Controlling Molecular Orientation of Naphthalenediimide-Based Polymer Acceptors for High Performance All-Polymer Solar Cells

Molecular orientation, with respect to donor/acceptor interface and electrodes, plays a critical role in determining the performance of all-polymer solar cells (all-PSCs), but is often difficult to rationally control. Here, an effective approach for tuning the molecular crystallinity and orientation of naphthalenediimide-bithiophene-based n-type polymers (P(NDI2HD-T2)) by controlling their number average molecular weights (M-n) is reported. A series of P(NDI2HD-T2) polymers with different M-n of 13.6 (PL), 22.9 (PM), and 49.9 kg mol(-1) (PH) are prepared by changing the amount of end-capping agent (2-bromothiophene) during polymerization. Increasing the M-n values of P(NDI2HD-T2) polymers leads to a remarkable shift of dominant lamellar crystallite textures from edge-on (PL) to face-on (PH) as well as more than a twofold increase in the crystallinity. For example, the portion of face-on oriented crystallites is dramatically increased from 21.5% and 46.1%, to 78.6% for PL, PM, and PH polymers. These different packing structures in terms of the molecular orientation greatly affect the charge dissociation efficiency at the donor/acceptor interface and thus the short-circuit current density of the all-PSCs. All-PSCs with PTB7-Th as electron donor and PH as electron acceptor show the highest efficiency of 6.14%, outperforming those with PM (5.08%) and PL (4.29%).