Photodiodes using amorphous-selenium (a-Se) have gained significant attention in medical imaging due to their avalanche phenomena. At electric fields above 70 V/μm, holes in a-Se undergo impact ionization which increases the effective sensitivity of the photodetector. However, at the high electric fields required for avalanche gain, dark current due to charge injection from metallic electrode can become a significant source of noise, limiting the dynamic range of the device. A non-insulating n-type hole blocking layer (HBL) is required to minimize the dark current noise from charge injection while conducting only electrons. Among previously reported HBLs, vacuum deposited bulk cerium dioxide (CeO2) have been successfully utilized in n-i-p avalanche a-Se sensors. However, bulk CeO2 cannot be directly deposited on a-Se and suffers from high dark current due to lower effective hole barrier for CeO2 (2.8 eV). In our work, we successfully synthesized colloidal CeO2 quantum dots (QDs) with a large bandgap (Eg = 3.77 eV), which are electronically passivated by surfactants/ligands and spin coated directly on a-Se at room temperature without inducing crystallization. Our prototype device achieved an ultra-low dark current of 30 pA/cm2 at 70 V/μm which is three orders of magnitude lower than any avalanche a-Se device previously reported and may be the lowest dark current ever recorded for an avalanche device operated at room temperature signifying the potential importance of utilization of solution-processed QDs as HBL.