Observing long-range non-fullerene backbone ordering in real-space to improve the charge transport properties of organic solar cells

To understand the dominance of 2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5] thiadiazolo[3,4-e]thieno[2 '',3 '' :4',5']thieno[2',3' :4,5]pyrrolo[3,2-g]thieno[2',3' :4,5]thieno[3,2-b]indole-2,10-diyl) bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile (Y6) and its derivatives leading to the rapid efficiency rise of organic solar cells (OSCs), solid film structures are of significant importance. Here, we employ cryo-transmission electron microscopy (Cryo-TEM) to resolve the landscape of Y6 packing in neat films (unlike single crystals) in relation to device performance, and reveal how processing with carbon disulfide and [6,6]-phenyl-C-71-butyric acid methyl ester (PC71BM) influences its molecular packing, prominently the backbone ordering. We show that Y6 prefers a face-on dominant packing structure with an in-plane long-range conjugated backbone packing in films. The long-range backbone ordering is beneficial for reducing disorders on the energy distribution of the electron transport level, thereby improving the carrier lifetime in heterojunctions. We confirm that long-range energy transfer assists poly[(2,6-(4,8-bis(5-(2-ethylhexyl)-4-fluorothiophen-2-yl)-benzo[1,2-b:4,5-b']dithiophene))-alt-(5,5-(1',3'-di2-thienyl-50,70 -bis(2-ethylhexyl)benzo[1',2'-c:4',5'-c']dithiophene-4,8-dione))] (PM6) excitons to reach the preferred Y6/PM6 interfaces without being quenched by PC71BM clusters, yielding no signs of bimolecular recombination and a high power conversion efficiency of 16.8%. Our results suggest an effective molecular packing structure in solid films, and the prominent role of backbone ordering in photoelectric conversion processes, which will outline the future development of OSCs.