I am guessing the problem with DLC is the high refractive index which gives high reflectivity.  Glass alone reflects enough on a sunny day but DLC would be worse.  Texturing the surface to add an anti-reflective effect wont work as the mechanical abrasion/scratch resistance requirements for cellphones and such is very high.  Any roughness of the surface would compromise the mechanical properties and as you lower reflectivity of the surface even very tiny scratches become highly visible.

Thanks Martin. Do you mean that for a single layer of DLC coating, one has to trade off between "anti-reflective" and "anti-scratch"? If so what if one puts the DLC coating layer under a conventional anti-reflective layer and above the protective glass for a cellphone screen? Could one take advantages of both layers?

Actually I have several pieces of cellphone screens with an outermost DLC coating layer(by magnetron sputtering), a right-beneath anti-reflective layer and the protective glass under the two. I don't notice uncomfortable reflections in daily use but according to you I should put it under serious reflection tests from tomorrow on.

Hi Hayk, this seems exactly what I'm looking for!

It is my understanding that DLC coatings may suffer from particularly difficult adhesion issues on many substrates due to stress induced by lattice mismatch, but I'm not sure if this is the case on glass.