ABSTRACT We investigate the influence of particle plasmons on exciton and charge generation and recombination processes in the blend of poly (9‐(1‐octylnonyl)‐9H‐carbazole‐benzothiadiazole‐4,7‐diyl‐2,5‐thiophenediyl) (PCDTBT) and [6,6]‐phenyl‐C 70 butyric acid methyl ester (PC 70 BM). The particle plasmons are generated from gold nanoparticles, which are embedded into PCDTBT:PC 70 BM blend. For the blend with gold nanoparticles, we observe enhance light harvesting. Despite the enhanced light collection, we find that the quasi‐steady‐state charge generation has not been influenced by the particle plasmons. However, the generation and recombination of long‐lived (sub‐millisecond) polaron paris have been significantly enhanced: from untrapped state in the pristine blend to the trapped state in the gold nanoparticle‐embedded blend. This result implies that the plasmon‐influenced polarons are trapped at the broadband geminate polaron pair (GPP) state. This state acts as an intermediate state, which either leads to the formation of charge transfer excitons (CTXs) or free charge carriers. In our case, the particle plasmon‐influenced polarons are trapped in the GPP state, which leads to the formation of CTXs. For this reason, we do not observe the enhanced charge generation in PCDTBT:PC 70 BM blend with particle plasmon resonance. Finally, we revealed that the long‐lived polarons mainly resulted from the localization by particle plasmons. The macroscopic modification in the blend film made negligible contributions to this influence. © 2017 Wiley Periodicals, Inc. J. Polym. Sci., Part B: Polym. Phys. 2017 , 55 , 940–947

Resonance Raman spectroscopy reveals conformation and morphology dependent electronic localization in push–pull donor–acceptor co-polymers.

In polymeric semiconductors, charge carriers are polarons, which means that the excess charge deforms the molecular structure of the polymer chain that hosts it. This results in distinctive signatures in the vibrational modes of the polymer. Here, we probe polaron photogeneration dynamics at polymer:fullerene heterojunctions by monitoring its time-resolved resonance-Raman spectrum following ultrafast photoexcitation. We conclude that polarons emerge within 300 fs. Surprisingly, further structural evolution on ≲ 50-ps timescales is modest, indicating that the polymer conformation hosting nascent polarons is not significantly different from that near equilibrium. We interpret this as suggestive that charges are free from their mutual Coulomb potential because we would expect rich vibrational dynamics associated with charge-pair relaxation. We address current debates on the photocarrier generation mechanism at molecular heterojunctions, and our work is, to our knowledge, the first direct probe of molecular conformation dynamics during this fundamentally important process in these materials.

We explore charge recombination dynamics at electron donor-acceptor heterojunctions, formed between a semiconductor polymer (PCDTBT) and a fullerene derivative (PC70BM), by means of combined time-resolved photoluminescence and transient absorption spectroscopies. Following prompt exciton dissociation across the heterojunction, a subset of bound electron-hole pairs recombines with a temperature-independent rate distribution spanning submicrosecond timescales to produce luminescent charge-transfer excitons (CTX). At 14 K, this slow mechanism is the dominant geminate charge recombination pathway, whereas we also observe CTX emission on subnanosecond timescales at 293 K. We thus find that at these temperatures, a fraction of the initial charge-pair population is trapped deeply such that they only recombine slowly over a broad distribution of timescales by quantum tunneling. We identify geminate polaron pairs (GPP) as a reservoir of long-lived localized states that repopulate the CTX up to microsecond timescales. The observation of such distributed geminate-charge recombination highlights the importance of the molecular nature of specific donoracceptor electronic interactions in defining the relaxation pathways of trapped GPP. (c) 2012 Wiley Periodicals, Inc. J Polym Sci Part B: Polym Phys, 2012

We examine the interdependence of structural and electronic properties of two substituted pyrene crystals by means of combined spectroscopic probes and density-functional theory calculations. Substituted pyrenes are useful model systems to unravel the interplay of crystal structure and electronic properties in organic semiconductors. To study the effect of steric encumbrance on the crystalline arrangement of two 1,3,6,8-tetraalkynylpyrene derivatives, one features linear n-hexyl side groups while the other contains branched trimethylsilyl groups. Both derivatives form triclinic crystal structures when grown from solution, but the electronic dispersion behavior is significantly different due to differences in π-π overlap along the π-stacking axis. Both systems display dispersion of around 0.45 eV in the valence band, suggesting a high intrinsic hole mobility. However, the direction of the dispersion is different: it is primarily along the π-stacking axis in the trimethylsilyl-substituted derivative, but less aligned with this crystal axis in the hexyl-substituted molecule. This is a direct consequence of the differences in co-facial π electron overlap revealed by the crystallographic studies. We find that photophysical defects, ascribed to excimer-like states, point to the importance of localized trap states.

In organic photovoltaic diodes, singlet intrachain excitons dissociate into geminate polaron pairs (GPP) at the heterojonction, which further dissociate into photocarriers or relax into charge transfer exciton (CTX) states. Our temperature-dependent, time-resolved spectroscopic approach unravels the dynamics of those species in films of polycarbazole/fullerene derivative blend. We find that GPP act as a dark reservoir that feeds the CTX. At low temperature, the GPP are trapped and feed the CTX via tunneling, but not the free photocarriers. At room temperature, some of the GPP can overcome the Coulomb barrier and feed the CTX promptly, or be deeply trapped and feed the CTX on several timescales. We find that at room temperature, 16% of the geminate recombination is accounted for by trapped geminate polaron pairs.

This work presents the physical characterization of conjugated organic molecules to reveal the interplay between molecular structure, molecular packing and electronic structure. The molecules used in this study are a perylene tetracarboxdiimide derivative (EPPTC) and pyrene derivatives which differ by their side chains, linear or branched. We characterized these materials mainly by means of X-ray diffraction, continuous and time-resolveeJ spectroscopy and ab initia calculations. The singlet state of molecular EPPTC has only one desexcitation rate whereas the cristalline form has a continuous distribution of rates due to energetic disorder in the solid state. Both pyrene derivatives show a similar band dispersion, around 0.45 eV, 1 despite their very different packing. Our results show that a cofacial stacking does not guarantee a good carrier mobility.

In this contribution, we present steady-state and time-resolved photoluminescence measurements, as well as absorbance measurements, on dilute solutions and on solution-grown crystals of a perylene tetracarboxdiimide derivative (EPPTC). We also present density functional theory (DFT) calculations of their electronic structure. These molecules self-assemble into highly anisotropic needle-shaped crystals with monoclinic structure, as revealed by X-ray diffraction techniques and polarized optical microscopy. We find experimentally that the Huang-Rhys factor for this molecule is 0.565, which is in good agreement with that extracted from ab initio calculations (0.678). We demonstrate that in a film, the nature of photoexcitations is a vibrationally dressed Frenkel exciton and that these recombine radiatively with a distribution of excited-state constants centered at similar to 1 ns(-1). (C) 2009 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim