Heat the olive oil in a skillet over medium-high heat. Brown tenderloins on all sides, about 10 minutes. Meanwhile, mix together teriyaki sauce, chicken broth, and brown sugar in a bowl. Stir in garlic, red chile pepper, onion, and black pepper.
Put browned tenderloins into slow cooker, cover with the teriyaki sauce mixture. Cook on High for about 4 hours, turning 2 to 3 times during the cooking time to ensure even doneness.
Remove tenderloins from the slow cooker and let rest for 5 minutes before slicing. If desired, spoon liquid over slices when serving.