What does 100% efficiency mean? Does that mean that for some power delivered to the LED load there is a (near) 100% efficient conversion to visible light ("photons")? Would that mean the LEDs operate with practically no heat generation?

Yup means little or no heat is generated so all the electricity is converted to light.

Yes, they claim that almost all states excited by electricity now emit visible light; previously 3/4 were in the IR spectrum.

The 3/4 may be true for oled . But for regular LEDs were near 100% already :

https://c1cleantechnicacom-wpengine.netdna-ssl.com/files/201...

The graph states lumen per watt, not percentage efficiency.

Indeed. Theoretical maximum is over 200 lm/W: https://en.m.wikipedia.org/wiki/Luminous_efficacy

Oh, that is nicely misleading. The combination of being totally awed by the close proximity to perfection implied by the misreading and being used all of SI fitting together like Legos completely got me (see downvoted comment below, I guess the yaysayers could be right once more - and even now I still think that a photon stream worth one Watt of power, constrained to the visual spectrum could be a very nice alternative unit for the value usually given in lumens).

And yet you won't find any shortage of people who will call you a terrible, terrible naysaying pessimist if you dare to voice the prediction that an efficiency jump like from incandescents to CFL or from CFL to LED won't ever happen on top of state of the art LED.

Unfortunately this (truly 100%) is impossible ipso facto the 2nd law of thermodynamics.

You're probably thinking of heat engines. Extracting 100% efficiency from a heat source is impossible. That doesn't mean that near-100% of non-heat energy can't be converted to another energy type. Eg resistors pretty much convert electricity to heat at a 100% "efficiency" rate..

Hm clearly I've got some more learning to do about this. Thanks!

In fact because of thermodynamics over 100% efficient LEDs are possible. This was practically demonstrated some years ago: https://phys.org/news/2012-03-efficiency.html

Converting light to electricity on the other hand cannot reach 100% efficiency.

It needs to be pointed out that the diode isn't converting over 100% of the electrical power into optical power. The extra optical power comes from Peltier heat drawn from the semiconductor lattice. The LEDs only had ">100% efficiency" when heated to 135°C. At 25°C the electrical efficiency was less than 1%.[1]

[1] Fig 1 (a) https://journals.aps.org/prl/pdf/10.1103/PhysRevLett.108.097...

That still counts. Ambient temperature and pressure are necessary for just about any 'engine'. In fact, extracting heat from the environment to do something useful is more impressive than just converting one form of energy into another.

Please explain how this is. It seems like something could convert all energy to light without generating heat and while conserving total energy & mass...

Well, the law they're referring to states that rather than just conserving energy and mass you also can't have a process that lowers entropy. Although it's unclear how this is relevant because, if anything, converting kinetic energy to light increases entropy, or at the very least doesn't decrease it.

This rules out superconductors, does it not?

But how do they get the light out? Mode density in air is only about half of what it is in organics.