Sam-Shajing Sun

Organic photovoltaics : mechanisms, materials, and devices

Recently developed organic photovoltaics (OPVs) show distinct advantages over their inorganic counterparts due to their lighter weight, flexible shape, versatile materials synthesis and device fabr ...

Photovoltaic enhancement of organic solar cells by a bridged donor-acceptor block copolymer approach

The authors show that a photovoltaic device composed of a -donor-bridge–acceptor-bridge- type block copolymer thin film exhibits a significant performance improvement over its corresponding donor/acceptor blend (Voc increased from 0.14to1.10V and Jsc increased from 0.017 to 0.058mA∕cm2) under identical conditions, where donor is an alkyl derivatized poly-p-phenylenevinylene (PPV) conjugated block, acceptor is a sulfone-alkyl derivatized PPV conjugated block, and bridge is a nonconjugated and flexible unit. The authors attribute such improvement to the block copolymer intrinsic nanophase separation and molecular self-assembly that results in the reduction of the exciton and carrier losses.

Design of a block copolymer solar cell

Abstract A photovoltaic device based on conjugated block copolymers has been designed and preliminarily examined by the recent development of target block copolymers. The unique feature of the device includes a bridge–donor–bridge–acceptor-type of block copolymer primary structure, where donor and acceptor are conjugated polymer blocks and bridge is a non-conjugated and flexible chain, a π orbital stacked and conjugated chain self-assembled and ordered secondary structure, and a donor/acceptor asymmetric layers sandwiched donor/acceptor columnar tertiary structure. This device is expected to improve the photovoltaic power conversion efficiency significantly in comparison to most existing organic-based donor/acceptor binary photovoltaic devices due to the reduction of exciton loss, the carrier loss, as well as the photon loss via three-dimensional space and energy level optimizations.

Poly(3-dodedyl-2,5-thienylenevinylene)s from the Stille coupling and the Horner–Emmons reaction

Regioregular (RR) conjugated polymers are critical for electronic and optoelectronic properties of polymer based semiconducting devices. Monosubstituted polythienylvinylene (PTV), a relatively low band-gap conjugated polymer, has been reportedly synthesized using the Stille coupling reaction between 3-dodecyl-2,5-dibromothiophene and (E)-1,2-bis(tributylstannyl)ethylene. Although a small fraction of the product shows good (∼90%) regioregularity, the whole product (S-C12-PTV) consists of mostly unidentified structures. To gain better control of the polymer structure and regioregularity, a difunctionalized C12-thiophene monomer, 3-dodecyl-5-formyl-thiophen-2-ylmethyl)-phosphonic acid diethyl ester, has been polymerized in near quantitative yield via the Horner–Emmons reaction. The full (100%) regioregularity of the resulting polymer (HE-C12-PTV) has been confirmed by its 1H and 13C NMR spectra. The full regioregularity is also reflected in its strong tendency to crystallize and practically no solubility in boiling hexane, in sharp contrast to S-C12-PTV. UV-vis absorption spectroscopy, fluorescence spectroscopy, cyclovoltammetry, thermal analysis (DSC & TGA) and X-ray diffraction have been used to characterize both polymers. UV-vis absorption spectra of the HE-C12-PTV chloroform solutions of different concentrations show well-resolved vibronic structures with an absorption maximum at 577 nm and a prominent shoulder at 614 nm. The optical bandgaps are 1.80 eV in chloroform solution and 1.65 eV in film. The HOMO/LUMO energy levels of the HE-C12-PTV film were found to be at −4.98 eV and −2.88 eV, respectively. The electrochemical bandgap of the polymer in the film is estimated to be 2.1 eV. The DSC curve shows a pronounced melting peak at 205 °C. XRD study shows that a decent crystalline structure is formed without any annealing of the as-cast films.

Vertically tapered polymer waveguide mode size transformer for improved fiber coupling

We describe a novel vertical taper structure fabricated at the ends of polymer optical waveguide devices to improve the coupling be- tween channel waveguides and single mode fibers. The taper smoothly converts a highly elliptical waveguide mode into a large and more circu- lar mode for low loss coupling and relaxed fiber alignment tolerances. A vertical taper 0.5 to 2 mm in length is made in the upper cladding to reduce its thickness from a few micrometers to zero. The taper is sub- sequently covered by an upper cladding. The new upper cladding has an index higher than that of the previous upper cladding but slightly lower than that of waveguide core. In the taper, the channel waveguide mode gradually loses confinement by the upper cladding so that the mode size grows larger as light propagates toward the end of the device, whereas the confinement by the lower cladding and the lateral confinement are not significantly affected. The waveguide mode grows upward away from the lossy ground electrode and substrate commonly found in many poly- mer devices; therefore, no trade-off between mode size and propagation loss is involved. Two special but simple reactive ion etching techniques, shadow masked etching and etching with a tapered photoresist mask, are developed to make the vertical taper. Mode expansion and a 1.8 dB reduction in coupling loss per tapered end are demonstrated experimen- tally. The performance of the mode size transformer is found to be in- sensitive to both waveguide width and polarization.

Conjugated Block Copolymers for Opto-Electronic Functions

A novel block copolymer system containing a conjugated donor block (RO-PPV) and a conjugated acceptor block (SF-PPV) coupled by a non-conjugated bridge unit has been synthesized and characterized. 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 -DBAB- block copolymer films were quenched by over 99%. Preliminary thin film electron microscopy studies revealed certain regular morphological pattern, possibly due to block copolymer microphase separation.

Integrated polymer waveguide mode size transformer with a vertical taper for improved fiber coupling

We describe a novel vertical taper structure fabricated at the ends of polymer optical waveguide devices to improve the coupling between channel waveguides and single-m,ode fibers. The taper smoothly converts a highly elliptical waveguide mode into a bigger and more circular mode for low loss coupling and relaxed fiber alignment tolerances. A vertical taper 0.5-2 mm in length is made in the low index upper cladding to reduce its thickness from several micrometers to zero, followed by the coating of a second upper cladding with index higher than that of the previous upper cladding but slightly lower than that of waveguide core. In the taper, the channel waveguide mode gradually loses confinement by the upper cladding so that the mode size grows bigger a light propagates, whereas the confinement by the lower cladding and lateral confinement are hardly affected. The waveguide mode grows in the vertical direction away from the lossy ground electrode and substrate; therefore no compromise between mode size and propagation loss is involved. Two special but simple reactive ion etching techniques, shadow masked etching and tapered photoresist etching mask, are develop for making this vertical taper. Mode expansion and a 1.8 dB reduction in coupling los, which is not sensitive to waveguide width and polarization, is obtained in our preliminary experiment.

Synthesis and characterisation of 1,3-bis(dicyanomethylidene)indane (BDMI)-based nonlinear optical polymers

Abstract New polymers containing nonlinear optical chromophores of electron donor-bridge moieties amino-phenylenethienylidene (APT) coupled with a strong electron acceptor 1,3-bis(dicyanomethylidene)indane (BDMI) were synthesised by coupling the aldehydic APT precursor polymers with acceptor BDMI in acetic anhydride. The aldehydic APT precursor polymers were prepared via traditional condensation or radical polymerisation schemes where the aldehydic group of APT was found intact. Thermal stability of these BDMI-based polymers determined by t.g.a. was up to 500°C depending mainly on the chromophore loading density. The electro-optic coefficient ( r 33 ) of 10 pm V −1 was obtained in a 30% by weight chromophore loading polymer.

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.