Investigation of 4,4′-bis[(N- carbazole) styryl] biphenyl (BSB4) for a pure blue fluorescent OLED with enhanced efficiency nearing the theoretical limit

Abstract

4,4′-bis[(N-carbazole) styryl] biphenyl (BSB4 or BSBCz) is one of the widely studied organic fluorescent materials for blue organic electroluminescent devices in the recent times. In this work, BSB4 is used as a guest material to construct the host-guest matrix for the emissive layer (EML) of a pure blue fluorescent organic light-emitting diode (OLED). A pure blue emission suitable for display application with a Commission Internationale de l’Eclairage (CIE) coordinate of (0.147, 0.070) is achieved by the blue-shift of the emission spectrum of the host-guest matrix from that of the pristine guest (BSB4) molecules. The optimization of OLED structures is carried out by considering (i) charge balance in the emissive layer for high exciton density, and (ii) optical interference of generated light in the organic layers for increased light outcoupling. A thorough comparative study on the use of different combinations of widely used hole and electron transport layers to obtain charge balance in the EML of the OLED, thereby enhancing the external quantum efficiency (EQE) is shown. Optical interference effects in the fabricated OLEDs are analyzed by optical simulation of each device structure by transfer matrix method (TMM). With the optimized device structures, we are able to overcome the 2% EQE limit that has been reported so far for blue fluorescent OLEDs with BSB4 as light emitting material and achieve a maximum EQE of 4.08%, which is near to the theoretical limit of EQE for fluorescent OLEDs.

Broadband white electroluminescence from a dopant-free OLED comprising pure electromer and electroplex emission

“…HATCN/TAPC interface has been reported as an efficient charge generation unit that aids the injection of holes into the device [34][35][36]. TmPyPB is chosen as the ETL to engineer charge balance in the device as it has electron mobility comparable to the hole mobility in TAPC [37,38]. Gaussian represents one specific electronic transition between two energy states.…”

“…To examine the contribution of TAPC in the white emis- 3). Here, NPB is chosen as a replacement for TAPC since the HOMO energy level of TAPC and NPB are reported to be the same (5.5 eV) [37,38]. Hence, it forbids any significant alteration of the hole injection barrier in Device 3 as compared to that in Device 2 or Device 1.…”

“…Hence, here we witness the much weaker electromer emissions over usually stronger exciton or excimer emission. The effect of recombination zone (RZ) also determines the shape of the EL spectra [37,48,49]. In this work, the presented device structure posses multiple RZs.…”

Study on low-temperature evaporation of Ag2O-based Ag electrode and electron injection layer and their application in OLEDs

“…Conventional OLEDs are easily eroded by water and oxygen in the air, causing black spots and greatly reducing the lifetime. In contrast, inverted OLEDs (IOLEDs), which place water-oxygen-sensitive electron transport layers (ETLs) at the bottom of the devices, can effectively reduce the speed of water-oxygen erosion to the devices, thereby improving the lifetime [9][10][11]. In addition, for the driver of active matrix OLEDs, amorphous silicon thin film transistors (a-Si TFTs) with high carrier mobility have become very mature since the 1970s [12].…”