I use the power on/off button for that.
From the i3lock manpage:
Users who wish to explicitly enable DPMS only when their screen is locked can use a wrapper script around i3lock like the following:
#!/bin/sh
revert() {
xset dpms 0 0 0
}
trap revert HUP INT TERM
xset +dpms dpms 5 5 5
i3lock -n
revert