Tritiated Amorphous Silicon: Insights into the Staebler-Wronski Mechanism

Hydrogen, though essential for device-quality amorphous silicon, likely contributes to the light-induced degradation process (Staebler-Wronski effect) that reduces the solar cell efficiency by about 4 absolute percent. We are testing the role of hydrogen by using its isotope tritium. When tritium bonded to Si spontaneously decays into inert helium-3, it should leave behind the Si dangling bonddefect. We have studied degradation due to tritium and note its resemblance to the Staebler-Wronski effect. Surprisingly, 100x fewer defects are created than expected from the tritium decay rate, suggesting a mechanism that heals most of the defects, even at temperatures down to 4 K. We consider different mechanisms for the thermal and athermal healing processes (e.g. motion of hydrogen, effectof beta-electrons, decay of hydrogen-tritium molecules). Our findings shed new light on the degradation mechanism in a Si:H and help reveal the role of hydrogen and structural rearrangements near a newly created defect.