Place chicken in the bottom of a lightly greased 6-quart slow cooker.
In a bowl, mix together lemonade concentrate, ketchup, brown sugar, vinegar, Worcestershire sauce, garlic powder, and red pepper flakes.
Pour mixture over chicken.Cover slow cooker and cook on LOW for 3 1/2 to 4 hours.
Pour liquid in slow cooker into a saucepan. Stir together cornstarch with 2 tablespoons cold water. Whisk into the saucepan and bring mixture to a boil. Simmer for a few minutes to thicken.
Pour sauce back over chicken. Sprinkle with parsley and serve.