Fullerenes have nearly isotropic conjugated cages and their derivatives have been widely used as acceptor materials in OPVs. Obviously, when a ball-like small molecular fullerene derivative such as (6, 6)-phenyl-C61-butyric acid methyl ester (PCBM) is blended with a conjugated polymer donor, the interaction between their respective π-orbits can be comparatively easily formed (see Figure 1a), which might be the main reason why fullerenes are still the predominant acceptor material in OPVs. The above speculation can be partially supported by recent progresses in nonfullerene acceptors, ie, when small molecular nonfullerene compounds are used as acceptors in BHJ OPVs, the molecules with weakly anisotropic π-orbits (highly twisted conjugated moieties) often showed better photovoltaic properties than their counterparts with highly anisotropic π-orbits (planar conjugated skeletons).[18–26] That is to say, if both the donor and acceptor materials in a BHJ layer have highly anisotropic conjugated chemical structure, the charge separation process will dependent on the molecular/segmental orientations of the donor and acceptor at the D/A interface. Although the molecular orientation at or relative to the D/A interface is correlated to device performance in a few cases of polymer: PCBM photovoltaic systems,[27] it is more challenging to correlate the anisotropic morphologies at or relative to D/A interfaces to charge separation processes and thus photovoltaic performance of all-anisotropic materials-based OPVs.[28] Moreover, manipulating the potentially critical parameter, ie, molecular orientation at the D/A interface in the anisotropic materials-based OPVs through materials design efforts has not yet been reported.

Here, one pair of donor polymers namely poly {1-(5-(4, 8-bis ((2-ethylhexyl) oxy)-6-methylbenzo [1, 2-b: 4, 5-b′] dithiophen-2-yl) thiophen-2-yl)-5, 7-bis (2-ethylhexyl)-3-(5-methylthiophen-2-yl) benzo [1, 2-c: 4, 5-c′] dithiophene-4, 8-dione}(PBDTBDD) and poly {1-(5-(4, 8-bis (5-(2-ethylhexyl) thiophen-2-yl)-6-methylbenzo [1, 2-b: 4, 5-b′] dithiophen-2-yl) thiophen-2-yl)-5, 7-bis (2-ethylhexyl)-3-(5-methylthiophen-2-yl) benzo [1, 2-c: 4, 5-c′] dithiophene-4, 8-dione}(PBDTBDD-T) were selected to respectively blend with an anisotropic polymer acceptor poly ((N, N′-bis (2-octyldodecyl)-1, 4, 5, 8-naphthalenedicarboximide-2, 6-diyl)-alt-5, 5′-(2, 2′-bithiophene))(PNDI2OD-T2)[29, 30] also known as ActivInk N2200 (abbreviated to PNDI hereafter, see Figure 1 b) in OPV devices. As illustrated in Figure 1 b, the backbones of both donor polymers are highly anisotropic and a slight difference exists in their side chains, ie, the alkoxyl group of PBDTBDD is replaced by alkylthienyl moiety in PBDTBDDT. Interestingly, although a small change exists in chemical structure, the molecular orientation relative to the D/A and electrode interfaces are distinct for PBDTBDD: PNDI and