Improvement of singlet exciton induced by spin flip in conjugated polymers

Improving the ratio of singlet exciton is an effective method to maximize the efficiency of organic light emitting diodes (OLEDs). The effects of spin conversion on exciton productivities remain hot and debating topics. In this work, based on the extended tight-binding Su-Schrieffer-Heeger model, we theoretically investigate the effect of spin flip on the proportion of exciton manifolds. By analyzing the composition of spin mixed excitons, we found that the spin flip from spin-orbit coupling and hyperfine interaction can make ratio of singlet exciton exceed the statistical value 25%. Moreover, the productivity of the singlet exciton is sensitive to the chain length and polymer softness. A large singlet ratio of 30% is obtained under suitable condition. This work gives a rational explanation for the singlets ratio over exciton statistics and provides theoretical basis for material selection and molecular design in OLEDs.