In spin-cast amorphous films, emission from intrachain singlet excitons has a longer lifetime than in annealed films and only high-energy-gap aggregates are populated.
1. Introduction In the past two decades conjugated polymers have attracted much attention in fundamental research and technology.1-3 Among them, polythiophenes are technologically promising materials since they are relatively stable, have well-established polymerization routines, and their electronic properties can be varied by changing side groups.4-7 Polythiophenes already have found applications as conductors in antistatic coatings and electrodes for electrolyte capacitors,4 but they also show great potential for applications in optoelectronic devices such as lightemitting diodes (LEDs),5-8 field-effect transistors (FETs)9, photodetectors and solar cells,10,11 and optically pumped lasers.12 The photophysics of conjugated polymers has turned out to be very complex and strongly dependent on chain packing. The lowest intrachain singlet excited state in conjugated polymers made of aromatic units is highly emissive,13-15 while a close contact of conjugated backbones usually reduces luminescence quantum yield, and the spectral signatures of less-emissive interchain excited states are observed, such as physical dimers or higher aggregates,16-19 excimers,20-24 and interchain charge pairs.13,25-30 On the other hand, a short interchain distance is essential for good mobility of charge carriers, which is crucial for the performance of LEDs, FETs, and other optoelectronic * Author to whom correspondence should be addressed. E-mail:
[email protected]. † Lund University. ‡ Linko ¨ ping University. § Chalmers University of Technology. | On leave from Department of Spectroscopy, Indian Association for the Cultivation of Science, Jadavpur, Calcutta 700032, India.
Figure 1. Molecular structure of POPT.
devices.31 Despite great interest, only a few systematic studies were reported on the photophysics of conjugated polymers with controlled morphology.17-20,28 In this paper we report time-resolved photoluminescence (PL) studies of pristine and annealed films of regioregular poly[3(4-octylphenyl)thiophene] (POPT) (Figure 1). It was shown by X-ray diffraction studies that spin-cast films of this polymer are essentially amorphous, but structural order increases significantly upon annealing32 and the crystalline phase amounts to about 50%, with an interchain stacking distance in the
10.1021/jp010511n CCC: $20.00 © 2001 American Chemical Society Published on Web 07/19/2001
Luminescence in Phenyl-Substituted Polythiophene
J. Phys. Chem. B, Vol. 105, No. 32, 2001 7625
Figure 2. Ground-state absorbance spectra of a spin-cast POPT film on a glass substrate before (solid line) and after (dotted line) annealing. Both spectra are recorded at room temperature (293 K).
crystallites of 5.06 Å. This feature provides a good opportunity to investigate the impact of structural ordering on photophysics of polythiophene films and of conjugated polymers in general. Using the annealed form of POPT as the emitting material, an infrared LED with emission peak at 1.55 eV (800 nm) was demonstrated.33 We show that the red-shifted emission originates from interchain aggregates, which are populated by excitation energy transfer. The long radiative lifetime and the dominating nonradiative decay in aggregates result in low efficiency of PL and electroluminescence in polymers with dense chain packing. We also found that the aggregate luminescence in semicrystalline films shows quite small inhomogeneous broadening, on the order of 0.1 eV, and pronounced coupling to a CdC stretch. 2. Experimental Section POPT was polymerized using FeCl3 from the isomerically pure monomer, which allowed preparation of a highly regioregular polymer with about 90% head-to-tail couplings.34 Films with a thickness of about 100 nm were prepared by spin-casting from concentrated chloroform solution. The annealing of the films was performed by heating the spin-cast film to 130 °C for 1 min. Time-resolved photoluminescence was measured with a streak camera using the second harmonic of a femtosecond mode-locked Ti:sapphire laser at 3.2 eV for excitation at a pulse repetition rate of 82 MHz. The response function of the streak camera was ∼2 ps (fwhm). Measurements were made at dynamical vacuum (
