How can I use the fn key as a standalone key on my Mac?

I want to use the fn key on my keyboard as a trigger for an AppleScript (.scpt file) via FastScripts.

The reason that I want to do this is because the fn key is occupying coveted real estate on the keyboard, and I don't think I've once used a fn key shortcut in my life.

I installed Karabiner and went through every option under the "Change Fn key" and "Fn To All Modifiers Mode" categories individually, to see if FastScripts would be able to pick up on me pressing the fn key and thus allow me to assign the fn key as a script shortcut.

FastScripts was able to pick up my keypress when the "Fn to F14" option in Karabiner was selected. But, F14 is apparently already in use, because when I pressed the key, my screen brightness setting decreased a notch, which is obviously undesirable.
FastScripts was also able to pick up my keypress when the "Fn 2x to F19" option in Karabiner was selected. But, this obviously requires me to press the fn key, twice in order to start the script. This is also undesirable.

These were the only two Karabiner options that FastScripts was able to perceive.

I am not particularly wedded to using any application in particular (e.g., Karabiner) to accomplish this.

_{OS X El Capitan, version 10.11.6.}