The paper analyses the interrelation between the signal frequency response in an organic planar photodetector and the distribution of traps of the active material playing a fundamental role in the transport of charge carriers. In particular, the response of a dithiolene-based photodetector spectrally matched to the near infrared has been acquired under optical pulses ranging from 10 Hz to 1 MHz in frequency. By means of modulated photocurrent spectroscopy, the trap density distribution of the dithiolene in the solid state has been extracted. The capture and release of photogenerated carriers exerted by a Gaussian density of trapping states, located 0.42 eV below the transport energy, is shown to be the responsible for the observed device frequency response.