Complex Cell Format, Maybe!!!

If you are fine with a bit of VBA you can make this happen.

Right-click the worksheet's tab and choose "View Code". Paste this code in the window that opens:

    Private Sub Worksheet_Change(ByVal Target As Range)
        If Target.Cells.Count = 1 Then
            TurnIntoSuperScript Target
        End If
    End Sub

Next, from the menu, choose Insert, Module. Paste in this code:

    Sub TurnIntoSuperScript(cl As Range)
        Dim val As Variant
        Dim pos As Long
        val = cl.Value
        pos = InStr(val, "-")
        If pos > 0 Then
            With cl.Characters(Start:=1, Length:=pos - 1).Font
                .Superscript = False
            End With
            With cl.Characters(Start:=pos, Length:=Len(val) - pos + 1).Font
                .Superscript = True
            End With
        Else
            cl.Font.Subscript = False
        End If
    End Sub

Now try entering a fraction into a cell on the worksheet.