Although this isn't related to expressions and should probably be posted in the generic after effects section, here is something that will hopefully help.
In my opinion the key to making this work is using a font that is suitable, then adding a levels adjustment and adjust the alpha channel in the levels effect to remove all unwanted anti aliasing. I am using a font from dafont called blocks to make my example work.
I can only add that normally TC is displayed in a monospaced font so it won't jump around.
but this is going to be unreadable anyways. What is it for, humans? (If not, you have some 6 bytes of data for a monochrome QR alike code, that is plenty.)