Enhanced Efficiency and Thermal Stability in All‐Polymer p‐i‐n Organic Photovoltaics With Synergistically Designed Second Polymer Acceptor

ABSTRACT A small amount of a synergistically designed and selenophene‐substituted second polymer acceptor, PYISe‐BO‐OD, was incorporated into the PYIT‐OD acceptor layer for enhancing interfacial interactions with the PM6 donor layer, resulting in high‐performance and superior thermal stability in all‐polymer p‐i‐n organic photovoltaics. The PM6/PYIT‐OD:PYISe‐BO‐OD (wt. 1:0.1) device displayed a champion power conversion efficiency (PCE) value of 19.1% and a superior T 80 (time for PCE = 0.8 PCE onset value at 80°C) value of 3415 h, greatly surpassing the PM6/PYIT‐OD device with PCE and T 80 values of 16.3% and 937 h, respectively. The PCE and thermal stability improvements resulted from PYISe‐BO‐OD's optimized intermolecular interactions with both PM6 and PYIT‐OD (binding energy value –2.73 and −2.71 eV by density functional theory; Flory–Huggins interaction parameter values of 0.014 and 0.652) that expanded the interfacial i‐region, inducing more efficient charge separation and prolonged carrier lifetime. The grazing‐incidence wide‐angle X‐ray scattering results revealed the crystalline coherence length values remained after 2050 h because of the stabilized morphology owing to strong intermolecular interactions of PYISe‐BO‐OD with PM6 and PYIT‐OD. The strategy of incorporating synergistical second polymer acceptor for enhancing both the efficiency and thermal stability of the devices lays the foundation for the realization of p‐i‐n organic photovoltaic devices.