  Temporal variations of Cu(In,Ga)Se$_2$ photovoltaic device properties during
light exposure at various temperatures and voltage biases for times up to 100 h
were analyzed using the kinetic theory of large lattice relaxations.
Open-circuit voltage and p-type doping increased with charge injection and
decreased with temperature at low injection conditions. Lattice relaxation can
account for both trends and activation energies extracted from the data were
approximately 0.9 and 1.2 eV for devices with lower and higher sodium content,
respectively. In these devices, increased sodium content resulted in higher
initial p-type doping with greater stability. First principles calculations
providing revised activation energies for the ($V_{Se}-V_{Cu}$) complex suggest
that this defect does not account for the metastability observed here.
