Chlorination Strategy-Induced Abnormal Nanomorphology Tuning in High-Efficiency Organic Solar Cells: A Study of Phenyl-Substituted Benzodithiophene-Based Nonfullerene Acceptors

A new heptacyclic core based on phenyl-substituted benzo[1,2-b:4,5-b']dithiophene (BDT) is designed and paired with 1,1-dicyano methylene-3-indanone (INCN) end group to construct a nonfullerene acceptor, BPIC. The strong aggregation and large phase separation in the poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl)-benzo[1,2-b:4,5-b′]dithiophene))-alt-(5,5-(1′,3′-di-2-thienyl-5′,7′-bis(2-ethylhexyl)benzo[1′,2′-c:4′,5′-c′]dithiophene-4,8-dione))]) (PBDB-T):BPIC blend cause inefficient exciton dissociation and ineffective charge transport, resulting in a low 11.12% power conversion efficiency (PCE) with low short-circuit current density (J_SC) and fill factor (FF). To finely control the active-layer nanomorphology, the chlorine atom is introduced into the INCN termini, and di-chlorinated BPIC-2Cl and tetra-chlorinated BPIC-4Cl are synthesized. It is an interesting phenomenon that, unlike other literature reports, while the di-chlorination reduces crystallinity and phase-separation scale, further chlorination increases crystallinity and phase separation. The PBDB-T:BPIC-2Cl device exhibits suitable molecular packing and nearly ideal nanoscale phase separation, which facilitates exciton dissociation and charge transport and thus yields the higher PCE of 12.63% with significantly improved J_SC and FF. PBDB-T:BPIC-4Cl device, however, exhibits strong stacking intensity and excessively large phase separation, leading to the clearly reduced J_SC, FF, and PCE of only 8.23%. This work demonstrates that novel phenyl-substituted BDT core and delicated chlorination strategy provides powerful tools for high-performance nonfullerene acceptors in organic solar cells.