This work is focused on developing novel techniques for characterizing organic bulk heterojunction (BHJ) materials for organic photovoltaic (OPV) applications. Polymer:fullerene BHJs are a promising class of photovoltaic materials, but an improved understanding of the charge transport processes and materials science of BHJs is required for them to effectively compete with other photovoltaic systems. Key parameters of BHJ systems that need to be evaluated include both electron and hole mobilities, the carrier concentrations, the recombination mechanism and the recombination coefficient. For these studies, poly(3-hexylthiophene) (P3HT):(6,6)-phenyl C₆₁-butyric acid methyl ester (PCBM) have been characterized due to its wide use among researchers. Traditional characterization techniques have focused on transient measurements in a vertical device configuration, but we demonstrate the use of lateral BHJ devices as materials diagnostic platforms. Lateral devices allow for direct access to the active layer for spatially resolved and environmental effect measurements. The devices are also measured under steady state operation, similar to a working OPV cell. Under these conditions, lateral BHJ devices exhibit space charge limited transport behavior. A detailed charge transport model is presented to describe the potential, electric field, and carrier concentration profiles of lateral BHJ devices, as well as the current versus voltage characteristics of different regions of the device. We are able to calculate the slower carrier mobility from photocurrent measurements of lateral devices and the carrier mobility ratio from the device potential profile, even in ambipolar BHJ systems. In situ potentiometry is used to construct detailed potential profiles of the device channel and calculate both carrier mobilities. The carrier concentration and recombination coefficient are calculated from lateral conductivity measurements, and we show that bimolecular recombination is the dominant mechanism in bulk P3HT:PCBM. A simplified in situ potentiometry and photocurrent measurement technique is presented to measure the time evolution of organic BHJ performance. Due to the open geometry of the lateral BHJ device, we are also able to monitor the change in key charge transport parameters, including the recombination mechanism, in response to environmental degradation, analyte exposure, and ambient temperature. We show increased geminate recombination in P3HT:PC₇₁BM after prolonged light exposure. Lateral BHJ device measurements offer a useful complement to measurements on vertical photovoltaic structures and provide a more complete and detailed picture of OPV materials. / text

The as-produced isomer mixture of the organic photovoltaic device acceptor material bisPC62BM has been purified into its constituents by peak-recycling HPLC, and those individual isomers were characterised by UV-Vis absorption spectroscopy and cyclic voltammetry. A total of 18 isomers were purified from the mixture to a standard exceeding 99.5% with respect to other isomers. The HOMOs, LUMOs, and HOMO-LUMO gaps of the purified isomers vary from (-5.673 to -5.402 eV), (-3.901 to -3.729 eV), and (1.664 to 1.883 eV), respectively. We also find a correlation between HPLC retention time and the relative positions of the addends; in that generally the closer the addends are to each other the longer the retention time of the isomer, and vice versa. The OPV acceptor molecule PC71BM was also purified into its constituent isomers to a standard of at least 99%. The total three purified isomers were each characterized by 13C NMR and UV-vis spectroscopy, and cyclic voltammetry. These characterizations were supported by HF/DFT ab-initio calculations. All three isomers are methano-fullerenes. The most abundant isomer (85% of the mixture) exists as a racemate involving the 8-25 bond of C70. The other two isomers both involved the 9-10 bond of C70, but are distinguished by opposing orientations of the addend with r and s pseudo-asymmetry about carbon atom 71. The r and s isomers comprised 9% and 6% of the as-produced of the mixture, respectively. In order of decreasing abundance, the LUMO levels of the isomers were -3.9316, -3.9194 and -3.9197 eV and the HOMO-LUMO gaps were 1,772, 1.754 and 1.748 eV.

With the need for the development of renewable sources of energy, organic photovoltaic (OPV) has been attracting researchers’ interest for the past decades. This solar technology utilizes carbon-based semiconductors instead of conventional inorganic materials which enables inexpensive, lightweight and flexible roll-to-roll fabrication of large area solar panels with a very short energy payback time. Device efficiencies have rapidly increased to above 18% within the last few years, becoming competitive with solar technologies available on the market. However, research has been focused on the maximization of efficiencies at all cost leading to synthetically challenging materials and processes with negligible commercial scalability. In this thesis, silicon phthalocyanines (SiPcs), synthetically facile molecules most known for their extensive use as dyes and pigments in the industry, were employed as low-cost and scalable active materials for OPV devices. We also report the use of layer-by-layer deposition of the donor and acceptor layer providing a more scalable process compared to the conventional blended heterojunction morphology. Different SiPc derivatives, both soluble and non-soluble, were used as acceptors, paired with different donor polymers (P3HT, PCDTBT, and PBDB-T) and integrated into hybrid evaporation-solution and all-solution layer-by-layer OPV devices. Significant device engineering and optimization was performed through the investigation of several processing conditions such as solvent choice, spin-speed, concentration and annealing temperature/time. In particular, all-solution processed SiPc-based bilayer OPV devices achieved PCEs above 3% with Voc above 1 V, which was similar to performances of corresponding BHJ OPVs. SiPc derivatives also demonstrated their ability to act as electron transport layers in perovskite solar cells. These results further establish the potential of SiPc derivatives as active materials in different solar technologies, while promoting the use of the bilayer structure in OPV devices.

This thesis reports the functionalization of metal nanoparticles to allow for solubility in organic solvents used in solar cell fabrication. Functionalization of the nanoparticles using poly(ethylene glycol) methyl ether thiol (PEG-SH) allows for the phase transfer of the nanoparticles from aqueous solution to organic solvents. Once functionalized it was found that nanoprisms will undergo a shape change. This change in morphology was investigated using UV-Vis measurements, transmission electron microscopy (TEM), and X-ray Absorption Near Edge Structure (XANES) measurements and a mechanism for the shape degradation is presented. The PEG functionalization procedure can be applied to other types of metal nanoparticles and once soluble, these particles were incorporated into the active layer of the BHJ cells. It has been found that the PEG functionalized particles do not improve the cell efficiency, but they do affect the cell performance. The addition of the particles does not influence the open circuit voltage, but it does affect the current density of the devices. This suggests that the particles may be acting as electron traps, not allowing current to flow efficiently through the device. This shows that while the PEG-ylation of the particles is effective at solubilising them into useful organic solvents, the thickness of the PEG layer on the nanoparticles may not provide protection from electrons and allow for effective charge transfer throughout the solar cell.

Polymer photovoltaic systems whose photoactive layer is a blend of a semiconducting polymer with a fullerene derivative in a bulk heterojunction configuration are amongst the most successful organic photovoltaic devices nowadays. The three-dimensional organization in these layers (the morphology) plays a crucial role in the performance of the devices. Detailed characterization of this organization at the nanoscale would provide valuable information for improving future material and architectural design and for device optimization. In this thesis, the results of morphology studies of blends of several polyfluorene copolymers (APFOs) blended with a fullerene derivative are presented. Near-Edge X-ray Absorption Fine Structure (NEXAFS) spectroscopy was combined with dynamic Secondary Ion Mass Spectrometry (dSIMS) for surface and in-depth characterization of the blend films. NEXAFS was performed using two different electron detection methods, partial (PEY) and total (TEY) electron yield, which provide information from different depth regimes. Quantitative compositional information was obtained by fitting the spectra of the blend films with a linear combination of the spectra of films of the pure components. In blends of APFO3 with PCBM in two different blend ratios (1:1 and 1:4 of polymer:fullerene) NEXAFS data show the existence of compositional gradients in the vertical direction for both blend ratios, with clear polymer enrichment of the free surface. A series of APFOs with systematic changes in the side-chains was studied and it was shown that those small modifications can affect polymer:fullerene interaction and induce vertical phase separation. Polymer-enrichment of the free surface was clearly identified, in accordance with surface energy minimization mechanisms, and a compositional gradient was revealed already in the first few nanometers of the surface of the blend films. dSIMS showed that this vertical phase separation propagates throughout the film. It was possible to determine that as the polar character of the polymer increases, and thus the polymer:fullerene miscibility is improved, the tendency for vertical phase separation becomes stronger. / <p>Paper II was not published at the time of the licentiate defence and had the title: NEXAFS spectroscopy study of the surface composition in APFO3:PCBM blend films</p>

Significant improvements in power conversion efficiency (>10%) of emerging thin-film photovoltaics have been achieved in the last 5 years. High efficiencies would not be possible without the development of new selective interfacial layers. However, a complete understanding of how interfacial layers function to improve the selectivity of charge extracting contacts in thin-film photovoltaics is still being sought. The goal of this work is to contribute to the understanding of the operation of selective interfacial layers based on the study of ionically functionalized fullerenes. Just as other ionically functionalized materials have shown promise as electron-selective interfacial layers in organic photovoltaics and mixed organic-inorganic halide perovskites, Chapter II demonstrates the utility of ionically functionalized fullerenes. High performing solar cells necessitate the use of conductive interfacial layers; anomalously high conductivity in ionically functionalized materials, which have been used as interfacial layers, has been ascribed to self-doping. This work demonstrates that less than 1% of an ionically functionalized fullerene is reduced in its highly conductive pristine state and is concurrent with the presence of distinct chemical species. These studies describe how the chemical origin of the high conductivity of ionically functionalized fullerenes does not require the invocation of direct anion reduction or significant chemical transformations such as Hofmann-like elimination reactions occurring to a stoichiometric degree. This work also addresses the question of how the selectivity of a charge extracting contact is improved by the presence of an interfacial layer. The quantification of energy barrier reduction, which is often discussed in terms of work function modification or energy-level alignment, is demonstrated using metal|semiconductor junctions modified with an ionically functionalized fullerene. The barrier height of high work function electrodes was reduced significantly, by as much as 0.45 V, and was correlated to thin (2–5 nm) portions of the film rather than fullerene aggregates. The studies that comprise this work form a coherent model for understanding the key factors that have resulted in the continued use of ionically functionalized interfacial layers, their high conductivity, and energy barrier modification of the charge extracting electrodes. This dissertation contains coauthored, previously published, and unpublished work. / 10000-01-01

An in depth study of the performance of thermally interdiffused concentration gradient polymer photovoltaic devices is carried out with particular attention to the effect of the thickness and the thermal treatments on the power conversion efficiency, short circuit current, open circuit voltage and other key electrical properties. Bilayer films of sequentially spin-cast donor and acceptor materials are exposed to various heat treatments in order to induce the interdiffusion. The depth profiles show concentration gradients in the donor and acceptor as a result of interdiffusion and these devices show an order of magnitude increase in the device performance compared to the bilayer devices. Dual spin-cast poly (3-octylthiophene-2,5-diyl) (P3OT)- [6,6] phenyl C61 butyric acid methyl ester (PCBM) and poly (3-hexylthiophene-2,5-diyl) (P3HT)-PCBM interdiffused devices are studied in detail by varying the thickness of the donor and acceptor layers as well as the annealing conditions for initial polymer layer and the time and temperature of the interdiffusion process. Auger spectroscopy and X-ray photoelectron spectroscopy along with ion beam milling are used to investigate the concentration gradient formed as a result of the interdiffusion. The sulfur signal present in the P3OT and P3HT backbone is detected to identify the concentration profiles in the P3OT-PCBM and P3HT-PCBM devices. The interdiffusion conditions and thickness of the active layers have been optimized to obtain the highest power conversion efficiency. The best device performance of the P3OT-PCBM interdiffused devices is achieved when the interdiffusion is carried out at 150°C for 20 minutes and the P3OT thickness is maintained at 70 nm and the PCBM thickness at 40-50 nm. The highest efficiency achieved for P3OT-PCBM interdiffused devices is 1.0% under AM1.5G solar simulated spectrum. In order to further increase the efficiency, P3OT is replaced by (P3HT) which has higher hole mobility. P3HT- PCBM based concentration gradient devices show improved device performance over P3OT-PCBM devices. Power conversion efficiency of the order of ~3.0% is obtained for P3HT-PCBM interdiffused devices when the interdiffusion is carried out at 150°C for 20 minutes. For both P3OT:PCBM and P3HT:PCBM devices, the optimum performance occurs when the concentration gradient extends across the entire film and is correlated with an increase in the short circuit current density and fill factor as well as a decrease in the series resistance. The results demonstrate that an interdiffused bilayer fabrication approach is a novel and efficient approach for fabrication of polymer solar cell devices. In addition, porphyrin derivative 5, 10, 15, 20-Tetraphenyl-21H, 23H-porphine zinc (ZnTPP) is studied as a new donor material for organic solar cells. ZnTPP: PCBM blend devices are investigated in detail by varying the weight ratio of the donor and acceptor materials in blend devices. The devices with ZnTPP: PCBM in 1:9 ratios showed the best device performance and the efficiency of the order of 0.2% is achieved under AM1.5G solar simulated conditions. Trimetallic Nitride Tempelated (TNT) endohedral fullerenes are also examined in this thesis as the novel acceptor materials. Bulk heterojunction or blend devices are fabricated with P3HT as the donor material and several TNT endohedral fullerenes as the acceptor material. Y3N@C₈₀PCBH based devices which are annealed both before and after the electrode deposition show improvement in the device performance compared to devices that are only annealed before the electrode deposition. The highest power conversion efficiency achieved for TNT endohedral fullerene devices is only 0.06%, suggesting that substantial additional work must be done to optimize the compatibility of the donor and acceptor as well as the device fabrication parameters. / Ph. D.

Organic Photovoltaic (OPV) solar cells provide a promising alternative for harnessing solar energy. However, the efficient design of OPV materials that achieve better performance requires support by better-tailored visualization tools than are currently available, which is the goal of this thesis. One promising approach in the OPV field is to control the effective material of the OPV device, which is known as the Bulk-Heterojunction (BHJ) morphology. The BHJ morphology has a complex composition. Current BHJ exploration techniques deal with the morphologies as black boxes with no perception of the photoelectric current in the BHJ morphology. Therefore, this method depends on a trial-and-error approach and does not efficiently characterize complex BHJ morphologies. On the other hand, current state-of-the-art methods for assessing the performance of BHJ morphologies are based on the global quantification of morphological features. Accordingly, scientists in OPV research are still lacking a sufficient understanding of the best material design. To remove these limitations, we propose a new approach for knowledge-assisted visual exploration and analysis in the OPV domain. We develop new techniques for enabling efficient OPV charge transport path analysis. We employ, adapt, and develop techniques from scientific visualization, geometric modeling, clustering, and visual interaction to obtain new designs of visualization tools that are specifically tailored for the needs of OPV scientists. At the molecular scale, the user can use semantic rules to define clusters of atoms with certain geometric properties. At the nanoscale, we propose a novel framework for visual characterization and exploration of local structure-performance correlations. We also propose a new approach for correlating structural features to performance bottlenecks. We employ a visual feedback strategy that allows scientists to make intuitive choices about fabrication parameters. We furthermore propose a visual analysis framework to help answer domain science questions through parameter space exploration for local morphology features. This framework is built on the shape-based clustering of local regions (patches), which for the first time enables local analysis of BHJ morphologies. Using our proposed system, domain experts can interactively create and visualize new BHJ structures of interest at both the molecular and nanoscale levels in a relatively short time.

No description available.

Charge transfer efficiency at the organic/transparent conducting oxide (TCO) interface is one of the key parameters controlling the overall efficiency of organic photovoltaics (OPVs). Modification of this interface with a redox-active organic surface modifier may further enhance the charge transfer across the interface by providing a charge-transfer pathway between the electrode and the organic active layer. Functionalized perylene diimide molecules (PDI) are useful for modifying metal oxide/acceptor interfaces for inverted solar cell devices because their LUMO energy level is close to some commonly used acceptor molecules. The effects of PDI structural parameters on the interfacial charge transfer processes across the organic/ITO interface were investigated. Six different PDI monolayers with different structural parameters were deposited on ITO surfaces to investigate the relationship between molecular orientation, linker length, aggregation and charge transfer process. The PDI orientation, degree of PDI aggregation and charge transfer process acrosses PDI/ITO interfaces were characterized by polarized ATR spectroscopy, PM-ATR spectroscopy and photoelectrochemistry. Both linker length and orientation affected the tunneling distance between PDI and ITO, therefore affecting the charge transfer rate constant across the PDI/ITO interfaces. PDI aggregation forced a more out-of-plane orientation of PDI molecules and increased the overall measured charge transfer rate constant. However, PDI aggregation also increased the excited state recombination rate which ultimately led to decrease of the charge collection efficiency. The first application of a PM-TIRF platform to characterize the electron-transfer processes of PDI monomeric films across the organic/electrode interface is presented. The PM-TIRF technique provides higher sensitivity as well as the capability to measure very fast charge transfer events, compared to other commonly used potential-modulated spectroscopy techniques. PDI-phenyl-PA monomeric films exhibited a more in-plane orientation compared with aggregated films and showed a smaller charge transfer rate constant across the PDI/ITO interfaces compared with PDI films with higher degrees of aggregation after normalizing the tunneling distance contributions.