We achieved the reduced the surface roughness of poly[9,9-dioctylfluorenyl-2,7-diyl]-co-1,4-benzo-(2,1,3)-thiadiazole (F8BT) thin film fabricated by the electrospray deposition technique, using two organic solvents. The root mean square (RMS) roughness of the F8BT thin film was reduced by the addition of dimethyl sulfoxide (DMSO) in tetrahydrofuran (THF). A minimum RMS roughness of 23 nm was obtained by optimizing the ratio of DMSO in THF. We found that the low evaporation pressure of the additional organic solvent is an important factor in attaining a smooth surface for F8BT thin film. The current density-voltage-luminance characteristics of the organic light-emitting diode was improved by reducing the surface roughness of the F8BT layer. A maximum luminance of 36.7 cd/cm(2) was obtained, and it was more than 14 times higher than the reference device fabricated by the conventional single organic solvent technique. (C) 2011 Elsevier B.V. All rights reserved.