How do Excess Carriers Affect the Steady-State Thermodynamic Stability of Point Defects?

2026 marks the 100^th anniversary of Frenkel introducing the concept of thermal excitations creating pairs of point defects in crystals. Yet to date the influence of excess carriers introduced by bias or illumination on the concentrations of point defects in crystals has not been properly developed. Under steady-state above-gap illumination, a semiconductor stores a portion of the incident chemical potential energy in the form of excess carrier pairs with measurable quasi-Fermi level splitting (i.e. the V_oc of a solar cell). Taking into account the requirement of charge balance, we demonstrate that above-gap illumination changes the steady-state thermodynamic equilibrium density of point defects coupled to the carrier system by capture/emission processes. In the simplest model of a material able to form one type of defect, we demonstrate that the concentration may increase slightly or be dramatically suppressed depending on the dark equilibrium Fermi energy. This can be viewed in terms of a portion of the energy stored in the excess carriers being transferred into excess defect concentrations via capture/emission processes. For impurity defects, this can be expressed as a change in the “equilibrium” solubility limit under illumination. We give expressions for the excess chemical potential and free energy change, and describe attempts to extend the theory to the general case with many defects present.

This bulk photoionic effect is obviously of interest in terms of engineering defect populations during crystal growth (although many other competing effects are also possible), and extends our understanding of what “non-equilibrium processing” truly means in the context of laser and rapid thermal annealing. Lastly, in scenarios such as halide perovskites with low migration barriers, or at long times in inorganic solar cells and in high-injection LEDs and lasers, these effects may help determine long-term degradation modes.