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Investigation on OLEDs efficiency roll-off with interfacial charge storage and their time-resolved emission spectra
摘要： Organic light-emitting diodes (OLEDs) are the next generation technique of display and lighting, confronted with the bottle-neck problem of OLEDs efficiency roll-off. The nature of luminescence and efficiency roll-off of OLEDs are processes of electrons and energy, during which charge injection and storage play important roles. In order to study the mechanism of OLEDs efficiency roll-off with interfacial charge storage, organic light-emitting devices with Ir(ppy)3 doped CBP as the light-emitting layer were prepared, and PMMA layers of 5 nm, 6 nm, 8 nm and 9 nm were inserted as charge storage layers to control hole injection and hole accumulation. The OLEDs efficiency roll-off is obvious with the increase of the current density. The storage of the interfacial charges is analyzed by transient electroluminescence measurement. Luminescence spikes occur at both forward and reverse turn-off voltages, which is caused by electron diffusion and trap carriers release, respectively. The time-resolved emission spectra prove the relationship of interfacial charges and exciton quenching. The OLEDs efficiency roll-off is positively correlated with the interfacial charge storage especially at low current density. This is due to the effect of the accumulation of the interfacial charge especially under unbalanced charge injection of OLEDs, which is of importance to improve the OLEDs performance and to decrease the OLEDs efficiency roll-off in industry.
关键词： OLEDs efficiency roll-off,Transient electroluminescence measurement,Interfacial accumulated charges
更新于2025-09-23 15:19:57
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Performance of Solar Photovoltaic panel using Forced convection of water-based CuO nanofluid: An Understanding
摘要： Most of the conventional Solar Photovoltaic module consists of a Silicon cell that converts sunlight into electric energy. The process of conversion into electricity is exothermic and all photons are not able to produce electricity due to insufficient energy. Depending upon efficiency to convert it into electricity only the small amount of radiations are used and rest all are involved in increasing the temperature of the module. Study shows that 80% of incident solar radiation are absorbed by a solar photovoltaic cell. This increases the temperature of the module, reduces its electrical efficiency. This increase in temperature affects the power output and lifespan of the PV module. So to maintain the temperature of the module various cooling methods such as air cooling, hydraulic cooling, heat pipe cooling, cooling with phase change materials and cooling with nanofluids have been reported in the literature. The use of suitable nanofluids is one of the effective methods to increase thermal capacitance and control the temperature rise of the PV module. To increase the performance of the system thermal properties of working fluid must be improved which is achieved by using suitable additives with the base fluid which are referred to as nanofluids. Using Copper oxide/water as a working fluid analysis was performed. It was concluded that performance can be improved significantly if we integrate the system with a good heat exchanger. In this paper, the effect of CuO based nanofluids as a cooling medium for a PV module has been reported.
关键词： Solar photovoltaic,Thermal efficiency,cut off voltage,Electrical efficiency
更新于2025-09-23 15:19:57
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Rotation-restricted thermally activated delayed fluorescence compounds for efficient solution-processed OLEDs with EQEs of up to 24.3% and small roll-off
摘要： Two triphenylamine or 4,4’-di(tert-butyl)triphenylamine groups are introduced at 1,8-positions of 3,6-di(tert-butyl)-9-(4-(4,6-diphenyl-1,3,5-triazin-2-yl)phenyl)carbazole to yield two emitters containing a cofacial donor-acceptor-donor chromophore, which exhibit strong TADF characteristic dominated by through-space charge-transfer. The solution-processed OLEDs achieve the maximum external quantum efficiencies of up to 17.4% and 24.3% with small efficiency roll-off rate.
关键词： Thermally Activated Delayed Fluorescence,solution-processed,efficiency roll-off,OLEDs
更新于2025-09-23 15:19:57
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Imidazo[1,2-a]pyridine as an electron acceptor to construct?? high-performance deep-blue organic light-emitting diodes with negligible efficiency roll-off
摘要： Two novel bipolar deep-blue fluorescent emitters, IP-PPI and IP-DPPI featuring different lengths of phenyl bridge, were designed and synthesized, in which imidazo[1,2-a]pyridine (IP) and phenanthroimidazole (PI) were proposed as an electron-acceptor and an electron donor respectively. Both of them exhibited outstanding thermal stability and high emission quantum yield. All the devices based on these two materials showed negligible efficiency roll-off with increasing current density. Impressively, non-doped organic light-emitting diodes (OLEDs) based on IP-PPI and IP-DPPI exhibited EQE of 4.85% and 4.74% with CIE coordinates of (0.153, 0.097) and (0.154, 0.114) at 10000 cd m-2, respectively. Besides, the 40 wt% IP-PPI doped device maintained high EQE of 5.23% with CIE coordinates of (0.154, 0.077) at 10000 cd m-2. The doped device based on 20 wt% IP-DPPI exhibited higher deep-blue electroluminescence (EL) performance with maximum EQE up to 6.13% at CIE of (0.153, 0.078) and remained EQE of 5.07% at 10000 cd m-2. To the best of our knowledge, these performances are among the state-of-the art devices with CIEy ≤ 0.08 at a high brightness of 10000 cd m-2. Furthermore, by doping a red phosphorescent dye Ir(MDQ)2 into IP-PPI and IP-DPPI hosts, high-performance red PhOLEDs with EQE of 20.8% and 19.1% were achieved, respectively. This work may provide a new approach for designing highly efficient deep-blue emitters with negligible roll-off for OLED applications.
关键词： negligible efficiency roll-off,imidazo[1,2-a]pyridine,OLEDs,deep-blue
更新于2025-09-23 15:19:57
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Efficient Aggregation-Induced Delayed Fluorescence Luminogens for Solution-Processed OLEDs With Small Efficiency Roll-Off
摘要： Purely organic small molecules with thermally-activated delayed fluorescence have a high potential for application in organic light-emitting diodes (OLEDs), but overcoming severe efficiency roll-off at high voltages still remains challenging. In this work, we design and synthesize two new emitters consisting of electron-withdrawing benzoyl and electron-donating phenoxazine and 9,9-dihexylfluorene. Their electronic structures, thermal stability, electrochemical behaviors, photoluminescence property, and electroluminescence performance are thoroughly investigated. These new emitters show weak fluorescence in dilute solution, but they can emit strongly with prominent delayed fluorescence in the aggregated state, indicating the aggregation-induced delayed fluorescence (AIDF) character. The solution-processed OLEDs based on the two emitters show high external quantum efficiency of 14.69%, and the vacuum-deposited OLEDs can also provide comparable external quantum efficiency of 14.86%. Significantly, roll-offs of the external quantum efficiencies are very small (down to 0.2% at 1,000 cd m?2) for these devices, demonstrating the evidently advanced efficiency stability. These results prove that the purely organic emitters with AIDF properties can be promising to fabricate high-performance solution-processed OLEDs.
关键词： thermally activated delayed fluorescence,aggregation-induced fluorescence,efficiency roll-off,delayed fluorescence,organic light-emitting diodes,electroluminescence
更新于2025-09-23 15:19:57
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Multiple epitaxial lift-off of stacked GaAs solar cells for low-cost photovoltaic applications
摘要： This paper presents a multilayer peeling from a stacked cell structure as an approach for the cost reduction of III–V solar cells. We demonstrate the separation of two-layer stacked GaAs solar cells with Al(Ga)As release layers on the GaAs substrate into individual layers without cracks. The cells in each layer peeled from the stacked structure show equivalent device performances. Thermal cycling tests with repeated heating to 85 °C and cooling to ?40 °C show that the flexible GaAs thin-film cell exhibits a high durability against temperature changes. Further, a damp heat test conducted at 85 °C and 85% humidity indicates that the cell has long-term stability. These results suggest that the flexible GaAs thin-film cells fabricated by peeling from stacked structures have a high reliability and prove that the separation of the stacked cell structures into individual layers is effective in fabricating low-cost III–V solar cells.
关键词： epitaxial lift-off,cost reduction,damp heat test,photovoltaic applications,thermal cycling,GaAs solar cells
更新于2025-09-23 15:19:57
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[IEEE 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Munich, Germany (2019.6.23-2019.6.27)] 2019 Conference on Lasers and Electro-Optics Europe & European Quantum Electronics Conference (CLEO/Europe-EQEC) - Topological Photonics
摘要： The network densi?cation of small cells (SCs) is a promising way to cope with the explosive growth of future traf?c demands in 5G networks. However, the overall power consumption and the backhaul limitation of the network have become the key factors affecting the network performance and users’ quality-of-experience, which have great importance in 5G wireless networks. Due to the complexity of 5G networks and the variety of user behaviors, the combination of software-de?ned networks and content delivery strategy could be a more ef?cient way to manage such networks. In this paper, a cache-enabled wireless heterogeneous network with the control-plane (C-plane) and user-plane (U -plane) split is proposed, where the macro cell and SCs with different cache abilities are overlaid and cooperated together in the backhaul scenario. Using an evaluation tool composed of stochastic processes and classical power consumption model, key performance indicators, e.g., the coverage probability, throughput, and energy ef?ciency (EE), are derived as closed-form expressions or the functions of the signal-to-interference-plus-noise ratio threshold, path loss exponent, transmission power and density of macro and SCs, cache ability, ?le popularity, and backhaul capacity. Fundamental trade-offs are illustrated between EE and transmission power, EE and SC density, as well as the throughput and density of SCs. Numerical results show that the proposed cache-enabled software-de?ned networks have much higher throughput and improved EE than current LTE networks, which shows a promising solution for future cellular networks.
关键词： Cache-enabled networks,green communications,trade-off,spectral and energy efficiency,coverage probability,software defined networks
更新于2025-09-23 15:19:57
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Femtosecond laser direct written off-axis fiber Bragg gratings for sensing applications
摘要： First order off-axis fiber Bragg gratings (FBGs) were fabricated in a standard single mode fiber (SMF-28e) through femtosecond laser direct writing. A minimum offset distance between the grating and core center of 2.5 μm was found to create a multimode section, which supports two separate fiber modes (LP0,1 and LP1,1), each split into two degenerate polarization modes. The resulting structure breaks the cylindrical symmetry of the fiber, introducing birefringence (≈10?4) resulting in a polarization dependent Bragg wavelength for each mode. Based on the modal and birefringence behavior, three off-axis FBGs were fabricated with 3.0, 4.5 and 6.0 μm offsets from the core center, and then characterized in strain, temperature, and curvature. The tested off-axis FBGs exhibited a similar strain sensitivity of ~1.14 pm/μ? and a temperature sensitivity of ~12 pm/C. The curvature and orientation angle were simultaneously monitored by analyzing the intensity fluctuation and the wavelength shift of the LP1,1 Bragg resonance. A maximum curvature sensitivity of 0.53 dB/m?1 was obtained for the off-axis FBG with a 3.0 μm offset.
关键词： optical fiber sensors,femtosecond laser direct writing,single mode fiber,off-axis fiber Bragg gratings,Birefringence
更新于2025-09-23 15:19:57
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Highly efficient non-doped blue fluorescent OLEDs with low efficiency roll-off based on hybridized local and charge transfer excited state emitters
摘要： Designing a donor-acceptor (D-A) molecule with a hybridized local and charge transfer (HLCT) excited state is a very effective strategy for producing an organic light-emitting diode (OLED) with a high exciton utilization efficiency and external quantum efficiency. Herein, a novel twisting D-π-A fluorescent molecule (triphenylamine-anthracene-phenanthroimidazole; TPAAnPI) is designed and synthesized. The excited state properties of the TPAAnPI investigated through photophysical experiments and density functional theory (DFT) analysis reveal that its fluorescence is due to the HLCT excited state. The optimized non-doped blue OLED using TPAAnPI as light-emitting layer exhibits a novel blue emission with an electroluminescence (EL) peak at 470 nm, corresponding to the Commission International de L’Eclairage (CIE) coordinates of (0.15, 0.22). A fabricated device termed Device II exhibits a maximum current efficiency of 18.09 cd A ?1, power efficiency of 12.35 lm W?1, luminescence of ≈29900 cd cm?2, and external quantum efficiency (EQE) of 11.47%, corresponding to a high exciton utilization efficiency of 91%. Its EQE remains as high as 9.70% at a luminescence of 1000 cd m?2 with low efficiency roll-off of 15%. These results are among the best for HLCT blue-emitting materials involved in non-doped blue fluorescent OLEDs. The performance of Device II highlights a great industrial application potential for the TPAAnPI molecule.
关键词： HLCT,efficiency roll-off,blue emission,non-doped,OLED
更新于2025-09-23 15:19:57
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[IEEE 2019 8th International Conference on Power Systems (ICPS) - Jaipur, India (2019.12.20-2019.12.22)] 2019 8th International Conference on Power Systems (ICPS) - An Efficient LLC Resonant Converter Design for Photovoltaic Application
摘要： This paper presents a novel design procedure to the LLC resonant converter for improve the ef?ciency of photovoltaic (PV) applications. State of the art LLC resonant converter operates either in buck or boost mode by operating at a switching frequency higher or lower than the resonant frequency respectively. However, such operation of the converter at a switching frequency other than the resonant frequency may result in increased turn off losses for MOSFET, conduction losses, and hard commutation for the recti?er diodes which reduces the ef?ciency of the converter. In this paper, a design procedure is proposed where the converter operates very close to the resonant frequency (10% margin) in buck and boost mode of operation to improve the ef?ciency of the converter. The design procedure mainly involves the control of the transformer tap ratio such that the converter operates near the resonant frequency. Since the transformer tap changing control is slow in nature, the proposed control strategy is mainly suited for photovoltaic application where the input voltage to the converter does not change rapidly during the daytime. The transformer tap control is suggested based on the historical data for solar energy, i.e. recorded temperature and solar irradiance. On changing the transformer tap ratio according to the input voltage, the switching frequency is observed to be moving towards the resonant frequency of the LLC resonant converter. Simulations for both the boost and buck mode operation of the resonant tank of the converter are performed, and with the proposed control higher ef?ciency is achieved for PV applications.
关键词： switching frequency,conduction losses,LLC Resonant Converter,turn-off losses
更新于2025-09-23 15:19:57