powershell export local store certificate to pfx without password enters password

using certutil is pretty straight forward:

certutil -exportpfx -p "" my serialnr path\to\hostname.pfx noroot

I need to convert it from pfx to pem, so I need to do some scripting. This is what I have so far:

   $output = ( certutil -store my | findstr /i seri )
   $separator = ":"
   $serialstring = $output.split( $separator )
   $serialnr = $serialstring[1]
   $exportpfx = "certutil -exportpfx -p `"`" my" + $serialnr + " path\to\"+$env:computername+".pfx noroot" 
   invoke-expression $exportpfx

This nearly works, but the resulting pfx file has a password. So apparently my $exportpfx var is saving more info than it should, but I cannot see what.

Any tips? Thanks!