Lowest excited states and optical absorption spectra of donor―acceptor copolymers for organic photovoltaics: a new picture emerging from tuned long-range corrected density functionals

• Published in: Physical chemistry chemical physics : PCCP Vol. 14; no. 41; pp. 14243 - 14248
• Main Authors: PANDEY, Laxman, DOIRON, Curtis, SEARS, John S, BREDAS, Jean-Luc
• Format: Journal Article
• Language: English
• Published: Cambridge Royal Society of Chemistry 07.11.2012
• Subjects: Absorption spectra; Backbone; Chemistry; Density; Exact sciences and technology; Excitation; Fragments; Functionals; General and physical chemistry; Photovoltaic cells; Solar cells; Energy spectrum; Experimental data; Separation; Theoretical study; Polymer; Potential; Fragment; Absorption spectrum; Optical system; Energy gap; Density functional; Optical properties; Solar energy; Density functional method; Copolymer; Excitation; Charge transfer; Optical absorption; Excited state
• ISSN: 1463-9076; 1463-9084
• DOI: 10.1039/c2cp41724c

Polymers with low optical gaps are of importance to the organic photovoltaics community due to their potential for harnessing a large portion of the solar energy spectrum. The combination along their backbones of electron-rich and electron-deficient fragments contributes to the presence of low-lying excited states that are expected to display significant charge-transfer character. While conventional hybrid functionals are known to provide unsatisfactory results for charge-transfer excitations at the time-dependent DFT level, long-range corrected (LRC) functionals have been reported to give improved descriptions in a number of systems. Here, we use such LRC functionals, considering both tuned and default range-separation parameters, to characterize the absorption spectra of low-optical-gap systems of interest. Our results indicate that tuned LRC functionals lead to simulated optical-absorption properties in good agreement with experimental data. Importantly, the lowest-lying excited states (excitons) are shown to present a much more localized nature than initially anticipated.