Charge carrier generation in a conjugated polymer studied via ultrafast pump-push-probe experiments

Conjugated polymers find rapidly growing application in electroluminescent displays and are extensively studied for use in photovoltaics and laser diodes. For a wide range of conjugated materials ultrafast pump-probe experiments have revealed the excited state dynamics of singlet and triplet excitons as well as positively and negatively charged polarons. Charge carriers play a key role in all the above mentioned applications. However, there is yet no clear picture of the mechanisms which lead to their generation. Photocurrent excitation cross-correlation measurement on methyl-substituted ladder-type poly(para)phenyl (m-LPPP), a prototypical conjugated polymer with very appealing properties for the above mentioned applications, have suggested that charge carrier generation occurs preferentially from higher lying states during energy migration. Our approach to examining this mechanism consists of an innovative modification of the ultrafast time-resolved pump-probe technique

Charge trapping and coalescence dynamics in few layer MoS2

The optoelectronic properties of a material are determined by the processes following light-matter interaction. Here we use femtosecond optical spectroscopy to systematically study photoexcited carrier relaxation in few-layer MoS2flakes as a function of excitation density and sample thickness. We find bimolecular coalescence of charges into indirect excitons as the dominant relaxation process in two- to three-layer flakes while thicker flakes show a much higher density of defects, which efficiently trap charges before they can coalesce

Strong Coupling of Coherent Phonons to Excitons in Semiconducting Monolayer MoTe2_2

The coupling of the electron system to lattice vibrations and their time-dependent control and detection provides unique insight into the non-equilibrium physics of semiconductors. Here, we investigate the ultrafast transient response of semiconducting monolayer 2$H$-MoTe$_2$ encapsulated with $h$BN using broadband optical pump-probe microscopy. The sub-40-fs pump pulse triggers extremely intense and long-lived coherent oscillations in the spectral region of the A' and B' exciton resonances, up to $\sim$20% of the maximum transient signal, due to the displacive excitation of the out-of-plane $A_{1g}$ phonon. Ab-initio calculations reveal a dramatic rearrangement of the optical absorption of monolayer MoTe$_2$ induced by an out-of-plane stretching and compression of the crystal lattice, consistent with an $A_{1g}$-type oscillation. Our results highlight the extreme sensitivity of the optical properties of monolayer TMDs to small structural modifications and their manipulation with light.Comment: 27 pages, 4 figures, supporting informatio

Ultrafast nonequilibrium dynamics of strongly coupled resonances in the intrinsic cavity of W S 2 nanotubes

This paper presents a femtosecond optical pump-probe study of the non-equilibrium behavior of the coupled optical resonances in semiconducting WS2. The authors focus on the transient optical response of WS2 nanotubes and show that it arises primarily from the photoinduced shifts of the exciton and trion resonances due to band gap renormalization and screening of the Coulomb interaction providing the exciton and trion binding energy

Photophysics of conjugated polymers: The contribution of ultrafast spectroscopy

The elementary processes in the conjugated polymers were presented using ultrafast techniques. The excitation energy migration in homopolymers, charge carrier generation both intrinsic and in donor-acceptor systems and the various mechanisms leading to triplet states were discussed. Time-resolved spectroscopy was used for the investigation of elementary excitation dynamics in the conjugated polymers

Large spectral shifts of electronic transitions in MoSI molecular wire dispersions as a function of bundle diameter

A systematic study of optical absorption spectra of Mo6S 9-xIx (x = 6) molecular wire dispersions in ethanol, fractionated into different bundle diameter populations shows that electronic transitions shift significantly as a function of bundle diameter. Two electronic transitions show significant shifts: the Mo-S charge transfer peak shifts from 1.8 to 1.5 eV and the next inter-band transition shifts from 2.7 to 2.4 eV with increasing bundle diameter d, in the range 5-100 nm. This empirical observation hugely simplifies characterization of Mo6S9-xIx wire dispersions according to diameter, opening the way to rapid advances in processing of these materials. We discuss the possible origin of the shift, dismissing quantum size effects, impurities and solvatochromism as well as stoichiometric variations between x = 6 and x = 4.5. © 2010 Elsevier B.V

Emission properties of pristine and oxidatively degraded polyfluorene type polymers

We present a detailed and comprehensive picture of the photophysics including device applications within the polyfluorene family of conjugated polymers. First, the photophysics of pristine polyfluorenes in solution and film is outlined, including a discussion of the so-called β-phase, which is characterised by a more planar ground state configuration. Particular attention is also dedicated to the occurence of low energy emission bands, which often deteriorate the initially blue emission of polyfluorenes, especially in electroluminescent devices. Although the origin of these emission features has been the object of a controversial discussion, strong evidence for our current ascription to emissive on-chain fluorenone defects is given also in contrast to previous assignments to aggregates, excimers, or exciplexes. According to the current attribution fluorenone-containing polyfluorenes can be described as a guest host system. Following this picture the photoexcitation dynamics from the fs to the ms regime is outlined. Finally, polymer light emitting diodes (PLEDs) based on polyfluorene-type emitters are discussed, especially related to their degradation mechanisms and possible remedies provided by chemistry to reduce the oxidative degradation of polyfluorene-based PLEDs. © 2004 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim

Mo6S9-xIx nanowires as additives for enhanced organic solar cell performance

The efficiency of organic bulk heterojunction solar cells depends crucially on charge transport in the composite. The charge carrier mobilities in common polymer:fullerene blends are rather low and depend strongly on the film morphology. Here we show the improvement of performance of canonical polymer:fullerene solar cells by embedding highly dispersed Mo6S 9-xIx nanowires within their active layer. Adding 0.5 wt% of nanowires raises both the fill factor and the open-circuit voltage, resulting in 18% higher power conversion efficiency. We attribute this increase to improved charge transport by the embedded nanowire mesh and envisage further improvement by using higher nanowire weight fractions and thinner nanowire bundles. © 2014 Elsevier B.V