Remarkable Electrical Connectivity in the Acceptor Phase of Y6-and Fullerene-Based Bulk Heterojunction Solar Cells

Molecular orientation and stacking motif have a major impact on charge transport within bulk heterojunction (BHJ) organic solar cell active layers. Unlike typical core pi-stacking organic semiconductors, fullerenes and the non-fullerene acceptor 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) exhibit highly interconnected three-dimensional packing arrangements. Technical challenges, however, have hindered direct experimental probing of the electrical connectivity within the acceptor phase of BHJs. Through the development of conductive atomic force microscopy (C-AFM) protocols, this study investigates local electron transport and lateral current spreading within the acceptor phase of fullerene- and Y6-based BHJs. These measurements reveal remarkable lateral electrical connectivity, evidenced by an interconnected filamentary electrical network in C-AFM current maps and lateral current spreading radii that are more than three times greater than those in the donor phase. The effective current spreading radius for Y6 was 278 nm at -4 V, versus 182 nm in the fullerene [6,6]phenyl-C-71-butyric acid methyl ester (PC71BM), owing to the interlocked, electronically coupled three-dimensional network formed by the curved Y6 molecules. These findings point to the promise of molecular stacking arrangements with three-dimensional electronic coupling as a means of promoting efficient electron and hole collection in BHJ organic solar cells.