We have recently applied SSL certificate on our website and we want all our url to have https:// protocol.
Once we moved our website to https://, our website broke down
Please use the following:
get_stylesheet_directory_uri();
it will be a web-address (starting with http:// or https:// for SSL). As such, it is most appropriately used for links, referencing additional stylesheets, or probably most commonly, images.
https://codex.wordpress.org/Function_Reference/get_stylesheet_directory_uri