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

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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 ( P L ), 22.9 ( P M ), and 49.9 kg mol −1 ( P H ) 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 ( P L ) to face‐on ( P H ) 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 P L , P M , and P H 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 P H as electron acceptor show the highest efficiency of 6.14%, outperforming those with P M (5.08%) and P L (4.29%).