rows, pullups (palms away, add weight when you can) and bent over laterals all hit the rear delts pretty well
When I first started lifting seriously I really neglected isolations and since adding the bent laterals my rear delts have grown alot. At least for me just doing pulling exercises works my back way more. If you want to focus on the delt do your rows with a dumbell and bring your elbow up high. Barbell is way more of a back exercise for me