Why do we need to add hydrogen to amorphous silicon?
Well, one either adds it on purpose: In order to reduce the defect volume - by passivating the dangling bonds. Unsaturated silicon atoms in a-Si is a serial killer of charge carriers. But then the issue is the SWE - you have to handle it somehow. Options are few but research is being done. The common way of handling it right now is to be honest about it. This pv cell will lose some efficiency the first months, but then stabilize. Not a good selling point, but that is where it is at.

Or one adds it accidentally / coincidentally by using a deposition technique or raw material that contains hydrogen. ($\ce{SiH4}$ or $\ce{SiH_xCl_{4-x}}$). But you can usually thermocycle these problems away, as long as your substrate can handle it. If you did it, accidentally, on a plastic surface - it is usually referred to as "scrap" and recycled...