Here's the thing, the Format-* commands are best left for displaying textual information inside your console — on the screen. When we start putting data into files, we typically (practically always) forgo this set of cmdlets. Enter my example below into your console and see what you get. Replace COMPUTER with the name of a single machine, as it's best to test with one, before you return them all (*).
Edit: Another thing to consider, as I've done above, and now below, is to use the -Filter parameter of Get-ADComputer to do your filtering, instead of filtering with Where-Object. There are some really sound reasons as to why you should do it this way. While you can't use -match, you can use other comparison operators, such as -eq and -like. Take a look.