Shahin Maaref

Design and synthesis of novel block copolymers for efficient optoelectronic applications

It has been predicted that nano-phase separated block copolymer systems containing electron rich donor blocks and electron deficient acceptor blocks may facilitate the charge carrier separation and migration in organic photovoltaic devices due to improved morphology in comparison to polymer blend system. This paper presents preliminary data describing the design and synthesis of a novel Donor-Bridge-Acceptor (D-B-A) block copolymer system for potential high efficient organic opto-electronic applications. Specifically, the donor block contains an electron donating alkyloxy derivatized polyphenylenevinylene (PPV), the acceptor block contains an electron withdrawing alkyl-sulfone derivatized polyphenylenevinylene (PPV), and the bridge block contains an electronically neutral non-conjugated aliphatic hydrocarbon chain. The key synthetic strategy includes the synthesis of each individual block first, then couple the blocks together. While the donor block stabilizes and facilitates the transport of the holes, the acceptor block stabilizes and facilitates the transport of the electrons, the bridge block is designed to hinder the probability of electron-hole recombination. Thus, improved charge separation and stability are expected with this system. In addition, charge migration toward electrodes may also be facilitated due to the potential nano-phase separated and highly ordered block copolymer ultra-structure.

Recent development of crosslinked NLO polymers for large bandwidth electro-optical modulations

A future polymer based electro-optical (EO) modulator has several advantages over a current commercial Lithium Niobate EO modulator, a key device that encodes electronic signals into optical signals. These advantages include larger bandwidth (over 100 GHz), lower drive voltage (less then 1 volt), more cost effective, etc. However, one major challenge for EO polymer development is materials stability, including short term and long term SHG (NLO chromophore orientation) stability. Since the modulator fabrication and packaging processes typically require heating temperatures of 200-250oC for up to 10 minutes, yet the SHG thermal stability of most NLO polymers developed so far only reach about 150oC. In order to overcome this challenge, either modulator processing temperatures are reduced, or SHG thermal stability of EO polymers be increased. Polymer crosslinking technique is one of the most versatile and effective methods for fabricating and stabilizing polymer nano structures at high temperatures. Among various NLO polymer crosslinking schemes developed so far, fumaryl chloride (FC) and maleic anhydride (MA) derived crosslinked polyester system seems to be a versatile and convenient scheme. This scheme also looks attractive for low loss applications at 1550 nm. FC/MA crosslinking systems also offer visible light photolithographic fabrication advantage during waveguide fabrication.

Synthesis and characterization of a novel -D-B-A-B- block copolymer system for potential light harvesting applications

Supra-molecular and nano-structured electro-active polymers are potentially useful for developing variety inexpensive and flexible shaped opto-electronic devices. In the case of organic photovoltaic materials or devices, for instance, photo induced electrons and holes need to be separated and transported in organic acceptor (A) and donor (D) phases respectively. In this paper, preliminary results of synthesis and characterizations of a coupled block copolymers containing a conjugated donor block (RO-PPV), a conjugated acceptor block (SF-PPV), and some of their electronic/optical properties are presented. While the donor block film has a strong PL emission at around 570 nm, and acceptor block film has a strong PL emission at around 590 nm, the PL emissions of final -D-B-A-B- block copolymer films were quenched by over 99%. Experimental results demonstrated an effective photo induced electron transfer and charge separation due to the interfaces of donor and acceptor blocks. The system is very promising for variety light harvesting applications, including “plastic” photovoltaic devices.

A low energy gap and fully regioregular poly(3-Dodecyl-2,5-thienylenevinylene) for photovoltaics

We report the development of a fully regioregular Poly(3-Dodecyl-2,5-thienylenevinylene) (HT-HT PDDTV) using the Horner-Emmons reaction, and studies using proton and carbon NMR spectroscopy, UV-vis absorption spectroscopy, fluorescence spectroscopy, cyclovoltametry, thermal analysis (DSC & TGA) and XRD. The HT-HT PDDTV developed has practically no solubility in boiling hexane, in sharp contrast to the literature PDDTV prepared from the Stille coupling reaction, which is mostly soluble in hexanes (an indication of high content of structural defects). The optical energy gaps are 1.80 eV in chloroform solution and 1.65 eV in film. The HOMO/LUMO of the film were -5.03 eV and -3.63eV, respectively. The electrochemical energy gap in the film is 1.4 eV. XRD study shows that a decent crystalline structure was formed without any annealing of the as-cast films. The lamellar sheets (formed from π. π. stacking) preferentially are oriented in parallel to the substrate surface with an interlayer spacing of 17.6 angstrom.

Polymer molecular environment effects of Disperse Red Vis-NIR absorption behavior

Spectral absorption behavior of Disperse Red-1 and Disperse Red-19 dyes incorporated into a series of polymers by covalent attachment, representing various chemical structures, is characterized by photothermal deflection spectroscopy. Of particular interest are the spectral characteristics of the red edge of the main dye electronic absorption peak, and the fine structure in the near-IR, dominated by overtones of fundamental C-H and O-H stretching modes. The spectral structure in these key regions can be influenced by inter- and intramolecular interactions, or conformational or configurational changes in the dye. The NIR structure, in turn, will dictate absorption loss in optical devices prepared from these materials at key transmission wavelengths (1.3 and 1.55 um) for waveguide devices. A well characterized dye-polymer system, DR1-PMMA, is compared with two other polymer systems. Differences in spectral absorption behavior is assessed in terms of polymer host structure and bonding environment.

Photocrosslinking and SHG stability of PDRMA/PDRFC thin films

A series of functional and photo-crosslinkable maleate or fumarate type polyesters containing azo-type NLO chromophores were studied. The result demonstrated this is a versatile and convenient method of fabricating crosslinked supramolecular ultra-structure polymer thin films for potential nonlinear optical (NLO) and other photonic applications. The unsaturated PDRMA/PDRFC polyester thin films are capable of crosslinking in air via photo polymerization to form a hardened lattice. A ratio of 1:1 of crosslinker double bonds to polyester double bonds was preferred for optimal crosslinking. Thermal stability of Second Harmonic Generation (SHG) signals for a photo crosslinked polymer thin film reached to about 150°C. Films from photo crosslinking of fumarate or maleate vs. vinyl crosslinkers have the advantages of avoiding NH/OH groups and their vibrational overtones that absorb at telecommunication wavelength of 1550 nm.

Synthesis and characterization of maleic anhydride derived crosslinkable polymers for nonlinear optical applications

The synthesis and characterization of maleate type crosslinkable nonlinear optical (NLO) polymers derived form maleic anhydride and fumaryl chloride is described. Preliminary results demonstrated this is a convenient, inexpensive, and versatile method of fabricating crosslinked NLO polymer thin films. These maleate type polyesters containing NLO chromophores such as Disperse Red 19 are capable of crosslinking to form a hardened lattice under thermal or high-energy radiation conditions. Crosslinking is a critical nano-scale technique for second order nonlinear optical as well as other potential photonic applications where molecular orientations need to be aligned and be stabilized against molecular thermal motions. Photolithographic techniques may be readily employed in this system to fabricate patterned polymer waveguide.

Molecular morphological effects to optoelectronics

A comparison study between a block copolymer and blend samples (D/A; donor/acceptor) revealed that their optoelectronic properties change significantly in different morphologies due to different processing conditions. The study shows that the photovoltaic performance of a block copolymer is better than that of corresponding donor/acceptor simple blend devices due to smaller scale (5-10 nanometers) donor/acceptor phase separation in the block copolymer, and that thermal annealing generally improves OE property due to potential better molecular packing.

New polythiophene block copolymers for potential light harvesting applications

This paper presents preliminary synthetic and physical study of a new polythiophene block copolymer. These are conjugated donor (D) block poly(hexylenedithiathiophene) copolymerized with an acceptor (A) block of fluorinated ester derivatized polythiophene via an aliphatic bridge (B) unit. Experimental results show that when -DBAB- type of block copolymer forms, there is strong photoluminescence (PL) quenching in -DBAB- relative to D/A blend or the pristine D or A blocks. PL quenching is attributed to both intra and inter chain photo induced electron transfer or charge separation. Since block copolymer can be easily tailored, this system appears attractive for light harvesting applications including photovoltaic applications.