Bimodal polarons and hole transport in poly(3-hexylthiophene):fullerene blend films.

The bimolecular recombination dynamics in blend films of poly(3-hexylthiophene) (P3HT) and [6,6]-phenyl-C(61)-butyric acid methyl ester (PCBM) has been studied by transient absorption spectroscopy. On a microsecond time scale, two polaron bands were observed at 700 and 1000 nm and exhibited different bimolecular recombination dynamics. The 700-nm band decayed with a time-independent bimolecular recombination rate of 10(-12) cm(3) s(-1). The activation energy was as small as approximately 0.078 eV independently of the carrier density. On the other hand, the 1000-nm band decayed with a time-dependent bimolecular recombination rate, which varied from 10(-12) to 10(-13) cm(3) s(-1), depending on time or carrier density. The activation energy decreased exponentially from 0.178 to 0.097 eV with the increase in the carrier density. Therefore, we assigned the 700-nm band to freely mobile delocalized polarons in crystalline P3HT domains and the 1000-nm band to localized polarons trapped in relatively disordered P3HT domains. At a charge density of 10(17) cm(-3), which corresponds to 1 sun open-circuit condition, some localized polarons exhibited trap-free bimolecular recombination due to trap-filling. These findings suggest that not only delocalized polarons but also some localized polarons play a crucial role in the efficient hole transport in P3HT:PCBM solar cells.

Charge generation and recombination dynamics in poly(3-hexylthiophene)/fullerene blend films with different regioregularities and morphologies.

The charge generation and recombination dynamics in blend films of a poly(3-hexylthiophene) (P3HT) and a methanofullerene derivative (PCBM) were comprehensively studied by transient absorption spectroscopy in the wavelength region from 450 to 1650 nm under various excitation intensities and different excitation wavelengths. In homogeneously mixed blend films of regiorandom P3HT (RRa-P3HT) and PCBM, virtually all the excitons can reach the interface of RRa-P3HT/PCBM and then form bound radical pairs. However, two-thirds of them geminately recombine to the ground state, and only one-third of them can be dissociated into free polarons that survive up to milliseconds. In phase-separated blend films of regioregular P3HT (RR-P3HT) and PCBM, almost all the excitons can reach the interface of RR-P3HT/PCBM, where most of them can be dissociated into free polarons efficiently and the rest of them form bound radical pairs. There are two pathways for the polaron generation: the prompt formation from hot excitons generated near the interface on a time scale of <100 fs and the delayed formation via the exciton migration to the interface on a time scale of approximately 10 ps. The thermal annealing improves the charge dissociation efficiency of bound radical pairs. On the basis of such spectroscopic data, a series of fundamental photovoltaic conversion processes are quantitatively analyzed. Consequently, it is concluded that there is not much difference in the charge generation yield between RRa-P3HT/PCBM(50 wt %) and RR-P3HT/PCBM(50 wt %) blend films. Rather, the charge dissociation and collection have a critical impact on the overall device performance of P3HT/PCBM solar cells, where the phase-separated blend structures have a high tendency to form free carriers and transport these free carriers to the electrode.

Near-IR femtosecond transient absorption spectroscopy of ultrafast polaron and triplet exciton formation in polythiophene films with different regioregularities.

The formation dynamics of polaron pairs, polarons, and triplet excitons in regiorandom and regioregular poly(3-hexylthiophene) (RRa-P3HT and RR-P3HT) films was comprehensively studied by transient absorption spectroscopy over the wide wavelength region from 500 to 1650 nm under various excitation intensities. In both RRa-P3HT and RR-P3HT films, polaron pairs were generated not from relaxed singlet exciton states but from hot excitons on a time scale of <100 fs and decayed monomolecularly by geminate recombination. In RRa-P3HT films, triplet excitons were rapidly generated on a picosecond time scale from higher exciton states produced by the singlet exciton-exciton annihilation as well as from the lowest singlet exciton states by the normal intersystem crossing. In RR-P3HT films, no triplet excitons were observed; polarons were also generated not from relaxed singlet exciton states but from hot excitons in competition with the formation of polaron pairs. The polarons formed in RR-P3HT can freely migrate and mainly recombine with other polarons bimolecularly in the nanosecond time domain. The ultrafast formation of triplet excitons can be explained by the singlet exciton fission into two triplets, and the ultrafast formation of polaron pairs and polarons can be explained on the basis of the hot-exciton dissociation model where the excess thermal energy of the initially formed hot excitons is necessary to overcome their Coulombic binding energy. The remarkably different formation dynamics in P3HTs with different regioregularities is discussed in terms of the film morphology of conjugated polymers.

Intramolecular singlet and triplet excimers of triply bridged [3.3.N](3,6,9)carbazolophanes.

Intermoiety electronic interactions in the singlet and triplet excimer states of triply bridged [3.3.n](3,6,9)carbazolophanes ([3.3.n]Cz, n=3-6) were studied by emission and transient absorption measurements. In these [3.3.n]Cz molecules, the dihedral angle and the separation distance r between fully overlapped two carbazole rings change systematically from nearly parallel (n=3, r=3.35 A) to oblique (n=6, r=4.03 A). In rigid glass at 77 K, [3.3.n]Cz (n=3, 4) (r<4 A) exhibited red-shifted and structureless excimer fluorescence and phosphorescence while [3.3.n]Cz (n=5, 6) (r>4 A) exhibited monomer-like vibrational fluorescence and phosphorescence. In solution at 130 K, all [3.3.n]Cz molecules exhibited an excimeric fluorescence band while [3.3.5]Cz still exhibited monomer-like phosphorescence. Transient absorption spectra measured at 294 K exhibited local excitation and charge-transfer bands for all [3.3.n]Cz molecules in the excited singlet and triplet states, suggesting that not only singlet but also triplet excimers of carbazole are formed at room temperature. Furthermore, the singlet-triplet energy gap decreased with the decrease in n, suggesting that electrons are effectively delocalized over the two carbazole moieties. These findings showed that both singlet and triplet excimers of carbazole are formed with a separation distance shorter than about 4 A and are most stable in the parallel-sandwich structure and that the configurational mixing between exciton resonance and charge resonance states plays an essential role in the formation of singlet and triplet excimers of carbazole.