宮前 孝行

Understanding the charge behavior inside organic layer interfaces in multilayer organic light-emitting diodes (OLEDs) is essential for improving device efficiency and lifetime.

Abstract To optimize carrier balance in organic light-emitting diodes (OLEDs), it is crucial to gather data on the charge transport properties of organic semiconductors as measured within a functioning device. In this study, we directly investigated the hole mobility of the N,N′-diphenyl-N,N′-bis(1-naphthyl)-1,1′-biphenyl-4,4′-diamine (α-NPD) layer in multilayer OLEDs with various hole injection layers (HILs). This was achieved using the metal–insulator–semiconductor charge extraction by linearly increasing voltage (MIS-CELIV) technique, which utilized hole accumulation at the α-NPD/8-hydroxyquinoline aluminum (Alq3) interface. Our findings confirmed that hole mobilities evaluated via MIS-CELIV with sufficiently large offset voltages were more reliable, as the space-charge-limited extraction was realized through adequate hole accumulation at the α-NPD/Alq3 interface. Moreover, the hole mobility in the multilayer OLED configuration, as assessed by MIS-CELIV, reflects not only the hole transport properties of the α-NPD layer but also the hole injection properties at the HIL/α-NPD interface.

Organic multilayer systems, which are stacked layers of different organic materials, are used in various organic electronic devices such as organic light-emitting diodes (OLEDs) and organic field-effect transistors (OFETs). In particular, OFETs are promising as key components in flexible electronic devices. In this study, we investigated how the inclusion of an insulating tetratetracontane (TTC) interlayer in ambipolar indigo-based OFETs can be used to alter the crystallinity and electrical properties of the indigo charge transport layer. We find that the inclusion of a 20-nm-thick TTC film thermally annealed at a low temperature of 70 °C acts to significantly increase the ambipolar electrical transport of the indigo layer. X-ray diffraction, atomic force microscopy, and vibrational sum frequency generation measurements showed that annealing the TTC film significantly improved its ordering. The electronic sum-frequency generation spectra of TTC/indigo bilayers show that this improved ordering of TTC films promotes the growth of crystalline indigo films that exhibit charge mobilities in OFET that are nearly an order of magnitude larger than those measured for devices grown on unannealed TTC layers. Furthermore, using vibrational sum-frequency generation spectroscopy, we found that pre-annealing the TTC layer prior to indigo deposition can suppress the formation of defects within the TTC layer during indigo film growth, which also contributes to enhanced charge transport. Our results highlight the importance of controlling the molecular ordering within the interlayer contacts in OFET structures to achieve an enhanced performance.