Place the chicken breasts, peppers, and onions in your slow cooker.
Drizzle with the oil, then sprinkle the chili powder, cumin, paprika, cayenne, garlic powder, salt, and pepper evenly over the top.
Cover and cook on HIGH for 3 hours or on LOW for 5-6 hours.
Check to ensure the chicken is fully cooked (internal temperature should reach 165°F/74°C). Carefully remove the chicken, slice it, and return it to the slow cooker.
Optional: Drain some of the excess liquid if you prefer a less wet mixture.
Serve warm with tortillas, toppings, or over rice or salad for a healthy meal.