  Modern electronic devices are unthinkable without the well-controlled
formation of interfaces at heterostructures. These often involve at least one
amorphous material. Modeling such interfaces poses a significant challenge,
since a meaningful result can only be expected by using huge models or by
drawing from many statistically independent samples. Here we report on the
results of high throughput calculations for interfaces between crystalline
silicon (c-Si) and amorphous silicon nitride (a-Si$_3$N$_{3.5}$:H), which are
omnipresent in commercially available solar cells. The findings reconcile only
partly understood key features. At the interface, threefold coordinated Si
atoms are present. These are caused by the structural mismatch between the
amorphous and crystalline part. The local Fermi level of undoped c-Si lies well
below that of a-SiN:H. To align the Fermi levels in the device, charge is
transferred from the a-SiN:H part to the c-Si part resulting in an abundance of
positively charged, threefold coordinated Si atoms at the interface. This
explains the existence of a positive, fixed charge at the interface that repels
holes.
