Abstract We report on the passivation of surface states of Al_0.5Ga_0.5N epilayers by employing self-assembled monolayers (SAMs) of organic molecules, which led to a significant improvement in the performance of Al_0.5Ga_0.5N based solar-blind photodetector. The formation of SAM of meso-(5-hydroxyphenyl)-10,15,20-tri(p-tolyl) porphyrin (ZnTPP(OH)) on the surface of Al_0.5Ga_0.5N was probed by contact angle measurement, x-ray photoelectron spectroscopy, and atomic force microscopy. The successful passivation of surface states was confirmed by Kelvin probe force microscopy as a significant decrease in the surface potential of Al_0.5Ga_0.5N by ∼280 mV was observed. The inference was supported by a four-fold increase in the photoluminescence intensity of the near-band edge emission peak upon passivation. As a result, the dark current of the as-fabricated solar-blind photodetector reduced by two orders of magnitude, without compromising with the magnitude of the photo current at 270 nm. The role of SAM was evident in improving the performance of the photodetector as the peak value of photo-to-dark current ratio enhanced by ∼36 times. The peak responsivity of the photodetector increased from 1.6 to 2.2 mA W⁻¹ at 10 V. The significant reduction in the dark current and enhancement in the responsivity led to an improvement in the specific detectivity by ∼10 times. Additionally, the response speed of the photodetector was found to improve significantly from 4 to 0.5 s.