We report subpicosecond carrier dynamics in photoexcited solid films of regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) and a blend of P3HT/[6,6]-phenyl-C 61 -butyric acid methyl ester (PCBM) using opticalpump terahertz-probe spectroscopy for excitations well above and below the generally accepted energy gap in P3HT. Both 400 nm excitation and 800 nm excitation yield similar results, suggesting the same photoproduct results from these two different excitation energies. The time evolution of the frequency-dependent complex conductivity after optical-pumping reveals characteristics of strong carrier localization. Carrier mobilities and photon-to-carrier yields are estimated based on the application of the Drude-Smith model. Low carrier yields (e1.5%) suggest that free carriers are not the primary photogenerated species. High hole mobilities (30-40 cm 2 /Vs) are consistent with the THz probing frequency, which selectively probes short-range highly ordered domains. Low-temperature measurements show a slight increase in mobility. Inclusion of PCBM led to an increase in carrier yield for both excitation wavelengths.

We report broad bandwidth, 0.1-10 THz time-domain spectroscopy of linear and electro-optic polymers. The common THz optical component materials high-density polyethylene, polytetrafluoroethylene, polyimide (Kapton), and polyethylene cyclic olefin copolymer (Topas) were evaluated for broadband THz applications. Host polymers polymethyl methacrylate, polystyrene, and two types of amorphous polycarbonate were also examined for suitability as host for several important chromophores in guest-host electro-optic polymer composites for use as broadband THz emitters and sensors.

The creation of a single electron-hole pair (i.e., exciton) per incident photon is a fundamental limitation for current optoelectronic devices including photodetectors and photovoltaic cells. The prospect of multiple exciton generation per incident photon is of great interest to fundamental science and the improvement of solar cell technology. Multiple exciton generation is known to occur in semiconductor nanostructures with increased efficiency and reduced threshold energy compared to their bulk counterparts. Here we report a significant enhancement of multiple exciton generation in PbSe quasi-one-dimensional semiconductors (nanorods) over zero-dimensional nanostructures (nanocrystals), characterized by a 2-fold increase in efficiency and reduction of the threshold energy to (2.23 ± 0.03)E(g), which approaches the theoretical limit of 2E(g). Photovoltaic cells based on PbSe nanorods are capable of improved power conversion efficiencies, in particular when operated in conjunction with solar concentrators.

Strong Coulomb attraction in monolayer transition metal dichalcogenides gives rise to tightly bound excitons and many-body interactions that dominate their optoelectronic properties. However, this Coulomb interaction can be screened through control of the surrounding dielectric environment as well as through applied voltage, which provides a potential means of tuning the bandgap, exciton binding energy, and emission wavelength. Here, we directly show that the bandgap and exciton binding energy can be optically tuned by means of the intensity of the incident light. Using transient absorption spectroscopy, we identify a sub-picosecond decay component in the excited-state dynamics of WS that emerges for incident photon energies above the A-exciton resonance, which originates from a nonequilibrium population of charge carriers that form excitons as they cool. The generation of this charge-carrier population exhibits two distinct energy thresholds. The higher threshold is coincident with the onset of continuum states and therefore provides a direct optical means of determining both the bandgap and exciton binding energy. Using this technique, we observe a reduction in the exciton binding energy from 310 ± 30 to 220 ± 20 meV as the excitation density is increased from 3 × 10 to 1.2 × 10 photons/cm. This reduction is due to dynamic dipolar screening of Coulomb interactions by excitons, which is the underlying physical process that initiates bandgap renormalization and leads to the insulator-metal transition in monolayer transition metal dichalcogenides.

Modeling of transient absorption spectra The TA spectrum is modeled as changes in the ground state absorption amplitude, line-width, and center wavelength as described in the main text. It is counter intuitive that a shift in the TA spectrum does not necessarily imply that the absorption spectrum has shifted. Instead, spectral shifts of the absorption features cause an oscillatory TA spectrum that changes sign depending on the direction of the shift, Figure S1.