Blocked impurity band (BIB) detectors, developed from extrinsic detectors, have long been employed for ground-based and airborne astronomical imaging and photon detections. They are the state-of-the-art choice for highly sensitive detection from mid-infrared to far-infrared radiation. In this work, we demonstrate the existence of an interfacial barrier in blocked impurity band structures by evidence of temperature-dependent dark currents, bias-dependent photocurrent spectra and corresponding theoretical calculations. The origin of the build-in field is studied. The temperature-dependent characteristics of space charge effects are also investigated in detail. It is found that at higher temperature (T 14 K), the space charge influence is negligible, and the interfacial barrier is mainly caused by bandgap narrowing effects. Based on interfacial barrier effects, a dual-excitation model is proposed to clarify the band structure of BIB detectors. The photocurrent spectra related to the two excitation processes, i.e., the direct excitation over the interfacial barrier and excitation to the band edge with subquent tunneling into blocking layer, are successfully extracted and agree reasonably well with the calculated band structure results. The effects of interfacial barrier on the photocurrent spectrum, peak responsivity and internal quantum efficiency of the devices are investigated. With the consideration of interfacial barrier effects, the calculated peak responsivity shows good agreement with the experimental result. It is suggested that interfacial barrier effects should be considered for successfully designing the BIB detectors. Additionally, the build-in field is found to equivalently lower the critical field for impact ionization. This study provides a better understanding of the working mechanism in BIB detectors and also a better device optimization.