Organic light-emitting diodes (OLED) are widely used in the display of smartwatches, cell phones, televisions, and laptop computer monitors due to OLED’s superior properties, such as being large-area, self-emissive, having a high contrast ratio, and being able to be made in flexible form[1]. In addition, OLED should be utilized in emerging areas such as augmented reality (AR), virtual reality (VR), indoor visible light communication, and bio-identifications[2, 3]. The brightness and efficiency of an OLED are crucial for various applications. They are closely linked to the radiative emission process. Electrons and holes are injected from the cathode and anode into the OLED and then transported to the luminescent material for radiative emission. The radiative emission recombines electrons and holes into photons. This process is evaluated by the quantum efficiency (QE), which indicates the percentage of electrons and holes transformed into photons, encompassing injection efficiency, transport efficiency, balance of electron and hole numbers, exciton formation efficiency, and radiative recombination efficiency of carriers [4].
The brightness depends on the number of emitted photons. The earliest fluorescent material emits photons by exciton recombination through the singlet state (S1) to ground state (S0) transition. The participation of S1 exciton is 25%, limiting the QE to be 25%. Another 75% of excitons belong to the triplet state (T1), which is not recombined radiatively due to the unpaired spin[5]. Several material systems are reported to achieve high efficiency and high brightness[6, 7], including but not limited to the guest-host phosphorescent materials[8], thermal activate delayed fluorescent (TADF), and type-II aligned luminescent materials for exciplex emissions[9, 10]. Phosphorescent materials are typically molecules with a transition metal at the center, facilitating metal-ligand charge transfer to ensure charge recombination through singlet and triplet states, thereby enhancing radiative recombination efficiency [11-15]. Theoretically, the allowance of both singlet and triplet transition in the phosphorescent material has the ultimate quantum efficiency (QE) of up to 100% [16-18]. The phosphorescent material is appropriately doped in a host material to prevent concentration quenching and form the guest-host luminescent system in a phosphorescent OLED[18, 19]. The TADF depends on a specially designed luminescent material or a material system that has a very small singlet and triplet states’ energy difference (DES-T). The excitons in the triplet states are transferred to singlet states through reverse inter-system crossing (RISC) by thermal energy[20-24]. By transferring the excitons in triplet states to singlet states, the theoretical QE is 100%.
To enhance the QE of an OLED, it is important to increase the number of charge carriers available for radiative recombination through device architecture, in addition to the complex material design and synthesis. One way to achieve this is by inserting a layer of indium-tin-oxide (ITO) or V2O5 between two luminescent layers to increase the number of charge carriers[25]. Electron-hole pairs generated within the V2O5 layer are separated and then injected into the corresponding luminescent layers by an applied voltage. Subsequently, they recombine radiatively with electrons or holes injected from the cathode or anode. Using an oxide semiconductor charge generation layer, the current efficiency increased from approximately 16 cd/A to 31 cd/A. With two V2O5 layers, the current efficiency increased to nearly 48 cd/A. By appropriately designing the emitting layer, considering material combinations, and replacing the V2O5 with MoO3, the QE can be improved to as high as 40% [26]. Besides the single-layer oxide semiconductor, various charge-generation layers have been reported. These include photovoltaic-type organic bulk heterojunction layers[27, 28] like CuPc-C60 or ZnPc-C60, an organic heterojunction donor-acceptor layer (HAT-CN: m-MTDATA)[29], MoO3-ZnO bilayers[30], organic-inorganic bilayers (C60/rubrene: MoO3)[31], and perovskite (CsPbBr3)-C60 bilayers[32]. The efficiency improved by these charge generation layers. It’s important to note that in some tandem devices, an increase in total thickness leads to a higher applied voltage. Along with the high operating voltage, the complexity of device fabrication also hinders the realization of CGLs. This work proposes and implements a device with a simple structure that leverages multiple CGLs to achieve high IQE without an increased operating voltage by using inorganic quantum dots.
We present a highly efficient blue organic light-emitting diode (OLED) that utilizes silicon quantum dots (SiQDs) as charge-generation centers (CGCs). This approach enables internal quantum efficiency (IQE) to be over 100%. The SiQDs, characterized by transmission electron microscopy (TEM), x-ray photoemission spectroscopy (XPS), and photoluminescence (PL) spectrum, have an average size of approximately 6 nm and are well-dispersed within the guest-host blue phosphorescent light-emitting materials. The XPS depth profile shows that the Si atoms are distributed throughout the layer. Moreover, the presence of SiO2 in the XPS indicates that the SiQDs have a core-shell structure, which allows them to be spatially separated within the luminous layer. While SiQDs exhibit red emission when excited by ultraviolet illumination, the blended SiQDs and blue phosphorescent exhibit sky-blue emission peaks at 436, 475, and 500 nm in the PL spectrum. To investigate the impact of SiQDs on OLED performance, we measured the electroluminescent spectrum on OLED devices with different SiQD concentrations. With only 5×10-3 % (in weight) of SiQDs in the precursor, the QE increased from 2% to 17.7%. Notably, the operation voltage remained almost unchanged at this concentration, indicating that SiQDs do not affect the operation of an OLED. Time-resolved photoluminescence was used to investigate charge generation. We found that the decay time in the time-resolved PL (TR-PL) increased from 1.68 to 5.97 ns. By the charge-transition mechanism, the increased radiative decay time indicates the charge transfer from the SiQDs to the luminous material.