RESUMO
Enforcing balanced electron-hole injection into the emitter layer of quantum-dot light-emitting diodes (QLEDs) remains key to maximizing the quantum efficiency over a wide current density range. This was previously thought not possible for quantum dot (QD) emitters because of their very deep energy bands. Here, we show using Mesolight® blue-emitting CdZnSeS/ZnS QDs as a model that its valence levels are in fact considerably shallower than the corresponding band maximum of the bulk semiconductor, which makes the ideal double-type-I injection/confinement heterostructure accessible using a variety of polymer organic semiconductors as transport and injection layers. We demonstrate flat external quantum efficiency characteristics that indicate near perfect recombination within the QD layer over several decades of current density from the onset of device turn-on of about 10 µA cm-2, for both normal and inverted QLED architectures. We also demonstrate that these organic semiconductors do not chemically degrade the QDs, unlike the usual ZnMgO nanoparticles. However, these more efficient injection heterostructures expose a new vulnerability of the QDs to in device electrochemical degradation. The work here opens a clear path towards next-generation ultra-high-performance, all-solution-processed QLEDs.
RESUMO
Self-compensated (SC) hole-doped conjugated polyelectrolytes with high work functions can provide efficient hole-injection and -collection layers for organic and other semiconductor devices. If these films can be photocrosslinked, the semiconductor overlayer can be deposited from a wider range of solvents, enabling flexibility in device design and fabrication. However, a generic photocrosslinking methodology for these materials is not yet available. Here, we demonstrate that sFPA82-TfO, the recently developed bis(fluorophenyl azide) photocrosslinker that is also i-line compatible, can surprisingly give 100% efficient photocrosslinking for SC hole-doped conjugated polyelectrolytes, i.e., one crosslink per reactive moiety, using mTFF-C2F5SIS-Na, a triarylamine-fluorene copolymer, as the model polyelectrolyte, without degrading its ultrahigh work function of 5.75 eV. The photocrosslinking efficiency is much higher than in the corresponding undoped polyelectrolyte and nonconjugated polyelectrolyte films, where the efficiency is only 20%. We attribute this improvement to the formation of smaller ion multiplet clusters in the hole-doped polyelectrolyte, as suggested by molecular dynamics simulations and infrared spectroscopy, which prevents occlusion of the ionic crosslinker. Photocrosslinking of the SC hole-doped mTFF-C2F5SIS-Na film used as a hole-injection layer in 100 nm-thick PFOP diodes suppresses the leakage current by over 3 orders of magnitude compared to those without crosslinking, to below 30 nA cm-2 at ±2 V. Photocrosslinking of the same film used as the hole-collection layer in PBDTTPD:PC61BM solar cells produces a higher photocurrent density, fill factor, and power conversion efficiency.
