A series of V-shaped small molecule non-fullerene electron acceptors for efficient bulk-heterojunction devices

Two simple semiconducting acceptor-acceptor1-donor-acceptor (A-A1-D-A) modular, small molecule non-fullerene electron acceptors, 2-(4-(7-hexadecyl-1,3,6,8-tetraoxo-3,6,7,8-tetrahydrobenzo[lmn][3,8]phenanthrolin-2(1H)-yl)phenyl)-3-(6-((4-(7-hexadecyl-1,3,6,8-tetraoxo-3,6,7,8-tetrahydrobenzo[lmn][3,8]phenanthrolin-2(1H)-yl)phenyl)ethynyl)-9-octyl-9H-carbazol-3-yl)buta-1,3-diene-1,1,4,4-tetracarbonitrile (NDICz-5) and 2-(4-(3,3-dicyano-2-(4-(7-hexadecyl-1,3,6,8-tetraoxo-3,6,7,8-tetrahydrobenzo[lmn][3,8]phenanthrolin-2(1H)-yl)phenyl)-1-(6-((4-(7-hexadecyl-1,3,6,8-tetraoxo-3,6,7,8-tetrahydrobenzo[lmn][3,8]phenanthrolin-2(1H)-yl)phenyl)ethynyl)-9-octyl-9H-carbazol-3-yl)allylidene)cyclohexa-2,5-dien-1-ylidene)malononitrile (NDICz-6), were designed, synthesized and characterized for application in solution-processable bulk-heterojunction solar cells. The optoelectronic and photovoltaic properties of NDICz-5 and NDICz-6 were directly compared with those of a structural analogue, 7,7'-(((9-octyl-9H-carbazole-3,6-diyl)bis(ethyne-2,1-diyl))bis(4,1-phenylene))bis(2-hexadecylbenzo[lmn][3,8]phenanthroline-1,3,6,8(2H,7H)-tetraone) (NDICz-4), which was designed based on an A-D-A format. All of these new materials were designed to be V-shaped and comprised an electron rich carbazole donor core (D) together with electron deficient naphthalene diimide terminal core (A). In the simple A-D-A system, naphthalene diimide was the terminal functionality, whereas in the A-A1-D-A system, tetracyanoethylene (TCNE) and tetracyanoquinodimethane (TCNQ) derived functionalities were incorporated as A1 units by keeping D/A units constant. The inclusion of A1 was mainly done to induce cross-conjugation within the molecular backbone and hence to improve light-harvesting. The physical and optoelectronic properties were characterized by ultraviolet–visible, thermogravimetric analysis, photo-electron spectroscopy in air and cyclic voltammetry. These new materials exhibited energy levels complementing those of the conventional donor polymer poly(3-hexyl thiophene) (P3HT). Solution-processable bulk-heterojunction devices were fabricated with NDICz-4, NDICz-5 and NDICz-6 as non-fullerene electron acceptors. Studies on the photovoltaic properties revealed that the best P3HT: NDI-Cz6-based device showed an impressive enhanced power conversion efficiency of 7.58%, an increase of around two-fold with respect to as-cast blend, after solvent vapor annealing using carbon disulfide. Not only are the result