Jenny Clark

Role of intermolecular coupling in the photophysics of disordered organic semiconductors: aggregate emission in regioregular polythiophene.

We address the role of excitonic coupling on the nature of photoexcitations in the conjugated polymer regioregular poly(3-hexylthiophene). By means of temperature-dependent absorption and photoluminescence spectroscopy, we show that optical emission is overwhelmingly dominated by weakly coupled H aggregates. The relative absorbance of the 0-0 and 0-1 vibronic peaks provides a powerfully simple means to extract the magnitude of the intermolecular coupling energy, of approximately 5 and 30 meV for films spun from isodurene and chloroform solutions, respectively.

Ultrafast long-range charge separation in organic semiconductor photovoltaic diodes.

Early Separation In photovoltaic devices, electrons excited by the absorption of light must travel across a junction, while the positively charged “holes” they leave behind effectively migrate in the opposite direction. If the electrons and holes do not separate efficiently, they can recombine and fail to produce any appreciable current. Gélinas et al. (p. 512, published online 12 December; see the Perspective by Bredas) studied this separation process by ultrafast optical absorption spectroscopy in thiophene-derived donor-fullerene acceptor systems common in organic photovoltaics and report a rate significantly faster than simple charge diffusion would suggest. The results implicate a coherent charge delocalization process, likely to involve fullerene π-electron states. Ultrafast spectroscopy shows electrons and holes separating faster than simple diffusion would imply in organic photovoltaics. [Also see Perspective by Bredas] Understanding the charge-separation mechanism in organic photovoltaic cells (OPVs) could facilitate optimization of their overall efficiency. Here we report the time dependence of the separation of photogenerated electron hole pairs across the donor-acceptor heterojunction in OPV model systems. By tracking the modulation of the optical absorption due to the electric field generated between the charges, we measure ~200 millielectron volts of electrostatic energy arising from electron-hole separation within 40 femtoseconds of excitation, corresponding to a charge separation distance of at least 4 nanometers. At this separation, the residual Coulomb attraction between charges is at or below thermal energies, so that electron and hole separate freely. This early time behavior is consistent with charge separation through access to delocalized π-electron states in ordered regions of the fullerene acceptor material.

Determining exciton bandwidth and film microstructure in polythiophene films using linear absorption spectroscopy

We analyze the linear absorption spectrum of regioregular poly(3-hexylthiophene) films spun from a variety of solvents to probe directly the film microstructure and how it depends on processing conditions. We estimate the exciton bandwidth and the percentage of the film composed of aggregates quantitatively using a weakly interacting H-aggregate model. This provides a description of the degree and quality of crystallites within the film and is in turn correlated with thin-film field-effect transistor characteristics.

Ultrafast Dynamics of Exciton Fission in Polycrystalline Pentacene

We use ultrafast transient absorption spectroscopy with sub-20 fs time resolution and broad spectral coverage to directly probe the process of exciton fission in polycrystalline thin films of pentacene. We observe that the overwhelming majority of initially photogenerated singlet excitons evolve into triplet excitons on an ∼80 fs time scale independent of the excitation wavelength. This implies that exciton fission occurs at a rate comparable to phonon-mediated exciton localization processes and may proceed directly from the initial, delocalized, state. The singlet population is identified due to the brief presence of stimulated emission, which is emitted at wavelengths which vary with the photon energy of the excitation pulse, a violation of Kashas Rule that confirms that the lowest-lying singlet state is extremely short-lived. This direct demonstration that triplet generation is both rapid and efficient establishes multiple exciton generation by exciton fission as an attractive route to increased efficiency in organic solar cells.

Determining exciton coherence from the photoluminescence spectral line shape in poly(3-hexylthiophene) thin films

The photoluminescence (PL) spectral line shape of regioregular poly(3-hexylthiophene) thin films is analyzed using a model which treats the polymer pi-stacks as H-aggregates with exciton-vibrational coupling and spatially correlated site disorder. The Stokes shift, linewidth, and relative vibronic peak intensities in the low-temperature PL spectrum (T=10 K) are accurately reproduced, allowing the coherence function corresponding to the lowest energy (emitting) exciton to be determined from the ratio of the 0-0 to 0-1 peak intensities. The exciton migration length is determined from the N-dependent Stokes shift, where N is the number of segments comprising the stack. Based on the temperature dependence of the PL spectrum it is concluded that emission arises from a low concentration of aggregates which are more disordered than the dominant species responsible for absorption. The emissive aggregates are characterized by shorter average conjugation lengths and hence greater exciton bandwidths. The coherence length of the emitting exciton is estimated to be only three lattice spacings ( approximately 1.1 nm) along the pi-stacking direction. By contrast, the exciton migration length for incoherent hopping between coherent domains is estimated to be approximately 15 nm.

Activated Singlet Exciton Fission in a Semiconducting Polymer

Singlet exciton fission is a spin-allowed process to generate two triplet excitons from a single absorbed photon. This phenomenon offers great potential in organic photovoltaics, but the mechanism remains poorly understood. Most reports to date have addressed intermolecular fission within small-molecular crystals. However, through appropriate chemical design chromophores capable of intramolecular fission can also be produced. Here we directly observe sub-100 fs activated singlet fission in a semiconducting poly(thienylenevinylene). We demonstrate that fission proceeds directly from the initial 1Bu exciton, contrary to current models that involve the lower-lying 2Ag exciton. In solution, the generated triplet pairs rapidly recombine and decay through the 2Ag state. In films, exciton diffusion breaks this symmetry and we observe long-lived triplets which form charge-transfer states in photovoltaic blends.

The Nature of Singlet Exciton Fission in Carotenoid Aggregates

Singlet exciton fission allows the fast and efficient generation of two spin triplet states from one photoexcited singlet. It has the potential to improve organic photovoltaics, enabling efficient coupling to the blue to ultraviolet region of the solar spectrum to capture the energy generally lost as waste heat. However, many questions remain about the underlying fission mechanism. The relation between intermolecular geometry and singlet fission rate and yield is poorly understood and remains one of the most significant barriers to the design of new singlet fission sensitizers. Here we explore the structure–property relationship and examine the mechanism of singlet fission in aggregates of astaxanthin, a small polyene. We isolate five distinct supramolecular structures of astaxanthin generated through self-assembly in solution. Each is capable of undergoing intermolecular singlet fission, with rates of triplet generation and annihilation that can be correlated with intermolecular coupling strength. In contrast with the conventional model of singlet fission in linear molecules, we demonstrate that no intermediate states are involved in the triplet formation: instead, singlet fission occurs directly from the initial 1Bu photoexcited state on ultrafast time scales. This result demands a re-evaluation of current theories of polyene photophysics and highlights the robustness of carotenoid singlet fission.

Control of intrachain charge transfer in model systems for block copolymer photovoltaic materials.

We report the electronic properties of the conjugated coupling between a donor polymer and an acceptor segment serving as a model for the coupling in conjugated donor-acceptor block copolymers. These structures allow the study of possible intrachain photoinduced charge separation, in contrast to the interchain separation achieved in conventional donor-acceptor blends. Depending on the nature of the conjugated linkage, we observe varying degrees of modification of the excited states, including the formation of intrachain charge transfer excitons. The polymers comprise a block (typically 18 repeat units) of P3HT, poly(3-hexyl thiophene), coupled to a single unit of F8-TBT (where F8 is dioctylfluorene, and TBT is thiophene-benzothiadiazole-thiophene). When the P3HT chain is linked to the TBT unit, we observe formation of a localized charge transfer state, with red-shifted absorption and emission. Independent of the excitation energy, this state is formed very rapidly (<40 fs) and efficiently. Because there is only a single TBT unit present, there is little scope for long-range charge separation and it is relatively short-lived, <1 ns. In contrast, when the P3HT chain and TBT unit are separated by the wider bandgap F8 unit, there is little indication for modification of either ground or excited electronic states, and longer-lived charge separated states are observed.

Blue polymer optical fiber amplifiers based on conjugated fluorene oligomers

We fabricated polymer optical fiber (POF) amplifiers operating between 440 and 480 nm, using POFs doped with a series of fluorene oligomers, including tri-, penta-(9,9-dioctylfluorene) and hepta-(9,9-dihexylfluorene). The gain properties of pure oligofluorene films demonstrate gain coefficients on the order of 250 dB/cm and amplified spontaneous emission thresholds between 1 and 8 μJ c m-2 , significantly lower than other fluorene gain media. The optical and morphological characteristics of PMMA thin films doped with the oligomers demonstrate that the oligomers are largely isolated within the PMMA. The optical and gain properties of POFs produced using an adapted preform-drawing technique and doped with the oligofluorenes provide gain values on the order of 0.07 dB for 2 mm of doped POF. The oligofluorenes are largely isolated within the POFs, paving the way for all optical gain-switching.

Nanoscale Imaging of the Interface Dynamics in Polymer Blends by Femtosecond Pump‐Probe Confocal Microscopy

Composites are at the heart of polymer applications in photonics and optoelectronics, and are indispensable in devices ranging from solar cells to all-optical modulators. In such samples, phenomena occurring at interfaces between two materials are of fundamental importance but are extremely complex and poorly understood. Their structural and electrical properties have been investigated by means of scanning probe techniques, which allow achieving atomic resolution. Optical properties too can be mapped, although with lower resolution. Dynamics, however, regarding the fate of the excited state and the relaxation paths which dictate the ultimate device performance, are rarely spatially imaged. Here we report on pump-probe measurements of phase-separated conjugated-polymer thin fi lms with ≈ 150-fs temporal resolution and ≈ 300-nm spatial resolution. Our results provide new insight into their complex structure and single out “dynamical” interfaces, i.e. border regions at the phase separated islands that behave differently in terms of transient absorption and relaxation dynamics. Blending different semiconducting polymers leads to samples with enhanced or new properties. Engineering such composites in order to obtain the desired functionalities is one of the major challenges in polymer technology, but it also raises fundamental questions about the physics and chemistry at the interfaces between different domains. Phenomena occurring at these interfaces often determine device performances, [ 1–5 ] but are poorly understood due to the variety of possible electronic states (excitons, excimers, charge transfer states) and processes (energy transfer and charge separation) [ 6–11 ] and to their complicated dynamics.