As electronic devices shrink in size to reach nanoscale dimensions, interfaces between different materials become increasingly important in defining the device characteristics. In this talk I will present first principles calculations of the electrical properties of interfaces, based on Non Equilibrium Greens Function calculations (NEGF) using the ATK software. I will present calculations of the Schottky barrier at a Silicon-Silver interface, and calculate the dependence of the barrier on the doping and the applied bias. In another example, I will present calculations for the band offset between CZTS and CdS in a solar cell device and use the ab initio data in a multi-scale simulation of the solar cell efficiency.