We introduce a vertical polymer phototransistor with low operational voltage (-1.5 V). A blended polymer layer with both acceptor and donor materials was used as a channel material in the vertical space-charge-limited transistor. Under illumination, we obtained external quantum efficiency (EQE) as high as 360% at 620 nm. We propose the effects of base-field shielding as a means to explain high EQE. This proposition has been supported by two-dimensional simulation of the device.