Whitney E. B. Shepherd

Aggregate formation and its effect on (opto)electronic properties of guest-host organic semiconductors

We quantify guest molecule aggregation and its effect on the photoconductive properties of guest-host thin films, depending on the guest concentration and host material. A high-performance anthradithiophene (ADT) derivative served as a guest, while functionalized benzothiophene (BTBTB) and polymethylmethacrylate (PMMA) were chosen as hosts. Aggregates exhibited redshifted optical absorption and photoluminescence (PL) spectra, as well as reduced PL quantum yields. Propensity toward guest aggregation differed for PMMA and BTBTB hosts. Photocurrents dramatically increased as the percentage of aggregated guest molecules increased due to considerably higher charge carrier mobility in the aggregates. At low guest concentrations, BTBTB films outperformed PMMA films.

Photophysical and photoconductive properties of organic semiconductor composites

We report on the effects of introducing guest molecules into a functionalized anthradithiophene (ADT) host on the photoluminescent (PL) and photoconductive properties of solution-deposited thin films. An addition of 0.1 wt % of an ADT derivative with cyano end groups (ADT-TIPS-CN) to a fluorinated ADT derivative (ADT-TES-F) resulted in the near complete quenching of the fluorescence spectrum of the ADT-TES-F host with an enhancement in the fluorescence spectrum of the ADT-TIPS-CN guest. A markedly longer PL lifetime was noted in films containing 10% ADT-TIPS-CN guest molecules compared to both pristine ADT-TES-F and ADT-TIPS-CN films. Stronger temperature dependence of the PL quantum yield was obtained in ADT-TIPSCN/ ADT-TES-F films at low ADT-TIPS-CN concentrations than in films of pristine material, with PL decreasing with increasing temperature. Significant changes in the photoexcited charge carrier dynamics were observed on nanosecond time-scales after 400 nm 100 fs pulsed photoexcitation upon adding ADT-TIPS-CN guest molecules to the ADT-TES-F host. In contrast, no considerable change in the photocurrent was detected under continuous wave 532 nm excitation for guest molecule concetrations up to 1% of ADT-TIPS-CN in ADT-TES-F.

Optical, photoluminescent, and photoconductive properties of functionalized anthradithiophene and benzothiophene derivatives

We present optical, photoluminescent (PL), and photoconductive properties of functionalized anthradithiophene (ADT) and benzothiophene (BTBTB) derivatives and their composites. Solution-deposited ADT films exhibit charge carrier mobilities of over 1.5 cm2/Vs, high PL quantum yields, and high photoconductivity at room temperature. We show molecular arrangement and intermolecular interactions significantly contribute to the (opto)electronic properties of thin films of these pi-stacked materials. In addition, these properties can be effectively manipulated through the addition of guest molecules to a host material. In particular, exciton and charge carrier dynamics can be varied using a competition between photoinduced charge and energy transfer in a guest-host system. To better understand these processes at a molecular level, we apply single-molecule fluorescence spectroscopy (SMFS) to probe the effects of intermolecular interactions on the molecular properties. Specifically, we demonstrate that ADT molecules exhibit high enough quantum yields and photostability to be imaged on a single-molecule level at room temperature. Moreover, we show that stability of single ADT molecules immobilized in a solid-state matrix are comparable to those of the best fluorophores utilized in SMFS.

Optical and electronic properties of functionalized pentacene and anthradithiophene derivatives

The optical, fluorescent, and photoconductive properties of solution-processable functionalized pentacene and anthradithiophene (ADT) derivatives are presented. Considerable fluorescence quantum yields of - 70-75% and ~ 40-50% were observed in several ADT derivatives in toluene solutions and in thin films, respectively. Using conventional wide-field fluorescence microscopy, ADT derivatives were successfully imaged in the polymethylmethacrylate (PMMA) matrix on a single molecule level, at 532 nm at room temperature. All films exhibited fast charge carrier photogeneration upon 100 fs 400 nm excitation and power-law decay dynamics of the transient photocurrent over many orders of magnitude in time. In solution-deposited ADT thin films, effective charge carrier mobilities calculated from the space-charge-limited currents reached ~ 0.1 cm2/Vs. In the same films, bulk photoconductive gains of up to 130 were observed at 532 nm continuous wave (cw) excitation with light intensity of 0.58 mW/cm2 at the applied electric field of 4 × 104 V/cm. Effects of metal-organic interfaces on the dark current and transient and cw photocurrent are also discussed.

Photophysics of organic semiconductors: from ensemble to the single-molecule level

We present photophysical properties of functionalized anthradithiophene (ADT) and pentacene (Pn) derivatives, as well as charge and energy transfer properties of donor-acceptor (D/A) pairs of these derivatives. All molecules studied were fluorescent and photostable enough to be imaged on the single-molecule level in a variety of polymeric and in a functionalized benzothiophene (BTBTB) crystalline host using room-temperature wide- field epifluorescence microscopy. Flexibility of functionalization of both guest (ADT, Pn) and host (BTBTB or polymer) molecules can be used for systematic studies of nanoscale morphology and photophysics of D/A organic semiconductor bulk heterojunctions, as well as in applications relying on FRET, using single-molecule fluorescence microscopy.

Effect of intermolecular interactions on charge and exciplex formation in high-performance organic semiconductors

We present optical, photoluminescent (PL), and photoconductive properties of functionalized anthradithiophene (ADT) derivatives and their composites. Solution-deposited ADT films exhibit charge carrier mobilities of over 1.5 cm2/(Vs), high PL quantum yields, and high photoconductivity. We show that molecular arrangement and intermolecular interactions significantly contribute to (opto)electronic properties of guest-host thin films of these pi-stacked materials. Specifically, the formation of aggregates plays an important role in establishing efficient conduction pathways in ADT derivatives dispersed in a host matrix. In addition, the extent and nature of aggregation and the resulting changes in PL and photoconductive behavior can be effectively manipulated through different film fabrications techniques. Furthermore, energy transfer and exciplex formation has been shown to occur between two different ADT derivatives, which also alters the photoconductive response of the systems. We explore the dependence of the photoconductive response of such guest-host systems on excitation wavelength.

Optical, Photoluminescent, and Photoconductive Properties of Novel High- Performance Organic Semiconductors

Organic semiconductors have been investigated as an alternative to inorganic semiconductors due to their low cost, ease of fabrication, and tunable properties (Forrest, 2004). Applications envisioned for organic semiconductors include xerography, thin-film transistors, light-emitting diodes, solar cells, organic lasers, and many others (Peumans et al., 2003; Murphy & Frechet, 2007; Samuel & Turnbull, 2007). Since most of these applications rely on the conductive and photoconductive properties of the materials, it is important to understand physical mechanisms of charge photogeneration, transport, trapping, and recombination. However, despite numerous theoretical and experimental studies of the optical and electronic properties of organic solids, these mechanisms are not well understood and are still the subject of debate in the literature (Sariciftci, 1997; Podzorov et al., 2004; Moses et al., 2006; Troisi & Orlandi, 2006; Coropceanu et al., 2007; Cheng & Silbey, 2008; Laarhoven et al., 2008). Indeed, it is a complicated task to reveal and utilize the intrinsic properties of organic materials, since they are often masked by the influence of impurities, the presence of which is sensitive to the methods of material purification and device fabrication. As a result, measurements performed in the same material using different experimental techniques often provide conflicting results (Nelson et al., 1998; Hegmann et al., 2002; Podzorov et al., 2003; Jurchescu et al., 2004; Lang et al., 2004; Thorsmolle et al., 2004; Ostroverkhova et al., 2005a; Ostroverkhova et al., 2006a; Koeberg et al., 2007; Laarhoven et al., 2008; Najafov et al., 2008; Marciniak et al., 2009). Experimental methods that probe charge carrier dynamics on picosecond (ps) time-scales after a 100femtosecond (fs) pulsed photoexcitation have had most success in revealing intrinsic properties of organic semiconductors (Hegmann et al., 2002; Thorsmolle et al., 2004; Hegmann et al., 2005; Ostroverkhova et al., 2005a; Ostroverkhova et al., 2005b). In contrast, techniques that probe equilibrium charge transport are much more sensitive to extrinsic effects (Nelson et al., 1998; Knipp et al., 2003; de Boer et al., 2004; Jurchescu et al., 2004). However, since properties under equilibrium conditions are relevant for most devices, it is necessary to understand how they are related to intrinsic properties and find ways to improve materials and device fabrication techniques in order to minimize extrinsic effects. Therefore, one of the emphases of this chapter is on photoexcited charge carrier dynamics from sub-ps (non-equilibrium) to many seconds (equilibrium) after photoexcitation in a variety of organic crystals and thin films (Sections 3.3-3.4).