  The thermodynamics of several reactions involving atomic and molecular
hydrogen with group-III acceptors is investigated. The results provide a
first-principles-level account of thermally- and carrier-activated processes
involving these species. Acceptor-hydrogen pairing is revisited as well. We
present a refined physicochemical picture of long-range migration, compensation
effects, and short-range reactions, leading to fully passivated
$\equiv\textrm{Si-H}\cdots X\equiv$ structures, where $X$ is a group-III
acceptor element. The formation and dissociation of acceptor-H and
acceptor-H$_{2}$ complexes is considered in the context of Light and elevated
Temperature Induced Degradation (LeTID) of silicon-based solar cells. Besides
explaining observed trends and answering several fundamental questions
regarding the properties of acceptor-hydrogen pairing, we find that the
BH$_{2}$ complex is a by-product along the reaction of H$_{2}$ molecules with
boron toward the formation of BH pairs (along with subtraction of free holes).
The calculated changes in Helmholtz free energies upon the considered defect
reactions, as well as activation barriers for BH$_{2}$ formation/dissociation
(close to $\sim1$ eV) are compatible with the experimentally determined
activation energies of degradation/recovery rates of Si:B-based cells during
LeTID. Dihydrogenated acceptors heavier than boron are anticipated to be
effective-mass-like shallow donors, and therefore, unlikely to show similar
non-radiative recombination activity.

Materials Science