Organic materials offer interesting properties for the realization of photoactive devices such as solar cells, photodetectors and phototransistors, including effective light absorption from ultraviolet to near-infrared, good photogeneration yield and low-temperature processability over large areas and flexible substrates. In particular, the high photo-responsivity shown recently by organic phototransistor are suggesting the possibility to implement this structure in a variety of optoelectronic applications e.g. image sensor, opto-isolator, optoelectronic switch and memory. In this work we present a broadband, and high efficient planar photodetector/phototransistor based on a semi-transparent active bilayer. Unlike bulk heterojunction, the bilayer structure allows phase continuity between the two gold electrodes and thus a pre-defined transport path for charge carriers. The electron acceptor consists in a nano-structured layer of P(NDI2OD-T2) while the donor material is based on a squaraine dye, namely LB-97, both deposited by spin-coating from solution. The n-type polymer, showing a good electron mobility, is acting as the electron transporting phase, while LB-97 is behaving as a light harvesting antenna, undergoing photoexcitation and subsequently transferring the electron to the polymer. Despite the percentage of photons absorbed by the active layer is lower than 10%, the device shows a high photosensitivity which is a result of the photoconductive mechanism occurring into the device. The magnitude of the photoconductive gain is dependent on the conductivity of the polymeric layer. The use of different solvents results in a higher electron mobility of the P(NDI2OD-T2) layer, leading to higher photoconductive gains. This effect can be further enhanced by adding a top gate electrode to the planar structure, thus exploiting the field effect of the phototransistor working in accumulation regime. This device can reach a remarkable value of responsivity of 12 A/W in saturation regime when illuminated at 570 nm, corresponding to the peak of the absorption band of the LB-97. Photocurrent mapping has been performed, giving fundamental information on how the photogeneration is arranged in the active layer, depending on the distance from the electrode and on the electric field applied. The device shows consistent responsivity values also in the near infrared (2 A/W @810 nm), suggesting the possibility to be implemented in imaging and biomedical sensing in which the optical signal intensity is relatively weak. This, combined with the simple architecture of the device and the processability from solution, make the phototransistor a promising component for future portable devices for diagnostics and health monitoring.