Question

why no current flows in a pnjunction if no battery is connected

Answer 1

Please refer to the attached diagram.

When a P type material and an N type material are appropriately joined holes from the P type material diffuse toward the N type and electrons from the N type diffuse toward the P type. The combine at the interface of the the junction and form a region of compensated charges (neutralizing ) called the depletion region. This migration of carriers leaves behind in the P type a region of fixed negative ions ( circled in the blue area)and in the N type there is a region of fixed positive ions ( circled in the red area) both next to the depletion region. The are referred to as space charges. The negative space charge keeps further electrons from the N type from migrating into the P type while the Positive space charge keeps additional holes from migrating into the N type.