The photogeneration quantum yield and dynamics of charge carriers and excitons in thin films of neat regioregular poly(3-hexylthiophene) (P3HT) and blends with [6,6]-phenyl-C 61-butyric acid methyl ester (PCBM) were studied with ultrafast optical pump-probe spectroscopy. In neat P3HT the quantum yield for direct photogeneration of charge carriers amounts to 0.15 per absorbed photon. The remaining fraction of absorbed photons leads to formation of excitons. Recombination of charges reduces the quantum yield to about 25% of its initial value on a time scale of 100 ps followed by decay to a no longer observable yield after 1 ns. Addition of 50% PCBM by weight leads to ultrafast (<200 fs) formation of charge pairs with a total quantum yield of 0.5. The presence of 50% PCBM causes exciton decay to be about an order of magnitude faster than in neat P3HT, which is expected to be at least in part due to interfacial exciton dissociation into charge carriers. The yield of charges in the blend has decayed to about half its initial value after 100 ps, while no further decay is observed within 1 ns. The small fraction (∼1%) of excitons in neat P3HT that is probed by photoluminescence measurements has a lifetime of 660 ps, which significantly exceeds the 200 ps lifetime of nonfluorescent excitons that are probed by transient absorption measurements. The nonfluorescent excitons have a diffusion coefficient of about 2 × 10-4 cm 2 /s, which is an order of magnitude smaller than reported values for fluorescent excitons. The interaction radius for second-order decay of photoexcitations is as large as 8-17 nm, in agreement with an earlier result in the literature.

Efficient carrier multiplication has been reported for several semiconductor nanocrystals: PbSe, PbS, PbTe, CdSe, InAs, and Si. Some of these reports have been challenged by studies claiming that carrier multiplication does not occur in CdSe, CdTe, and InAs nanocrystals, thus raising legitimate doubts concerning the occurrence of carrier multiplication in the remaining materials. Here, conclusive evidence is given for its occurrence in PbSe nanocrystals using femtosecond transient photobleaching. In addition, it is shown that a correct determination of carrier-multiplication efficiency requires spectral integration over the photobleach feature. The carrier multiplication efficiency we obtain is significantly lower than what has been reported previously, and it remains an open question whether it is higher in nanocrystals than it is in bulk semiconductors.

Multiplex coherent anti-Stokes Raman scattering (CARS) microscopic imaging is demonstrated for the first time and used to visualize the thermodynamic state, either liquid crystalline or gel phase, of lipid membranes. Whereas in spontaneous Raman scattering the low scattering cross section is generally prohibitive for highresolution microscopic imaging, the signal enhancement in multiplex CARS, by more than 4 orders of magnitude, enables microscopic imaging with realistic frame rates (∼min -1 ). Because of the broad spectral window over which the CARS signal is detected simultaneously, the identification of the thermodynamic state of the lipid membranes can be accomplished directly, without any additional tuning of the lasers. We demonstrate that the experimental CARS spectral data are highly consistent with those obtained with spontaneous Raman scattering, with a signal-to-noise ratio determined only by Poisson noise. In addition, the inherent intensity normalization renders multiplex CARS a robust imaging technique.

We have determined the Auger recombination kinetics of electrons and holes in colloidal CdSe-only and CdSe/CdS/ZnS core/shell nanoplatelets by time-resolved photoluminescence measurements. Excitation densities as high as an average of 18 electron−hole pairs per nanoplatelet were reached. Auger recombination can be described by second-order kinetics. From this we infer that the majority of electrons and holes are bound in the form of neutral excitons, while the fraction of free charges is much smaller. The biexciton Auger recombination rate in nanoplatelets is more than 1 order of magnitude smaller than for quantum dots and nanorods of equal volume. The latter is of advantage for application in lasers, light-emitting diodes, and photovoltaics.

Solid films of colloidal quantum dots show promise in the manufacture of photodetectors and solar cells. These devices require high yields of photogenerated charges and high carrier mobilities, which are difficult to achieve in quantum-dot films owing to a strong electron-hole interaction and quantum confinement. Here, we show that the quantum yield of photogenerated charges in strongly coupled PbSe quantum-dot films is unity over a large temperature range. At high photoexcitation density, a transition takes place from hopping between localized states to band-like transport. These strongly coupled quantum-dot films have electrical properties that approach those of crystalline bulk semiconductors, while retaining the size tunability and cheap processing properties of colloidal quantum dots.

The nature and decay dynamics of photoexcited states in CdSe core-only and CdSe/CdS core/shell nanoplatelets was studied. The photophysical species produced after ultrafast photoexcitation are studied using a combination of time-resolved photoluminescence (PL), transient absorption (TA), and terahertz (THz) conductivity measurements. The PL, TA, and THz exhibit very different decay kinetics, which leads to the immediate conclusion that photoexcitation produces different photophysical species. It is inferred from the data that photoexcitation initially leads to formation of bound electron-hole pairs in the form of neutral excitons. The decay dynamics of these excitons can be understood by distinguishing nanoplatelets with and without exciton quenching site, which are present in the sample with close to equal amounts. In absence of a quenching site, the excitons undergo PL decay to the ground state. In nanoplatelets with a quenching site, part of the initially produced excitons decays by hole trapping at a defect site. The electron that remains in the nanoplatelet moves in the Coulomb potential provided by the trapped hole.

We determine the efficiencies for the formation of excitons and charge carriers following ultrafast photoexcitation of a semiconducting polymer (MEH-PPV). The simultaneous, quantitative determination of exciton and charge photoyields is achieved through subpicosecond studies of both the real and the imaginary components of the complex conductivity over a wide frequency range. Predominantly excitons, with near-unity quantum efficiency, are generated on excitation, while only a very small fraction ( < 10 ÿ2 ) of free charges are initially excited, consistent with rapid ( 100 fs) hot exciton dissociation. These initial charges are very short lived, decaying on subpicosecond time scales. DOI: 10.1103/PhysRevLett.92.196601 PACS numbers: 72.80.Le, 71.35.Aa, 73.50.Gr, 73.61.Ph Since their discovery in the 1970s, semiconducting conjugated polymers have received considerable interest owing to their potential in technological applications, particularly in electronics [1]. These materials have many advantages over conventional semiconductors: They are low cost, easy to process, lightweight and malleable, and have shown significant promise in lightemitting diodes [2], photovoltaics, and laser optics [3]. Despite their widespread optical applications, the nature of the photoexcitation physics in these materials is currently subject to intense debate [4 -6]. One of the key questions that has remained controversial is whether, initially upon photoexcitation, excitons or charge carriers are primarily formed. From photoconductivity measurements [7], it is evident that free charges are formed, but the mechanism and efficiency of free charge formation remains polemical: Some studies suggest that excitons (Coulombically bound electron-hole pairs) are the primary photoexcitation product, which may dissociate into free charges after migration to electron (or hole) accepting defects, by absorption of a second photon or by bimolecular exciton-exciton annihilation [8,9]. Other studies suggest electron-hole pairs as the direct and predominant initial excitation product [10]. This apparent contradiction may be related to the selective sensitivity of different experimental approaches: For example, photoconductivity (PC) measurements [7] are sensitive only to free charges (responsible for real conductivity), whereas transient absorption (TA) and stimulated emission (SE) measurements [11][12][13] can probe excitons (bound electron-hole pairs responsible for imaginary conductivity). However, the simultaneous detection of the real and imaginary conductivity with sufficient time resolution is necessary to allow a quantitative comparison of both free and bound charges upon photoexcitation. The relatively recent technique of terahertz time domain spectroscopy [14] (THz-TDS) provides this opportunity.In this Letter we present the first investigation of a semiconducting polymer using THz-TDS. This technique allows us to monitor the evolution of free and bound charges on subpicosecond time scales following photoexcitation, through the time-depen...

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