Blends and doping of organic semiconductors are generally employed to improve effectively the charge transfer and dissociation performance. The absorption spectrum may be optimized making use of the different energy states of the components in the blends, which may favor the development of the photovoltaic or solar cell devices. Excellent type-II heterojunction structures can be produced by mixing the small-molecule perylene (EPPTC) and a copolymer of polyfluorene (F8BT). Actually, F8BT and EPPTC exhibit absorptions in the blue region and in the green region, respectively. Thus, the blend will have a much broadened absorption spectrum. In the experiment, the blend solution of these two materials in chloroform is spin-coated onto a piece of glass substrate, so that EPPTC is doped into the polymer of F8BT and the heterojunction structure forms in the final solid film. Then, steady-state absorption and fluorescence spectroscopy, as well as the transient photoluminesence spectroscopy (time-correlated single-photon counting), is used to investigate the formation and the photoluminescence properties of exciplex in the heterojunction film of F8BT doped with EPPTC. The photoluminscence (PL) spectrum and the life-time are measured to characterize the exciplex in the blend film, where the longer life-time of the red-shifted PL spectrum confirms the formation of the exciplex. This provides various experimental data for understanding the formation and the photophysical properties of the heterojunction structures in organic semiconductor blends. Futhermore, the absorption of the blend film covers a large range of the visible spectrum. Therefore, this kind of doping of organic semiconductor is important for the development of photovoltaic and solar cell devices.