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Several experimental groups have reported recently an intriguing high level of gain (Photo-Multiplication) in vertical organic photo-resistance (as well as in other technologies, such as perovskite for instance). This mechanism is sometimes named as "Trap-Assisted Photo-Multiplication". This paper investigates the origin of this mechanism by the mean of drift diffusion simulations, analytical theory and experiments, considering the particular case of PCDTBT:PC 60 BM photoresistors, although some conclusions are likely to apply in other technologies. It turns out that an excess of charges (induced by electron-holes carrier generation) may trigger additional carrier injection, leading to photomultiplication, under specific circumstances. We call this mechanism "gain by injection enhancement". Electron (resp. hole) trapping for P only (resp. N only) device can play this role efficiently. As these additional carriers came from contacts, significant dark current injection is thus needed to achieve large value of gain, explaining why this mechanism can occur only in P (or N) only photoresistors (and not photodiodes or intrinsic photoresistor, i.e. with mid gap contacts). In such devices however, the detectivity remains intrinsically limited by the high level of dark injection currents required to get gain, and consequently this type of device may be interesting in particular technologies where it is not possible to achieve low dark currents using photodiodes. However, penalized by the slow traps dynamic, the cut off frequency of these devices remains extremely low (< 100 Hz). Also, this gain takes high value only at low irradiance, making photoresistor responsivity light dependent. All these results bring new light in analysing and optimising photoresistor, opening a large field of investigation to take advantage of gain by injection enhancement.

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