I recently purchased and received two R2 bin LEDs. They have a forward voltage of about 3.80V and 3.56V at a current of 905mA. This is measured shortly after turn on, and they are well heatsunk, so these voltages are a little higher than if they were hot.
I'm an electrical engineer, and I work in the semiconductor industry. That said, I don't make LEDs, nor do I work for one of the LED manufacturers. Keep that in mind when reading the following...
As I understand it Indium Gallium Nitride (the semiconductor material of choice used to make todays blue LEDs) is not especially compatible with the dopants used to make P type and N type semiconductor material.
When pure, semiconductor materials (such as silicon, or indium gallium nitride) do not conduct electricity. When they are "doped", they can be made to conduct electricity. The stronger the doping the better they conduct, and the lower the resistivity of the resulting material. The doping process involves forcefully implanting, or diffusing at high temperature, special types of impurity atoms into the semiconductor crystal lattice structure. Phosphorous is usually used as a "P" type dopant, and there are a number of "N" type dopants used, such as boron.
The problem with Indium Gallium Nitride is that when you introduce impurity atoms(the dopants) into it, it distrupts the perfect crystal structure of the semiconductor, thereby creating imperfections in the material. These local sites of imperfection are potentially regions where dark recombination can occur in the LED. In other words, you can get current flow through this region of the junction, but no photons are emitted. The heavier you dope the material, the more imperfections in the crystal, and therefore the "current efficiency" of the LED decreases.
On the other hand, if you dope the LED heavily, you make the semiconductor conduct electricity better, and therefore you end up with an LED that has a lower forward voltage at a given current than a lighter doped LED. If you dope the LED heavily, you get an improvement in the "voltage efficiency", but at the expense of the "current efficiency." It is a constant trade off. All LED manufacturers want to make the most efficient LED overall, so they have to find the best possible doping level for their particular manufacturing process and technology.
In all probability, CREE decreased the average doping level on the Q5 bin LEDs, relative to the P4 or other lower flux, lower voltage devices. This decrease in doping reduced the number of imperfections in the die, thereby improving optical output, but at the expense of increasing forward voltage somewhat. Overall however, the Q5 is most certainly a superior product and produces more light from a given package and at higher overall efficiency. My recommendation is to be happy with the higher flux bins. They are, overall, better products than the lower bin devices.
These R2 bin LEDs are really awesome! Extremely bright, very significantly better than the P4. I can't wait for the R4 and beyond.
