ES6 Factory Pattern this is undefined

I am getting my feet wet with ES6 and implementing design patterns in Javascript for live projects. I have the following example snippet of code which throws an error saying that the this keyword evaluates to undefined:

export let LifecycleFactoryObj = (() => {
  this.myCollection = new WeakSet();
  //.... More here
  this.add = function (opts) {
    let tempObject = new MyCollection(opts);
    this.myCollection.add(tempObject);
    if (this.myCollection.has(tempObject)) {
      return tempObject;
     }
    return null;
  };
})();

The error that is thrown is:

undefined.lifecycles = new WeakSet(); ^ TypeError: Cannot set property 'lifecycles' of undefined

Looks like this inside the IIFE is being evaluated to undefined but I can't seem to trace why. I thought ES6 had scoped the this keyword correctly when using let for variable instantiation.

Thanks!