Thesis (Ph.D.)--University of Delaware, 2008. / Principal faculty advisor: Robert W. Birkmire, Dept. of Materials Science & Engineering. Includes bibliographical references.

Thesis (Ph. D.)--University of Hong Kong, 2009. / Includes bibliographical references. Also available in print.

Thesis (Ph.D.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2008. / Committee Chair: Cressler, John D.; Committee Member: Laskar, Joy; Committee Member: Lee, Chin-Hui; Committee Member: Morley, Thomas; Committee Member: Papapolymerou, John

Thesis (M. S.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2009. / Committee Member: John Cressler; Committee Member: John Papapolymerou; Committee Member: Joy Laskar; Committee Member: Thomas Morley; Committee Member: William Hunt.

This thesis presents the transport, material characterization, and device applications of photovoltaic polymers used in bulk heterojunction solar cells. These three areas were found to be well correlated. Materials properties affect charge transport behaviors. Charge transport behaviors affect organic photovoltaic (OPV) cell performances. Two typical PV polymers were selected for investigation. They were poly(3-hexylthiophene) (P3HT) and poly[N-9-hepta- decanyl-2,7- carbazole-alt-5,5-(4’,7’-di-2- thienyl-2’,1’,3’- benzothiadiazole)] (PCDTBT). Different charge transport measurement techniques were employed to study how charge carriers move in OPV materials including space-charge-limited current (SCLC) measurement, dark-injection space-charge-limited current (DI-SCLC) measurement, and admittance spectroscopy (AS). For hole transport measurement on P3HT, electron leakages were found in a presumed hole-only device structure resulting in ill-defined DI-SCLC and AS signals. After inserting a thin electron blocking and trapping (EBT) layer between the active layer and the Au cathode, the electron leakages can be significantly suppressed leading to well-defined transport measurement signals. Applying the EBT layer to the polymer:fullerene bulk heterojunction (BHJ) blends, the transport properties can also be studied. Charge transport measurements were carried out at different temperatures for Gaussian Disorder Model (GDM) analysis to extract energetic disorders σ and high-temperature limit mobilities μ_∞. For P3HT BHJ films, σ were found to be much smaller than PCDTBT BHJ films. Within the same polymer system, similar σ were extracted. σ can be correlated to the device parameters such as open-circuit voltage V_OC and fill factor FF. Large σ was found to limit both V_OC and FF. With the experience of transport measurement for PV materials gained, we focused on a common problem of batch-to-batch variations in device performance. Five batches of amorphous polymers PCDTBT were purchased from two vendors. From gel permeation chromatography, bimodal distributions of molecular weight were observed in all five batches of PCDTBT with different fraction of small molecular weight component. The corresponding charge carrier mobilities and device performances drop significantly with the small molecular weight component. From GDM, all five batches of polymers have similar σ. However, μ_∞ for each batch of PCDTBT appear to have significant differences. The differences originate from the variation of charge carrier hopping distances caused by different amounts of the small molecular weight component of PCDTBT. At last, ZnO prepared by low temperature annealing sol gel method was used as functional layers for OPV cells and charge transport measurements. Structural, elemental, energetic, optical, and electrical characterizations were performed to examine the ZnO. The results suggested that the ZnO should be suitable for organic device applications. The applications of the ZnO on inverted OPV cells and charge transport measurements were demonstrated.

Fast carrier cooling is important for high power graphene based devices. Strongly coupled optical phonons play a major role in the relaxation of photo-excited carriers in graphene. Heterostructures of graphene and hexagonal boron nitride (hBN) have shown exceptional mobility and high saturation current, which makes them ideal for applications, but the effect of the hBN substrate on carrier cooling mechanisms is not understood. We track the cooling of hot photo-excited carriers in graphene-hBN heterostructures using ultrafast pump-probe spectroscopy. We find that the carriers cool down four times faster in the case of graphene on hBN than on a silicon oxide substrate thus overcoming the hot phonon bottleneck that plagues cooling in graphene devices. (C) 2017 Author(s).

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Renewable forms of energy are becoming increasingly important as the world quickly depletes its current energy reserves, and rapidly increases the concentration of pollutants in our environment. Solar technology based on organic semiconductors provides a promising candidate to fulfill a portion of our future energy needs in an environmentally sustainable manner. Organic semiconductors are a collection of pi-conjugated small molecules and polymers which can be implemented in photovoltaic cells that are potentially quite low cost. Currently, however, their commercial applications are limited due to a relatively low efficiency in converting sunlight into usable power. The fundamental physics of such devices must be clarified if these materials are to compete with traditional inorganic solar cells. In this dissertation, two emerging experimental tools are implemented in investigations of the internal electric fields present within operating organic photovoltaic cells. The first set of investigations utilizes the vibrational Stark effect to quantify the electric fields which often form at the interfaces between two organic semiconducting materials. Such interfaces are at the heart of the photocurrent generation process in these devices and any electric fields formed crucially alter device performance. We quantitatively determine the interfacial field present in blends of poly(3-hexylthiophene) (P3HT) and phenyl-C61-butyric acid methyl ester (PCBM) and show that this field depends strongly on annealing conditions. Finally we discuss a correlation between this interfacial electric field, crystalinity and device performance. The second set of investigations take advantage of electric field induced second harmonic generation microscopy to examine the electric potential across active organic solar cells. We again investigate blends of PCBM and P3HT as well as poly(4,4-dioctyldithieno(3,2-b:2',3'-d)silole)-2,6-diyl-alt-(2,1,3-benzothiadiazole)-4,7-diyl) (PSBTBT) and PCBM. In the former we find that the potential drop across the device shifts dramatically over time under illumination, while in the latter we find a nearly linear drop which remains constant through device operation. We then extend our examinations of PSBTBT:PCBM with EFISH by quantifying the extent of space charge accumulation throughout such devices. / text

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