Quantum dot light emitting diode (QLED) display is the new generation of display after organic light emitting diode (OLED) display. Just like the introduction of inkjet printing into OLED display panel manufacturing, QLED display can also employ inkjet printing to deposit red-green-blue (RGB) pixels precisely and to make great saving in materials. However, the performances of inkjet-printed QLEDs are far below those of spin-coated QLEDs at present, which has hindered its further development into a viable manufacturing process.
There are two main reasons affecting the efficiency of inkjet-printed QLEDs,including the lack of hole transport materials (HTMs) which have matched energy bands and suitable for inkjet printing, and the erosive effect of solvents in solution form of materials on the underlying film during the inkjet process. Current HTMs such as poly(9-vinylcarbazole) (PVK), TFB or Poly-TPD have not only poor solvent resistance, but also low hole mobility or high hole injection barrier at hole transport layer/quantum dots (HTL/QDs) interface (more than 1 eV).
Recently, professor SU Wenming’s group at the Printable Electronics Research Center (PERC), Suzhou Institute of Nano-tech and Nano-bionics (SINANO) of Chinese Academy of Sciences (CAS), has developed a novel cross-linkable HTM based on the small molecule ((4,4’-bis(3-vinyl-9H-carbazol-9-yl)-1,1’-biphenyl, CBP-V). The cross-linked CBP-V film has a sufficiently low highest occupied molecular orbital (HOMO) energy level (~ -6.2 eV), high hole mobility better than PVK, excellent solvent resistance and long-term stability.
For spin-coated red QLEDs using octane-dispersed CdSe-ZnS core-shell QDs, the CBP-V showed maximum external quantum efficiency (EQE) of 15.0% compared to 11.5% of with PVK. For inkjet-printed bilayer (CBP-V/QDs) QLED in air, a maximum EQE of 11.6% has been achieved, which almost equated to the spin-coated reference device (12.6%). A group of 30 devices were fabricated, showing highly reproducible efficiency with the maximum relative deviation no more than 3%.
This is the first report of such high-efficiency inkjet-printed multilayer QLEDs and demonstrates a unique and effective approach to inkjet printing fabrication of high-performance QLEDs. This study has been published in Small.
The work was financially supported by the National Natural Science Foundation of China, National Program on Key Research Project and Youth Innovation Promotion Association CAS.
The Journal cover (Image by Small)
Prof. Su Wenming, Printable Electronics Research Center, Suzhou Institute of Nano-tech and Nano-bionics, Chinese Academy of Sciences.