CharacterController doesn't work with collider

Hi I make a 2d game. The Player moves up and I can move him to the left and the right side. Now I want that he can't move out of the view so I want to create cubes at the left and right sides. But everytime he moves through the cubes. Here is my movement script:

 var moveDirection : Vector3 = Vector3.zero;
 
 function Update () 
 {
 
 var controller : CharacterController = GetComponent(CharacterController);
 
 controller.Move(moveDirection*Time.deltaTime);
 
 if(Input.GetKey("d"))
 {
    //transform.position = Vector3(transform.position.x+5*Time.deltaTime,transform.position.y,transform.position.z);
 }
 
 if(Input.GetKey("a"))
 {
    //transform.position = Vector3(transform.position.x-5*Time.deltaTime,transform.position.y,transform.position.z);
 }
 
 }