Controlling 7W 12-14V power LED with Arduino
A Darlington has great current amplification but doesn't switch on with a zero volt drop: -

 

You can put a decent drive voltage on the base that will make the input transistor turn on and only drop 0.2 (ish) volts. This is recognized as the saturation limit for a BJT generally. Some are lower of course.

So, the input transistor is turned on and the voltage on its emitter is 0.2V below the voltage on the common collector point. The final transistor needs about 0.7V across its base and emitter but it can only get this voltage via the 1st transistor hence, the best volt drop across the final stage is 0.7V + 0.2V - you lose about a volt of drive.

This is the basic problem and maybe consider using a MOSFET instead.