The characteristics of violet-sensitive organic photodetectors (OPDs) utilizing polyalkylfluorene and triplet materials have been studied as a host and a dopant material, respectively. For the photo absorption layer, poly(9, 9-dioctylfluorene) [PFO] and a phosphorescent iridium complex (Iridium (III) bis(2-(4, 6-difluorophenyl)pyridinato-N, C²) [FIrpic] or Iridium (III) bis(2-(2'-benzothienyl)pyridinato-N, C³')(acetyl-acetonate) [(btp)₂Ir(acac)]) were used as a host and a dopant material, respectively. PFO: (btp)₂Ir(acac) device showed less photocurrent than PFO device because (btp)₂Ir(acac) enhances recombination of the photo generated carriers in the photo absorption layer. On the other hand, PFO : FIrpic device showed larger photocurrent than PFO device due to triplet energy transfer from FIrpic to PFO. A cutoff frequency of 20MHz was observed using a sinusoidal modulated violet laser light illumination under the reverse bias of 8V.