There is interest in green and yellow LEDs with high radiance and high wall plug efficiency (WPE) for diverse applications such as displays (e.g., micro-displays), architectural lighting, and general illumination systems based on mixing the emission of direct color LEDs rather than phosphor conversion. These applications may be limited by the relatively poor WPE of conventional green and yellow LEDs. The so-called efficiency droop phenomenon may be much more severe in conventional green LEDs as compared to blue LEDs. Green LEDs may be particularly inefficient when they are driven at the high current densities required for projection display applications. The higher operating voltage of green LEDs relative to blue and red LEDs further complicates the design of driver circuitry and heat sinks.

The photoluminescence (PL) of green InGaN multi-quantum wells (MQWs) excited by absorption of shorter wavelength photons may be more efficient than the electroluminescence (EL) excited by electrical injection the same MQWs sandwiched in a p-n junction. This may be explained at least in part by a more even distribution of carriers between the MQWs when carriers are generated by optical absorption instead of electrical injection. The efficiency droop in EL applications may be exacerbated by an uneven distribution of carriers among the MQWs resulting from differences in the electrical transport behavior of holes and electrons. Using the PL from green MQWs excited by absorption of the EL of a shorter wavelength may be a promising method to improve the efficiency of high-radiance green LEDs. This concept may also benefit from the typically lower operating voltage of blue or near-ultraviolet LEDs compared to state-of-the-art electrically-injected green LEDs.