Mix together the broth, peppercorns and thyme in the insert of a 5- to 7-quart slow cooker. Place the chicken breasts in the slow cooker, stacking them in an even layer.
Cover and cook on low for 4 to 5 hours, until the chicken is cooked through and tender. Let the chicken cool and remove it from the slow cooker. Refrigerate for 2 days or freeze for up to 10 weeks.