you want to target your lower/middle traps.
The best thing i found that gave me really good back thickness is doing underhand yates rows. I like to go as heavy as i possibly can and squeeze out a solid 10 reps. Make sure your elbows don't travel to closely to your body otherwise your lats will come into play. Keep your body tight and imagine that you are pushing something back with your elbows instead of pulling something close to your chest. That way you use less of the bicep and more of your back.
Also very important to overall back thickness. REAR DELTS. I isolate these hard on shoulder day i do about 8 sets total and 12 reps each set. This is the most important muscle for proper posture and shoulder girdle positioning and general shoulder health.