The origin of the empirical linear dependence of E-int the electric field at the charge-injecting metal/poly(3-hexylthiophene), P3HT, interfaces on the externally applied bias, E-a, that governs its room temperature negative field charge drift mobility is investigated. The published electrostatic model is modified in the sense that the energetic shift phi of the Gaussian disordered hole and electron energy states is determined to be a linear function of the externally applied electric field, E-a. On this basis the distorted Gaussian shaped interfacial electric field E-int is obtained and the empirical E-int is identified as the linear interpolation function of positive slope through the inflection point of the calculated curve. The disordered energy states extend throughout the P3HT charge transport gap and the populations of interface charges follow unequal E-a dependences. The coupling between the deduced interfacial electric field E-int and the P3HT effective mobility enables the predictions to be compared to the published time-of-flight room temperature negative field drift mobility data of holes and electrons in various metal/P3HT sandwich-type organic structures.