Ultrafast Relaxation of the Poly(3-hexylthiophene) Emission Spectrum

The optoelectronic properties of conjugated polymers combined with “plastic” mechanical behavior and solution processability make them highly interesting materials with applications in organic electronics. 5 In bulk heterojunction (BHJ) solar cells, an electron-donating conjugated polymer is blended with a fullerene electron acceptor, such as [6,6]-phenyl C60 butyric acid methyl ester (PCBM). 12 This yields a nanoscale morphology in the thin film active layer with a high donor acceptor interfacial area for efficient photoinduced charge separation and with phaseseparated fullerene and polymer networks for charge transport to the electrodes. Regioregular poly(3-hexylthiophene), abbreviated as P3HT throughout the text, has been thoroughly investigated and is considered a state of the art material. High power conversion efficiencies, around 5%, can be obtained in thermally annealed P3HT:PCBM devices. 15 A key to optoelectronic device functioning is the interaction of the conjugated polymer with light. Numerous steady-state and time-resolved spectroscopic studies have been conducted in pristine P3HT and P3HT:PCBM blends in order to understand the nature and evolution of neutral and charged excitations, 56 but many aspects of the photophysics remain unclear. This is explained by the general complexity of light-induced processes in conjugated polymers, 60 as well as by additional complications that arise because of the strong interchain interactions in the partially microcrystalline film morphology of P3HT. Indeed, the P3HT chains in thin films π-stack into two-dimensional lamellae sheets perpendicular to the substrate with interchain distances of only 3.8 Å. Amorphous conjugated polymers can be treated as one-dimensional semiconductors where excitations are localized on single polymer chains. On the other hand, it has been shown in P3HT films that both neutral excitations and charged polarons 51 are at least partially delocalized over neighboring polymer chains and become quasi-two-dimensional interchain species. This infers unique properties to the polymer, such as a relatively high charge carrier mobility [approximately 0.1 cm/(V s)] and a high yield, up to 30%, of photogenerated charge carriers in pristine P3HT films. We report here an ultrafast emission study of pristine P3HT in spin-cast thin films and dilute chlorobenzene (CB) solution, as measured by femtosecond-resolved fluorescence up-conversion (FU). This technique selectively probes neutral singlet excitations and is not concerned with charged polarons or triplet states, in contrast to transient absorption spectroscopy. Data recorded