Internal photoemission (IPE) is a promising phenomenon for sub-bandgap photodetection at near-infrared wavelengths using large bandgap semiconductor materials. To improve the photon-to-electron conversion efficiency in silicon based sub-bandgap Schottky barrier photodetectors (SBPDs), previous studies have mainly focused on subwavelength-scale nanostructures to enhance the electric fields and optical absorption. Here, in a different way from the previous approaches, we theoretically and experimentally demonstrate a rigorous quantum efficiency analysis framework that can quantitatively explain the hot carrier transport processes. Guided by the design principles from the hot carrier loss mechanism analysis, we experimentally demonstrate patternless thin-film SBPDs that can surpass the performance of conventional nanostructured SBPDs, exceeding the external quantum efficiency of 10-3. Our work shows that optical absorption and hot carrier generation are responsible for only a part of the entire IPE process and other transport mechanisms should be carefully considered in a wholistic manner, indicating the importance of quantitative quantum efficiency analysis.