Efficient As‐Cast Polymer Solar Cells with High and Stabilized Fill Factor

Abstract

Molecular ordering and miscibility of donor and acceptor materials play critical roles in developing high‐performance as‐cast polymer solar cells (PSCs). In this work, a highly crystalline nonfullerene small molecular acceptor, namely, C8‐IT‐4F, based on alkylated indacenodithieno[3,2‐b]thiophene as the aromatic core and 2‐(5,6‐difluoro‐3‐oxo‐2,3‐dihydro‐1H‐inden‐1‐ylidene)malononitrile moieties as end groups, is selected and synthesized. The π–π stacking distance in C8‐IT‐4F film can be tuned from 3.88 Å to a more compact (3.48 Å) state by a film‐formation process and the confinement induced by the preaggregated polymer donor PM7, leading to broadened absorption and fine phase separation in the blend film. The optimal morphology with a framework of preaggregated polymer donor, J‐type face‐on π–π stacked acceptor, and appropriate donor/acceptor miscibility facilitates charge generation and transport and reduces charge recombination. As a result, the best PSC based on the as‐cast PM7:C8‐IT‐4F blend film exhibits power conversion efficiency of 14.3%, with an open‐circuit voltage of 0.82 V, a short‐circuit current density of 22.7 mA cm−2, a fill factor of 77.1%, and good photostability with a stabilized fill factor.