Abstract
Solvent annealing induces the instability of conjugated polymer ultrathin films and the changes in optoelectronic properties at room temperature was investigated in this report. At first, we dispersed MEH-PPV molecules in inert polystyrene (PS) in the concentration (c) of 5%. Solvent annealing-driven dewetting created massive PL enhancement when polymers flowed into dewetting droplets and residual layer. This molecular shear flow suppressed electron-phonon interaction in polymer chains and the PL enhancement is up to ~ 10 folds at the residual layer. The blue shift accompanying with dewetting was recorded. As annealing continued on, the blue shift reversed, illustrating the increase of conjugation length during solvent annealing. The transient blue shift became smaller by poor solvent dewetting and larger as we used entangling long chains PS matrix. These results indicated solvent-polymer interactions and alteration of inter-segmental chain entanglements influences up to the stage of dewetting. Before dewetting, solvent molecules penetrated into the polymer film and saturated the underlayer. All the film instability process too place on the loosen underlayer. As c decreases, molecular shear stress could effectively transferred to individual MEH-PPV molecule and created huge blue shift (~ 30 nm). However, high MEH-PPV concentration film (c = 25%) wouldn’t dewetting during solvent annealing due to sufficient percolation of MEH-PPV. As a result, only few bumps emergence on the surface in order to decrease system free energy. The PL spectrum red shift and the intensity drop after solvent annealing. Interestingly, solvent-annealed polymer films transformed to wrinkle structure after aging in the room temperature and accompanying with blue shift.