Heat olive oil in a skillet over medium-high heat. Brown tenderloins on all sides, about 10 minutes.

Meanwhile, mix together chicken broth, teriyaki sauce, and brown sugar in a bowl. Stir in onion, garlic, red chile peppers, and black pepper.

Put browned tenderloins in a slow cooker and cover with teriyaki sauce mixture. Cook on Low for about 4 hours, turning 2 to 3 times during cooking time to ensure even doneness. The internal temperature of tenderloins should reach 145 degrees F (63 degrees C).

Remove tenderloins from the slow cooker and let rest for 5 minutes before slicing. Spoon liquid over slices when serving, if desired.