Hi,
Use hidden field because data stored in hidden field does not get lost when page is get post back.
Before getting page post back store yr value in hidden field
<asp:HiddenField ID="HiddenField1" runat="server" />
HiddenField1.Value = "Yr Value in Div";
For reading from it just
divId.InnerHtml= HiddenField1.Value;
Hope this will work.