The material had to work on both sides of a junction
A large band gap permits blue photons, but a bright LED also needs good crystals and effective electron and hole injection. Akasaki and Amano developed buffer-layer growth and p-type gallium nitride; Nakamura developed complementary growth and activation methods.
Bright nitride LEDs emerged in the early 1990s. The 2014 award recognizes the efficient blue sources that enabled new white-light technology.
Injected carriers can take several routes
An electron and a hole can recombine radiatively, producing a photon. Defects offer nonradiative routes, and high carrier densities introduce further losses. Light extraction then determines how much internally generated light reaches the outside.
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Efficiency measures need their denominators
Internal quantum efficiency compares radiative recombination with total recombination. External quantum efficiency also includes injection and extraction. Wall-plug efficiency compares optical power with electrical power, so electrical voltage matters as well as the number of photons.
Sources: [1]
The Lab isolates competing recombination channels
The new LED Lab changes injection in a synthetic recombination model and separates emitted light from losses. It explains why more current need not improve the fraction converted to light. Its coefficients do not identify the unique microscopic cause of droop in a real nitride device.