<p>To fully optimize devices for solar energy conversion, a comprehensive understanding of how excitons migrate in materials for solar cell devices is crucial. Understanding the mechanisms behind exciton diffusion and energy transfer will enable the fabrication of highly efficient devices. However to thoroughly study exciton properties, techniques implementing high spatial (nm sizes) and temporal (fs time scales) resolution is required. Herein, we utilize transient absorption microscopy (TAM) with 50 nm spatial resolution and 200 fs temporal resolution to elucidate exciton diffusion in polymeric materials for solar energy conversion.</p> <p>While organic devices are inexpensive and require simpler fabrication procedures than inorganic materials, their device efficiencies often suffer due to their semi-crystalline nature, lending to short diffusion lengths which lead to trap sites and inevitably recombination. It has been demonstrated that achieving long-range exciton diffusion lengths is possible through coherence effects. Coherence can be found in an intermediate electronic coupling region where delocalization and localization compete.</p> <p>To exploit coherence effects, we study polymeric systems in which polymer chains are highly aligned via simple and scalable procedures; semiconducting fibers and solution coated films. In studying the fiber, we first implement polarized photoluminesce (PL) to determine optical ansitropy. From the polarized PL and PL images, it is observed that emission intensities are largest when probing along the transition dipole moment of the polymer. This suggests a type of Förester Resonance Energy Transfer mechanism in which excitons hop from one polymer chain to another.</p> <p>Solution coated polymer films are also studied to understand exciton diffusion as a function of deposition methods. By varying the solution concentration as well as coating rate, we are able to tune the morphology of the film. We observe a strong dependence between diffusion constant and deposition parameters, with diffusion constants of <i>ca.</i> 9, 13 and 33 cm²/s for three different films. The results obtained in this thesis are preliminary steps in an effort to elucidate energy transfer mechanisms and rates.</p><br>

A theory of the carrier transport, optical emission, and photoconductivity from optoelectronic devices based on ambipolar long-channel carbon-nanotube (CNT) field-effect transistors (FETs) is presented in this dissertation. In optical emitters based on ambipolar long-channel CNT FETs, an analytic diffusive-transport model for various recombination mechanisms is provided for the first time. The relationship and the scaling of emitted light-spot size and emitted optical power are clearly depicted for the first time as well. We also implement a numerical diffusive-transport approach for the light emission, in which the focus is on the effects of radiative and nonradiative recombination in the channel, with the movement of the spatial recombination profile in response to the gate and drain voltages. For the first time, we find that the emitted light-spot size and the emitted optical power depend sensitively on the operative nonradiative recombination mechanisms. We implement a numerical diffusive-transport approach including exciton photogeneration as well for photoconductors based on ambipolar long-channel CNT FETs with uniform and near-field photoexcitation. We show that the photocurrents are typically much smaller than the dark currents, and explain some possible reasons. Moreover, the exciton densities in CNTs are calculated and the effect of exciton diffusion is presented.

No description available.

The exciton diffusion length, which is the distance an exciton can diffuse in its lifetime, is an important parameter that has a critical impact on the operation of many organic optoelectronic devices, including organic solar cells, light emitting diodes and lasers. Knowledge of the exciton diffusion length can be a powerful aid for the design and optimisation of these devices. This thesis details the development of techniques based on time-resolved fluorescence for measuring the exciton diffusion in organic semiconductors. Two main methods were used to investigate exciton diffusion in the conjugated polymers P3HT, MEH-PPV and F8BT: the surface quenching technique and exciton-exciton annihilation. In particular, the surface quenching technique was adapted to avoid some of the potential pitfalls that have plagued earlier measurements. Using a titania quencher, measurements were performed using the surface quenching technique and fitted with an exciton diffusion model, allowing the calculation of the exciton diffusion length. Results from measurements of the exciton-exciton annihilation rate, which is a diffusion controlled process, where in good agreement with those from surface quenching, confirming the robustness of this twofold approach. A novel method for the control of the β-phase conformation in PFO films was used to produce films containing varying concentrations of β-phase. Exciton-exciton annihilation was used to investigate exciton diffusion in these films, revealing a gradual rise with increasing β-phase fraction due to improved interconnectivity. This work demonstrates how simple processing techniques can be used to control both film morphology and the exciton diffusion. The thickness dependence of the photoluminescence lifetime in conjugated polymers is a phenomenon that has so far received little attention and, thus, remained unexplained. This study demonstrates that it is not due to exciton quenching by external factors, but can be explained by a change in the morphology with decreasing film thickness.

This thesis presents studies of exciton diffusion in organic semiconductors measured using exciton-exciton annihilation and the measurements were performed on materials important for organic solar cells. In the conjugated polymer poly(3-hexylthiophene) (P3HT), the effect of molecular weight (4-76 kgmol⁻¹) was explored. Using exciton-exciton annihilation measurements, the highest diffusion coefficient was observed in the intermediate molecular weight region and was correlated with long conjugation lengths, higher fraction of aggregated states and more delocalised excitons within the aggregate. The results demonstrated that the molecular weight dependence is due to a complex relationship between intermolecular interactions, aggregate size and Boltzmann statistics. This thesis also includes an investigation of exciton diffusion in diketopyrrolopyrrole(DPP)-based small molecules as a function of molecular structure. Significant changes in photophysical and exciton diffusion properties were observed due to minor changes in molecular structure. Long conjugation lengths, bulky side chains or reduced steric hindrance due to absence of end alkyl chains correlated with reduced film crystallinity and reduced diffusion coefficients. The increase in disorder observed due to large conformational torsions resulted in inhomogeneous broadening of density of states and as a result exciton diffusion becomes dispersive. In this case, a slowdown of exciton diffusion is observed. This study demonstrates that enhanced exciton diffusion can be achieved by designing more rigid and planar conjugated backbones with smaller conjugation lengths. Finally, exciton diffusion measurements were used to rationalise the performance of T3 truxene oligomers as explosive sensors. Side chain lengths were found to have a subtle influence on exciton diffusion. Time-resolved PL quenching measurements were used to estimate the quencher concentration. Differences in quencher concentration were observed suggesting different interaction strengths of the quencher with the truxene oligomer which help explain the explosive sensing performance.

Kyoto University (京都大学) / 0048 / 新制・課程博士 / 博士(工学) / 甲第17595号 / 工博第3754号 / 新制||工||1572(附属図書館) / 30361 / 京都大学大学院工学研究科高分子化学専攻 / (主査)教授 伊藤 紳三郎, 教授 赤木 和夫, 教授 金谷 利治 / 学位規則第4条第1項該当

京都大学 / 0048 / 新制・課程博士 / 博士(工学) / 甲第18593号 / 工博第3954号 / 新制||工||1608(附属図書館) / 31493 / 京都大学大学院工学研究科高分子化学専攻 / (主査)教授 伊藤 紳三郎, 教授 木村 俊作, 教授 辻井 敬亘 / 学位規則第4条第1項該当 / Doctor of Philosophy (Engineering) / Kyoto University / DFAM

This thesis deals with exciton relaxation processes in thin polycrystalline films and matrix-isolated molecules of the perylene derivatives PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) and MePTCDI (N,N'-dimethylperylene-3,4,9,10-dicarboximide). Using femtosecond pump-probe spectroscopy, transient absorption spectra, excitonic relaxation in the lowest excited state subsequent to excitation, and exciton-exciton interaction and annihilation at high excitation densities have been addressed. Transient absorption spectroscopy in the range 1.2eV-2.6eV has been applied to thin polycrystalline films of PTCDA and MePTCDI and to solid solutions of PTCDA and MePTCDI molecules (monomers) in a SiO2 matrix. We are able to ascribe the respective signal contributions to ground state bleaching, stimulated emission, and excited state absorption. Both systems exhibit broad excited-state absorption features below 2.0eV, with dominant peaks between 1.8eV and 2.0eV. The monomer spectra can be consistently explained by the results of quantum-chemical calculations on single molecules, and the respective experimental polarization anisotropies for the two major transitions agree with the calculated polarizations. Dimer calculations allow to qualitatively understand the trends visible in the experimental results from monomers to thin films. The broad excited state absorption band between 1.8eV and 2.0eV allows to probe the population dynamics in the first excited state of thin films. We show that excitons created at the Gamma point relax towards the border of the Brillouin zone on a 100fs time scale in both systems. Excitonic relaxation is accelerated by increase of temperature and/or excitation density, which is attributed to stimulated phonon emission during relaxation in k-space. Lower and upper limits of the intraband relaxation time constants are 25fs (resolution limit) and 250fs (100fs) for PTCDA (MePTCDI). These values agree with the upper limit for the intraband relaxation time of 10ps, evaluated from time-resolved luminescence measurements. While the luminescence anisotropy is in full accordance with the predictions made by a luminescence anisotropy model being consistent with the exciton model of Davydov-split states, the pump-probe anisotropy calls for an explanation beyond the models presently available. At excitation densities 10^(19)cm^(-3), the major de-excitation mechanism for the relaxed excitons is exciton-exciton annihilation, resulting in a strongly reduced exciton life time. Three different models for the microscopic behavior have been tested: a diffusion-limited annihilation model in both three and one dimensions (with diffusion constant D as fit parameter) as well as a long-range single-step Förster-type annihilation model (with Förster radius RF as fit parameter). For PTCDA, the latter two, being structurally equivalent, allow to fit a set of multiexponential decay curves for multiple initial exciton densities with high precision. In contrast, the three-dimensional diffusion-limited model is clearly inferior. For all three models, we extract annihilation rates, diffusion constants and diffusion lengths (or Förster radii), for both room and liquid helium temperature. Temperature dependence and orders of magnitude of the obtained parameters D or RF correspond to the expectations. For MePTCDI, the 1D and the Förster model are in good agreement for a smaller interval of excitation densities. For a initial exciton densities higher than 5 x 10^(19)cm^(-3), the 3D model performs significantly better than the other two.

This thesis deals with exciton relaxation processes in thin polycrystalline films and matrix-isolated molecules of the perylene derivatives PTCDA (3,4,9,10-perylenetetracarboxylic dianhydride) and MePTCDI (N,N'-dimethylperylene-3,4,9,10-dicarboximide). Using femtosecond pump-probe spectroscopy, transient absorption spectra, excitonic relaxation in the lowest excited state subsequent to excitation, and exciton-exciton interaction and annihilation at high excitation densities have been addressed. Transient absorption spectroscopy in the range 1.2eV-2.6eV has been applied to thin polycrystalline films of PTCDA and MePTCDI and to solid solutions of PTCDA and MePTCDI molecules (monomers) in a SiO2 matrix. We are able to ascribe the respective signal contributions to ground state bleaching, stimulated emission, and excited state absorption. Both systems exhibit broad excited-state absorption features below 2.0eV, with dominant peaks between 1.8eV and 2.0eV. The monomer spectra can be consistently explained by the results of quantum-chemical calculations on single molecules, and the respective experimental polarization anisotropies for the two major transitions agree with the calculated polarizations. Dimer calculations allow to qualitatively understand the trends visible in the experimental results from monomers to thin films. The broad excited state absorption band between 1.8eV and 2.0eV allows to probe the population dynamics in the first excited state of thin films. We show that excitons created at the Gamma point relax towards the border of the Brillouin zone on a 100fs time scale in both systems. Excitonic relaxation is accelerated by increase of temperature and/or excitation density, which is attributed to stimulated phonon emission during relaxation in k-space. Lower and upper limits of the intraband relaxation time constants are 25fs (resolution limit) and 250fs (100fs) for PTCDA (MePTCDI). These values agree with the upper limit for the intraband relaxation time of 10ps, evaluated from time-resolved luminescence measurements. While the luminescence anisotropy is in full accordance with the predictions made by a luminescence anisotropy model being consistent with the exciton model of Davydov-split states, the pump-probe anisotropy calls for an explanation beyond the models presently available. At excitation densities 10^(19)cm^(-3), the major de-excitation mechanism for the relaxed excitons is exciton-exciton annihilation, resulting in a strongly reduced exciton life time. Three different models for the microscopic behavior have been tested: a diffusion-limited annihilation model in both three and one dimensions (with diffusion constant D as fit parameter) as well as a long-range single-step Förster-type annihilation model (with Förster radius RF as fit parameter). For PTCDA, the latter two, being structurally equivalent, allow to fit a set of multiexponential decay curves for multiple initial exciton densities with high precision. In contrast, the three-dimensional diffusion-limited model is clearly inferior. For all three models, we extract annihilation rates, diffusion constants and diffusion lengths (or Förster radii), for both room and liquid helium temperature. Temperature dependence and orders of magnitude of the obtained parameters D or RF correspond to the expectations. For MePTCDI, the 1D and the Förster model are in good agreement for a smaller interval of excitation densities. For a initial exciton densities higher than 5 x 10^(19)cm^(-3), the 3D model performs significantly better than the other two.