I don't recomended to make autonumber to an Id like this but you can split it.
Get the Last Invoice number from database and Split it by "/". Extract the second value in array and you will get the existing invoice num. Add it by 1 and there are new Invoice Num.
Private Sub InvoiceNum()
Set rs = New ADODB.Recordset
' Select last row data using MAX() function
rs.Open "SELECT max(Id)from Address", Conn, adOpenDynamic, adLockBatchOptimistic
Dim temp() As String
temp = Split(rs.Fields(0), "/") 'Split data by "/" sign
' temp (0) will contain DLP
' temp (1) will contain 003
' temp (2) will contain 2013-2014
MsgBox "DLP/00" & temp(1) + 1 & "/2013-2014" ' Show new invoice number