where ( 1 − − f k e − − f k h ) weakens the Coulomb interaction via the so-called phase-space filling factor that stems from the Pauli exclusion principle preventing multiple excitations of fermions. Due to the phase-space filling factor, the Coulomb attraction becomes repulsive for excitations levels f k e + f k h > 1 . At this regime, the generalized Wannier equation produces only unbound solutions which follow from the excitonic Mott transition from bound to ionized electron–hole pairs.