Interface engineering, especially the realisation of ohmic contacts at the interface between organic semiconductors and metal contacts, is one of the essential preconditions to achieve high efficiency organic electronic devices. Here, the interface structure of polymer/fullerene blends are correlated with the charge extraction/injection properties of working organic solar cells. The model system - P3HT:PCBM - is fabricated using three different degrees of P3HT regioregularity to alter the blend interchain order and molecular packing, resulting in different device performance. Investigations by electroabsorption (EA) spectroscopy on these devices indicate a significant reduction (≈ 1 V) in the built-in potential with an increase in the P3HT regioregularity. This observation is also supported by a change in the WF of high regioregular polymer blends from photoelectron spectroscopy measurement. These results confirm the presence of a strong dipole layer acting as a δ-hole transporting layer at the polymer/MoO3/Ag electrode interface. Unipolar hole-only devices show an increase in the magnitude of the hole current in high regioregular P3HT devices, suggesting an increase in the hole injection/extraction efficiency inside device with a δ-hole transporting layer. Microscopically, near edge X-ray absorption fine structure (NEXAFS) spectroscopy was conducted to probe the surface microstructure in these blends finding a highly edge-on orientation of P3HT chains in blends made with high regioregular P3HT. This edge-on orientation of P3HT chains at the interface results in a layer of oriented alkyl side chains capping the surface which favors the formation of a dipole layer at the polymer/MoO3 interface. The increase in the charge extraction efficiency due to the formation of a δ-hole transporting layer thus results in higher short circuit currents and fill factor values, eventually increasing the device efficiency in high regioregular P3HT devices despite a slight decrease in cell open circuit voltage. These findings emphasise the significance of work function control as a tool for improved device performance, and pave the way towards interfacial optimisation based on the modulation of fundamental polymer properties, such as polymer regioregularity.