Semi-random vs Well-Defined Alternating Donor–Acceptor Copolymers

• Published in: ACS macro letters 2014-07, Vol.3 (7), p.622-627
• Main Authors: Braunecker, Wade A, Oosterhout, Stefan D, Owczarczyk, Zbyslaw R, Kopidakis, Nikos, Ratcliff, Erin L, Ginley, David S, Olson, Dana C
• Format: Article
• Language: English
• ISSN: 2161-1653; 2161-1653
• DOI: 10.1021/mz5002977

The influence of backbone composition on the physical properties of donor–acceptor (D–A) copolymers composed of varying amounts of benzodithiophene (BDT) donor with the thienoisoindoledione (TID) acceptor is investigated. First, the synthesis of bis- and tris-BDT monomers is reported; these monomers are subsequently used in Stille copolymerizations to create well-defined alternating polymer structures with repeating (D–A), (D–D–A), and (D–D–D–A) units. For comparison, five semi-random D–A copolymers with a D:A ratio of 1.5, 2, 3, 4, and 7 were synthesized by reacting trimethyltin-functionalized BDT with various ratios of iodinated BDT and brominated TID. While the HOMO levels of all the resultant polymers are very similar, a systematic red shift in the absorbance spectra onset of the D–A copolymer films from 687 to 883 nm is observed with increasing acceptor content, suggesting the LUMO can be fine-tuned over a range of 0.4 eV. When the solid-state absorbance spectra of well-defined alternating copolymers are compared to those of semi-random copolymers with analogous D:A ratios, the spectra of the alternating copolymers are significantly more red-shifted. Organic photovoltaic device efficiencies show that the semi-random materials all outperform the well-defined alternating copolymers, and an optimal D:A ratio of 2 produces the highest efficiency. Additional considerations concerning fine-tuning the lifetimes of the photoconductance transients of copolymer:fullerene films measured by time-resolved microwave conductivity are discussed. Overall, the results of this work indicate that the semi-random approach is a powerful synthetic strategy for fine-tuning the optoelectronic and photophysical properties of D–A materials for a number of systematic studies, especially given the ease with which the D:A ratios in the semi-random copolymers can be tuned.