Organic ultraviolet (UV) photodetectors with bulk-heterojunction structure were fabricated via a simple spin-coating method by using blend films of poly(9,9-dihexylfluorene) (PFH) and a soluble naphthalene diimide derivative, N,N'-Bis(phenylmethyl)naphthalene-1,4,5,8-tetracarboxylic diimide (NDI-BA), which respectively act as the donor and acceptor. The effect of thermal annealing temperature together with the components blend ratio on the morphology of the composite films, and further the performance of the devices was investigated. The results indicate that balanced charge transfer and transportation can be achieved by controlling the extent of phase separation. By optimization, the device with the 1:1 (by weight) blend layer annealed at 80 degrees C for 15 min demonstrates obvious photoresponse under UV radiation. A photo-to-dark current ratio of over two orders of magnitude, fast response with decay time (decay to 1% of the value before removing of the radiation) of less than 200 ms, and a responsivity of 224 mA/W at 4 V can be obtained under 1 mW/cm(2) UV radiation at 365 nm. The spectral response covering the region from 300 to 420 nm endows the device potential application in near-UV region as low-cost optical switches or photodetectors. (C) 2010 Elsevier B.V. All rights reserved.