You need to use a CellEditorBinding in order to bind the content of the RichTextBox to the cell's; however, if you want to use the WPF RichTextBox, you might be in for a surprise as it is not like the WinForms one and can't be easily bound to.

For example, in the WPF RTB, there is no Text property to which you can bind but rather a Document property, which exposes a FlowDocument an that is not bindable. You would need to create a "BindableFlowDocument" as demonstrated here and then bind to that property using the CellEditorBinding.