Generowanie QR Code dla miniaturki postu

Please help because I do not know much about PHP. I found a piece of code that I generate QR Code for each post

<img src="https://chart.apis.google.com/chart?cht=qr&chs=150x150&chld=L|4&chl=<?php the_permalink(); ?>" width="150" height="150" alt="QR code" />

I would like rewrite it so that for each post fetch the URL to the post thumbnails.