A series of organic photodetectors (OPDs) is prepared with two donor materials as active layers, with the only difference being the weight ratio of the two donors (one polymer and one small molecule). The OPDs work according to a photodiode model with an external quantum efficiency (EQE) of less than 10% at -10 V when the weight ratio of the two materials is 1 : 1 (wt/wt). The EQE of an OPD with P3HT:DRCN5T (100 : 2, wt/wt) as the active layer reaches 1400% at -10 V, exhibiting the photomultiplication (PM) phenomenon. The EQE values of PM-type OPDs can be markedly improved along with a bias increase, and the champion EQE reaches 10 600% at -20 V. The small number of small molecules can be used as electron traps due to the different lowest unoccupied molecular orbital (LUMO) levels of the two donors, and photogenerated electrons can be trapped in the small molecules surrounded by P3HT. The trapped electrons near the Al electrode can induce interfacial band bending for efficient hole tunneling injection from an external circuit. This work provides a new strategy for realizing acceptor-free PM-type OPDs, which may inspire us to further develop organic electronic devices with single type organic semiconducting materials.