Title: Integrating Contorted Aromatic Molecules into Molecular Electronics and Optoelectronic Devices
Description: This thesis has focused on the optical and electronic properties of organic semiconductors and their application in molecular electronic and optoelectronic devices. The studies have featured new and useful properties from a series of perylene diimide (PDI) nanoribbons and conjugated macrocycles. These novel strained carbon-based materials are highly promising as n-type semiconductors in organic gas sensor, organic solar cells and organic photodetectors. In Chapter 2, I describe a new molecular design that enables high performance organic photodetectors. We use a rigid, conjugated macrocycle as the electron acceptor in devices to obtain high photocurrent and low dark current. We make a direct comparison between the devices made with the macrocyclic acceptor and an acyclic control molecule; we find that the superior performance of the macrocycle originates from its rigid, conjugated, and cyclic structure. The macrocycle’s rigid structure reduces the number of charged defects originating from deformed sp2 carbons and covalent defects from photo/thermo-activation.

With this molecular design we are able to suppress dark current density while retaining high responsivity in an ultra-sensitive non-fullerene OPD. Importantly, we achieve a detectivity of ~1014 Jones at near zero bias voltage. This is without the need for extra carrier blocking layers commonly employed in fullerene-based devices. Our devices are comparable to the best fullerene-based photodetectors, and the sensitivity at low working voltages (< 0.1 V) is a record for non-fullerene OPDs. In Chapter 3, I describe a capsule-shaped molecule that assembles itself into a cellular semiconducting material. The interior space of the capsule with a volume of ~415 Å3 is a nanoenvironment that can accommodate a guest. To self-assemble these capsules into electronic materials, we functionalize the thiophene rings with bromines, which encode self-assembly into two-dimensional layers held together through halogen bonding interactions. In the solid state and in films, these two-dimensional layers assemble into the three-dimensional crystalline structure.

This hollow material is able to form the active layer in field effect transistor devices. We find that the current of these devices has strong response to the guest’s interaction within the hollow spaces in the film. These devices are remarkable in their ability to distinguish, through their electrical response, between small differences in the guest. In Chapter 4, I describe a new molecular design for the efficient synthesis of donor-acceptor, cove-edge graphene nanoribbons and their properties in solar cells. These nanoribbons are long (~5 nm), atomically precise, and soluble. The design is based on the fusion of electron deficient perylene diimide oligomers with an electron rich alkoxy pyrene subunit. This strategy of alternating electron rich and electron poor units facilitates a visible light fusion reaction in >95% yield, while the cove-edge nature of these nanoribbons results in a high degree of twisting along the long axis. The rigidity of the backbone yields a sharp longest wavelength absorption edge.

These nanoribbons are exceptional electron acceptors, and organic photovoltaics fabricated with the ribbons show efficiencies of ~8% without optimization. In Chapter 5, I describe a new molecular design that yields ultra-narrowband organic photodetectors. The design is based on a series of helically-twisted molecular ribbons as the optoelectronic material. We fabricate charge collection narrowing photodetectors based on four different helical ribbons that differ in the wavelength of their response. The photodetectors made from these materials have narrow spectral response with full-width at half maxima of < 20 nm. The devices reported here are superior by approximately a factor of 5 to those from traditional organic materials due to the narrowness of their response. Moreover, the active layers for the helical ribbon-based photodetectors are solution cast but have performance that is comparable to the state-of-the-art narrowband photodetectors made from methylammonium lead trihalide perovskite single crystals.

The ultra-narrow bandwidth for detection results from the helical ribbons’ high absorption coefficient, good electron mobility, and sharp absorption edges that are defined by the twisted molecular conformation. In Chapter 6, I describe the direct connection between the molecular conformation of a conjugated macrocycle and its macroscopic charge transport properties. The macrocycles studied here are new examples of a growing class of electronically active, conjugated macrocycles that have been utilized in materials applications. Here, we incorporate chiral, helical perylene diimide ribbons into the two separate macrocycles as the n-type, electron transporting material. As the macrocycles’ films and electronic structures are analogous, the important finding is that the macrocycles’ molecular structures and their associated dynamics determine device performance in organic field effect transistors. We show the more flexible macrocycle has a four-fold increase in electron mobility in field effect transistor devices.

Using a combination of 1H-NMR, spectroscopy, and density functional theory calculations, we find that the origin of the difference in device performance is the ability of more flexible isomer to make intermolecular contacts relative to the more rigid counterpart. In Chapter 7, I discuss that intramolecular conductivity can play a role in controlling device characteristics of organic field effect transistors made with macrocycle building blocks. We use two isomeric macrocyclic semiconductors that consist of perylene diimides linked with bithiophenes and find that the trans-linked macrocycle has a higher mobility than the cis-based device. Through a combination of single molecule junction conductance measurements of the components of the macrocycles, control experiments with acyclic counterparts to the macrocycles, and analyses of each of the materials using spectroscopy, electrochemistry, and density functional theory, we attribute the difference in electron mobility of the OFETs created with the two isomers to the difference in intramolecular conductivity of the two macrocycles.