James H. Haliburton

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.

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.

Conjugated Block Copolymers for Photovoltaics: Optimization of Photoelectric Power Conversion Efficiency in Both Space and Energy/Time Domains

The photoelectric power conversion efficiency of organic or polymeric solar cells can be improved via optimizations in both space and energy/time domains. Specifically, at spatial domain, a ‘tertiary’ block copolymer supra-molecular nano structure has been designed, and a series of –DBABtype of block copolymers, where D is a conjugated donor block, A is a conjugated acceptor block, and B is a non-conjugated and flexible bridge unit, have been synthesized, characterized, and preliminarily examined for target photovoltaic functions. For instance, in comparison to simple donor/acceptor (D/A) blend film, a corresponding DBABblock copolymer film exhibited much better photoluminescence (PL) quenching and photo conductivity. These are attributed mainly to spatial domain improvement for charge carrier generation and transportation. At materials energy levels and electron transfer dynamic regime, theoretical simulation shows the photo induced charge separation appears most efficient when the donor/acceptor frontier orbital energy offset approaches the sum of two major energy costs: the charge separation reorganization energy and the exciton binding energy. Other donor/acceptor frontier orbital energy offsets are also identified where the charge recombination becomes most severe, and where the charge separation rate constant over charge recombination rate constant become largest.

Optical limiting properties of derivatized carbothiacyanine dyes

The nonlinear optical absorption of a range of dialkyl- carbothiacyanine dyes has been measured using intensity dependent transmission measurements of the absorption cross- section at 532 nm. The ground and effective excited state absorption cross-section for each of the dyes has been determined. It was observed that with increasing bridge conjugation length between the carbothiacyanine end groups, the absorption cross section of ground state decreases slightly while the absorption cross-section of the excited state increases leading to an overall increase in the absorption cross-section ratio by an order of magnitude from 0.3 to 5.29. The absorption cross-section ratio tracks linearly with the difference between the energy of the pump laser and the energy of the ground state absorption. The counter ions of the dyes also affect the nonlinear absorption significantly. These organic materials exhibit good potential for optical limiting applications.