Solar Cell Efficiency, Self-Assembly, and Dipole–Dipole Interactions of Isomorphic Narrow-Band-Gap Molecules

We examine the correlations of the dipole moment and conformational stability to the self-assembly and solar cell performance within a series of isomorphic, solution-processable molecules. These charge-transfer chromophores are described by a D1-A-D-A-D1 structure comprising electron-rich 2-hexylbithiophene and 3,3′-di-2-ethylhexylsilylene-2,2′-bithiophene moieties as the donor units D1 and D, respectively. The building blocks 2,1,3-benzothiadiazole (BT) and [1,2,5]thiadiazolo[3,4-c]pyridine (PT) were used as the electron-deficient acceptor units A. Using a combination of UV–visible spectroscopy, field-effect transistors, solar cell devices, grazing incident wide-angle X-ray scattering, and transmission electron microscopy, three PT-containing compounds (1–3) with varying regiochemistry and symmetry, together with the BT-based compound 5,5′-bis{(4-(7-hexylthiophen-2-yl)thiophen-2-yl)-[1,2,5]thiadiazolobenzene}-3,3′-di-2-ethylhexylsilylene-2,2′-bithiophene (4), are compared and contrasted in solution, in thin films, and as blends with the electron acceptor [6,6]-phenyl-C70-butyric acid methyl ester. The molecules with symmetric orientations of the PT acceptor, 1 and 2, yield highly ordered blended thin films. The best films, processed with the solvent additive 1,8-diiodooctane, show donor “crystallite” length scales on the order of 15–35 nm and photovoltaic power conversion efficiencies (PCEs) of 7.0 and 5.6%, respectively. Compound 3, with an unsymmetrical orientation of PT heterocycles, shows subtle differences in the crystallization behavior and a best PCE of 3.2%. In contrast, blends of the BT-containing donor 4 are highly disordered and give PCEs below 0.2%. We speculate that the differences in self-assembly arise from the strong influence of the BT acceptor and its orientation on the net dipole moment and geometric description of the chromophore.