The critical role of molecular nanomorphology on organic semiconductor devices

Abstract

Organic semiconductors have a huge potential for low cost, scalable, lightweight, flexible, and semi-transparent electronic devices; which can revolutionize the semiconductor device industry upon their technological maturity. Semiconducting properties and device performances are critically dependent upon their molecular attributes such as conformation, order, and vibronic coupling. In this respect, this thesis aims to identify the critical role of molecular scale morphology on device performances (including efficiency and stability), with the focus on molecular nanomorphology (includes molecular-level structures and processes). Molecular vibrational Raman spectroscopy, photoluminescence, and several other advanced structural/energetic probes are used extensively in conjunction with electronic device characterizations to deduce the morphology and performance relationships in organic devices such as photovoltaics (OPVs), photodetectors (OPDs) and chemiresistors (organic diodes) within the context of organic small molecules and conjugated polymers.

Firstly, the impact of a model polymer:fullerene nanomorphology on long term device performances (i.e. operational stability) is investigated demonstrating that fine-tuning the nanomorphology of the photoactive layer of OPV devices is vital not only for power conversion efficiency but also for operational stability. The study proposes that a thermal annealing preconditioning of a general polymer:fullerene layer below the molecular scale phase segregation temperature (T_ps) is essential to develop a stable bulk-heterojunction (BHJ) morphology for improved operational stability whilst maintaining optimum efficiency.

Secondly, molecular morphology and associated photophysics of highly intermixed all small-molecule BHJ blend system comprising of a novel dipolar donor and buckminsterfullerene yielding state-of-the-art OPD performances are investigated. It is found that such highly intermixed small molecule blends morphology is poorly optimized for OPV operation due to dominant geminate recombination and strong emissive charge-transfer states, however, it shows high photocurrent generation for OPD operation due to efficient field-driven dissociation of interfacial charge-transfer (CT) states. The molecular conformation of the dipolar donor critically determines the device energetic landscape and thus the device parameters such as dark current, CT state binding energy, and photoresponse time. The study highlights key differences in ideal BHJ morphology required for optimum OPV and OPD operations. While OPVs require a fine balance of mixed and pure phases via optimized donor/acceptor phase segregations, OPDs can be efficient even when entirely finely mixed.

Finally, the electronic properties of organic diodes comprising of a model π-conjugated polymer blended with several modified ionic liquids (MIL, solid-state in room temperature) is investigated revealing an application as chemiresistors; showing sensitive and selective detection of polar and non-polar chemical analytes; operating at low power (µW) and room temperature. The study proposes that molecular-level electrochemical doping of the π-conjugated polymer by MIL is responsible for the conductivity tuning of the diode while gas-specific interaction between the polymer and MIL results in specific transduction of the dielectric environment into specific electronic signals.

This thesis highlights the key role of molecular properties responsible for determining the operational mechanism of the associated organic electronic devices. As such, optimizations at the molecular scales are paramount which can include new molecular structural designs, more compatible organic blends, or conformational optimizations at molecular heterointerfaces.