Historically, Cd-based photoluminescent quantum dot technology was superior to Cd-free, but that gap has narrowed significantly. That gap is also narrowing on the ELQD side, especially for external quantum efficiency (EQE), but lifetime still lags Cd-based ELQDs.
Development of ELQDs also has parallels to OLED technology as the fundamental device structures are similar and many of the same processing equipment can be used for both. Naturally, some of the materials and processing needs to be different, but like OLEDs, red and green are more mature than blue.
In the paper, the team reported the latest results for red, green and blue ELQD devices (not displays). Figure 1 shows the EQE development history along with the current results for Cd-based and Cd-free devices. Note that the EQE for Cd-free is now quite close to the Cd-based results.
Is this EQE good enough, we asked Hartlove?
“Not quite,” he said. “We think an EQE of 20% is the threshold for commercial viability and we expect to reach that in about a year. Display as opposed to device EQE can be higher as optical engineering can be applied to extract more light.” Nanosys exhibited an RGB demo at DisplayWeek.
What about lifetime, we asked next?
“There is more work that needs to be done here, and like OLED, blue is the tough nut to crack. Some of the red and green Cd-based ELQD materials may now have the lifetime and EQE to be viable commercially, but blue still lags for a full color display. We think our Cd-free lifetimes and EQE are now some of the best in the industry,” said Hartlove.