Nanosys is working with LG Display to perfect an all solution-processed approach to device fabrication. According to Hartlove, LGD has a complete solution processing facility for OLEDs that they are also using to develop ELQDs on. The goal is to use the same equipment but to develop new materials and processing to optimize performance for ELQD. Some of the layers and materials can be similar but others will be different.
Development is a complex process with interactions between materials and equipment and between layers above and below – all impacting performance, lifetime and manufacturability. The mobility of holes and electrons are different for red, green and blue and for each device structure, but maintaining charge balance is a critical goal. Suitable solution-processed charge transport materials is a particular challenge, for example.
In their paper, the team described experiments that varied the shell thickness of red ELQDs and its impact on EQE as well as the solution quantum yield. As the QD shell thickness increased a peak in solution quantum yield and EQE were noted. With the increase of shell thickness, the device EQE initially improves due to the better charge confinement and reduced energy transfer among the quantum dots. This is demonstrated by the QD film quantum yield behavior. However, when the shell thickness passes an upper bound, the large strain caused by lattice mismatch between core and shell has a detrimental effect to device EQE and solution QY.