Export gridview to excel file

Hi Friends,

 I want to export gridview to excel file. In template field i have AjaxModalPopupExtender control so that it showing error like

" Extender control 'AjaxModalPopupViewPhoto' is not a registered extender control. Extender controls must be registered using RegisterExtenderControl() before calling RegisterScriptDescriptors().
Parameter name: extenderControl"

  Now i dont know what i need to do. Any one can help me..
source code :-

<ItemTemplate>
<asp:Label ID="lblIssueNo" CssClass="itemtemplateblue" runat="server" Text='<%#Bind("Issue_number") %>'>
asp:Label>
<cc1:ModalPopupExtender ID="AjaxModalPopupViewPhoto" BackgroundCssClass="modalTextBackground" runat="server" TargetControlID="imgPhoto" PopupControlID="divphoto">cc1:ModalPopupExtender>ItemTemplate>asp:TemplateField>
ItemTemplate>