Hi Christian,
in order to provide an ID for a component, you have to set it at component instantiation, as e.g. in the following code snippet
var oComp = sap.ui.getCore().createComponent({
name: "my.component.class",
id: "myComp",
height: "100%"
});
var oComponentContainer = new sap.ui.core.ComponentContainer({
component : oComp
});
oComponentContainer.placeAt("content");
then you can access the component later on via
sap.ui.getCore().getComponent( "myComp")