  Hydrogen, introduced into crystalline (c-) or amorphous (a-) silicon (Si),
plays an important role in improving Si properties for photovoltaics
application. Low temperature proton implantation in c-Si and a-Si or H-doping
of Si films introduces metastable hydrogen in the bond-centered-position (BCH),
which dissociates with increasing temperatures into new metastable complexes.
Using ab-initio molecular dynamics we report on the stability of BCH, a
convenient hydrogen model system, in crystalline Si and its temperature-induced
dissociation products in a wide temperature range, which includes temperatures
close to solar cell operating conditions. Particular attention was paid to the
newly experimentally discovered H2** dimer, and the H2* dimer, as well as
isolated interstitial hydrogen and monohydrides. Each complex leaves a
characteristic signature in the frequency spectrum, the density of states (DOS)
and in the imaginary part of the dielectric constant that agrees well with
experiments. All complexes modify the vicinity of the energy gap of pure c-Si.
BCH introduces characteristic donor levels, causing a strong peak in the DOS
just below the intrinsic conduction band. The Fermi energy is raised, filling
these donor states with two electrons and causing a strong peak in the
imaginary part of the dielectric constant in the infrared. The H2** and H2*
dimers introduce a low energy tail in the imaginary part of the dielectric
constant. The results are important for experimental in-situ optical
characterization of Si film growth that often involves hydrogen.
