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Instead of using a dumbbell, try kickbacks using a low cable. The dumbbell triceps kickback isn't bad. To see why doing a kickback using a low cable is better, you first need to know about "length-tension relationship" or what many lifters call the strength curve. This is the relationship between the length of the muscle fiber and the force the fiber produces at that length. Muscles have the lowest potential to generate force when they're either fully elongated (stretched) or fully shortened (contracted). They generate the highest possible tension in the middle – halfway through the range of motion. But using the dumbbell gives you no loading demand on the triceps at the bottom of the exercise, when your elbow is bent at 90-degrees, which is where you're strongest. Same thing at the top of the range of motion, when your elbow is straight and your triceps are shortened (contracted), which is where you're weaker – the weight is at is heaviest because the lever-arm is at its longest.

This is why so many people cheat at the top-end of the range of motion when doing dumbbell kickbacks by using momentum or dipping their arm or torso farther forward. So when doing dumbbell kickbacks, you're getting weaker as the weight is getting heavier. Sure, you could just go lighter, but doing kickbacks using a low cable does two things: The cable gives you more tension through the range of motion, and it involves a resistance curve that matches the strength curve better. Start the exercise with the cable at roughly a 45-degree angle to the floor. This will load the bottom position of the exercise and create the longest lever-arm in the mid-range aspect of the motion. It'll also create a short lever-arm at the top of the action. – Nick Tumminello