Turn off macbook screen when connected to a monitor [duplicate]

When connecting my 2017 15' macbook pro to a 4k external monitor it gets a little bit warmer (65° celsius, while it's only at 35 without the monitor) than I want it to be, even if it's just idling. Since I don't ever use the macbook screen as a secondary screen when connected, I thought I might turn it off completely and hopefully cool the machine down a little bit. Is there any way of doing this? I figured out I could just set the brightness to 0 but will this prevent the machine from actually rendering images?

If the idea was to just hide doubled picture on laptop display, you can just lower brightness level to 0 using fn + F1