Enhancing the efficiency of solution-processable bulk-heterojunction devices via a three-dimensional molecular architecture comprising triphenylamine and cyanopyridone

Through the employment of a three-dimensional molecular geometry, a novel, solution-processable small molecular organic chromophore was designed, synthesized and characterized for application in bulk-heterojunction solar cells. The new chromophore, [(5Z,5′Z,5″Z)-5,5′,5″-(((nitrilotris(benzene-4,1-diyl))tris(thiophene-5,2-diyl))tris(methanylylidene))tris(1-(2-ethylhexyl)-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile)] (coded as 3D), was based on a donor–acceptor (D–A) module where a simple triphenylamine unit served as an electron donor, cyanopyridone as an electron acceptor, and a thiophene unit as π-bridge embedded between the donor and acceptor functionalities. The optoelectronic and photovoltaic properties of 3D were directly compared with those of a structural analogue, [(Z)-5-((5-(4-(diphenylamino)phenyl)thiophen-2-yl)methylene)-1-(2-ethylhexyl)-4-methyl-2,6-dioxo-1,2,5,6-tetrahydropyridine-3-carbonitrile)], namely 1D. The three-dimensional (otherwise termed as non-linear) design, compared to one-dimensional (or linear) design, demonstrated (1) an enhancement of light-harvesting ability by about 50%; (2) an increase in wavelength of the longest wavelength absorption maximum of thin film (633 nm vs. 581 nm) and (3) a narrower optical band-gap (1.53 eV vs. 1.65 eV). Solution-processable bulk-heterojunction devices were fabricated with 3D and 1D as donor materials. Studies on the photovoltaic and hole charge mobility properties revealed that the best 3D- [6,6]-phenyl-C61-butyric acid methyl ester (PC61BM)-based device showed an impressive enhanced power conversion efficiency of 4.76%, an increase of greater than two-fold with respect to the efficiency of the best 1D-based device (2.11%), and a higher hole mobility of the order of 3.4 × 10−4cm2V−1s−1. Not only is 3D the first reported example in the literature where cyanopyridone acceptor functionality has been used to generate a three-dimensional donor molecule but the power conversion efficiency reported here is the highest value reported so far for non-linear, star-shaped molecular architectures. These results clearly illustrated that the use of a three-dimensional geometry helped to extend molecular conjugation, thus enhancing the light-harvesting ability and short-circuit current density, while further improving the bulk-heterojunction device performance.