High gain and low dark current solution-processed colloidal PbS quantum dots infrared (IR) PIN photodetectors with IR sensitivity up to 1500 nm are demonstrated. The low dark current is due to the P-I-N structure with both electron and hole blockers. The high gain in our IR photodiodes is due to the enhancement of electron tunneling injection through the 1,1-bis[(di-4-tolylamino)phenyl]cyclohexane (TAPC) electron blocker under IR illumination resulting from a distorted electron blocking barrier in the presence of photo-generated holes trapped in the TAPC electron blocker. It is further found that the trap states in the TAPC layer are generated by the Ag atoms penetrated in the TAPC layer during the thermal evaporation process. The resulting photodetectors have a high detectivity value of 7 x 10(13) Jones, which is even higher than that of a commercial InGaAs photodiode.