Joshua A. Davies

Increased open circuit voltage in fluorinated benzothiadiazole-based alternating conjugated polymers

Small band-gap conjugated polymers based on monofluoro- and difluoro-substituted benzothiadiazole were developed. Highly efficient polymer solar cells (PCE as high as 5.40%) could be achieved for devices made from these polymers.

Nonlinear Polymer-Clad Silicon Slot Waveguide Modulator with a Half Wave Voltage of 0.25 V

We report on an electro-optic modulator fabricated from a silicon slot waveguide and clad in a nonlinear polymer. In this geometry, the electrodes form parts of the waveguide, and the modulator driving voltage drops across a 120nm slot. As a result, a half wave voltage of 0.25V is achieved near 1550nm. This is one of the lowest values for any modulator obtained to date. As the nonlinear polymers are extremely resistive, our device also has the advantage of drawing almost no current, suggesting this type of modulator could operate at exceedingly low power.

Rational Enhancement of Second-Order Nonlinearity: Bis-(4-methoxyphenyl)hetero-aryl-amino Donor-Based Chromophores: Design, Synthesis, and Electrooptic Activity

Two new highly hyperpolarizable chromophores, based on N,N- bis-(4-methoxyphenyl) aryl-amino donors and phenyl-trifluoromethyl-tricyanofuran (CF3-Ph-TCF) acceptor linked together via pi-conjugation through 2,5-divinylenethienyl moieties as the bridge, have been designed and synthesized successfully for the first time. The aryl moieties on the donor side of the chromophore molecules were varied as to be thiophene and 1-n-hexylpyrrole. The linear and nonlinear optical (NLO) properties of all compounds were evaluated in addition to recording relevant thermal and electrochemical data. The properties of the two new molecules were comparatively studied. These results are critically analyzed along with two other compounds, reported earlier from our laboratories and our collaborators, that contain (i) aliphatic chain-bearing aniline and (ii) dianisylaniline as donors, keeping the bridge (2,5-divinylenethienyl-), and the acceptor (CF3-Ph-TCF), constant. Trends in theoretically (density functional theory, DFT) predicted, zero-frequency gas-phase hyperpolarizability [beta(0;0,0)] values are shown to be consistent with the trends in beta HRS(-2omega;omega,omega), as measured by Hyper-Rayleigh Scattering (HRS), when corrected to zero-frequency using the two-level model (TLM) approximation. Similarly, trends in poling efficiency data (r33/E(p)) and wavelength dispersion measured by reflection ellipsometry (using a Teng-Man apparatus) and attenuated total reflection (ATR) are found to fit the TLM and DFT predictions. A 3-fold enhancement in bulk nonlinearity (r33) is realized as the donor subunits are changed from alkylaniline to dianisylaminopyrrole donors. The results of these studies provide insight into the complicated effects on molecular hyperpolarizability of substituting heteroaromatic subunits into the donor group structures. These studies also demonstrate that, when frequency dependence and electric-field-induced ordering behavior are correctly accounted for, ab initio DFT generated beta(0;0,0) is effective as a predictor of changes in r33 behavior based on chromophore structure modification. Thus DFT can provide valuable insight into the electronic structure origin of complex optical phenomena in organic media.

n-Doping of thermally polymerizable fullerenes as an electron transporting layer for inverted polymer solar cells

A novel [6,6]-phenyl-C61-butyric acid methyl styryl ester (PCBM-S) was synthesized and employed as an electron transporting interfacial layer for bulk heterojunction polymer solar cells with an inverted device configuration. After the deposition of PCBM-S film from solution, the styryl groups of PCBM-S were polymerized by post-thermal treatment to form a robust film which is resistive to common organic solvents. This allows the solution processing of upper bulk heterojunction film without eroding the PCBM-S layer. Additionally, the PCBM-S was n-doped with decamethylcobaltocene (DMC) to increase the conductivity of the film, which resulted in a significantly improved power conversion efficiency from 1.24% to 2.33%. The improved device performance is due to the decrease of series resistance and improved electron extraction property of the n-doped PCBM-S film.

Push–pull tetraene chromophores derived from dialkylaminophenyl, tetrahydroquinolinyl and julolidinyl moieties: optimization of second-order optical nonlinearity by fine-tuning the strength of electron-donating groups

A series of highly polarizable chromophores 1–3 has been synthesized based on three different types of electron donors, including diethylaminophenyl, tetrahydroquinolinyl and julolidinyl groups respectively, with the same isophorone-derived tetraene bridges and strong CF3–TCF acceptors. The progressively increased electron-donating strength for these chromophores allows for the fine-tuning of their molecular ground-state polarization, being very close to optimal for very large hyperpolarizability (β), as a function of the local dielectric environment and poling-induced acentric ordering. The solvatochromic study and DFT calculations suggested that more dipolar chromophores 2 and 3 with stronger donating groups can be polarized quite close to the cyanine limit, or even beyond that into the zwitterionic regime in the most polar solvents to give the inverted solvatochromism and diminished β values. This is in stark contrast to the polyene-like characteristic of 1 with the diethylaminophenyl donor in all the tested solvents. Most intriguingly, the electric field poling has induced significant changes in the position, intensity and shape of the chromophoric charge-transfer absorption band in their poled thin films. It is thus indicated that the nontrivial intermolecular interaction in solid state can vary the reaction field that acts on the chromophores in poled and depoled films, and significantly affect their achievable electro-optic (EO) activities. As a result, the guest–host polymers containing 10 wt% of chromophores in PMMA showed EO coefficients of ∼80 pm V−1 for 1/PMMA and 2/PMMA, while dramatically dropping to 20 pm V−1 for 3/PMMA. These systematic analyses led to the rational design of a new guest–host EO polymer incorporating 35 wt% of a spatially modified chromophore AJLZ55, which gave ultrahigh EO coefficients of 218 pm V−1 at 1.31 μm.

Facile structure and property tuning through alteration of ring structures in conformationally locked phenyltetraene nonlinear optical chromophores

A series of phenyltetraene-based nonlinear optical (NLO) chromophores 1a–c with the same donor and acceptor groups, but different tetraene bridges that are partly connected by various sizes of aliphatic rings, have been synthesized and systematically investigated. The interposed conjugated tetraene segments in three chromophores studied are based on isophorone, (1S)-(−)-verbenone, and 3,4,4-trimethyl-2-cyclopentenone, respectively. This kind of structural alteration has significant effect on the intrinsic electronic structures and physical properties of these highly polarizable chromophores as revealed by a variety of characterization techniques. The introduction of the verbenone- and trimethylcyclopentenone-based tetraene bridges could significantly improve the glass-forming ability of chromophores 1b and 1c in comparison with the highly crystalline characteristics of isophorone-based chromophores 1a. More importantly, chromophores 1a–c exhibited distinct optical features in absorption band shape, solvatochromic behavior, as well as energy band gap from the UV-vis-NIR absorption measurements. Quantum mechanical calculations using density functional theory (DFT) were also used to evaluate second-order NLO properties of these chromophores. The electro-optic (EO) coefficients of 1a–c in poled polymers with the 10 wt% chromophore content showed an apparent decrease from 78 pm V−1 for 1a to 42 pm V−1 for 1c. This decrease is attributed to the gradual decrease of the molecular hyperpolarizability (β) of the chromophores which is associated with the progressive cyanine-like electronic structure from the isophorone-based 1a to the cyclopentenone-based 1cchromophore.

Direct electron beam writing of electro-optic polymer microring resonators

Electro-optic polymer waveguides in electron beam sensitive polymethyl methacrylate (PMMA) polymer matrix doped with organic nonlinear chromophores could be directly patterned by electron beam exposure with high resolution and smoothness. The polymer in the exposed regions was removed with standard electron beam resist developer and without damaging the chromophore containing polymer waveguides. Feature sizes on the order of 100 nm could be clearly resolved. High quality microring resonators made of YL124/PMMA electro-optic polymer were successfully fabricated with this technique. The measured resonance extinction ratios were more than 16 dB and quality factors were in the range of 10(3) approximately 10(4).

Measuring Order in Contact-Poled Organic Electrooptic Materials with Variable-Angle Polarization-Referenced Absorption Spectroscopy (VAPRAS)

Organic nonlinear electrooptical (ONLO) chromophores must be acentrically ordered for the ONLO material to have electrooptic (EO) activity. The magnitude of the order is characterized by the acentric order parameter, , where β is the major Euler angle between the main axis of the chromophore and the poling field which imposes the acentric order. The acentric order parameter, which is difficult to measure directly, is related to the centrosymmetric order parameter, defined as = ½(3-1), through the underlying statistical distribution. We have developed a method to determine centrosymmetric order of the ONLO chromophores when the order is low (i.e., < 0.1). We have extended the method (begun by Graf et al. J. Appl. Phys. 1994, 75, 3335.) based on the absorption of light to determine the centrosymmetric order parameter induced by a poling field on a thin film sample of ONLO material. We find that the order parameters, analyzed by two different methods, are similar and also consistent with theoretical estimates from modeling of the system using coarse-grained Monte Carlo statistical mechanical methods.

Microring resonators fabricated by electron beam bleaching of chromophore doped polymers

Decomposition of chromophore molecules under direct electron beam irradiation reduces the refractive index of chromophore containing polymers. The induced refractive index contrast between the exposed and unexposed regions is high enough for waveguide bends of small radius and thus microring resonator devices. This electron beam bleaching of chromophore-containing polymers provides a fabrication approach for nonlinear polymer optical waveguide devices. Fabrication of high quality microring resonators with critical feature size on the order of 100nm was demonstrated with this technique in an electro-optic polymer that contains YL124 chromophores.

Polarization selective electro-optic polymer waveguide devices by direct electron beam writing

A novel technique for the fabrication of polarization selective electro-optic polymer waveguide devices with direct electron beam writing was described. Birefringence induced by the electric field poling in the electro-optic polymer film was erased in the electron beam exposed regions. The formed waveguides had stronger confinement for the light polarized along the poling direction. High fabrication resolution on the 100 nm scale or smaller could be achieved. Fabrication of polymer polarizer and polarization selective microring resonators with this technique was reported. The highest polarization extinction ratio was measured to be 21.4 dB.