Optimization of bulkheterojunction organic photovoltaics: Structure/property study with oxadiazole contained poly(p-phenylene)s [OXA-PPVs] and device system engineering

This thesis is focused on investigating organic semiconducting materials and photophysical phenomenon to build high efficient polymer light emitting diodes (PLEDs) and organic photovoltaic cells (OPVs) through material engineering and process engineering.;We have synthesized and characterized three electroactive polymers of oxadiazole containing poly(p-phenylenevinylene)s [OXA-PPV3-1s] with different solubilizing alkoxy side chains. They have hybrid electronic characteristics of hole transporting and electron transporting properties in a molecule. By utilizing their novel properties we expect high PLED and OPV device performance. First, we seek to resolve the structure-property relationships by looking at the effects of side groups through photophysical studies such as UV/Vis spectroscopy, photoluminescence spectroscopy (PL) and also by morphological characterization with atomic force microscopy (AFM).;High quantum efficiencies have been observed from solution OXA-PPVs. To investigate the electric field induced photogeneration characteristic in OXA-PPVs, we fabricated single layer PLEDs using three OXA-PPV3-1s as the active material. Balanced charge injection will be discussed based on device performances.;At the same time, to investigate the photoactivated charge separation phenomenon in OXA-PPV3-1s, we prepared several bulkheterojunction OPVs. The active layer was formed from a solution mixture of OXA-PPV3-1s as an electron acceptor and well known semicrystalline poly(3-hexylthiophene) (P3HT) as an electron donor. As a pair, the well matched HOMO and LUMO levels, as well as significant oxidative stability in OXA-PPV3-1s and high carrier mobility in P3HT motivated our OPV study.;When OXA-PPV3-1 is blended with P3HT and formed into films, significant photoluminescence quenching (PL quenching) is observed from the films using a wavelength that corresponds to the absorption maximum of OXA-PPV3-1s. Such PL quenching is evidence for the pre-requisite of photoactivated charge generation essential for efficient OPV devices.;However inefficient PL quenching was observed from the films when the absorption maximum of P3HT was used as an excitation source. This can be attributed to improperly aligned lowest unoccupied molecular orbital (LUMO) levels between the two components.;Another bulkheterojunction system with strong electron acceptor PCBM was investigated, but the device performance was not efficient in the system of P3HT/OXA-PPV3-1s even though they have well balanced highest occupied molecular orbital (HOMO) and LUMO energy levels. These results motivate us to engineer the energy levels of OXA-PPV3-1s to utilize them as electron acceptors instead of as electron donors, thus taking advantage of their intrinsically strong electron transporting property.;The design and synthesis of cyano group incorporated OXA-PPV3-1s (CNOXA-PPV3-1s) will be discussed for the purpose of reducing the LUMO level to build an energetically well balanced system between CNOXA-PPV3-1 as an electron donor and P3HT an electron acceptor. Additionally the design and synthesize of cyano group incorporated OXA-PPV5-1s will be discussed. These polymers were designed to increase the solubility of the polymers by increasing the number of solubilizing units in the polymer backbone. Though the material engineering was successful with respect to energy level engineering, the OPV device performance is still poor. Although charge carriers were generated with high yield, both electron and hole carrier transport was slow. To overcome this limited slow carrier mobility, another material was engineered to achieve a new p-type X-shape thiophene based material (XT1), which was designed for improving carrier mobility through two dimensional self-assembled p-p stacking. The synthesis and characterization of this novel material will be discussed too.;Along with the study of material engineering through energy level engineering and a molecular geometry engineering to improve OPV performance, another study of "energy level gradient bulkheterojunction (EGB) OPV" will be discussed as an approach to system engineering. The development and optimization of EGB-OPV is envisaged through a spray process.