This experiment addresses the role of hydrogen in determining both quality and stability of surface passivation materials for silicon solar cells. Surface recombination causes losses in such solar cells, but can be reduced by depositing passivating surface films of amorphous hydrogenated silicon (a-Si:H). Long term stability and thermal stability is however a challenge due to hydrogen desorption. It is not easy to measure hydrogen concentration gradients in thin films using other experimental methods than neutron reflectometry. Our samples are precharacterized with X-ray reflectometry and photoconductance minority carrier lifetime measurements. We ask to measure 10 a-Si:H/Si bilayers and expect the measuring time based on earlier experiments at ILL to be approximately 12 hours, including setup time and alignment. Thus we ask for 10 x 12 = 120 hours of beam time, or 5 days.