In a large skillet over medium heat, brown the ground beef until fully cooked, breaking it apart as it cooks. Drain any excess fat.
Add the diced onion and green bell pepper to the skillet and sauté for about 5 minutes until they are softened.
Transfer the beef mixture to a slow cooker.
Add the ketchup, brown sugar, Worcestershire sauce, mustard, garlic powder, onion powder, salt, and black pepper. Stir until well combined.
Cover the slow cooker and cook on low for 4-6 hours or on high for 2-3 hours, stirring occasionally.
Once cooked, serve the sloppy joe mixture on hamburger buns.