Face-on orientation and vertical phase separation of p-DTS(FBTTh2)2/PC70BM induced by epitaxial crystallization of polymer interface layer

Abstract

Abstract It is important to arrange the donor and acceptor phases and molecules within those phases in a way that optimizes relevant optoelectronic processes required for efficient operation of bulk-heterojunction (BHJ) organic solar cells (OSCs). 7,7′-(4,4-bis(2-ethylhexyl)-4H-silolo[3,2-b:4,5-b′]dithiophene-2,6-diyl)bis(6-fluoro-4(5′-hexyl-[2,2′-bithiophen]-5-yl)benzo[c][1,2,5]thiadiazole) (p-DTS(FBTTh2)2) tends to adopt edge-on orientation with respect to PEDOT/PSS substrate, which is adverse to charge and exciton transport in the vertical direction. Besides, [6,6]-phenyl-C71-butyric acid methyl ester (PC70BM) tends to deposit on PEDOT/PSS substrate, while p-DTS(FBTTh2)2 is inclined to accumulate at surface, which blocks charge transport and increases injection barrier of charge. Here, we tried to optimize both the molecular orientation and vertical phase separation of p-DTS(FBTTh2)2/PC70BM with proper crystal size and bicontinuous network structure. Poly[4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)benzo[1,2-b:4,5-b′]dithiophene-co-3-fluorothieno[3,4-b]thiophene-2-carboxylate], (PTB7-Th) was used as the interfacial layer to induce p-DTS(FBTTh2)2 to adopt face-on orientation because of epitaxial crystallization. At the same time, vertical phase separation can be improved due to that p-DTS(FBTTh2)2 tends to deposit on PTB7-Th. Consequently, the efficiency of exciton separation of devices with PTB7-Th as the interfacial layer is improved and both the hole and electron mobility were increased and became more balanced. As a result, the power conversion efficiency (PCE) increased from 5.47% to 6.74%.