How to search the function in Excel files

Hello everyone!

I have Script bellow, and it’s works for string, but I need search the function into the excel cell, not string. It´s possible?
For exemple: $keyword = “SUM” or $keyword = “=CONCAT”
Whem the Script finds key ($keyword ), the file (excel) will move to other directory ($destination)

Get-Process -Name EXCEL | Stop-Process -ErrorAction SilentlyContinue

$source = 'C:\'
$destination = 'C:\WithFunction'
$keyword = "SUM"

$files = Get-ChildItem -Path $source | Where-Object {$_.Name -like '*.xls*' -and '*.xlsx*'}
foreach ($file in $files)
    Write-Progress -Activity "Checking: $file" -Status "File $counter of $($files.count)" -PercentComplete ($counter*100/$files.count)
    $path = $file.FullName
    $objExcel = New-Object -ComObject Excel.Application
    $objExcel.Visible = $false
    $workBook = $objExcel.Workbooks.Open($path)
    $sheets = $workBook.Sheets
    $results = @()
    foreach ($sheet in $sheets)
        $sheetName = $sheet.Name
        $filter1 = $workBook.Sheets.Item("$sheetName").UsedRange.Find("$keyword")
        if ($filter1.Text -eq $keyword -or $filter1.Text -match '^OT[- ].*' )
          $results = $true
        $close = $workBook.Close()
    if ($results -eq $true)
        Move-Item -Path $path -Destination $destination -Verbose
    If ($results -ne $true)
        Write-Host $path do not contain $keyword1 -ForegroundColor Yellow