We report the effect of a poly[(9,9-bis(3'-(N,N-dimethylamino)propyl)-2,7-fluorene)-alt-2,7-(9,9-dioctyl fluorene)] (PFN) interlayer on the performance of PTB7:PC71BM-based organic thin-film solar cells (OTFSCs) prepared using the spray-coating method. The active layer shows a broad absorption, but the effective contact area in the bulk hetrojunction (BHJ) is small because of the formation of aggregates on the surface. The surface aggregates dissolved by the addition of 1,8-diiodooctane (DIO) to the PTB7:PC71BM matrix, which results in an increase in the contact area between the donor and acceptor in the active layer and, thereby , an improvement in the device performance. The PFN interlayer enhances the charge-transport properties and suppresses exciton-recombination processes. This results in increases in the short-circuit current density (J(sc) = 13.87 mA/cm(2)), open-circuit voltage (V-oc = 0.747 V), and fill factor (FF = 57.97%), as a result of a high power conversion efficiency (PCE, eta = 6.01%) compared to a device without a PFN layer (eta = 4.84%). The thickness of the PFN interlayer was found to be of siginificant importance, as thicker layers exhibit an insulating nature instead of improving the electron transfer.