We investigate the electron-injecting properties of a ZnO substrate modified by a thin layer of solution-processed Cs₂ CO₃ into poly(9, 9′ -dioctylfluorene)-co-benzothiodiazole (F8BT). Ultraviolet photoemission spectroscopy studies reveal that in the case of coannealed F8BT/ Cs₂ CO₃ films, the Fermi level is pinned at the lowest unoccupied molecular orbital of the F8BT, indicating significant n-doping of the polymer. X-ray photoemission spectroscopy measurements reveal the presence of Cs₂ CO₃ in the coannealed F8BT/ Cs₂ CO ₃ films. I-V measurements in single carrier devices show a significant improvement in the electron injection of the coannealed F8BT/ Cs₂ CO₃, in agreement with the observed n-doping of the polymer upon coannealing.