Place the chicken breasts into the bottom of the slow cooker.
In a medium bowl, stir together the brown sugar, vinegar, juice, garlic, soy sauce, and pepper.
Pour the liquid over the chicken.
Cover the slow cooker and cook on low for 6 hours.
30 minutes before serving, whisk together the cornstarch and water and pour into the slow cooker, stirring in to combine with the cooking liquid. Allow to thicken 30 minutes.
Serve the chicken with the liquid spooned over the top.
To prepare as a freezer meal:
Place all of the ingredients EXCEPT the cornstarch and water into a gallon size freezer bag. Freeze for up to 6 months. Thaw in the refrigerator overnight prior to cooking according to the instructions above.