Question

Why p-n junction allows current flow in one direction ?

Answer 1

In the PN junction, one side is made only for the negative charge and the other side is made for the positive charge and there are no carriers in the center thanks to electron/hole recombination. A forward bias will push the holes forward and pull the electrons back, generating a forward current.  A reverse bias will not push any carriers across the depleted junction. In this way, the electrons only move in one direction either forward or reverse.