Spectroscopic investigation of excitonic and charge photogeneration processes in organic photovoltaic cells

<p>Organic photovoltaic (OPV) cells show significant promise as a renewable energy resource capable of meeting the world’s large and growing energy needs. Increasing device efficiency is central to achieving an economically viable option for widespread applications. To this end, a better understanding of the structure and dynamics of the electronic excited states is needed. In particular, the mechanism by which excitons (electron-hole pairs) escape their Coulombic attraction and generate photocurrent is yet to be established. In this thesis ultrafast laser spectroscopy, in particular transient absorption and time-resolved photoluminescence, are used to study: exciton relaxation, morphological effects on charge separation, and the pathway leading to triplet exciton states. In Chapter 3, a series of oligothiophenes are synthesised with well-defined conjugation lengths to act as molecular models of polymer backbone sub-units, and thereby probe exciton relaxation processes. Time-resolved photoluminescence (TRPL) and transient absorption (TA) spectroscopy measurements presented in Chapter 4 reveal emission signatures evolve from a mirror image of absorption - which lacks vibronic structure - towards a spectrally narrower and vibronically structured species on the hundreds of femtosecond to early picosecond timescale. Analysis of this spectral evolution shows that a broad distribution of torsional conformers is driven to rapidly planarize in the excited state, including in solid films. This provides evidence that both torsional relaxation and energy migration could contribute to the non-mirror image absorption-emission spectra observed in polymer thin films. Recently, long lived TA signatures have been attributed to triplet excited states with the suggested formation pathway being similar to organic light emitting diodes, whereby non-geminate (bimolecular) charge recombination leads to the formation of both singlet and triplet states. Isolated oligothiophenes in solution provide an ideal model system to investigate the role of structural relaxation on triplet exciton formation. Through analysis of TA spectral dynamics in Chapter 5, singlet and triplet exciton populations were tracked. Restriction of the torsional relaxation increased triplet yield suggesting vibrational hot states could drive triplet formation. This model could aid in understanding triplet exciton formation in polymer-based solar cells via spin-mixing instead of non-geminate recombination. In a series of polymer:fullerene blends, the link between the nature of polymerfullerene intermixing and charge generation pathways was investigated. It is shown in Chapter 6 that free charge generation is most efficient in a 3-phase morphology that features intimately mixed polymer:fullerene regions amongst neat polymer and fullerene phases. Distinct spectroscopic signatures made it possible to determine whether holes occupy disordered or crystalline polymer chains. TA spectral dynamics reveal the migration of holes from intermixed to pure olymer regions in 3-phase morphology blends, which contrasted with observations in 2-phase blends. The energy gradient between the intermixed and phase-pure regions may be sufficient to drive efficient separation of charge pairs initially generated in intermixed regions, with free charges subsequently percolating through these phase-pure domains. The photophysics of a high performance polymer:polymer blend is studied in Chapter 7 in an effort to elucidate how these blends can rival their polymer:fullerene counterparts. Optical spectroscopy reveals incomplete exciton dissociation and rapid geminate recombination in the blends. This is shown to result from a largely phase-separated morphology with domains greater than the exciton diffusion length. Significant loss of charge carriers on early timescales highlights increasing polymer: polymer solar cell efficiency requires optimizing blend morphology to realise facile charge separation. Taken together, this thesis presents a valuable spectroscopic insight into the pathway of efficient charge separation and the importance of both blend morphology and polymer structure.</p>

Spectroscopic investigation of excitonic and charge photogeneration processes in organic photovoltaic cells

<p>Organic photovoltaic (OPV) cells show significant promise as a renewable energy resource capable of meeting the world’s large and growing energy needs. Increasing device efficiency is central to achieving an economically viable option for widespread applications. To this end, a better understanding of the structure and dynamics of the electronic excited states is needed. In particular, the mechanism by which excitons (electron-hole pairs) escape their Coulombic attraction and generate photocurrent is yet to be established. In this thesis ultrafast laser spectroscopy, in particular transient absorption and time-resolved photoluminescence, are used to study: exciton relaxation, morphological effects on charge separation, and the pathway leading to triplet exciton states. In Chapter 3, a series of oligothiophenes are synthesised with well-defined conjugation lengths to act as molecular models of polymer backbone sub-units, and thereby probe exciton relaxation processes. Time-resolved photoluminescence (TRPL) and transient absorption (TA) spectroscopy measurements presented in Chapter 4 reveal emission signatures evolve from a mirror image of absorption - which lacks vibronic structure - towards a spectrally narrower and vibronically structured species on the hundreds of femtosecond to early picosecond timescale. Analysis of this spectral evolution shows that a broad distribution of torsional conformers is driven to rapidly planarize in the excited state, including in solid films. This provides evidence that both torsional relaxation and energy migration could contribute to the non-mirror image absorption-emission spectra observed in polymer thin films. Recently, long lived TA signatures have been attributed to triplet excited states with the suggested formation pathway being similar to organic light emitting diodes, whereby non-geminate (bimolecular) charge recombination leads to the formation of both singlet and triplet states. Isolated oligothiophenes in solution provide an ideal model system to investigate the role of structural relaxation on triplet exciton formation. Through analysis of TA spectral dynamics in Chapter 5, singlet and triplet exciton populations were tracked. Restriction of the torsional relaxation increased triplet yield suggesting vibrational hot states could drive triplet formation. This model could aid in understanding triplet exciton formation in polymer-based solar cells via spin-mixing instead of non-geminate recombination. In a series of polymer:fullerene blends, the link between the nature of polymerfullerene intermixing and charge generation pathways was investigated. It is shown in Chapter 6 that free charge generation is most efficient in a 3-phase morphology that features intimately mixed polymer:fullerene regions amongst neat polymer and fullerene phases. Distinct spectroscopic signatures made it possible to determine whether holes occupy disordered or crystalline polymer chains. TA spectral dynamics reveal the migration of holes from intermixed to pure olymer regions in 3-phase morphology blends, which contrasted with observations in 2-phase blends. The energy gradient between the intermixed and phase-pure regions may be sufficient to drive efficient separation of charge pairs initially generated in intermixed regions, with free charges subsequently percolating through these phase-pure domains. The photophysics of a high performance polymer:polymer blend is studied in Chapter 7 in an effort to elucidate how these blends can rival their polymer:fullerene counterparts. Optical spectroscopy reveals incomplete exciton dissociation and rapid geminate recombination in the blends. This is shown to result from a largely phase-separated morphology with domains greater than the exciton diffusion length. Significant loss of charge carriers on early timescales highlights increasing polymer: polymer solar cell efficiency requires optimizing blend morphology to realise facile charge separation. Taken together, this thesis presents a valuable spectroscopic insight into the pathway of efficient charge separation and the importance of both blend morphology and polymer structure.</p>

Optimization of polycaprolactone - based nanofiber matrices for the cultivation of corneal endothelial cells

Posterior lamellar transplantation of the eye’ s cornea (DSAEK, DMEK) currently is the gold standard for treating patients with corneal endothelial cell and back surface pathologies resulting in functional impairment. An artificial biomimetic graft carrying human corneal endothelium could minimize the dependency on human donor corneas giving access to this vision-restoring surgery to large numbers of patients, thus reducing current long waiting lists. In this study, four groups of electrospun nanofibrous scaffolds were compared: polycaprolactone (PCL), PCL/collagen, PCL/gelatin and PCL/chitosan. Each of the scaffolds were tissue-engineered with human corneal endothelial cells (HCEC-B4G12) and analyzed with regard to their potential application as artificial posterior lamellar grafts. Staining with ZO-1 and Na+/K+-ATPase antibodies revealed intact cell functionalities. It could be shown, that blending leads to decreasing contact angle, whereby a heterogeneous blend morphology could be revealed. Scaffold cytocompatibility could be confirmed for all groups via live/dead staining, whereby a significant higher cell viability could be observed for the collagen and gelatine blended matrices with 97 ± 3% and 98 ± 2% living cells respectively. TEM images show the superficial anchoring of the HCECs onto the scaffolds. This work emphasizes the benefit of blended PCL nanofibrous scaffolds for corneal endothelial keratoplasty.

Ultra-Low Percolation Threshold Induced by Thermal Treatments in Co-Continuous Blend-Based PP/PS/MWCNTs Nanocomposites

The effect of the crystallization of polypropylene (PP) forming an immiscible polymer blend with polystyrene (PS) containing conductive multi-wall carbon nanotubes (MWCNTs) on its electrical conductivity and electrical percolation threshold (PT) was investigated in this work. PP/PS/MWCNTs composites with a co-continuous morphology and a concentration of MWCNTs ranging from 0 to 2 wt.% were obtained. The PT was greatly reduced by a two-step approach. First, a 50% reduction in the PT was achieved by using the effect of double percolation in the blend system compared to PP/MWCNTs. Second, with the additional thermal treatments, referred to as slow-cooling treatment (with the cooling rate 0.5 °C/min), and isothermal treatment (at 135 °C for 15 min), ultra-low PT values were achieved for the PP/PS/MWCNTs system. A 0.06 wt.% of MWCNTs was attained upon the use of the slow-cooling treatment and 0.08 wt.% of MWCNTs upon the isothermal treatment. This reduction is attributed to PP crystals’ volume exclusion, with no alteration in the blend morphology.

Numerical simulations of the polydisperse droplet size distribution of disperse blends in complex flow

The blend morphology model developed by Wong et al. (Rheologica Acta, 2019), based on Peters et al. (J Rheol 45(3):659–689, 2001), is used to investigate the development of the polydispersity of the disperse polymer blend morphology in complex flow. First, the model is extended with additional morphological states. The extended model is tested for simple shear flow, where it is found that the droplet size distribution does not simply scale with the shear rate, because this scaling does not hold for coalescing droplets. Subsequently, the model is applied to Poiseuille flow, showing formation of distinct layers, which occurs in realistic pressure-driven flows. Finally, the model is applied on an eccentric cylinder flow, where histograms are made of the average droplet size throughout the domain. It is observed that outer cylinder rotation results in narrow distributions where the small droplets are relatively large, whereas inner cylinder rotation results in broad distributions where the small droplets are significantly smaller than in the case of outer cylinder rotation. Eccentricity seems to only have a minor effect if the maximum shear rate is held constant. The flow profile and history in combination with the maximum shear rate strongly determine how the polydisperse droplet size distribution develops.