Louis A. Perez

High‐Mobility Field‐Effect Transistors Fabricated with Macroscopic Aligned Semiconducting Polymers

A record high OFET hole mobility, as high as 23.7 cm(2) /Vs, is achieved in macroscopic aligned semiconducting polymers. The high mobility is insensitive to the polymer molecular weight. Polymer chains are aligned along the fiber to facilitate intrachain charge transport.

Regioregular pyridal[2,1,3]thiadiazole π-conjugated copolymers

π-Conjugated, narrow band gap copolymers containing pyridal[2,1,3]thiadiazole (PT) were synthesized via starting materials that prevent random incorporation of the PT heterocycles relative to the backbone vector. Two regioregular structures could be obtained: in one the PTs are oriented in the same direction, and in the other the orientation of the PTs alternates every other repeat unit. Compared to their regiorandom counterparts, the regioregular polymers exhibit a 2 orders of magnitude increase of the hole mobilites, from 0.005 to 0.6 cm(2) V(-1) s(-1), as determined by field-effect transistor measurements.

A modular molecular framework for utility in small-molecule solution-processed organic photovoltaic devices

We report on the design, synthesis and characterization of light harvesting small molecules for use in solution-processed small molecule bulk heterojunction (SM-BHJ) solar cell devices. These molecular materials are based upon an acceptor/donor/acceptor (A/D/A) core with donor endcapping units. Utilization of a dithieno(3,2-b;2′,3′-d)silole (DTS) donor and pyridal[2,1,3]thiadiazole (PT) acceptor leads to strong charge transfer characteristics, resulting in broad optical absorption spectra extending well beyond 700 nm. SM-BHJ solar cell devices fabricated with the specific example 5,5′-bis{7-(4-(5-hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolo[3,4-c]pyridine}-3,3′-di-2-ethylhexylsilylene-2,2′-bithiophene (6) as the donor and [6,6]-phenyl-C71-butyric acid methyl ester (PC71BM) as the acceptor component showed short circuit currents above −10 mA cm−2 and power conversion efficiencies (PCEs) over 3%. Thermal processing is a critical factor in obtaining favorable active layer morphologies and high PCE values. A combination of UV-visible spectroscopy, conductive and photo-conductive atomic force microscopies, dynamic secondary mass ion spectrometry (DSIMS), and grazing incident wide angle X-ray scattering (GIWAXS) experiments were carried out to characterize how thermal treatment influences the active layer structure and organization.

Narrow-Band-Gap Conjugated Chromophores with Extended Molecular Lengths

The influence of extending the molecular length of donor-acceptor chromophores on properties relevant to organic optoelectronic devices has been studied by using two new narrow-band-gap systems. Most significantly, we find that the higher molecular weight systems exhibit higher thermal stabilities (beyond 200 °C) when introduced into field effect transistor devices. It is also possible to fabricate bulk heterojunction solar cells using PC(61)BM with power conversion efficiencies >6%. These high values are not heavily influenced by the blend composition and are achieved without the influence of solvent additives or postdeposition thermal annealing.

Solvent Additive Effects on Small Molecule Crystallization in Bulk Heterojunction Solar Cells Probed During Spin Casting

Solvent additive processing can lead to drastic improvements in the power conversion efficiency (PCE) in solution processable small molecule (SPSM) bulk heterojunction solar cells. In situ grazing incidence wide-angle X-ray scattering is used to investigate the kinetics of crystallite formation during and shortly after spin casting. The additive is shown to have a complex effect on structural evolution invoking polymorphism and enhanced crystalline quality of the donor SPSM.

Molecular orientation within thin films of isomorphic molecular semiconductors

Four isomorphic organic semiconductors are compared to map out how the precision of chemical structures determines solid state molecular organization. Intramolecular electronic structure and intermolecular packing preference in the solid state were shown to be exclusively dependent on the relative location of the electron accepting fragments within a given molecule.

CHAPTER 8:New Science and New Technology in Semiconducting Polymers

New Science: The field of bulk heterojunction (BHJ) solar cells was created as a result of the discovery of ultrafast charge transfer. The length scale for the wavefunction describing the probability amplitude for finding a photoexcitation at a particular point in space was estimated using position–momentum uncertainty as expressed by the uncertainty principle. The problem can also be considered semi-classically and a very similar estimate of the length scale of the photoexcitation wavefunction is obtained via the resolution limit of a microscope; (λ/2πn) where n is the index of refraction. Finally, the BHJ solar cell should be especially sensitive to the effective interaction volume of the photon because it is comprised of a densely packed collection of strong absorbers. The photoexcitation process, therefore, generates a delocalized coherent superposition of the eigenfunctions of the Schrodinger equation that describes the nanostructured blend, with an immediate probability amplitude for finding a photoexcitation near a BHJ boundary enabling charge transfer in the femtosecond regime over relatively long distances. New Technology: A general strategy is presented to self-assemble unidirectional alignment and efficient charge transport for semiconducting polymer films deposited on textured Si/SiO2 substrates. By employing sandwich casting in a tilted tunnel system, we utilize capillary action, generated by functionalized spacers, to self-assemble semiconducting polymers along uniaxial nano-grooves on the substrate. The strength of capillary action can be tailored by different surface treatments of the glass spacers. PTS functionalization yields highly oriented crystalline films with compact structure with µh = 25.4 cm2 V−1 s−1 and µh = 22.2 cm2 V−1 s−1 for PCDTPT and CDTBTZ, respectively. These values are limited by the S–D contact resistance, Rc. Using longer channels, Rc is significantly less than the channel resistance and µ = 36.3 cm2 V−1 s−1 was measured. Extrapolating to infinite channel length, the intrinsic mobility for PCDTPT is obtained at this degree of chain alignment and structural order: µ = 47 cm2 V−1 s−1. The mobility is strongly anisotropic with 13.6- and 17.6-fold higher values parallel to the direction of polymer alignment.