Black polarizer foil damaged after seperating the digitizer

Hi,

I want to fix my Xperia Z by changing the digitizer part of the screen. I have already disassembled the Phone and I seperated the screen and the digitizer. There is a black foil between the screen and the digitizer which is now damaged. What is this foil good for? Do I actually need it or did I just destroy my screen?

Thanks for your help.