With the aim to enhance the energy conversion efficiency of the rainbow shape piezoelectric transducer, an analysis model of energy conversion efficiency is established based on the elastic mechanics theory and piezoe...With the aim to enhance the energy conversion efficiency of the rainbow shape piezoelectric transducer, an analysis model of energy conversion efficiency is established based on the elastic mechanics theory and piezoelectricity theory. It can be found that the energy conversion efficiency of the rainbow shape piezoelectric transducer mainly depends on its shape parameters and ma- terial properties from the analysis model. Simulation results show that there is an optimal length ratio to generate maximum en- ergy conversion efficiency and the optimal length ratios and energy conversion efficiencies of beryllium bronze substrate trans- ducer and steel substrate transducer are （0.65, 2.21%） and （0.65, 1.64%） respectively. The optimal thickness ratios and energy conversion efficieneies of beryllium bronze substrate transducer and steel substrate transducer are （1.16, 2.56%） and （1.49, 1.57%） respectively. With the increase of width ratio and initial curvature radius, both the energy conversion efficiencies de- crease. Moreover, beryllium bronze flexible substrate transducer is superior to the steel flexible substrate transducer.展开更多
Harvesting energy from ambient vibration sources with ultrathin flexible piezoelectric energy harvesters(PEHs)for battery-free electronics has received attention in recent years.However,the excitation modes in the env...Harvesting energy from ambient vibration sources with ultrathin flexible piezoelectric energy harvesters(PEHs)for battery-free electronics has received attention in recent years.However,the excitation modes in the environment and human body motion are more complicated than the ideal harmonic modes employed in previous theoretical analyses,and their influence on the efficiency of PEHs has received little attention.In this study,these environmental excitation modes are classified into three types,i.e.,the triangular,sinusoidal,and square wave modes,with varied duty ratios.We derived theoretically the output power of flexible PEHs under these excitation modes and establish a simple scaling law,in which the normalized output power depends only on two combined normalized parameters,i.e.,the intrinsic system parameter and the excitation mode.Results reveal that the output power of PEHs changes dramatically for different excitation modes with varied duty ratios even when all the other parameters including excitation ampli tude,excitation frequency,elec trical parameter,and geometrical and material parameters of the PEHs are identical.This paper may provide a systematic understanding in the effect of exc计ation modes on the output power of flexible PEHs and promote the realization of energy harvesting from the complex environmental and human body motions.展开更多
A nanowire （NW） structure provides an alternative scheme for deep ultraviolet light emitting diodes （DUV-LEDs） that promises high material quality and better light extraction efficiency （LEE）. In this report, we...A nanowire （NW） structure provides an alternative scheme for deep ultraviolet light emitting diodes （DUV-LEDs） that promises high material quality and better light extraction efficiency （LEE）. In this report, we investigate the influence of the tapering angle of closely packed AIGaN NWs, which is found to exist naturally in molecular beam epitaxy （MBE） grown NW structures, on the LEE of NW DUV-LEDs. It is observed that, by having a small tapering angle, the vertical extraction is greatly enhanced for both transverse magnetic （TM） and transverse elec- tric （TE） polarizations. Most notably, the vertical extraction of TM emission increased from 4.8% to 24.3%, which makes the LEE reasonably large to achieve high-performance DUV-LEDs. This is because the breaking of symmetry in the vertical direction changes the propagation of the light significantly to allow more coupling into radiation modes. Finally, we introduce errors to the NW positions to show the advantages of the tapered NW structures can be projected to random closely packed NW arrays. The results obtained in this paper can provide guidelines for designing efficient NW DUV-LEDs.展开更多
Indoor organic and perovskite photovoltaics(PVs)have been attracting great interest in recent years.The theoretical limit of indoor PVs has been calculated based on the detailed balance method developed by Shockley–Q...Indoor organic and perovskite photovoltaics(PVs)have been attracting great interest in recent years.The theoretical limit of indoor PVs has been calculated based on the detailed balance method developed by Shockley–Queisser.However,realistic losses of the organic and perovskite PVs under indoor illumination are to be understood for further efficiency improvement.In this work,the efficiency limit of indoor PVs is calculated to 55.33%under indoor illumination(2700 K,1000 lux)when the bandgap(E_(g))of the semiconductor is 1.77 eV.The efficiency limit was obtained on the basis of assuming 100%photovoltaic external quantum efficiency(EQ_(EPV))when E≥E_(g),there was no nonradiative recombination,and there were no resistance losses.In reality,the maximum EQEPV reported in the literature is 0.80–0.90.The proportion of radiative recombination in realistic devices is only 10^(−5)–10^(−2),which causes the open-circuit voltage loss(ΔV_(loss))of 0.12–0.3 V.The fill factor(FF)of the indoor PVs is sensitive to the shunt resistance(R_(sh)).The realistic losses of EQE_(PV),nonradiative recombination,and resistance cause the large efficiency gap between the realistic values(excellent perovskite indoor PV,32.4%;superior organic indoor PV,30.2%)and the theoretical limit of 55.33%.In reality,it is feasible to reach the efficiency of 47.4%at 1.77 eV for organic and perovskite photovoltaics under indoor light(1000 lux,2700 K)with V_(OC)=1.299 V,J_(SC)=125.33μA/cm^(2),and FF=0.903 when EQE_(PV)=0.9,EQE_(EL)=10^(−1),R_(s)=0.5Ωcm^(2),and R_(sh)=10^(4) kΩcm^(2).展开更多
The importance of optical resonance in enhancing light outcoupling efficiency(OCE)is frequently overlooked in conventional bottom-emitting quantum-dot light-emitting diodes(QLEDs)due to their weak microcavity effect.H...The importance of optical resonance in enhancing light outcoupling efficiency(OCE)is frequently overlooked in conventional bottom-emitting quantum-dot light-emitting diodes(QLEDs)due to their weak microcavity effect.Herein,we show that by synergistically optimizing the optical and the electrical performances,QLEDs with efficiency approaching the theoretical limit can be realized.By introducing a high refractive index indium zinc oxide(IZO)electrode and optimizing its thickness,the light OCE is significantly improved and consequently the red QLEDs exhibit an external quantum efficiency(EQE)of 33.2%,which is 1.4-fold higher than that of the reference devices with conventional indium tin oxide(ITO)electrodes.Moreover,with a high refractive index plastic substrate and a microlens array,the EQE can further be improved to a record value of 37.5%.Similar results are obtained in green and blue devices,which show an EQE of 18.8%and 14.4%,respectively.We also predict that the theoretical EQE limit of red,green,and blue QLEDs can reach 35.4%-36.5%,24.8%-34.0%,and 25.1%-35.8%,respectively,without using any light outcoupling structures.The proposed synergistic optimization strategy enables the efficiencies of red,green,and blue QLEDs to approach their theoretical limits.展开更多
基金National Natural Science Foundation of China (10972 102)Research Fund for the Doctoral Program of Higher Education of China (200802870007)Technology Research and Development Program of Jiangsu Province (BE2009163)
文摘With the aim to enhance the energy conversion efficiency of the rainbow shape piezoelectric transducer, an analysis model of energy conversion efficiency is established based on the elastic mechanics theory and piezoelectricity theory. It can be found that the energy conversion efficiency of the rainbow shape piezoelectric transducer mainly depends on its shape parameters and ma- terial properties from the analysis model. Simulation results show that there is an optimal length ratio to generate maximum en- ergy conversion efficiency and the optimal length ratios and energy conversion efficiencies of beryllium bronze substrate trans- ducer and steel substrate transducer are （0.65, 2.21%） and （0.65, 1.64%） respectively. The optimal thickness ratios and energy conversion efficieneies of beryllium bronze substrate transducer and steel substrate transducer are （1.16, 2.56%） and （1.49, 1.57%） respectively. With the increase of width ratio and initial curvature radius, both the energy conversion efficiencies de- crease. Moreover, beryllium bronze flexible substrate transducer is superior to the steel flexible substrate transducer.
文摘Harvesting energy from ambient vibration sources with ultrathin flexible piezoelectric energy harvesters(PEHs)for battery-free electronics has received attention in recent years.However,the excitation modes in the environment and human body motion are more complicated than the ideal harmonic modes employed in previous theoretical analyses,and their influence on the efficiency of PEHs has received little attention.In this study,these environmental excitation modes are classified into three types,i.e.,the triangular,sinusoidal,and square wave modes,with varied duty ratios.We derived theoretically the output power of flexible PEHs under these excitation modes and establish a simple scaling law,in which the normalized output power depends only on two combined normalized parameters,i.e.,the intrinsic system parameter and the excitation mode.Results reveal that the output power of PEHs changes dramatically for different excitation modes with varied duty ratios even when all the other parameters including excitation ampli tude,excitation frequency,elec trical parameter,and geometrical and material parameters of the PEHs are identical.This paper may provide a systematic understanding in the effect of exc计ation modes on the output power of flexible PEHs and promote the realization of energy harvesting from the complex environmental and human body motions.
基金King Abdullah University of Science and Technology(KAUST)(KAUST Baseline Fund BAS/1/1614-01-01,KAUST Baseline Fund BAS/1/1664-01-01,KAUST Equipment Fund BAS/1/1664-01-07)National Natural Science Foundation of China(NSFC)(61774065)
文摘A nanowire （NW） structure provides an alternative scheme for deep ultraviolet light emitting diodes （DUV-LEDs） that promises high material quality and better light extraction efficiency （LEE）. In this report, we investigate the influence of the tapering angle of closely packed AIGaN NWs, which is found to exist naturally in molecular beam epitaxy （MBE） grown NW structures, on the LEE of NW DUV-LEDs. It is observed that, by having a small tapering angle, the vertical extraction is greatly enhanced for both transverse magnetic （TM） and transverse elec- tric （TE） polarizations. Most notably, the vertical extraction of TM emission increased from 4.8% to 24.3%, which makes the LEE reasonably large to achieve high-performance DUV-LEDs. This is because the breaking of symmetry in the vertical direction changes the propagation of the light significantly to allow more coupling into radiation modes. Finally, we introduce errors to the NW positions to show the advantages of the tapered NW structures can be projected to random closely packed NW arrays. The results obtained in this paper can provide guidelines for designing efficient NW DUV-LEDs.
基金supported by the National Natural Science Foundation of China(Nos.52273180 and 51973074)the China Postdoctoral Science Foundation(Nos.2019M662614 and 2020M682404)the WNLO Funds for Innovation.
文摘Indoor organic and perovskite photovoltaics(PVs)have been attracting great interest in recent years.The theoretical limit of indoor PVs has been calculated based on the detailed balance method developed by Shockley–Queisser.However,realistic losses of the organic and perovskite PVs under indoor illumination are to be understood for further efficiency improvement.In this work,the efficiency limit of indoor PVs is calculated to 55.33%under indoor illumination(2700 K,1000 lux)when the bandgap(E_(g))of the semiconductor is 1.77 eV.The efficiency limit was obtained on the basis of assuming 100%photovoltaic external quantum efficiency(EQ_(EPV))when E≥E_(g),there was no nonradiative recombination,and there were no resistance losses.In reality,the maximum EQEPV reported in the literature is 0.80–0.90.The proportion of radiative recombination in realistic devices is only 10^(−5)–10^(−2),which causes the open-circuit voltage loss(ΔV_(loss))of 0.12–0.3 V.The fill factor(FF)of the indoor PVs is sensitive to the shunt resistance(R_(sh)).The realistic losses of EQE_(PV),nonradiative recombination,and resistance cause the large efficiency gap between the realistic values(excellent perovskite indoor PV,32.4%;superior organic indoor PV,30.2%)and the theoretical limit of 55.33%.In reality,it is feasible to reach the efficiency of 47.4%at 1.77 eV for organic and perovskite photovoltaics under indoor light(1000 lux,2700 K)with V_(OC)=1.299 V,J_(SC)=125.33μA/cm^(2),and FF=0.903 when EQE_(PV)=0.9,EQE_(EL)=10^(−1),R_(s)=0.5Ωcm^(2),and R_(sh)=10^(4) kΩcm^(2).
基金This work was supported by the National Natural Science Foundation of China(No.62174075)the Shenzhen Science and Technology Program(Nos.JCYJ20210324105400002 and JCYJ20220530113809022)the Guangdong University Research Program(No.2020ZDZX3062).
文摘The importance of optical resonance in enhancing light outcoupling efficiency(OCE)is frequently overlooked in conventional bottom-emitting quantum-dot light-emitting diodes(QLEDs)due to their weak microcavity effect.Herein,we show that by synergistically optimizing the optical and the electrical performances,QLEDs with efficiency approaching the theoretical limit can be realized.By introducing a high refractive index indium zinc oxide(IZO)electrode and optimizing its thickness,the light OCE is significantly improved and consequently the red QLEDs exhibit an external quantum efficiency(EQE)of 33.2%,which is 1.4-fold higher than that of the reference devices with conventional indium tin oxide(ITO)electrodes.Moreover,with a high refractive index plastic substrate and a microlens array,the EQE can further be improved to a record value of 37.5%.Similar results are obtained in green and blue devices,which show an EQE of 18.8%and 14.4%,respectively.We also predict that the theoretical EQE limit of red,green,and blue QLEDs can reach 35.4%-36.5%,24.8%-34.0%,and 25.1%-35.8%,respectively,without using any light outcoupling structures.The proposed synergistic optimization strategy enables the efficiencies of red,green,and blue QLEDs to approach their theoretical limits.
