Well, there are several possible responses to that...

There is certainly a theoretical limit to efficiency in converting electricity to light.

I'm not at all sure that means that any current technology is anywhere near it's limit. We're going to see some pretty impressive advances in LEDs, cells, controlling circuitry etc. for a long time to come... and, I think, with increasing rapidity.

If you mean whether the technical advance is theoretically unbounded, well, maybe not... but I remember reading the indisputable (at the time) mathematical demonstration that the cube-square law prevented there from being anything like a "ray gun", as the heat generated at the source would destroy any such device long before the heat generated at the target was effective. No matter how efficient your reflectors, etc, you hit the beam-spread limitations much faster than the limitations of your materials.

That was before the laser... then we just threw out all that theory, and acted like we never thought that, just like we always do when a theoretical limit is transcended.