In order to improve the performance of amorphous selenium (a-Se) based detectors, it is beneficial to operate the device at high electric field (≥10 V/μm). Increasing the electric field reduces the ionization energy and increases the hole mobility within the a-Se detector. In order for a practical a-Se detector to be capable of working at a high electric field, injection of holes from the positively biased electrode and injection of electrons from the negatively biased electrode should be prevented. We have investigated different organic materials with high ionization potential as hole-blocking contacts for a-Se based photodetectors. The effect of the organic layer thickness on the dark current and photocurrent performance of the detector was examined. It was found that the injection of holes could be reduced at high electric fields by increasing the thickness of the organic layer.