Adsorption of Se, Na, and O on the Mo(110) surface: modeling atomic mechanisms at the back contact of thin film chalcopyrite solar cells.

Guido Roma1,2, Letizia Chiodo3, Janos Kiss2 1SRMP/CEA-Saclay, Gif sur Yvette, France /2Joh. Gutenberg Universität, Mainz, Germany /3Italian Institut of Technology, CBN@unlie, Lecce, Italy

Adsorption of Se, Na, and O on the Mo(110) surface: modeling atomic mechanisms at the back contact of thin film chalcopyrite solar cells. Guido Roma1,2, Letizia Chiodo3, and Janos Kiss2 1SRMP/CEA-Saclay, Gif sur Yvette, F-91191, France 2Institute for Inorganic and Analytical Chemistry, Joh. Gutenberg Universität, Mainz, D-55128, Germany 3IIT Italian Institute of Technology - CBN@Unile, Lecce, I-73010, Italy The deposition of the chalcopyrite light absorbers onto the back contact of thin films solar cells involves the adsorption of Se on Mo surfaces, to form an intermediate MoSe2 layer. Impurities like oxygen and sodium are also present in common processing technologies. As a support to the understanding of the atomic mechanisms related to the formation of MoSe2 during deposition, as well as the proposed catalytic role of sodium, we predict the adsorption energies and most stable adsorption sites of Se, O and Na on the Mo(110) surface using first principles calculations, based on density functional theory. We have checked the high symmetry adsorption sites and found that the long bridge site is the most stable one for Se, while the hollow site is preferred for oxygen and Na adsorption. Oxygen has the highest absolute value of adsorption energy, followed by selenium, then by sodium. The latter can easily be desorbed at the processing temperature of thin films CIGSe solar cells, while the barrier for the desorption of oxygen and selenium is much to high. The calculated adsorption energy allows also an estimation of the migration barrier for surface diffusion at low coverage, which is expected to be extremely low for sodium (~0.1 eV) for Na, and somewhat higher for Se and O (~0.4-0.6 eV). With these results all the three elements are expected to be highly mobile on the Mo(110) surface at usual processing temperatures.