Controlled phase separation of polyfluorene blends via inkjet printing

We report the use of inkjet printing (IJP) to produce thin polymer films with controlled phase separation of binary polymer blends. Photovoltaic devices comprising a blend of poly(9,9'-dioctyfluorene-co-bis-N,N'-(4-butylphenyl)-bis-N,N-phenyl-1,4-phenylenediamine) (PFB) with poly(9,9'-dioctylfluorene-co-benzothiadiazole) (F8BT) and light-emitting diodes (LEDs) constituting a blend of F8BT with poly(9,9-dioctylfluorene-co-N-(4-butylphenyl)diphenylamine) (TFB) were fabricated. The phase separation in these polymer blends was analyzed using atomic force microscopy and photoluminescent optical microscopy. The lateral phase separation in both blend films spin-coated from p-xylene solution is typically a few microns; however, by inkjet printing a solution of an identical composition, the feature size can be reduced to similar to 300 nm when combined with an elevated substrate temperature during the printing process. This results from the rapid solvent evaporation of the inkjet-printed droplets. LEDs made by IJP show well-behaved I-V-L characteristics, with lower current density and higher luminescent efficiency than spin-coated devices at high voltages. The finer-scale phase separation from IJP also leads to a factor of 2 improvement in the external quantum efficiencies of photovoltaic diodes.