In a medium bowl, combine lemon juice, garlic, cilantro, soy sauce, olive oil, onion powder, and black pepper, mixing well. Place the chicken breasts in a large sealable plastic bag, pour the marinade over the chicken, close the bag and marinate for at least two hours or overnight in the refrigerator. Remove the chicken from the plastic bag, and place on a foil lined baking pan. Broil for 6 minutes on each side or grill until the chicken reaches 160º.