Enhancing external quantum efficiency in a sky-blue OLED by charge transfer via Si quantum dots

Enhancing external quantum efficiency in a sky-blue OLED by charge transfer via Si quantum dots | Research Square window.SnipcartSettings = { analytics: { enabled: false } }; (function() { var accessVector = localStorage.getItem('access_vector') || ''; window.dataLayer = window.dataLayer || []; if (accessVector) { window.dataLayer.push({ user: { profile: { profileInfo: { snid: accessVector } } } }); } })(); (function(w,d,s,l,i){w[l]=w[l]||[];w[l].push({'gtm.start':new Date().getTime(),event:'gtm.js'});var f=d.getElementsByTagName(s)[0],j=d.createElement(s),dl=l!='dataLayer'?'&l='+l:'';j.async=true;j.src='https://www.googletagmanager.com/gtm.js?id='+i+dl;f.parentNode.insertBefore(j,f);})(window,document,'script','dataLayer','GTM-K279D39R'); Browse Preprints In Review Journals COVID-19 Preprints AJE Video Bytes Research Tools Research Promotion AJE Professional Editing AJE Rubriq About Preprint Platform In Review Editorial Policies Our Team Advisory Board Help Center Sign In Submit a Preprint Cite Share Download PDF Research Article Enhancing external quantum efficiency in a sky-blue OLED by charge transfer via Si quantum dots ZINGWAY PEI, HAN YUN WEI, YI CHUN LIU, THIYAGU SUBRAMANI, NAOKI FUKATA This is a preprint; it has not been peer reviewed by a journal. https://doi.org/ 10.21203/rs.3.rs-4466701/v1 This work is licensed under a CC BY 4.0 License Status: Published Journal Publication published 14 Dec, 2024 Read the published version in Discover Nano → Version 1 posted 11 You are reading this latest preprint version Abstract Organic light-emitting diodes (OLEDs) aim to achieve high efficiency by using excitons to achieve a 100% quantum efficiency (QE). However, developing functional organic materials for this purpose can be time-consuming. To address this challenge, a new method has been proposed to incorporate inorganic quantum dots into the organic luminescent layer to enable unlimited exciton formation and approach the 100% QE limit. Inorganic quantum dots are clusters of atoms that contain numerous thermally generated electrons and holes at conduction and valence bands. Immersed quantum dots act as charge generation centers, providing electrons and holes with unlimited amounts to form excitons. After radiative recombination, these excitons generate photons that cause internal QE to nearly 100%. This concept has been demonstrated using Silicon quantum dots (SiQDs) and phosphorescent materials. The average size of SiQDs is approximately 6 nm, and they are well-dispersed within the guest-host blue phosphorescent light-emitting materials. With only 5×10 -3 % (in weight) of SiQDs in the precursor, external QE increased from 2% to 17.7%, nearly a nine-fold enhancement. The prolonged decay time from 1.68 to 5.97 ns indicates that electrons are transferred from SiQDs to the luminescent materials. This universal method can be applied to green and red emissions with various inorganic quantum dots in different organic luminescent material systems. Figures Figure 1 Figure 2 Figure 3 Figure 4 Figure 5 1. Introduction 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 (S 1 ) to ground state (S 0 ) transition. The participation of S 1 exciton is 25%, limiting the QE to be 25%. Another 75% of excitons belong to the triplet state (T 1 ), 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 (DE S-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 V 2 O 5 between two luminescent layers to increase the number of charge carriers[25]. Electron-hole pairs generated within the V 2 O 5 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 V 2 O 5 layers, the current efficiency increased to nearly 48 cd/A. By appropriately designing the emitting layer, considering material combinations, and replacing the V 2 O 5 with MoO 3 , 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], MoO 3 -ZnO bilayers[30], organic-inorganic bilayers (C60/rubrene: MoO 3 )[31], and perovskite (CsPbBr 3 )-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 SiO 2 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. 2. Materials and Methods 2.1 Concept The proposed structure, shown in Fig. 1 a, exemplifies a significant deviation from the traditional approach. Rather than forming a charge generation layer (CGL), the Si quantum dots (SiQDs) are dispersed throughout the luminescent layer, allowing for electrons or holes provided by the SiQDs to form excitons at neighboring luminescent molecules (guest) and recombine radiatively to emit light. The SiQDs are covalent-bonded inorganic materials with a high density of states that contribute many carriers, as depicted in Fig. 1 b. Ensuring the carriers in SiQDs transfer effectively to the luminescent molecules is essential. Therefore, the carrier transition rate of a host, guest, and SiQDs in a phosphorescent OLED (PhOLED) should be considered. The relationship of transition rate in the guest-host system has been studied previously[ 33 ]. We included SiQDs in the conventional guest-host transition system. The transition rate inside a proposed system contains K H , K F , K R , K G , K FF , K RR, and K QDs , as displayed in Fig. 1 c. Where K H , K G , and K QDs are the radiative recombination rate in the host, guest, and the SiQDs, respectively. The K F and K R are carriers’ forward and reverse transition rates from the host to the guest material. K FF and K RR are carriers’ forward and reversed transition rates from the guest to the SiQDs. K G should be maximized to achieve the highest luminescent efficiency. Therefore, either K G should be more significant than K H or K F , or it should be more effective than K R and K H between host and guest materials, as shown in Fig. 1 d. Furthermore, K G should either be higher than KQDs or KRR should be more significant than K FF and K QDs to prevent radiative recombination in the SiQDs. 2.2 Silicon Quantum Dots Preparation To prepare SiQDs, the hydrogen silsesquioxane (HSQ) was used as a precursor and was used as received. The solvent was removed from the HSQ stock solution using a rotary evaporator in a water bath at 40°C, resulting in gel formation and drying overnight under vacuum. After it dried, the white solid was placed in a quartz crucible and transferred in an inert atmosphere to a high-temperature furnace, where it was annealed at 1100°C for one hour in the atmosphere of 95% Ar and 5% H 2 . After grinding, 200 mg of the fine powder was added to a mixture of 2 mL of de-ionized (DI) water, 2 mL of ethanol, and 2 mL of HF to etch the SiO 2 matrix and decrease the size of the Si. After solvent removal and centrifugation, the product was dried under a dry N 2 flow to obtain 20 mg of powder and quickly transferred to a round-bottom flask containing 10 mL of 1-dodecane. The solution was then heated at 190°C overnight in Ar ambient in the same round-bottom flask. After the reaction and removal of the excess 1-dodecane, 5 mL of toluene was added to obtain a 1-dodecane-capped, hydrogen-terminated SiQDs solution. The solution is further diluted to 0.1 wt % in toluene for device preparation. 2.3 Quantum dot characterization The TEM images of SiQDs are shown in Fig. 1 e, and the SiQDs are well separated. The size of the SiQDs ranged from 3.1 to 9.0 nm and had the highest population at around 6.6-7.0 nm, as shown in Fig. 1 f, according to the TEM images. The photoluminescence of the Si QD is shown in Fig. 1 g. It peaks at around 680 nm, a red emission corresponding to the bandgap of 1.82 eV. After blending the SiQDs with a 5×10 − 3 wt.% concentration to the host (4,4’-Bis(N-carbazolyl)-1,1’-biphenyl (CBP)) and the guest Bis[2-(4,6-difluorophenyl) pyridinato-C2, N] (picolinato) iridium (III)(FIrpic)) materials in the precursor. Three PL emissions at 436 nm, 470 nm, and 510 nm were found for the blending solution with and without SiQDs, corresponding to the CBP and FIrpic emissions, respectively. The PL emission associated with SiQDs was not observed in this system. Further, excite the solution by 254 nm UV light, the SiQDs exhibit a red color, and FIrpic + CBP and FIrpic + CBP + SiQDs solutions display a sky-blue color, as shown in Fig. 1 h, which coincides with the PL spectrum. This indicates the additive of the SiQDs does not alter the emission of the guest and host system in the phosphorous OLED. These observations support our assumption that the additive SiQDs have small KQDs and observable radiative luminescence and are used to provide the carriers to the emitting molecules, the FIrpic. 2.4 SiQDs OLED device fabrication The layer structure and energy alignments of the PhOLED were depicted in Fig. 2 a and b , respectively. Indium-tin-oxide (ITO) coated glass was used as a substrate to make SiQDs OLED devices. The ITO was patterned into two mm-wide slots. After cleaning, organic layers were applied. The conducting polymer, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT: PSS), was used as the hole injection layer, spin-coating on the ITO at 5,000 rpm for the 30s. After coating, the PEDOT: PSS was thermally dried on a hot plate at 120 o C for 10 min. The thickness is approximately 30 nm. After PEDOT: PSS, the precursor of the emission layer was also spin-coated on the PEDOT: PSS at 2,000 for the 30s and was then dried on a hotplate at 40 o C for 240s. FIrpic is dissolved with SiQDs in the CBP as the emitting layer. The 4,4’-Bis(N-carbazolyl)-1,1’-biphenyl (CBP) and Bis[2-(4,6-difluorophenyl) pyridinato-C2, N] (picolinato) iridium (III)(FIrpic) were dissolved in the chlorobenzene (CB) in a total concentration of 2.4 wt. % in 1 ml as precursors. The weight of CBP is 21.12 mg, which is 2.88 mg for FIrpic. All chemicals in the precursors of the emitting layer (EML) were used as received. The SiQDs in the solution were added into the precursors by a micropipette with different amounts. The PEDOT: PSS and light-emitting layer thickness are approximately 30 and 68 nm measured by a stylus profiler (BRUKER DektakXT) and was confirmed by an atomic force microscopy (Digital Instruments Dimension 3100 Controller). After the emitting layer, 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), Tris(8-hydroxyquinolinolato)-aluminum (III)(Alq3), Lithium Fluoride (LiF), Aluminum (Al) were sequentially coated in 10, 20, 1 and 100 nm thick, respectively, by thermal evaporation. The device area is 2 mm by 3 mm. 3. Results and discussion 3.1 Characterization of OLED emission The photograph of the sky-blue emission from the PhOLED is shown in Fig. 2 c. The current-voltage characteristics of the PhOLED device with different SiQDs concentrations are shown in Fig. 2 d, which was measured using an Agilent B2912A semiconductor parameter analyzer in dark conditions. The PhOLED were made of the SiQDs in the precursor from 5×10 − 4 to 0.33 wt.%, a significant range of doping concentration. With a high SiQDs concentration (0.33 wt. %), the turn-on voltage is around 3.3 V, which is a 7.9 V reduction compared to the reference device. This indicates the injected carriers may transport through the SiQDs, causing a concentration quench. Therefore, no emissions are recorded until the SiQD concentration is reduced to 0.11 wt.%. In contrast to OLED with layer CGLs, the PhOLED with SiQDs CGCs has reduced operation voltage instead of increased. The conventional peripheral circuit can operate the SiQDs CGCs PhOLED in low voltage. The dependence of the turn-on voltage on the SiQDs concentration is plotted in Fig. 2 e. The turn-on voltage increased with decreased SiQDs concentration in the precursors. With 5×10 − 4 wt. % of the SiQDs, the turn-on voltage is almost identical to the reference device, indicating that the injected carriers will not transport through SiQDs. The emission spectrum of the OLED from the reference and SiQDs doped (5×10 − 3 wt. %) device was shown in Fig. 2 f taken by a spectroradiometer (Optronic Laboratories, OL-770) equipped with a 6-inch integrated sphere powered by a Keithley 2400 source meter. The applied voltage is 14 V. They have almost identical peaks in the emission, 470 and 510 nm. However, PhOLED with SiQDs CGCs exhibits lower turn-on voltage and higher external quantum efficiency (EQE). The EQE increased with the increase of the SiQDs concentration from 5×10 − 4 wt. % to 5×10 − 3 wt. %. Meanwhile, the SiQDs concentration is beyond 5×10 − 3 wt. %, the EQE starts to decrease, as shown in Fig. 2 g. The highest EQE for the PhOLED with SiQDs is 17.7% at low current density (2 mA/cm 2 ), as shown in Fig. 2 h. In comparison, the EQE for the PhOLED without SiQDs is only 2% in this study. The luminous efficiency for the device is 19.6 cd/A at 500 cd/m 2 for the PhOLED with SiQDs, as shown in Fig. 2 i. In comparison, it is 3.08 cd/m 2 for the reference device at a much lower luminance. The EQE enhanced about eight times from 2 to 17.7% for the SiQDs doping. 3.2 Distribution of SiQDs The additive of quantum dots or nano-flasks into the precursor solution of the luminescent materials may be thought to condense at the hole injection layer (HIL) and emitting layer (EML)[ 34 ], which enhances the hole injection efficiency and hence improves the EQE. To clarify that the SiQDs are well-dispersed inside the emitting materials instead of forming a layer at the HIL/EML interface, the areal and vertical distribution of the SiQDs in the EML were explored. The precursor solution contains CBP, FIrpic, and 5×transmission electron microscopy TEM investigated 10 − 3 wt. % of the SiQDs). As shown in Fig. 3 a, the dropped solution on the Cu mesh tends to aggregate together. The elements analysis indicates the Si atoms have a distribution shape the same as the TEM image, as shown in Fig. 3 b. It is worth noting that the O atoms have the same distribution as the Si atoms. Although the N, F, Ir, and C atoms cannot be observed to have the same distribution shape as the TEM image, they are distributed well. These atoms are associated with the CBP and FIrpic. The TEM images indicate the SiQDs are well dispersed in the CBP and FIrpic blended materials. Further exploring the SiQD, the shape of TEM images (Fig. 3 c), the distribution of the Si atoms (Fig. 3 d), and the oxygen atoms (Fig. 3 e) are identical, which indicates the SiQDs contain large amounts of oxygen. The atomic percentage of oxygen is like the Si atoms (Fig. 3 f). The SiQDs doped (5×10 − 3 wt. %) Firpic and CBP thin film was investigated by the x-ray photoemission (XPS) depth profile to understand the vertical distribution. The depth is deduced from the XPS’s sputter time and film thickness. The atomic signals of Si and O exhibit stable distribution from the top of the emitting layer to the bottom, as shown in Fig. 4 a. The signals of F and Ir were also analyzed. They are indicators of the FIrpic. Si, O, F, and Ir coexistence indicates the SiQDs are distributed well along the film thickness with the Firpic. The TEM and XPS studies show that the SiQDs are well-dispersed spatially inside the EML and do not pile up at the HTL/EML interface. In addition to the SiQDs distribution, the binding status of the Si is studied by XPS, as shown in Fig. 4 b. The Si 2p spectroscopy can be deconvoluted into two peaks at 99.6 eV and 102.36 eV, which corresponds to the Si and SiOx. The existence of the SiOx indicates the SiQDs may partially oxidize at its outer surface, forming a SiOx/Si core-shell structure that prevents the coalescence of SiQDs. 3.3 Carrier dynamics in the OLED The donating of carriers from SiQDs to the FIrpic can be proved by the time-resolved PL (TRPL). The transient behavior in the TRPL can be modeled by the exponential decay with a time constant[ 35 ]. The time constant represents the lifetime of carriers in the material system that could contribute to the radiative light emission. The TRPL from the FIrpic + CBPP and SiQDs doped FIrpic + CBPP are depicted in Fig. 4 c. The TRPL presents two distinct decay behavior. Therefore, it is modeled by the following equation with two lifetimes. $${I}_{PL}={A}_{1}{e}^{-\left(\raisebox{1ex}{$t-{t}_{0}$}\!\left/ \!\raisebox{-1ex}{${\tau }_{1}$}\right.\right)}+{A}_{2}{e}^{-\left(\raisebox{1ex}{$t-{t}_{0}$}\!\left/ \!\raisebox{-1ex}{${\tau }_{2}$}\right.\right)}$$ 1 The τ 1 and τ 2 for referenced and SiQDs doped devices at 510 nm are fitted by this equation. The τ 1 and τ 2 are 1.68 and 95.99 ns for the referenced device and 5.97 and 103.71 ns for the device with SiQDs doping. The τ 1 is 3.5 times enhanced after SiQDs doping. The lifetime can be further correlated with the transition rate by the following equation, $${\tau }_{PL}=\frac{1}{{K}_{r}+{K}_{nr}}$$ 2 in which the K r is the transition rate of radiative recombination, and the K nr is the transition rate of non-radiative recombination. According to Fig. 1 c and d , the difference in transition rate between reference and SiQDs-doped material systems is the forward and backward carrier transition rates, K FF and K RR . Therefore, the \({\tau }_{PL}\) in the reference can be expressed as $${\tau }_{PL}=\frac{1}{{K}_{G}+{K}_{nr}}$$ 3 The \({\tau }_{PL, Si QDs}\) for the materials containing SiQDs can be expressed as $${\tau }_{PL, Si QDs}=\frac{1}{{K}_{G}+{K}_{nr}+{K}_{FF}-{K}_{RR}}$$ 4 If K RR is larger than K FF , the denominator in the (4) will be smaller than in the (3). Consequently, the \({\tau }_{PL, Si QDs}\) is larger than \({\tau }_{PL}\) , which coincides with the TRPL results in this work. The more extensive lifetime in the SiQDs doped layer indicates the carriers in the SiQDs are donating to the FIrpic, as our assumption. 3.4 Carrier transport path in the OLED The ultra-violet photoemission spectroscopy (UPS) is used to study the actual band alignment between SiQDs and the Firpic. The UPS for SiQDs and Firpic are shown in Fig. 4 d. The energy of the input UV photons is 21.2 eV, and the Fermi levels extracted for SiQDs and Firpic are 4.68 eV and 4.88 eV, respectively. We assume either SiQDs or Firpic are undoped, and the measured Fermi levels are the mid-gap. The conduction band and valence band edges of the SiQDs are calculated as 3.73 and 5.63 eV, respectively. With the same method, the calculated Lowest unoccupied molecule orbit (LUMO) and highest occupied molecule orbit (HOMO) of FIrpic are 3.64 and 6.12 eV, respectively. The alignment between SiQDs and the FIrpic is plotted in Fig. 5 a. The barrier for electron transfer from SiQDs to the FIrpic is 0.09 eV. And the energy barrier is 0.49 eV for the holes in the SiQDs. The average thermal energy of the carrier is 3/2 kT, which is around 0.04 eV at room temperature. These barriers are higher than the thermal energy, and the carriers in the SiQDs in the conduction and valence band edge cannot easily overcome these barriers. Fortunately, some carriers exhibit higher energy if the carrier concentration is high enough. These energetic carriers can be transferred from SiQDs to the FIrpic at room temperature. The reason for there is an appropriate concentration of the SiQDs doping can be understood by the mechanism of carrier transfer from SiQDs to the surrounding emitting molecules, FIrpic. With high concentrations of the SiQDs, large amounts of charges are transported through SiQDs, proved by the increased current density as explored. As we demonstrated, the transport between SiQDs causes a concentration quenching because the SiQDs have a low radiative recombination rate. Reducing the SiQDs’ concentration can help reduce the transport path between SiQDs. Therefore, the isolated charges inside SiQDs can donate to the neighbor luminescent molecules, the FIrpic. The EQE reaches a maximum when the transport path through SiQDs is prohibited, and most of the luminescent molecules are ranged in the donation range of the SiQDs. Two mechanisms are responsible for the charge transfer in the organic system, as shown in Fig. 5 b: Dexter charge transfer and Föster resonant energy transfer (FRET)[ 36 , 37 ]. By the Dexter charge transfer, the electron in the SiQDs may transfer directly to the closed-surrounded luminesce molecules, setting up the charge donation[ 37 ]. By the FRET, the energy that recombines an electron in the SiQDs from the conduction band to the valence transfers to the other luminescent molecule by exciting an electron from the ground to the excited state[ 37 ]. We marked part of the SiQDs in the TEM image, as shown in Fig. 5 c, to verify a possible transfer mechanism. Each SiQD is very close, within 2 nm, for the doping level of SiQDs is 5×10 − 3 wt. %. Therefore, the dominant carrier transfer mechanism is most possibly the Dexter direct transfer. The electrons in the SiQDs are transferred directly to luminescent molecules. 4. Conclusion Silicon quantum dots are immersed in this work into phosphorescent luminescent materials as charge generation centers for unlimited electrons and holes. The SiQDs have an average diameter of 6 nm and are well dispersed in the luminescent materials by TEM and XPS analysis. The PhOLED exhibits 17.7% external QE by the SiQDs doping in minimal amounts of 5×10 − 3 %, which is approximately 9 times enhancement. TRPL measured the decay time of radiative emission on the luminescent organic with SiQDs, and the increased decay time at the luminescent wavelength (500 nm) reveals the supply of excitons from the neighbors, which are SiQDs. The proposed and demonstrated concept solely involved electron transfer that may not limited by the categories of inorganic quantum dots, the emission wavelength, and the selection of luminescent organics. Abbreviations EQE External quantum efficiency OLED Organic light-emitting diodes PhOLED Phosphorescent OLED SiQDs Silicon quantum dots CGCs charge-generation centers EML Emitting layer Firpic Bis[2-(4,6-difluorophenyl) pyridinato-C2, N] (picolinato) iridium (III) CBP (4,4’-Bis(N-carbazolyl)-1,1’-biphenyl PEDOT PSS:poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) TRPL Time-resolved photoluminescence UPS Ultraviolet photoelectron spectroscopy LUMO Lowest unoccupied molecule orbit HOMO highest occupied molecule orbit FRET Föster resonant energy transfer Declarations The authors declare no conflicts of interest. Ethics approval and consent to participate: Not applicable Consent for publication: All authors consent to the publication of this manuscript. Availability of data and material: The data that support the plots and other findings within this report are available from the corresponding authors upon reasonable request. Competing interests: The authors declare that they have no competing interests. Funding: The authors Z. P., H. Y. W., and Y. C. L acknowledge the financial support of the National Science and Technology Council, Taiwan by grants NSTC-112-2221-E-005-066-, NSTC-111-2622-E-005-008- and supported by the “Innovation and Development Center of Sustainable Agriculture” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education MOEE) in Taiwan. The authors T.S. and N. F. acknowledge the support of the World Premier International Research Center Initiative WPII), MEXT, Japan Authors’ contributions: Z. P. conceived the idea, designed experiments, analyzed data, and wrote the paper. T.S. performed material synthesizing under the supervision of N. F., and H.-Y. W. and Y.-C. L. performed structure characterization and fabricated device characterization. All authors discussed the results and co-wrote the manuscript. 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Recent Advances in Thermally Activated Delayed Fluorescent Polymer—Molecular Designing Strategies. 2020, 8 (725), DOI: 10.3389/fchem.2020.00725. Kim, J. H.; Lee, D. R.; Han, S. H.; Lee, J. Y. Over 20% external quantum efficiency in red thermally activated delayed fluorescence organic light-emitting diodes using a reverse intersystem crossing activating host. Journal of Materials Chemistry C 2018, 6 (20), 5363-5368, DOI: 10.1039/C7TC05811J. Matsumoto, T.; Nakada, T.; Endo, J.; Mori, K.; Kawamura, N.; Yokoi, A.; Kido, J. 27.5L: Late-News Paper: Multiphoton OrganicELL device having Charge Generation Layer.SID D Symposium Digest of Technical Papers 2003, 34 (1), 979-981, DOI: https://doi.org/10.1889/1.1832449. Sasabe, H.; Minamoto, K.; Pu, Y.-J.; Hirasawa, M.; Kido, J. Ultra high-efficiency multi-photon emission blue phosphorescent OLEDs with external quantum efficiency exceeding 40%. Organic Electronics 2012, 13 (11), 2615-2619, DOI: https://doi.org/10.1016/j.orgel.2012.07.019. Chen, Y.; Chen, J.; Ma, D.; Yan, D.; Wang, L.; Zhu, F. High power efficiency tandem organic light-emitting diodes based on bulk heterojunction organic bipolar charge generation layer. Applied Physics Letters 2011, 98 (24), 243309, DOI: 10.1063/1.3599557. Zhao, D.; Liu, H.; Miao, Y.; Wang, H.; Zhao, B.; Hao, Y.; Zhu, F.; Xu, B. A red tandem organic light-emitting diode based on organic photovoltaic-type charge generation layer. Organic Electronics 2016, 32 , 1-6, DOI: https://doi.org/10.1016/j.orgel.2015.12.029. Sun, H.; Guo, Q.; Yang, D.; Chen, Y.; Chen, J.; Ma, D. High Efficiency Tandem Organic Light Emitting Diode Using an Organic Heterojunction as the Charge Generation Layer: An Investigation into the Charge Generation Model and Device Performance.ACS S Photonics 2015, 2 (2), 271-279, DOI: 10.1021/acsphotonics.5b00010. Yang, H.; Kim, J.; Yamamoto, K.; Hosono, H. Efficient charge generation layer for tandem OLEDs: Bi-layered MoO3/ZnO-based oxide semiconductor. Organic Electronics 2017, 46 , 133-138, DOI: https://doi.org/10.1016/j.orgel.2017.03.041. Liu, Y.; Wu, X.; Xiao, Z.; Gao, J.; Zhang, J.; Rui, H.; Lin, X.; Zhang, N.; Hua, Y.; Yin, S. Highly efficient tandem OLED based on C60/rubrene: MoO3 as charge generation layer and LiF/Al as electron injection layer. Applied Surface Science 2017, 413 , 302-307, DOI: https://doi.org/10.1016/j.apsusc.2017.04.038. Bao, C.; Chen, C.; Muhammad, M.; Ma, X.; Wang, Z.; Liu, Y.; Chen, P.; Chen, S.; Liu, B.; Wang, J.; Duan, Y. Hybrid perovskite charge generation layer for highly efficient tandem organic light-emitting diodes. Organic Electronics 2019, 73 , 299-303, DOI: https://doi.org/10.1016/j.orgel.2019.06.022. Baldo, M. A.; Forrest, S. R. Transient analysis of organic electrophosphorescence: I. Transient analysis of triplet energy transfer. Physical Review B 2000, 62 (16), 10958-10966, DOI: 10.1103/PhysRevB.62.10958. Pei, Z.; Chiang, W.; Shih, H.; Chang, H.; Yang, J. Using Distributed Energy States of Graphene Quantum Dots for an Efficient Hole-Injection Media in an Organic Electroluminescent Device. IEEE Electron Device Letters 2018, 39 (12), 1912-1915, DOI: 10.1109/LED.2018.2874445. Pei, Z. W., H.-Y.; Liu, Y.-C. Exciton Up-Conversion by Well-Distributed Carbon Quantum Dots in Luminescent Materials for an Efficient Organic Light-Emitting Diode. Nanomaterials 2022, 12 , 1174, DOI: https://doi.org/10.3390/nano12071174. Lakowicz, J. Principles of Fluorescence Spectroscopy , 2006; Vol. 1. Martins, T.; Ribeiro, A.; Souza, G.; Cordeiro, D.; Silva, R.; Colmati, F.; Lima, R. B.; Aguiar, L.; Carvalho, L.; Reis, R.; Santos, W. New Materials to Solve Energy Issues through Photochemical and Photophysical Processes: The Kinetics Involved. 2018. Salehi, A.; Fu, X.; Shin, D.-H.; So, F. Recent Advances in OLED Optical Design. Advanced Functional Materials 2019, 29 (15), 1808803, DOI: https://doi.org/10.1002/adfm.201808803. Brütting, W.; Frischeisen, J.; Schmidt, T. D.; Scholz, B. J.; Mayr, C. Device efficiency of organic light-emitting diodes: Progress by improved light outcoupling. physica status solidi (a) 2013, 210 (1), 44-65, DOI: https://doi.org/10.1002/pssa.201228320. Additional Declarations No competing interests reported. Cite Share Download PDF Status: Published Journal Publication published 14 Dec, 2024 Read the published version in Discover Nano → Version 1 posted Editorial decision: Revision requested 10 Jul, 2024 Reviewers agreed at journal 07 Jul, 2024 Reviewers agreed at journal 01 Jul, 2024 Reviews received at journal 06 Jun, 2024 Reviews received at journal 31 May, 2024 Reviewers agreed at journal 31 May, 2024 Reviewers agreed at journal 31 May, 2024 Reviewers invited by journal 26 May, 2024 Editor assigned by journal 26 May, 2024 Submission checks completed at journal 26 May, 2024 First submitted to journal 23 May, 2024 You are reading this latest preprint version Research Square lets you share your work early, gain feedback from the community, and start making changes to your manuscript prior to peer review in a journal. As a division of Research Square Company, we’re committed to making research communication faster, fairer, and more useful. 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Also discoverable on Platform About Our Team In Review Editorial Policies Advisory Board Help Center Resources Author Services Accessibility API Access RSS feed Manage Cookie Preferences © Research Square 2026 | ISSN 2693-5015 (online) Privacy Policy Terms of Service Do Not Sell My Personal Information {"props":{"pageProps":{"initialData":{"identity":"rs-4466701","acceptedTermsAndConditions":true,"allowDirectSubmit":false,"archivedVersions":[],"articleType":"Research Article","associatedPublications":[],"authors":[{"id":310761971,"identity":"a6f70065-6bb7-4f75-9878-24d898bf36f1","order_by":0,"name":"ZINGWAY PEI","email":"data:image/png;base64,iVBORw0KGgoAAAANSUhEUgAAAZAAAAAyAQMAAABI0h/eAAAABlBMVEX///8AAABVwtN+AAAACXBIWXMAAA7EAAAOxAGVKw4bAAABC0lEQVRIiWNgGAWjYDACZgglx3AAJiIBItgIazHG1MJDwLLEBqK1yLfzHn7NU3M4ve/24ccvPu5gyOOf3WPA8KHsMIO9RAJWLYzNfGnWPMcO5848l2ZmOfMMQ7HEnTMGjDPOHWbgwaGFmZnHzDiH7XDuhjMMZsa8bQyJGyRyDJh524BapLFrYQNr+Xc43eAM+zeElr94tPAw8xg/zm07nGBwBsiAa2HEo0UCaAvz3750w5lneMoYZ7ZJJM64kVZwsOdcOg/P/QfYQ6z/jPHHGd+s5fnOsG/+8LHNJrF/RvLGBz/KrOXYew5g1QLyDjAimmEMcKSAoxVfTDJ/YGCogzFGwSgYBaNgFGACAJ8YWmZtuJlOAAAAAElFTkSuQmCC","orcid":"","institution":"National Chung Hsing University","correspondingAuthor":true,"prefix":"","firstName":"ZINGWAY","middleName":"","lastName":"PEI","suffix":""},{"id":310761972,"identity":"0b23bd3a-b48d-4eb2-8695-57f0050aaba3","order_by":1,"name":"HAN YUN WEI","email":"","orcid":"","institution":"National Chung Hsing University","correspondingAuthor":false,"prefix":"","firstName":"HAN","middleName":"YUN","lastName":"WEI","suffix":""},{"id":310761973,"identity":"dd07f0ad-a1cd-48d2-8b4f-fa3ffa592ec7","order_by":2,"name":"YI CHUN LIU","email":"","orcid":"","institution":"National Chung Hsing University","correspondingAuthor":false,"prefix":"","firstName":"YI","middleName":"CHUN","lastName":"LIU","suffix":""},{"id":310761974,"identity":"6aebb83a-41fd-4bdd-aa91-f9f563ec4158","order_by":3,"name":"THIYAGU SUBRAMANI","email":"","orcid":"","institution":"National Institute for Materials Science (NIMS)","correspondingAuthor":false,"prefix":"","firstName":"THIYAGU","middleName":"","lastName":"SUBRAMANI","suffix":""},{"id":310761975,"identity":"9c74694e-f267-4a50-a07c-14870f5e39da","order_by":4,"name":"NAOKI FUKATA","email":"","orcid":"","institution":"National Institute for Materials Science (NIMS)","correspondingAuthor":false,"prefix":"","firstName":"NAOKI","middleName":"","lastName":"FUKATA","suffix":""}],"badges":[],"createdAt":"2024-05-23 12:02:23","currentVersionCode":1,"declarations":"","doi":"10.21203/rs.3.rs-4466701/v1","doiUrl":"https://doi.org/10.21203/rs.3.rs-4466701/v1","draftVersion":[],"editorialEvents":[{"content":"https://doi.org/10.1186/s11671-024-04171-w","type":"published","date":"2024-12-14T15:57:25+00:00"}],"editorialNote":"","failedWorkflow":false,"files":[{"id":57893336,"identity":"50e104dc-d7ae-4560-a200-06c0ab84baaa","added_by":"auto","created_at":"2024-06-07 07:01:07","extension":"png","order_by":1,"title":"Figure 1","display":"","copyAsset":false,"role":"figure","size":112503,"visible":true,"origin":"","legend":"\u003cp\u003eThe proposed structure and physical properties of SiQDs. (\u003cstrong\u003ea)\u003c/strong\u003e The proposed system contains well-dispersed SiQDs as the unlimited CGCs. \u003cstrong\u003e(b)\u003c/strong\u003e The carrier donation path from SiQDs to the guest-host materials. \u003cstrong\u003e(c)\u003c/strong\u003e the expression of charge transfer inside a proposed system by transition rate. \u003cstrong\u003e(d)\u003c/strong\u003e The criteria for the transition rate between guest-host and SiQDs is to achieve high efficiency in the proposed SiQDs CGCs system. \u003cstrong\u003e(e)\u003c/strong\u003e The schematic expression and the TEM images of the SiQD. \u003cstrong\u003e(f)\u003c/strong\u003e The distribution of the diameter of the observed SiQDs. \u003cstrong\u003e(g)\u003c/strong\u003e The photoluminescence (PL) spectrum, and \u003cstrong\u003e(h)\u003c/strong\u003e the visible appearance of the SiQDs and SiQDs blended guest-host material system excited by UV light.\u003c/p\u003e","description":"","filename":"1.png","url":"https://assets-eu.researchsquare.com/files/rs-4466701/v1/b096748b18a6b8918fd4ed02.png"},{"id":57894628,"identity":"2c830328-10bd-4efd-9108-e6680ce296b2","added_by":"auto","created_at":"2024-06-07 07:17:07","extension":"png","order_by":2,"title":"Figure 2","display":"","copyAsset":false,"role":"figure","size":86596,"visible":true,"origin":"","legend":"\u003cp\u003eThe structure and performance of the SiQDs doped CGCs PhOLED.\u003cstrong\u003e (a, b\u003c/strong\u003e) The layer structure and energy alignments of the PhOLED. The thickness of PEDOT PSSS, EMI+ SiQDs, BCP, Alq\u003csub\u003e3\u003c/sub\u003e, LiF, and Al are 30, 68, 10, 20, 1, and 100 nm, respectively. \u003cstrong\u003e(c)\u003c/strong\u003e The photograph of the sky-blue emission from the PhOLED. \u003cstrong\u003e(d)\u003c/strong\u003e The current-voltage characteristics of the PhOLED device with different SiQDs concentrations. \u003cstrong\u003e(e)\u003c/strong\u003e The dependence of the turn-on voltage on the SiQDs concentration. \u003cstrong\u003e(f)\u003c/strong\u003e The emission spectrum of the OLED from the referenced and SiQDs doped (5´10\u003csup\u003e-3\u003c/sup\u003ewt. %) device. \u003cstrong\u003e(g)\u003c/strong\u003e The dependence of EQE on the concentration of SiQDs in the precursor. \u003cstrong\u003e(h)\u003c/strong\u003e the EQE of the SiQDs doped CGCs devices to the current density, and \u003cstrong\u003e(i)\u003c/strong\u003e the luminescent efficiency of the SiQDs doped CGCs devices to the luminance.\u003c/p\u003e","description":"","filename":"2.png","url":"https://assets-eu.researchsquare.com/files/rs-4466701/v1/251a1f904c9bf888904c8a73.png"},{"id":57893337,"identity":"bdf62308-43bb-4a01-8ee1-377135a51414","added_by":"auto","created_at":"2024-06-07 07:01:07","extension":"png","order_by":3,"title":"Figure 3","display":"","copyAsset":false,"role":"figure","size":2909802,"visible":true,"origin":"","legend":"\u003cp\u003eThe TEM images of SiQDs and luminescent precursors containing CBP, FIrpic, and SiQDs\u003cstrong\u003e.\u003c/strong\u003e \u003cstrong\u003e(a)\u003c/strong\u003e The TEM images of the luminescent precursor containing CBP, FIrpic, and SiQDs.\u003cstrong\u003e (b\u003c/strong\u003e) The areal distribution of element atoms (N, F, Ir, C, O, and Si). \u003cstrong\u003e(c)\u003c/strong\u003e The TEM images of the SiQDs. \u003cstrong\u003e(d, e)\u003c/strong\u003e The distribution of Si and O atoms in the SiQDs. \u003cstrong\u003e(f)\u003c/strong\u003e The atomic percentage of Si and O in the SiQDs obtained from EDS.\u003c/p\u003e","description":"","filename":"3.png","url":"https://assets-eu.researchsquare.com/files/rs-4466701/v1/b9e6ea5462c8674c7bee8e29.png"},{"id":57894003,"identity":"a85ff096-6fcc-4bdf-a75c-ad40558060ca","added_by":"auto","created_at":"2024-06-07 07:09:07","extension":"png","order_by":4,"title":"Figure 4","display":"","copyAsset":false,"role":"figure","size":132260,"visible":true,"origin":"","legend":"\u003cp\u003eThe spectroscopy analysis of the SiQDs CGCs luminescent materials.\u003cstrong\u003e \u003c/strong\u003e(\u003cstrong\u003ea)\u003c/strong\u003e The depth profile of N 1s, O 1s, Si 2p, F 1s, Ir 4f, and S 2p signals studied by XPS. \u003cstrong\u003e(b)\u003c/strong\u003e The binding energy spectrum of Si 2p signal. It was deconvoluted into two distributions corresponding to the Si-Si and Si-O bonds. \u003cstrong\u003e(c)\u003c/strong\u003e The TR-PL spectrum of the Firpic and Firpic blended with SiQDs at 500 nm wavelength. \u003cstrong\u003e(d)\u003c/strong\u003e The UPS spectrum of SiQDs and the Firpic with the calibration of Au signal.\u003c/p\u003e","description":"","filename":"4.png","url":"https://assets-eu.researchsquare.com/files/rs-4466701/v1/a54f8a326f562852ce2225da.png"},{"id":57893340,"identity":"60774892-cc02-42cb-8bd7-41a0c3af61ee","added_by":"auto","created_at":"2024-06-07 07:01:07","extension":"png","order_by":5,"title":"Figure 5","display":"","copyAsset":false,"role":"figure","size":70129,"visible":true,"origin":"","legend":"\u003cp\u003eThe mechanism of charge transfer in the SiQDs CGCs system. (\u003cstrong\u003ea)\u003c/strong\u003e The energy alignments between SiQDs and FIrpic according to the UPS measurement. \u003cstrong\u003e(b)\u003c/strong\u003e The schematic expression of the Dexter and Föster energy transfer. The Föster transfer is a Coulombic resonant energy transfer, and the Dexter transfer corresponds to electron transfer.\u003cstrong\u003e \u003c/strong\u003e(\u003cstrong\u003ec)\u003c/strong\u003e The SiQDs (marked by the red circle) are identified in the TEM images, with the distance between them being 2 nm on average. \u003cstrong\u003e(d)\u003c/strong\u003e The schematic diagram shows how carriers in the SiQDs are transferred to Firpic through Dexter electron transfer.\u003c/p\u003e","description":"","filename":"5.png","url":"https://assets-eu.researchsquare.com/files/rs-4466701/v1/ee2d83c29096684ab8dcf207.png"},{"id":71552433,"identity":"0099d158-af55-4888-ab15-8c248edd9e38","added_by":"auto","created_at":"2024-12-16 16:06:03","extension":"pdf","order_by":0,"title":"","display":"","copyAsset":false,"role":"manuscript-pdf","size":4548941,"visible":true,"origin":"","legend":"","description":"","filename":"manuscript.pdf","url":"https://assets-eu.researchsquare.com/files/rs-4466701/v1/d981da48-e727-40ed-b7a3-e1954d8b8a32.pdf"}],"financialInterests":"No competing interests reported.","formattedTitle":"Enhancing external quantum efficiency in a sky-blue OLED by charge transfer via Si quantum dots","fulltext":[{"header":"1.\tIntroduction","content":"\u003cp\u003eOrganic 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\u0026nbsp;emission.\u0026nbsp;The radiative emission recombines electrons and holes into photons. This process is evaluated by the\u0026nbsp;quantum\u0026nbsp;efficiency (QE), which\u0026nbsp;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\u0026nbsp;[4].\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eThe brightness depends on the number of emitted photons. The earliest fluorescent material emits photons by exciton recombination through the singlet state (S\u003csub\u003e1\u003c/sub\u003e) to ground state (S\u003csub\u003e0\u003c/sub\u003e) transition. The participation of S\u003csub\u003e1\u003c/sub\u003e exciton is 25%,\u0026nbsp;limiting\u0026nbsp;the QE\u0026nbsp;to be 25%. Another 75% of excitons belong to the triplet state (T\u003csub\u003e1\u003c/sub\u003e), 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\u0026nbsp;[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%\u0026nbsp;[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 (DE\u003csub\u003eS-T\u003c/sub\u003e). 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%.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eTo enhance the\u0026nbsp;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 V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e between two luminescent layers to increase the number of charge carriers[25]. Electron-hole pairs generated within the V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e 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 V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e layers, the current efficiency increased to nearly 48 cd/A. By appropriately designing the emitting layer, considering material combinations, and replacing the V\u003csub\u003e2\u003c/sub\u003eO\u003csub\u003e5\u003c/sub\u003e with MoO\u003csub\u003e3\u003c/sub\u003e, the\u0026nbsp;QE can be improved to as high as 40%\u0026nbsp;[26]. Besides the single-layer oxide semiconductor, various charge-generation layers have been reported. These include photovoltaic-type organic bulk heterojunction layers[27, 28]\u0026nbsp;like CuPc-C60 or ZnPc-C60, an organic heterojunction donor-acceptor layer (HAT-CN: m-MTDATA)[29], MoO\u003csub\u003e3\u003c/sub\u003e-ZnO bilayers[30], organic-inorganic bilayers (C60/rubrene: MoO\u003csub\u003e3\u003c/sub\u003e)[31], and perovskite (CsPbBr\u003csub\u003e3\u003c/sub\u003e)-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.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003eWe 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 SiO\u003csub\u003e2\u003c/sub\u003e 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\u003csup\u003e-3\u0026nbsp;\u003c/sup\u003e% (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.\u0026nbsp;\u003c/p\u003e"},{"header":"2. Materials and Methods","content":"\u003cdiv id=\"Sec2\" class=\"Section2\"\u003e \u003ch2\u003e2.1 \u003cem\u003eConcept\u003c/em\u003e\u003c/h2\u003e \u003cp\u003eThe proposed structure, shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ea, exemplifies a significant deviation from the traditional approach. Rather than forming a charge generation layer (CGL), the Si quantum dots (SiQDs) are dispersed throughout the luminescent layer, allowing for electrons or holes provided by the SiQDs to form excitons at neighboring luminescent molecules (guest) and recombine radiatively to emit light. The SiQDs are covalent-bonded inorganic materials with a high density of states that contribute many carriers, as depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eb. Ensuring the carriers in SiQDs transfer effectively to the luminescent molecules is essential. Therefore, the carrier transition rate of a host, guest, and SiQDs in a phosphorescent OLED (PhOLED) should be considered. The relationship of transition rate in the guest-host system has been studied previously[\u003cspan citationid=\"CR33\" class=\"CitationRef\"\u003e33\u003c/span\u003e]. We included SiQDs in the conventional guest-host transition system. The transition rate inside a proposed system contains K\u003csub\u003eH\u003c/sub\u003e, K\u003csub\u003eF\u003c/sub\u003e, K\u003csub\u003eR\u003c/sub\u003e, K\u003csub\u003eG\u003c/sub\u003e, K\u003csub\u003eFF\u003c/sub\u003e, K\u003csub\u003eRR,\u003c/sub\u003e and K\u003csub\u003eQDs\u003c/sub\u003e, as displayed in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec. Where K\u003csub\u003eH\u003c/sub\u003e, K\u003csub\u003eG\u003c/sub\u003e, and K\u003csub\u003eQDs\u003c/sub\u003e are the radiative recombination rate in the host, guest, and the SiQDs, respectively.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe K\u003csub\u003eF\u003c/sub\u003e and K\u003csub\u003eR\u003c/sub\u003e are carriers\u0026rsquo; forward and reverse transition rates from the host to the guest material. K\u003csub\u003eFF\u003c/sub\u003e and K\u003csub\u003eRR\u003c/sub\u003e are carriers\u0026rsquo; forward and reversed transition rates from the guest to the SiQDs. K\u003csub\u003eG\u003c/sub\u003e should be maximized to achieve the highest luminescent efficiency. Therefore, either K\u003csub\u003eG\u003c/sub\u003e should be more significant than K\u003csub\u003eH\u003c/sub\u003e or K\u003csub\u003eF\u003c/sub\u003e, or it should be more effective than K\u003csub\u003eR\u003c/sub\u003e and K\u003csub\u003eH\u003c/sub\u003e between host and guest materials, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ed. Furthermore, K\u003csub\u003eG\u003c/sub\u003e should either be higher than KQDs or KRR should be more significant than K\u003csub\u003eFF\u003c/sub\u003e and K\u003csub\u003eQDs\u003c/sub\u003e to prevent radiative recombination in the SiQDs.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec3\" class=\"Section2\"\u003e \u003ch2\u003e2.2 \u003cem\u003eSilicon Quantum Dots Preparation\u003c/em\u003e\u003c/h2\u003e \u003cp\u003eTo prepare SiQDs, the hydrogen silsesquioxane (HSQ) was used as a precursor and was used as received. The solvent was removed from the HSQ stock solution using a rotary evaporator in a water bath at 40\u0026deg;C, resulting in gel formation and drying overnight under vacuum. After it dried, the white solid was placed in a quartz crucible and transferred in an inert atmosphere to a high-temperature furnace, where it was annealed at 1100\u0026deg;C for one hour in the atmosphere of 95% Ar and 5% H\u003csub\u003e2\u003c/sub\u003e. After grinding, 200 mg of the fine powder was added to a mixture of 2 mL of de-ionized (DI) water, 2 mL of ethanol, and 2 mL of HF to etch the SiO\u003csub\u003e2\u003c/sub\u003e matrix and decrease the size of the Si. After solvent removal and centrifugation, the product was dried under a dry N\u003csub\u003e2\u003c/sub\u003e flow to obtain 20 mg of powder and quickly transferred to a round-bottom flask containing 10 mL of 1-dodecane. The solution was then heated at 190\u0026deg;C overnight in Ar ambient in the same round-bottom flask. After the reaction and removal of the excess 1-dodecane, 5 mL of toluene was added to obtain a 1-dodecane-capped, hydrogen-terminated SiQDs solution. The solution is further diluted to 0.1 wt % in toluene for device preparation.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec4\" class=\"Section2\"\u003e \u003ch2\u003e2.3 Quantum dot \u003cem\u003echaracterization\u003c/em\u003e\u003c/h2\u003e \u003cp\u003eThe TEM images of SiQDs are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ee, and the SiQDs are well separated. The size of the SiQDs ranged from 3.1 to 9.0 nm and had the highest population at around 6.6-7.0 nm, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ef, according to the TEM images. The photoluminescence of the Si QD is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eg. It peaks at around 680 nm, a red emission corresponding to the bandgap of 1.82 eV. After blending the SiQDs with a 5\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e wt.% concentration to the host (4,4\u0026rsquo;-Bis(N-carbazolyl)-1,1\u0026rsquo;-biphenyl (CBP)) and the guest Bis[2-(4,6-difluorophenyl) pyridinato-C2, N] (picolinato) iridium (III)(FIrpic)) materials in the precursor. Three PL emissions at 436 nm, 470 nm, and 510 nm were found for the blending solution with and without SiQDs, corresponding to the CBP and FIrpic emissions, respectively. The PL emission associated with SiQDs was not observed in this system. Further, excite the solution by 254 nm UV light, the SiQDs exhibit a red color, and FIrpic\u0026thinsp;+\u0026thinsp;CBP and FIrpic\u0026thinsp;+\u0026thinsp;CBP\u0026thinsp;+\u0026thinsp;SiQDs solutions display a sky-blue color, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003eh, which coincides with the PL spectrum. This indicates the additive of the SiQDs does not alter the emission of the guest and host system in the phosphorous OLED. These observations support our assumption that the additive SiQDs have small KQDs and observable radiative luminescence and are used to provide the carriers to the emitting molecules, the FIrpic.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec5\" class=\"Section2\"\u003e \u003ch2\u003e2.4 SiQDs OLED \u003cem\u003edevice fabrication\u003c/em\u003e\u003c/h2\u003e \u003cp\u003eThe layer structure and energy alignments of the PhOLED were depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ea and \u003cb\u003eb\u003c/b\u003e, respectively. Indium-tin-oxide (ITO) coated glass was used as a substrate to make SiQDs OLED devices. The ITO was patterned into two mm-wide slots. After cleaning, organic layers were applied. The conducting polymer, poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate) (PEDOT: PSS), was used as the hole injection layer, spin-coating on the ITO at 5,000 rpm for the 30s. After coating, the PEDOT: PSS was thermally dried on a hot plate at 120 \u003csup\u003eo\u003c/sup\u003eC for 10 min. The thickness is approximately 30 nm. After PEDOT: PSS, the precursor of the emission layer was also spin-coated on the PEDOT: PSS at 2,000 for the 30s and was then dried on a hotplate at 40\u003csup\u003eo\u003c/sup\u003eC for 240s. FIrpic is dissolved with SiQDs in the CBP as the emitting layer. The 4,4\u0026rsquo;-Bis(N-carbazolyl)-1,1\u0026rsquo;-biphenyl (CBP) and Bis[2-(4,6-difluorophenyl) pyridinato-C2, N] (picolinato) iridium (III)(FIrpic) were dissolved in the chlorobenzene (CB) in a total concentration of 2.4 wt. % in 1 ml as precursors. The weight of CBP is 21.12 mg, which is 2.88 mg for FIrpic. All chemicals in the precursors of the emitting layer (EML) were used as received. The SiQDs in the solution were added into the precursors by a micropipette with different amounts. The PEDOT: PSS and light-emitting layer thickness are approximately 30 and 68 nm measured by a stylus profiler (BRUKER DektakXT) and was confirmed by an atomic force microscopy (Digital Instruments Dimension 3100 Controller). After the emitting layer, 2,9-Dimethyl-4,7-diphenyl-1,10-phenanthroline (BCP), Tris(8-hydroxyquinolinolato)-aluminum (III)(Alq3), Lithium Fluoride (LiF), Aluminum (Al) were sequentially coated in 10, 20, 1 and 100 nm thick, respectively, by thermal evaporation. The device area is 2 mm by 3 mm.\u003c/p\u003e \u003c/div\u003e"},{"header":"3. Results and discussion","content":"\u003cdiv id=\"Sec7\" class=\"Section2\"\u003e \u003ch2\u003e3.1 Characterization of OLED emission\u003c/h2\u003e \u003cp\u003eThe photograph of the sky-blue emission from the PhOLED is shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ec. The current-voltage characteristics of the PhOLED device with different SiQDs concentrations are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ed, which was measured using an Agilent B2912A semiconductor parameter analyzer in dark conditions. The PhOLED were made of the SiQDs in the precursor from 5\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e to 0.33 wt.%, a significant range of doping concentration. With a high SiQDs concentration (0.33 wt. %), the turn-on voltage is around 3.3 V, which is a 7.9 V reduction compared to the reference device. This indicates the injected carriers may transport through the SiQDs, causing a concentration quench. Therefore, no emissions are recorded until the SiQD concentration is reduced to 0.11 wt.%. In contrast to OLED with layer CGLs, the PhOLED with SiQDs CGCs has reduced operation voltage instead of increased. The conventional peripheral circuit can operate the SiQDs CGCs PhOLED in low voltage.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe dependence of the turn-on voltage on the SiQDs concentration is plotted in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ee. The turn-on voltage increased with decreased SiQDs concentration in the precursors. With 5\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e wt. % of the SiQDs, the turn-on voltage is almost identical to the reference device, indicating that the injected carriers will not transport through SiQDs. The emission spectrum of the OLED from the reference and SiQDs doped (5\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e wt. %) device was shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ef taken by a spectroradiometer (Optronic Laboratories, OL-770) equipped with a 6-inch integrated sphere powered by a Keithley 2400 source meter. The applied voltage is 14 V. They have almost identical peaks in the emission, 470 and 510 nm. However, PhOLED with SiQDs CGCs exhibits lower turn-on voltage and higher external quantum efficiency (EQE). The EQE increased with the increase of the SiQDs concentration from 5\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;4\u003c/sup\u003e wt. % to 5\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e wt. %. Meanwhile, the SiQDs concentration is beyond 5\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e wt. %, the EQE starts to decrease, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eg. The highest EQE for the PhOLED with SiQDs is 17.7% at low current density (2 mA/cm\u003csup\u003e2\u003c/sup\u003e), as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003eh. In comparison, the EQE for the PhOLED without SiQDs is only 2% in this study. The luminous efficiency for the device is 19.6 cd/A at 500 cd/m\u003csup\u003e2\u003c/sup\u003e for the PhOLED with SiQDs, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig2\" class=\"InternalRef\"\u003e2\u003c/span\u003ei. In comparison, it is 3.08 cd/m\u003csup\u003e2\u003c/sup\u003e for the reference device at a much lower luminance. The EQE enhanced about eight times from 2 to 17.7% for the SiQDs doping.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec8\" class=\"Section2\"\u003e \u003ch2\u003e3.2 Distribution of SiQDs\u003c/h2\u003e \u003cp\u003eThe additive of quantum dots or nano-flasks into the precursor solution of the luminescent materials may be thought to condense at the hole injection layer (HIL) and emitting layer (EML)[\u003cspan citationid=\"CR34\" class=\"CitationRef\"\u003e34\u003c/span\u003e], which enhances the hole injection efficiency and hence improves the EQE. To clarify that the SiQDs are well-dispersed inside the emitting materials instead of forming a layer at the HIL/EML interface, the areal and vertical distribution of the SiQDs in the EML were explored. The precursor solution contains CBP, FIrpic, and 5\u0026times;transmission electron microscopy TEM investigated 10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e wt. % of the SiQDs). As shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ea, the dropped solution on the Cu mesh tends to aggregate together. The elements analysis indicates the Si atoms have a distribution shape the same as the TEM image, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003eb. It is worth noting that the O atoms have the same distribution as the Si atoms. Although the N, F, Ir, and C atoms cannot be observed to have the same distribution shape as the TEM image, they are distributed well. These atoms are associated with the CBP and FIrpic. The TEM images indicate the SiQDs are well dispersed in the CBP and FIrpic blended materials. Further exploring the SiQD, the shape of TEM images (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ec), the distribution of the Si atoms (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ed), and the oxygen atoms (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ee) are identical, which indicates the SiQDs contain large amounts of oxygen. The atomic percentage of oxygen is like the Si atoms (Fig.\u0026nbsp;\u003cspan refid=\"Fig3\" class=\"InternalRef\"\u003e3\u003c/span\u003ef).\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eThe SiQDs doped (5\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e wt. %) Firpic and CBP thin film was investigated by the x-ray photoemission (XPS) depth profile to understand the vertical distribution. The depth is deduced from the XPS\u0026rsquo;s sputter time and film thickness. The atomic signals of Si and O exhibit stable distribution from the top of the emitting layer to the bottom, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ea. The signals of F and Ir were also analyzed. They are indicators of the FIrpic. Si, O, F, and Ir coexistence indicates the SiQDs are distributed well along the film thickness with the Firpic. The TEM and XPS studies show that the SiQDs are well-dispersed spatially inside the EML and do not pile up at the HTL/EML interface. In addition to the SiQDs distribution, the binding status of the Si is studied by XPS, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003eb. The Si 2p spectroscopy can be deconvoluted into two peaks at 99.6 eV and 102.36 eV, which corresponds to the Si and SiOx. The existence of the SiOx indicates the SiQDs may partially oxidize at its outer surface, forming a SiOx/Si core-shell structure that prevents the coalescence of SiQDs.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec9\" class=\"Section2\"\u003e \u003ch2\u003e3.3 Carrier dynamics in the OLED\u003c/h2\u003e \u003cp\u003eThe donating of carriers from SiQDs to the FIrpic can be proved by the time-resolved PL (TRPL). The transient behavior in the TRPL can be modeled by the exponential decay with a time constant[\u003cspan citationid=\"CR35\" class=\"CitationRef\"\u003e35\u003c/span\u003e]. The time constant represents the lifetime of carriers in the material system that could contribute to the radiative light emission. The TRPL from the FIrpic\u0026thinsp;+\u0026thinsp;CBPP and SiQDs doped FIrpic\u0026thinsp;+\u0026thinsp;CBPP are depicted in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ec. The TRPL presents two distinct decay behavior. Therefore, it is modeled by the following equation with two lifetimes.\u003cdiv id=\"Equ1\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ1\" name=\"EquationSource\"\u003e\n$${I}_{PL}={A}_{1}{e}^{-\\left(\\raisebox{1ex}{$t-{t}_{0}$}\\!\\left/ \\!\\raisebox{-1ex}{${\\tau }_{1}$}\\right.\\right)}+{A}_{2}{e}^{-\\left(\\raisebox{1ex}{$t-{t}_{0}$}\\!\\left/ \\!\\raisebox{-1ex}{${\\tau }_{2}$}\\right.\\right)}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e1\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThe τ\u003csub\u003e1\u003c/sub\u003e and τ\u003csub\u003e2\u003c/sub\u003e for referenced and SiQDs doped devices at 510 nm are fitted by this equation. The τ\u003csub\u003e1\u003c/sub\u003e and τ\u003csub\u003e2\u003c/sub\u003e are 1.68 and 95.99 ns for the referenced device and 5.97 and 103.71 ns for the device with SiQDs doping. The τ\u003csub\u003e1\u003c/sub\u003e is 3.5 times enhanced after SiQDs doping. The lifetime can be further correlated with the transition rate by the following equation,\u003cdiv id=\"Equ2\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ2\" name=\"EquationSource\"\u003e\n$${\\tau }_{PL}=\\frac{1}{{K}_{r}+{K}_{nr}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e2\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003ein which the K\u003csub\u003er\u003c/sub\u003e is the transition rate of radiative recombination, and the K\u003csub\u003enr\u003c/sub\u003e is the transition rate of non-radiative recombination. According to Fig.\u0026nbsp;\u003cspan refid=\"Fig1\" class=\"InternalRef\"\u003e1\u003c/span\u003ec and \u003cb\u003ed\u003c/b\u003e, the difference in transition rate between reference and SiQDs-doped material systems is the forward and backward carrier transition rates, K\u003csub\u003eFF\u003c/sub\u003e and K\u003csub\u003eRR\u003c/sub\u003e. Therefore, the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\tau }_{PL}\\)\u003c/span\u003e\u003c/span\u003e in the reference can be expressed as\u003cdiv id=\"Equ3\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ3\" name=\"EquationSource\"\u003e\n$${\\tau }_{PL}=\\frac{1}{{K}_{G}+{K}_{nr}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e3\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eThe \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\tau }_{PL, Si QDs}\\)\u003c/span\u003e\u003c/span\u003e for the materials containing SiQDs can be expressed as\u003cdiv id=\"Equ4\" class=\"Equation\"\u003e\u003cdiv format=\"TEX\" class=\"mathdisplay\" id=\"FileID_Equ4\" name=\"EquationSource\"\u003e\n$${\\tau }_{PL, Si QDs}=\\frac{1}{{K}_{G}+{K}_{nr}+{K}_{FF}-{K}_{RR}}$$\u003c/div\u003e\u003cdiv class=\"EquationNumber\"\u003e4\u003c/div\u003e\u003c/div\u003e\u003c/p\u003e \u003cp\u003eIf K\u003csub\u003eRR\u003c/sub\u003e is larger than K\u003csub\u003eFF\u003c/sub\u003e, the denominator in the (4) will be smaller than in the (3). Consequently, the \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\tau }_{PL, Si QDs}\\)\u003c/span\u003e\u003c/span\u003e is larger than \u003cspan class=\"InlineEquation\"\u003e\u003cspan class=\"mathinline\"\u003e\\({\\tau }_{PL}\\)\u003c/span\u003e\u003c/span\u003e, which coincides with the TRPL results in this work. The more extensive lifetime in the SiQDs doped layer indicates the carriers in the SiQDs are donating to the FIrpic, as our assumption.\u003c/p\u003e \u003c/div\u003e \u003cdiv id=\"Sec10\" class=\"Section2\"\u003e \u003ch2\u003e3.4 Carrier transport path in the OLED\u003c/h2\u003e \u003cp\u003eThe ultra-violet photoemission spectroscopy (UPS) is used to study the actual band alignment between SiQDs and the Firpic. The UPS for SiQDs and Firpic are shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig4\" class=\"InternalRef\"\u003e4\u003c/span\u003ed. The energy of the input UV photons is 21.2 eV, and the Fermi levels extracted for SiQDs and Firpic are 4.68 eV and 4.88 eV, respectively. We assume either SiQDs or Firpic are undoped, and the measured Fermi levels are the mid-gap. The conduction band and valence band edges of the SiQDs are calculated as 3.73 and 5.63 eV, respectively. With the same method, the calculated Lowest unoccupied molecule orbit (LUMO) and highest occupied molecule orbit (HOMO) of FIrpic are 3.64 and 6.12 eV, respectively. The alignment between SiQDs and the FIrpic is plotted in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ea. The barrier for electron transfer from SiQDs to the FIrpic is 0.09 eV. And the energy barrier is 0.49 eV for the holes in the SiQDs. The average thermal energy of the carrier is 3/2 kT, which is around 0.04 eV at room temperature. These barriers are higher than the thermal energy, and the carriers in the SiQDs in the conduction and valence band edge cannot easily overcome these barriers.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003cp\u003eFortunately, some carriers exhibit higher energy if the carrier concentration is high enough. These energetic carriers can be transferred from SiQDs to the FIrpic at room temperature. The reason for there is an appropriate concentration of the SiQDs doping can be understood by the mechanism of carrier transfer from SiQDs to the surrounding emitting molecules, FIrpic. With high concentrations of the SiQDs, large amounts of charges are transported through SiQDs, proved by the increased current density as explored. As we demonstrated, the transport between SiQDs causes a concentration quenching because the SiQDs have a low radiative recombination rate. Reducing the SiQDs\u0026rsquo; concentration can help reduce the transport path between SiQDs. Therefore, the isolated charges inside SiQDs can donate to the neighbor luminescent molecules, the FIrpic. The EQE reaches a maximum when the transport path through SiQDs is prohibited, and most of the luminescent molecules are ranged in the donation range of the SiQDs. Two mechanisms are responsible for the charge transfer in the organic system, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003eb: Dexter charge transfer and F\u0026ouml;ster resonant energy transfer (FRET)[\u003cspan citationid=\"CR36\" class=\"CitationRef\"\u003e36\u003c/span\u003e, \u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e].\u003c/p\u003e \u003cp\u003eBy the Dexter charge transfer, the electron in the SiQDs may transfer directly to the closed-surrounded luminesce molecules, setting up the charge donation[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. By the FRET, the energy that recombines an electron in the SiQDs from the conduction band to the valence transfers to the other luminescent molecule by exciting an electron from the ground to the excited state[\u003cspan citationid=\"CR37\" class=\"CitationRef\"\u003e37\u003c/span\u003e]. We marked part of the SiQDs in the TEM image, as shown in Fig.\u0026nbsp;\u003cspan refid=\"Fig5\" class=\"InternalRef\"\u003e5\u003c/span\u003ec, to verify a possible transfer mechanism. Each SiQD is very close, within 2 nm, for the doping level of SiQDs is 5\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e wt. %. Therefore, the dominant carrier transfer mechanism is most possibly the Dexter direct transfer. The electrons in the SiQDs are transferred directly to luminescent molecules.\u003c/p\u003e \u003cp\u003e \u003c/p\u003e \u003c/div\u003e"},{"header":"4. Conclusion","content":"\u003cp\u003eSilicon quantum dots are immersed in this work into phosphorescent luminescent materials as charge generation centers for unlimited electrons and holes. The SiQDs have an average diameter of 6 nm and are well dispersed in the luminescent materials by TEM and XPS analysis. The PhOLED exhibits 17.7% external QE by the SiQDs doping in minimal amounts of 5\u0026times;10\u003csup\u003e\u0026minus;\u0026thinsp;3\u003c/sup\u003e%, which is approximately 9 times enhancement. TRPL measured the decay time of radiative emission on the luminescent organic with SiQDs, and the increased decay time at the luminescent wavelength (500 nm) reveals the supply of excitons from the neighbors, which are SiQDs. The proposed and demonstrated concept solely involved electron transfer that may not limited by the categories of inorganic quantum dots, the emission wavelength, and the selection of luminescent organics.\u003c/p\u003e"},{"header":"Abbreviations","content":"\u003cdiv class=\"DefinitionList\"\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eEQE\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eExternal quantum efficiency\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eOLED\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eOrganic light-emitting diodes\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003ePhOLED\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003ePhosphorescent OLED\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eSiQDs\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eSilicon quantum dots\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eCGCs\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003echarge-generation centers\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eEML\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eEmitting layer\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eFirpic\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eBis[2-(4,6-difluorophenyl) pyridinato-C2, N] (picolinato) iridium (III)\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eCBP\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003e(4,4\u0026rsquo;-Bis(N-carbazolyl)-1,1\u0026rsquo;-biphenyl\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003ePEDOT\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003ePSS:poly(3,4-ethylenedioxythiophene)-poly(styrenesulfonate)\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eTRPL\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eTime-resolved photoluminescence\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eUPS\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eUltraviolet photoelectron spectroscopy\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eLUMO\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eLowest unoccupied molecule orbit\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eHOMO\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003ehighest occupied molecule orbit\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003cdiv class=\"DefinitionListEntry\"\u003e \u003cdiv class=\"Term\"\u003eFRET\u003c/div\u003e \u003cdiv class=\"Description\"\u003e \u003cp\u003eF\u0026ouml;ster resonant energy transfer\u003c/p\u003e \u003c/div\u003e \u003c/div\u003e \u003c/div\u003e"},{"header":"Declarations","content":"\u003cp\u003eThe authors declare no conflicts of interest.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eEthics approval and consent to participate:\u003c/strong\u003e Not applicable\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eConsent for publication:\u003c/strong\u003e All authors consent to the publication of this manuscript.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAvailability of data and material:\u003c/strong\u003e The data that support the plots and other findings within this report are available from the corresponding authors upon reasonable request.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eCompeting interests:\u003c/strong\u003e The authors declare that they have no competing interests.\u0026nbsp;\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eFunding:\u003c/strong\u003e The authors Z. P., H. Y. W., and Y. C. L acknowledge the financial support of the National Science and Technology Council, Taiwan by grants NSTC-112-2221-E-005-066-, NSTC-111-2622-E-005-008- and supported by the “Innovation and Development Center of Sustainable Agriculture” from The Featured Areas Research Center Program within the framework of the Higher Education Sprout Project by the Ministry of Education MOEE) in Taiwan. The authors T.S. and N. F. acknowledge the support of the World Premier International Research Center Initiative WPII), MEXT, Japan\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAuthors’ contributions:\u003c/strong\u003e Z. P. conceived the idea, designed experiments, analyzed data, and wrote the paper. T.S. performed material synthesizing under the supervision of N. F., and H.-Y. W. and Y.-C. L. performed structure characterization and fabricated device characterization. All authors discussed the results and co-wrote the manuscript.\u003c/p\u003e\n\u003cp\u003e\u003cstrong\u003eAcknowledgements:\u0026nbsp;\u003c/strong\u003eAll authors would like to thank funding supports.\u003c/p\u003e"},{"header":"References","content":"\u003col\u003e\n\u003cli\u003eHong, G.; Gan, X.; Leonhardt, C.; Zhang, Z.; Seibert, J.; Busch, J. M.; Br\u0026auml;se, S. A Brief History of OLEDs\u0026mdash;Emitter Development and Industry Milestones. \u003cem\u003eAdvanced Materials \u003c/em\u003e\u003cstrong\u003e2021,\u003c/strong\u003e\u003cem\u003e33\u003c/em\u003e (9), 2005630, DOI: https://doi.org/10.1002/adma.202005630.\u003c/li\u003e\n\u003cli\u003eHuang, Y.; Hsiang, E.-L.; Deng, M.-Y.; Wu, S.-T. 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Device efficiency of organic light-emitting diodes: Progress by improved light outcoupling. \u003cem\u003ephysica status solidi (a) \u003c/em\u003e\u003cstrong\u003e2013,\u003c/strong\u003e\u003cem\u003e210\u003c/em\u003e (1), 44-65, DOI: https://doi.org/10.1002/pssa.201228320.\u003c/li\u003e\n\u003c/ol\u003e"}],"fulltextSource":"","fullText":"","funders":[],"hasAdminPriorityOnWorkflow":false,"hasManuscriptDocX":true,"hasOptedInToPreprint":true,"hasPassedJournalQc":"","hasAnyPriority":false,"hideJournal":false,"highlight":"","institution":"","isAcceptedByJournal":true,"isAuthorSuppliedPdf":false,"isDeskRejected":"","isHiddenFromSearch":false,"isInQc":false,"isInWorkflow":false,"isPdf":false,"isPdfUpToDate":true,"isWithdrawnOrRetracted":false,"journal":{"display":true,"email":"[email protected]","identity":"discover-nano","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"narl","sideBox":"Learn more about [Discover Nano](https://www.springer.com/journal/11671)","snPcode":"11671","submissionUrl":"https://submission.nature.com/new-submission/11671/3","title":"Discover Nano","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Discover Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true},"keywords":"","lastPublishedDoi":"10.21203/rs.3.rs-4466701/v1","lastPublishedDoiUrl":"https://doi.org/10.21203/rs.3.rs-4466701/v1","license":{"name":"CC BY 4.0","url":"https://creativecommons.org/licenses/by/4.0/"},"manuscriptAbstract":"\u003cp\u003eOrganic light-emitting diodes (OLEDs) aim to achieve high efficiency by using excitons to achieve a 100% quantum efficiency (QE). However, developing functional organic materials for this purpose can be time-consuming. To address this challenge, a new method has been proposed to incorporate inorganic quantum dots into the organic luminescent layer to enable unlimited exciton formation and approach the 100% QE limit. Inorganic quantum dots are clusters of atoms that contain numerous thermally generated electrons and holes at conduction and valence bands. Immersed quantum dots act as charge generation centers, providing electrons and holes with unlimited amounts to form excitons. After radiative recombination, these excitons generate photons that cause internal QE to nearly 100%. This concept has been demonstrated using Silicon quantum dots (SiQDs) and phosphorescent materials. The average size of SiQDs is approximately 6 nm, and they are well-dispersed within the guest-host blue phosphorescent light-emitting materials. With only 5×10\u003csup\u003e-3\u003c/sup\u003e % (in weight) of SiQDs in the precursor, external QE increased from 2% to 17.7%, nearly a nine-fold enhancement. The prolonged decay time from 1.68 to 5.97 ns indicates that electrons are transferred from SiQDs to the luminescent materials. This universal method can be applied to green and red emissions with various inorganic quantum dots in different organic luminescent material systems.\u003c/p\u003e","manuscriptTitle":"Enhancing external quantum efficiency in a sky-blue OLED by charge transfer via Si quantum dots","msid":"","msnumber":"","nonDraftVersions":[{"code":1,"date":"2024-06-07 07:01:02","doi":"10.21203/rs.3.rs-4466701/v1","editorialEvents":[{"type":"communityComments","content":0},{"type":"decision","content":"Revision requested","date":"2024-07-10T07:09:17+00:00","index":"","fulltext":""},{"type":"reviewerAgreed","content":"38195666056315701228077867466843020830","date":"2024-07-07T07:06:16+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"329470548128210078222361849791295201732","date":"2024-07-01T10:52:08+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-06-07T03:19:49+00:00","index":"hide","fulltext":""},{"type":"editorInvitedReview","content":"","date":"2024-05-31T15:18:27+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"140834485248496941430231729152622425265","date":"2024-05-31T13:36:12+00:00","index":"hide","fulltext":""},{"type":"reviewerAgreed","content":"119755551248874890861972168775328588397","date":"2024-05-31T07:10:39+00:00","index":"hide","fulltext":""},{"type":"reviewersInvited","content":"","date":"2024-05-27T02:48:26+00:00","index":"","fulltext":""},{"type":"editorAssigned","content":"","date":"2024-05-26T05:06:27+00:00","index":"","fulltext":""},{"type":"checksComplete","content":"","date":"2024-05-26T05:04:28+00:00","index":"","fulltext":""},{"type":"submitted","content":"Discover Nano","date":"2024-05-23T12:01:11+00:00","index":"","fulltext":""}],"status":"published","journal":{"display":true,"email":"[email protected]","identity":"discover-nano","isNatureJournal":false,"hasQc":true,"allowDirectSubmit":false,"externalIdentity":"narl","sideBox":"Learn more about [Discover Nano](https://www.springer.com/journal/11671)","snPcode":"11671","submissionUrl":"https://submission.nature.com/new-submission/11671/3","title":"Discover Nano","twitterHandle":"@SpringerOpen","acdcEnabled":true,"dfaEnabled":true,"editorialSystem":"stoa","reportingPortfolio":"Discover Series","inReviewEnabled":true,"inReviewRevisionsEnabled":true}}],"origin":"","ownerIdentity":"879a9778-26fa-4020-8d09-a747665c5929","owner":[],"postedDate":"June 7th, 2024","published":true,"recentEditorialEvents":[],"rejectedJournal":[],"revision":"","amendment":"","status":"published-in-journal","subjectAreas":[],"tags":[],"updatedAt":"2024-12-16T16:00:58+00:00","versionOfRecord":{"articleIdentity":"rs-4466701","link":"https://doi.org/10.1186/s11671-024-04171-w","journal":{"identity":"discover-nano","isVorOnly":false,"title":"Discover Nano"},"publishedOn":"2024-12-14 15:57:25","publishedOnDateReadable":"December 14th, 2024"},"versionCreatedAt":"2024-06-07 07:01:02","video":"","vorDoi":"10.1186/s11671-024-04171-w","vorDoiUrl":"https://doi.org/10.1186/s11671-024-04171-w","workflowStages":[]},"version":"v1","identity":"rs-4466701","journalConfig":"researchsquare"},"__N_SSP":true},"page":"/article/[identity]/[[...version]]","query":{"redirect":"/article/rs-4466701","identity":"rs-4466701","version":["v1"]},"buildId":"qtupq5eGEP_6zYnWcrvyt","isFallback":false,"isExperimentalCompile":false,"dynamicIds":[84888],"gssp":true,"scriptLoader":[]}