Charge injection barrier height between semiconductors and electrodes is one of the key parameters in determining the dark current in organic photodetectors (OPDs), which should be minimized in order to enhance the sensitivity of the devices. It was often stated that this barrier height is mostly given by the apparent Schottky barrier heights, while the impact of mirror image potential and barrier reduction caused by material disorder was usually neglected for simplicity. Here we present a comprehensive study of charge injection in OPDs by using a low bandgap small molecule and its fluorinated derivatives with varying energy levels as electron acceptors, considering the influence of various parameters on the effective barrier for injection. As compared to the devices based on fluorinated acceptors, the devices based on non-fluorinated acceptors exhibit similar external quantum efficiency but lower dark current density, and hence a higher specific detectivity (D*). By combining the analysis of effective injection barriers, trap states, and activation energy, we reveal that the origin of the dark current of three OPDs is the charge injection from the contact to the active layer via subgap traps. And the lower dark current density in ITIC-based OPDs is explained using a model that gives a correct description of the effective injection barrier under reverse bias.