Fabrication of Organic Semiconductor Nanomaterials and Their Optoelectronic Applications

Abstract

One-dimensional (1-D) organic nanomaterials synthesized from organic semiconductors have recently received considerable attention due to their potential applications in cost-effective, light weight, flexible, high-performance electronics. Structural perfection of single-crystalline organic 1-D nanowires (NW) enables efficient charge transport and results in enhanced electronic and optoelectronic properties compared to thin film counterparts. Furthermore, in-depth study on the fundamental charge transport can be effectively performed in 1-D NWs. Due to their novel optical, chemical, and electrical properties, they are regarded as a promising class of nanomaterials for use in diverse applications. In addition to the single-component 1-D NWs, the concepts of multi-component NWs are of great importance for optoelectronic applications as well as for understanding their charge transport mechanisms. Due to their functional interplay between two different components, synergetic optoelectronic properties are exhibited from multi-component NW-devices. In chapter 1, typical properties and fabrication methods of 1-D organic semiconducting NWs are introduced with their optoelectronic applications. In chapters 2, 3, and 4, single component 1-D and multicomponent organic semiconductor nanomaterials are fully discussed in terms of property, fabrication, processing, device application, and investigation on the fundamental mechanisms governing the charge transport behaviors. In detail, in chapter 2, the photoelectronic characteristics of single-crystalline NW organic phototransistors (NW-OPTs) are studied using a high-performance n-channel organic semiconductor, N,N′-bis(2-phenylethyl)-perylene-3,4:9,10-tetracarboxylic diimide (BPE-PTCDI), as the photoactive layer. The optoelectronic performances of the NW-OPTs are analyzed by way of their current-voltage (I-V) characteristics on irradiation at different wavelengths, and comparison with corresponding thin-film organic phototransistors (OPTs). Significant enhancement in the charge-carrier mobility and external quantum efficiency (EQE) are observed upon light irradiation as compared with when performed in the dark. In addition, an approach is devised to analyze the charge-transport behaviors using charge accumulation/release rates from deep traps under on/off switching of external light sources. In chapter 3, the electrical charge transport through individual strands of single-crystalline dipentyl perylene tetracarboxylic diimide (PTCDI-C5) and dioctyl perylene tetracarboxylic diimide (PTCDI-C8) nanowires is investigated. Temperature-dependent mobility measurements (100-280 K) reveal distinct electrical transport characteristics in the two types of nanowire. Our results demonstrate the importance of attaining good molecular ordering and orientations within the molecular active layer with a high electronic purity for achieving superior electrical transport. In chapter 4, core/shell p-n heterojunction NWs are fabricated using BPE-PTCDI and reduced graphene oxide (rGO) in solution phase. BPE-PTCDI/rGO core/shell NWs exhibit significantly enhanced photocurrent and faster charge compensation rate under irradiation, compared with pure BPE-PTCDI NWs. The results obtained herein demonstrate great promise for use of carbon-based multicomponent core/shell nanomaterials in photodetectors, and the developed methodology provides insights into the quantitative analysis of the photogenerated charge-carrier dynamics of multicomponent semiconducting systems. Furthermore, fundamental study on the charge transport mechanism is of great importance ahead of fabrication of nanomaterials. In chapters 5, 6, and 7, structure-property relationship are investigated with small molecule and polymer semiconductors, providing the optimal molecular geometry for the most efficient charge transport in device application. In chapter 5, a series of o-xylene and indene fullerene derivatives with varying frontier molecular orbital energy levels are utilized for assessing the impact of the number of solubilizing groups on the electrical performance of fullerene-based organic field-effect transistors (OFETs). Our findings systematically demonstrate the relationship between the energy level and charge carrier polarity and provide insights into molecular design and processing strategies towards high-performance fullerene-based OFETs. In chapter 6, systematic side-chain engineering is performed for diketopyrrolopyrrole-selenophene vinylene selenophene (DPP-SVS) polymers to determine the optimal side-chain geometries for the most efficient charge transport, and the structure-property relationship is thoroughly investigated using a range of analyses. A series of DPP-SVS polymers, ranging from 25-DPP-SVS to 32-DPP-SVS, with branched alkyl groups containing linear spacer groups from C2 to C9 is synthesized, and the electrical performance of these polymers is significantly dependent on both the length of the spacer group and its odd-even characteristics. The results obtained herein provide new insight into the molecular design of high-performance polymer semiconductors. In chapter 7, for systematic investigation of the structure-property relationship, a series of diketopyrrolopyrrole-thiophene vinylene thiophene (DPP-TVT) polymers, ranging from 25-DPP-TVT to 32-DPP-TVT, with branched alkyl groups containing linear spacer groups from C2 to C9, is synthesized and fully investigated. The results obtained herein demonstrate the intriguing odd-even effects induced by the length of the side chain alkyl spacers for DPP-TVT polymers, and provide insight into the side chain engineering for the most efficient charge transport in DPP-based polymer semiconductors.