Why do charges accumulate at grain boundaries?

[citw]

The concept of "space charge" around grain boundaries keeps coming up in my reading. I understand that charges tend to build up at grain boundaries, but I'm not sure why. Can someone explain what "draws" charges to boundaries?

 

[UltrafastPED]

The internal potential for the neighboring grains differ due to strain and imperfections. This generates an effective field between the grains.

 

[DrDu]

I think the example best understood is that of a pn junction in semiconductors.