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Internal quantum efficiency (IQE) of AlGaN‐based multiple quantum wells (MQWs) on face‐to‐face‐annealed sputter‐deposited AlN templates is examined by photoluminescence spectroscopy. The excitation power density dependence of IQE is evaluated as a function of temperature under the selective excitation of the quantum wells. The temperature dependences of the maximum IQE and the corresponding excitation power density (EPD) are analyzed based on the rate equation models for carriers and excitons. The decrease of the maximum IQE and increase of the corresponding EPD are mainly due to the increase in the nonradiative recombination rate via nonradiative recombination centers. Furthermore, the nonradiative recombination rate for the MQW with an emission wavelength around 220 nm is activated at a lower temperature than the other samples, which is expected to lead to the lower IQE of the MQW with an emission wavelength around 220 nm. [ABSTRACT FROM AUTHOR]

Mn5+ emission is a promising candidate for imaging deep tissue structures (e.g., vessels, tumors) in the second near‐infrared (NIR‐II, 1000–1350 nm) region. However, its practical application is impeded by the limited quantum efficiency of the available phosphors due to the unstable valence state of Mn5+. Herein, a novel strategy involving site competition is proposed to stabilize the Mn5+ state by the introduction of valence‐unstable Bi2+/3+. The results demonstrate that Bi3+ ions tend to occupy two different Ca2+ ion sites within the Ca6Ba(PO4)4O lattice. The incorporation of a small amount of Bi3+ effectively suppresses the amount of Mn2+ in Ca2+ sites. This is also confirmed by spectroscopic experiments and density function theory calculations. Notably, an ultra‐high internal quantum efficiency of 82.3% is achieved under excitation at 653 nm, surpassing more than twofold the previously reported value of 37.5% in Ca6Ba(PO4)4O: Mn5+. As a proof of concept, deep tissue imaging with a penetration depth of ≈2.8 cm is achieved using a self‐produced NIR‐II light‐emitting diodes device embedded with Ca6Ba(PO4)4O: 0.003Mn5+/0.003Bi3+ powder. These findings provide valuable insights into improving the luminescent properties associated with Mn5+ ions and pave the way for deep tissue imaging with high spatiotemporal resolution. [ABSTRACT FROM AUTHOR]

Owing to their energy-saving and environmentally friendly characteristics, rare-earth-free ultraviolet-excited green-luminescent materials have garnered significant attention. To improve the luminescence efficiencies of green-luminescent materials, this study synthesized Ta-doped CsV 1-x O 3 green-light-emitting phosphors through a solid-state-reaction method, adopting an optimal Ta doping concentration of 10%. Theoretical calculations and reflection spectra revealed an intrinsic indirect band gap of 2.86 eV for CsV 0.9 Ta 0.1 O 3 , lower than that of CsVO 3 (3.63 eV). Under 365 nm excitation, the as-prepared phosphors exhibited broadband emissions ranging from 400 nm to 700 nm, attributed to the 3T 2 →1A 1 and 3T 1 →1A 1 transitions within VO 4 tetrahedra. Notably, adding Ta5+ ions to CsVO 3 increased its emission intensity by 1.39 times and induced a redshift in its excitation spectrum from 369 to 375 nm. Furthermore, Ta5+ doping significantly distorted the VO 4 tetrahedra and influenced the optical properties of the synthesized phosphors. Specifically, the nearest of Cs–V bond length increased, and weak interactions between V5+ ions and Cs + ions led to a 1.5-fold higher internal quantum efficiency (60.49%) of CsV 0.9 Ta 0.1 O 3 compared to that of CsVO 3. Furthermore, CsV 0.9 Ta 0.1 O 3 demonstrated high thermal stability, retaining 75.57% of its initial intensity even at 140 °C. Overall, these results indicate the potential applicability of CsV 0.9 Ta 0.1 O 3 as a green-emitting phosphor for white-light-emitting diode applications. [ABSTRACT FROM AUTHOR]

To address the drawbacks of severe electron spillover and low hole injection in AlGaN-based deep ultraviolet (DUV) light-emitting diodes (LEDs), we report a novel tapered superlattice p-AlInGaN layer which improves the optoelectronic properties of AlGaN-based DUV LEDs. The output power of the proposed structure is significantly improved by 337.8% compared to the conventional structure; it also has an internal quantum efficiency (IQE) of up to 96% with no efficiency droop. The simulation calculation results show that the introduction of tapered superlattice p-AlInGaN layer increases the concentration of carriers within the multiple quantum wells (MQWs) and reduces the electric field, leading to a higher radiative recombination rate and providing an attractive solution for improving the performance of DUV LEDs. [ABSTRACT FROM AUTHOR]

The theoretical expectation of semi‐polar light emitting diodes (LEDs) is reviewed and compared it to the experimental data. The reported peak internal quantum efficiency (IQE) of non‐polar, and semi‐polar InGaN LEDs are always much lower than the standard polar (0001) oriented LEDs. Calculating the band structure and states including the n‐ and p‐doped layer of LEDs in many orientations, It is found that the inverted polarization‐induced fields for most common semi‐polar orientations like (112¯$\bar{2}$2) or (202¯${\bar{2}}$1) causes the hole ground state to leak out of the quantum well (QW) into the p‐doped GaN above and by this a low wavefunction overlap and a strongly increased probability for non‐radiative recombination with point defect outside the QW. Based on the calculations, (112¯$\bar{2}$1), (101¯$\bar{1}$2), or (101¯$\bar{1}$3) are predicted as best candidates for LEDs with higher IQE at higher current densities than (0001). LEDs in the uncommon semi‐polar (101¯$\bar{1}$3) orientation on sapphire have been realized. The (101¯$\bar{1}$3) LEDs showed the same IQE as (0001) LEDs but at a much higher current density due to the higher overlap of electron and hole wave functions, which is especially useful for micro‐LEDs. [ABSTRACT FROM AUTHOR]

The aluminum gallium nitride (AlGaN)-based deep-ultraviolet light-emitting diode (DUV-LED) has been a prominent device due to its contribution in various fields. The electron blocking layer (EBL) is an additional layer in the epitaxy of the DUV-LED with the aim of reducing the overflow of electrons and improving the hole injection, consequently increasing the performance of the DUV-LED. However, the threshold of the EBL thickness and its influence on the electrical and optical properties is still not fully understood. Hence, the purpose of this research is to investigate the effects of varying the EBL thickness, ranging from 5 nm up to 60 nm, and investigate the threshold of EBL thickness for the AlGaN-based DUV-LED. The analysis includes the internal quantum efficiency (IQE), luminescence spectrum, band diagram behavior, and the current density of the carrier. It is found that EBL thickness of 15 nm produces the highest IQE (39.69%) for the DUV-LED with a single quantum well structure, where the wavelength emitted is ~ 257 nm, which is within the ultraviolet C (UVC) range. [ABSTRACT FROM AUTHOR]

This study attempts to understand and elucidate the factors limiting/determining the external quantum efficiency (EQE) of light‐emitting diodes (LEDs) depending on material systems, i.e., III‐arsenide (GaAs), III‐phosphide (AlGaInP), and III‐nitride (GaInN), via the temperature measurements (30–500 K). The behaviors of EQEs are investigated carefully in terms of the thermal droop and efficiency droop, revealing that the thermal droop in the AlGaInP and GaAs LEDs, while the efficiency droop in the GaInN LEDs, is a critical factor limiting the EQE. To deepen the insight, the EQE is separated into internal quantum efficiency (IQE) and light‐extraction efficiency (LEE). Further, the IQE is separated into radiative efficiency (RE) and injection efficiency (IE). The analysis shows that the LEE plays a significant role in the thermal droop for the AlGaInP and GaAs LEDs. Meanwhile, the IE and RE play a significant role in the EQE reduction of the blue and red LEDs at high temperatures and high current injection. [ABSTRACT FROM AUTHOR]

For the laser-driven lighting, traditional phosphor ceramics (PCs), such as Y 3 Al 5 O 12 : Ce3+ (YAG: Ce) PC and Lu 3 Al 5 O 12 : Ce3+ (LuAG: Ce) PC, usually show a wide "cyan cavity" due to the ultra-narrow emission bandwidth (∼2 nm) of the blue laser diode (LD). In this regard, the attainable color rendering (Ra) is always <90 even via the combination of "YAG: Ce/LuAG: Ce + CaAlSiN 3 :Eu2+". Evidently, the "cyan gap" is the bottleneck issue to attain high-color-quality laser-driven lighting. As such, we develop a novel cyan-green-emitting garnet-structured solid-solution Lu 3- x Sc x Al 2- y Sc y Al 3- z Sc z O 12 : Ce3+ (abbreviated as LuScAG: Ce) PCs, derived from the LuAG: Ce PC. A comparative study is conducted to unearth its microstructure, luminescence properties and promising availability. It can be found that the LuScAG: Ce PCs shows large full width at half maximum (FWHM) and abundant cyan emission. Upon material optimization, bright cyan-green emission can be generated with internal quantum efficiency (IQE) of 87.4 % and FWHM of 87.17 nm. Remarkably, this new material yields high-power emitting light with luminous flux (LF) of 3230 lm and luminous efficacy (LE) of 89.7 lm/W upon 36.0 W blue LD driven in rotatory excitation mode, which can serve as an efficient laser color converter. As expected, due to the achieved affluent cyan emissive components of developed LuScAG: Ce PCs, the lighting source based on "LuScAG: Ce PC + CASN: Eu phosphor-in-glass film (PiGF)" shows high color quality with Ra of 90.6 and R12 (related with the cyan cavity) of 85.0. [ABSTRACT FROM AUTHOR]

ABSTRACTSeparated evaluation of factors in the external quantum efficiency (EQE) is important in order to improve the characteristics of semiconductors optical devices. Especially, the internal quantum efficiency (IQE) is an important value which indicates crystal quality of the active layers, and an accurate method for estimating the IQE values is required. The IQE is usually estimated from temperature dependence of photoluminescence (PL) intensity by assuming that the IQE at cryogenic temperature is 100%. However, III-nitride semiconductor materials, used in many optical devices, usually have large defect density, and the assumption is not necessarily valid. In our previous report, we demonstrated the simultaneous photoacoustic (PA) and PL measurements to accurately estimate the IQE values in GaN films with various qualities and obtained reasonable results. In this work, we have successfully realized reproducible measurements with high accuracy for an InGaN-QW sample by suppressing the background noise significantly. Furthermore, we have also measured the values of EQE by using an integrating-sphere. Since the light extraction efficiency (LEE) can be obtained by the values of IQE and EQE, it has been shown that the overall picture of emission efficiency can be provided by our method.

Visible light communications (VLC) utilizing LEDs for transmissions have been widely considered a revolutionary solution for next-generation networks such as 6G. Frequency response (FR) is of great importance for LEDs, but the inherent internal quantum efficiency (IQE) of LEDs is often disregarded, resulting in imprecise FR models. Although it is widely known that IQE varies with the injected direct current (DC), the impact of IQE on the frequency response has not been thoroughly analyzed. To address this issue, we have developed a precise electro-optical (E-O) FR model by incorporating the ABC model of IQE, named IQE-FR. To validate the accuracy of IQE-FR, we conducted the experiments using commercial off-the-shelf (COTS) LEDs including a tri-color LED and three monochromatic LEDs. The IQE-FR model aligns well with the experimental findings in terms of the maximum FR value, while state-of-the-art FR models that ignore the IQE effects do not yield consistent results. Specifically, we discovered that the red LEDs exhibit a stable DC gain when the biasing current rises to 300 mA, but the highest DC gain for green and blue LEDs can only be achieved under 100 mA. For green and blue LEDs, the biasing currents are much lower than their typical working currents (350 mA and 500 mA). To address the trade-off between luminance and DC gain, we propose cascading multiple LEDs operating at a low current corresponding to the maximum response. Experimental results demonstrate that four cascaded monochromatic LEDs exhibit similar bandwidth and response gain larger than 11 dB compared to a single LED at the same biasing current while delivering four times the emission power, which is very close to the theoretical value of 12 dB. Furthermore, we find that the DC gain improvement by cascading multiple LEDs is nonlinear and bounded by an upper limit as the number of LEDs increases, and as a cost-effective solution, a few tens of LEDs is recommended. To the best of our knowledge, this is the first attempt to apply the IQE model to study the LEDs’ precise FR performances for VLC. We believe that the precise IQE-FR model opens new avenues for optimizing VLC system performance by considering the effects of IQE.

The metal–dielectric cermet (metal‐rich) material is utilized for turning the dispersion properties of surface plasmon in GaN‐light‐emitting diode. The designed structure contains a cermet layer coated on p‐GaN layer. The optimized ingredients in the cermet material and thickness of the cermet layer are obtained by the simulations. For enhancing the extraction of surface plasmon, the designed structure is further optimized by inserting a layer of low refractive index. The calculated results indicate that the Purcell factor in optimized structure is greater than 100 over the emission spectrum and the extraction efficiency of surface plasmon is effectively improved. The photoluminescence (PL) experiment by bottom excitation demonstrates that peaks of PL spectra of the designed and optimized samples are enhanced by 1.6 and 3.4 times, respectively, compared with the sample covered by Ag film. Compared with the naked sample, peaks of PL spectra of bottom pumping of optimized and designed samples are enhanced by 36.3 and 17.2 times, respectively. Peaks of normalized PL spectra by top excitation of optimized and designed samples are enhanced by 29.0 and 9.1 times, respectively. The results of numerical calculations are substantially consistent with the results of PL tests. [ABSTRACT FROM AUTHOR]

Tunnel nanojunctions based on inelastic electron tunneling (IET) have been heralded as a breakthrough for ultra-fast integrated light sources. However, the majority of electrons tend to tunnel through a junction elastically, resulting in weak photon-emission power and limited efficiency, which have hindered their practical applications to date. Resonant tunneling has been proposed as a way to alleviate this limitation, but photon-emissions under resonant tunneling conditions have remained unsatisfactory for practical IET-based light sources due to the inherent contradiction between high photon-emission efficiency and power. In this work, we introduce a novel approach that leverages much stronger resonant tunneling enhancement achieved by multiple metallic quantum wells, which has enabled the internal quantum efficiency to reach ∼1 and photon-emission power to reach ∼0.8 µW/µm2. Furthermore, this method is applicable with different electronic lifetimes ranging from 10 fs to 100 fs simultaneously, bringing practical implementation of IET-based sources one step closer to reality.

In the past, low-pressure mercury (Hg)-based deep-ultraviolet (DUV) lamps have been widely used for real-world applications. The exposure of pathogen to DUV radiation at 254 nm emission has been proven to be an effective way for their inactivation in unmanned environment. However, the low-pressure mercury (Hg) DUV lamps are heavy as well as toxic in nature and it consumes a lot of power. Therefore, in this work, smart and clean AlGaN-based DUV light-emitting diode (LED) at 254 nm emission wavelength has been proposed. The performance of the conventional LED can be improved by employing a 5 nm-thin tunnelling assisted Si-doped n-AlGaN layer on p-side of the device. The simulation results show that the n-AlGaN layer DUV LED has a maximum internal quantum efficiency (IQE) of 62% with an efficiency droop of 15% when compared to the conventional DUV LED device, which has a maximum IQE of 50% with an efficiency droop of 18% under 200 A/cm2. In addition, the Si-doped n-AlGaN layer-based LED emitted power has been improved by 34% compared to the conventional LED structure. The improved performance is mainly contributed by the quantum tunnelling phenomena using 5 nm-thin tunnelling assisted Si-doped n-AlGaN layer on p-side of the device. Such efficient tunnelling assisted by Si-doped AlGaN Layer on p-side of DUV LED at 254 nm emission wavelength could be an alternative to the toxic Hg-DUV lamp to be used in the unmanned environment for disinfection applications. [ABSTRACT FROM AUTHOR]

GaAs-based nanowires are among the most promising candidates for realizing a monolithic integration of III–V optoelectronics on the Si platform. To realize their full potential for applications as light absorbers and emitters, it is crucial to understand their interaction with light governing the absorption and extraction efficiency as well as the carrier recombination dynamics determining the radiative efficiency. Here, we study the spontaneous emission of zincblende GaAs/(Al,Ga)As core/shell nanowire arrays by μ-photoluminescence spectroscopy. These ordered arrays are synthesized on patterned Si(111) substrates using molecular beam epitaxy and exhibit an exceptionally low degree of polytypism for interwire separations exceeding a critical value. We record emission spectra over more than five orders of excitation density for both steady-state and pulsed excitation to identify the nature of the recombination channels. An abrupt Mott transition from excitonic to electron–hole plasma recombination is observed, and the corresponding Mott density is derived. Combining these experiments with simulations and additional direct measurements of the external quantum efficiency using a perfect diffuse reflector as a reference, we are able to extract the internal quantum efficiency as a function of carrier density and temperature, as well as the extraction efficiency of the nanowire array. The results vividly document the high potential of GaAs/(Al,Ga)As core/shell nanowires for efficient light emitters integrated on the Si platform. Furthermore, the methodology established in this work can be applied to nanowires of other materials systems of interest for optoelectronic applications. [ABSTRACT FROM AUTHOR]

The internal quantum efficiency (IQE) decay resultant lifetime performance of ultraviolet C‐band (UVC) light‐emitting diodes (LEDs) is investigated, and an LED lifetime model is deduced assuming that defect generation is the sole cause for IQE decay and the defect generation process is to maximize the entropy of the defects. The proposed model reveals that the lifetime of an LED is proportional to its radiative recombination coefficient and inversely proportional to the product of its initial nonradiative recombination coefficient and defect growth interest rate, thus providing explicit dependence of the lifetime on the LED junction temperature and current density. The model fits excellently well to full sets of lifetime measurement data, which were collected from UVC LEDs stressed at different currents and different ambient temperatures for 2500 up to ≈10 000 h. The sensitivity of the lifetime to the junction temperature and current density is revealed for some UVC LEDs. Using the model, a lifetime of ≈96 000 h is projected to maintain 70% of the initial optical output power for a UVC LED based on a stress test for ≈10 000 h. [ABSTRACT FROM AUTHOR]

Bulk heterojunction polymer solar cells (PSCs) blended with non-fullerene-type acceptors (NFAs) possess good solar power conversion efficiency and compatibility with flexible electronics, rendering them good candidates for mobile photovoltaic applications. However, their internal absorption performance and mechanism are yet to be fully elucidated because of their complicated interference effect caused by their multilayer device structure. The transfer matrix method (TMM) is ideal for analyzing complex optical electric fields by considering multilayer interference effects. In this study, an active layer (AL) thickness-dependent TMM is used to obtain accurate information on the photon-capturing mechanisms of NFA-based PSCs for comparison with experimental results. Devices with AL thicknesses of 40–350 nm were prepared, and the AL-thickness-dependent device parameters with incident photon-to-current efficiency spectra were compared with the calculated internal absorption spectra of the TMM. The spectrally and spatially resolved spectra as a function of the AL thickness and excitation wavelength revealed that the power conversion efficiency of the NFA-blended PSC decreased with the increasing AL thickness after reaching a maximum of ~100 nm; by contrast, the internal absorption efficiency showed the opposite trend. Furthermore, the TMM spectra indicated that the spatial distribution of the photogenerated charge carriers became significantly imbalanced as the AL thickness increased, implying that the AL-dependent loss stemmed from the discrepancy between the absorption and the extracted charge carriers. [ABSTRACT FROM AUTHOR]

The performance improvements of AlGaN‐based deep ultraviolet light‐emitting diodes (DUV‐LEDs) with multigradient electron blocking layer (EBL) and triangular last quantum barrier (LQB) are investigated. The results show that the maximum internal quantum efficiency (IQE) is improved by 44.9% and light output power (LOP) is improved by 58.1% at 200 A cm−2, exhibiting a tremendous improvement compared to the conventional structure. These improvements are mainly attributed to the fact that both multigradient EBL and triangular LQB can generate negative polarization charges in the graded composition layer, which leads to a significant increase in hole concentration compared to the conventional EBL and LQB structures. Meanwhile, the multigradient EBL can transform the conventional triangular barrier into an arc‐shaped barrier, increasing the electron potential barrier height and decreasing the hole potential barrier height. This unique design suppresses electron leakage and enhances hole injection, leading to a significant enhancement of the IQE and alleviation of the efficiency droop. [ABSTRACT FROM AUTHOR]

This article is devoted to a theoretical evaluation of the transient behavior of a light emitting diode with a resonant cavity called the resonant cavity enhanced light emitting diode (RCELED). The used analytical model is based on applying the convolution theorem for a step input signal and the transfer function of RCELED in the presence of photon recycling. Influence of the efficiency of extraction due to photon recycling on the output optical power is analyzed. The target parameters characterizing the transient behavior are investigated. A traditional light emitting diode with no photon recycling is compared to a diode with photon recycling. The obtained results show the improvement of the output optical power and the rise time with the increase of extraction efficiency and in the presence of photon recycling in the light emitting diodes. The light emitting diode considered here reaches the highest steady state output power within 2 ns. Therefore this diode model may be used for fast speed and high optical gain applications such as in thermal imaging systems and short reach optical interconnects. [ABSTRACT FROM AUTHOR]

The serious separation of electron–hole wavefunctions, which is caused by the built-in electric field, prevents electron–hole radiative recombination in quantum wells (QWs) in high-In-content InGaN-based red light-emitting diodes (LEDs). Here, we propose a staggered structure that inserts an ultrathin InN layer in the single quantum well (SQW) to reduce the piezoelectric polarization and suppress the quantum confined Stark effect (QCSE). We have numerically simulated the effects of SQW with the InN insertion layer (IL) on the energy band structure and electron–hole wavefunctions of the red LED. Owing to alleviated piezoelectric polarization and improved overlaps of electron–hole wavefunctions, the simulation results have revealed that the internal quantum well (IQE) of the red LED with InN IL exhibits 42% higher than that of the red LED with a square-shaped QW (SSQW) at 60 A/cm2, and the efficiency droop ratio of red LED with InN IL is 48% lower than that of red LEDs with SSQW. Furthermore, we have found that the position of InN IL can affect the energy states of carriers, which has a great influence on the IQE and peak emission wavelength of red LEDs. [ABSTRACT FROM AUTHOR]

The sidewall condition is a key factor determining the performance of micro‐light emitting diodes (µLEDs). In this study, equilateral triangular III‐nitride blue µLEDs are prepared with exclusively m‐plane sidewall surfaces to confirm the impact of sidewall conditions. It is found that inductively coupled plasma‐reactive ion etching (ICP‐RIE) causes surface damages to the sidewall and results in rough surface morphology. As confirmed by time‐resolved photoluminescence (TRPL) and X‐ray photoemission spectroscopy (XPS), tetramethylammonium hydroxide (TMAH) eliminates the etching damage and flattens the sidewall surface. After ICP‐RIE, 100 µm2‐µLEDs yield higher external quantum efficiency (EQE) than 400 µm2‐µLEDs. However, after TMAH treatment, the peak EQE of 400 µm2‐µLEDs increases by ≈10% in the low current regime, whereas that of 100 µm2‐µLEDs slightly decreases by ≈3%. The EQE of the 100 µm2‐µLEDs decreases after TMAH treatment although the internal quantum efficiency (IQE) increases. Further, the IQE of the 100 µm2‐µLEDs before and after TMAH treatment is insignificant at temperatures below 150 K, above which it becomes considerable. Based on PL, XPS, scanning transmission electron microscopy, and scanning electron microscopy results, mechanisms for the size dependence of the EQE of µLEDs are explained in terms of non‐radiative recombination rate and light extraction. [ABSTRACT FROM AUTHOR]

AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) have great application prospects in sterilization, UV phototherapy, biological monitoring and other aspects. Due to their advantages of energy conservation, environmental protection and easy miniaturization realization, they have garnered much interest and been widely researched. However, compared with InGaN-based blue LEDs, the efficiency of AlGaN-based DUV LEDs is still very low. This paper first introduces the research background of DUV LEDs. Then, various methods to improve the efficiency of DUV LED devices are summarized from three aspects: internal quantum efficiency (IQE), light extraction efficiency (LEE) and wall-plug efficiency (WPE). Finally, the future development of efficient AlGaN-based DUV LEDs is proposed. [ABSTRACT FROM AUTHOR]

In the past decade, intensive studies on monolayer MoS2-based phototransistors have been carried out to achieve further enhanced optoelectronic characteristics. However, the intrinsic optoelectronic characteristics of monolayer MoS2 have still not been explored until now because of unintended interferences, such as multiple reflections of incident light originating from commonly used opaque substrates. This leads to overestimated photoresponsive characteristics inevitably due to the enhanced photogating and photoconductive effects. Here, we reveal the intrinsic photoresponsive characteristics of monolayer MoS2, including its internal responsivity and quantum efficiency, in fully transparent monolayer MoS2 phototransistors employing a van der Waals heterostructure. Interestingly, as opposed to the previous reports, the internal photoresponsive characteristics do not significantly depend on the wavelength of the incident light as long as the electron-hole pairs are generated in the same k-space. This study provides a deeper understanding of the photoresponsive characteristics of MoS2 and lays the foundation for two-dimensional materials-based transparent phototransistors.

The maximum internal quantum efficiency (IQE) of light‐emitting diodes (LEDs) can be obtained using the so‐called ABC model. Since the ABC model is based on the experimentally not directly accessible carrier density, the light over current data is usually fitted as a function of the light output to yield the maximum IQE. This article shows a method to directly obtain the IQE from experimental data without fitting, which also reflects changes of the A, B, or C parameters over the current range. [ABSTRACT FROM AUTHOR]

High-power, broad-area, semiconductor lasers are attractive sources for material processing, aerospace, and laser pumping. The design of the active region is crucial to achieve the required high power and electro-optical conversion efficiency, since the temperature significantly affects the performance of the quantum well, including the internal quantum efficiency and mode gain. In this work, the temperature effects on the active region of a 808 nm high-power semiconductor laser were investigated theoretically and experimentally. The simulations were performed with a Quasi-3D model, which involved complete steady-state semiconductor and carrier confinement efficiency combined with a new mathematical method. The critical aluminum content of the quantum barrier was proposed and the relationship between temperature and various loss sources was disclosed in the temperature range of 213 to 333 K, which provides a reliable reference for the design of epitaxial structures of high-power semiconductor lasers in different operating conditions. Subsequently, the optimized epitaxial structure was determined and used to fabricate standard laser bar chips with a cavity length of 2 mm. The experimental electro-optical conversion efficiency of 71% was demonstrated with a slope efficiency of 1.34 W/A and an injection current of 600 A at the heatsink temperature of 223 K. A record high electro-optical conversion efficiency of 73.5% was reached at the injection current of 400 A, while the carrier confinement efficiency was as high as 98%. [ABSTRACT FROM AUTHOR]

InGaAs/GaAs surface quantum dot (SQD) heterostructures have long been viewed as having great potential for realizing environmental gas detection devices. The research has recently been steered into discovering ways to inject carriers into the available SQD energy states to facilitate readable devices. In the present research, carrier injection from buried QDs (BQDs) to SQDs was controlled by changing the interlayer coupling. Photoluminescence (PL) measurements show that, by increasing the stacking period of BQDs, the carrier collection efficiency and subsequently the luminescence intensity for SQDs can be effectively improved. A rate equation simulation of the carrier recombination process is used to quantitatively describe the luminescent quantum efficiency (I QE) for the SQDs, indicating an increase with additional BQD layers due to an increase in carrier injection. The PL spectra along with the rate equation simulations further indicated that for these SQD hybrid structures nonradiative recombination dominates, with Auger recombination becoming significant at high excitation intensities. These results may help to understand the carrier dynamics in coupled SQDs hybrid structures. In addition, they provide a path to manipulating the properties of InGaAs SQDs for the development of gas sensors. • Architecture carrier injection heterostructures for InGaAs surface quantum dots (SQDs). • Modified rate equation model is used to quantitatively describe carrier recombination of SQDs. • Nonradiative recombination dominates for these SQD hybrid structures, with Auger recombination becoming significant at high excitation intensities. • The PL measurement provides an in-depth assessment of the InGaAs SQDs heterostructures. [ABSTRACT FROM AUTHOR]

The poor efficiency of the aluminum gallium nitride (AlGaN)-based ultraviolet (UV)-C light emitting diode (LED) remains as the major hindrance towards its commercialization. Thus, to enhance the internal quantum efficiency (IQE) and light output power (LOP) and to mitigate the efficiency droop of AlGaN-based UV-C LED, this simulation work proposes and examines a new engineering strategy, i.e. , inverted linearly graded quantum wells along with quantum barriers over a series of samples A, B and C. To further improve the characteristics of AlGaN-based UV-C LED, an engineered linearly graded p-AlGaN layer has been proposed in the device structure. This increases the electron and hole concentrations by ∼2-fold and ∼8-fold, respectively. The 1.2-fold reduction in hole effective potential barrier height improves the hole injection. Also, the 1.5-fold increase in electron effective potential barrier height limits electron spill-off from the active region. The proposed structural-engineering reduces quantum confined stark effect (QCSE) confirmed by the reduction of quantum well slopes. Reducing carrier spill-off and QCSE increases average radiative recombination rate by ∼8-fold. The maximum IQE is improved by ∼44 % and the efficiency droop is also reduced to a promising level of ∼2 % in case of the sample under consideration. Finally, the LOP increases 10-fold, promisingly. Thus, the proposed structure is supposed to be highly potential for improving the UV-C LED performance. The electroluminescence spectra show that despite the gradual differences introduced in the structure, all the samples emit at ∼276 nm with similar full width at half maximum providing a reference for the comparison. [ABSTRACT FROM AUTHOR]

The internal quantum efficiency (IQE) of conventional AlGaN-based deep-ultraviolet (DUV) light-emitting diodes (LEDs) is seriously limited by the poor and inhomogeneous carrier injection. The typical solution is to optimize the structure parameters of p-type region and active region. In this work, however, we try to address this issue by introducing an n-type electron deceleration layer (EDL) underneath multiple quantum wells (MQWs). On one hand, the electron deceleration layer helps to decrease the electron velocity and thus increase the electron capture rate. On the other hand, it can also reduce barrier heights in the band valence and thus enhance the hole transport in the multiple quantum wells. As a consequence, the concentrations of electrons and holes in the multiple quantum wells were significantly increased, resulting in the enhancement of radiative recombination. Compared to the conventional structure, the DUV-LED structure with an electron deceleration layer achieves a higher internal quantum efficiency, leading to a 39% improvement in the light output power. It is believed that performing energy-band engineering in n-type region has great application prospects for high-performance DUV-LEDs.

In deep ultraviolet light-emitting diodes (DUV LEDs), we numerically investigate the effect of p-doped AlInN last quantum barrier (LQB). The p-doped AlInN LQB not only suppresses the electron overflow but also facilitates hole transport. This is attributed to the decreased lattice mismatching between epilayers, i.e., LQB and p-AlGaN. Our proposed p-doped AlInN LQB LED shows reduced efficiency droop at 200 A/cm2. Additionally, the peak emission spectra (at 266 nm) of p-doped AlInN LQB LED are enhanced by almost twice that of the conventional LED. Our proposed LED has no electron blocking layer (EBL) because the p-doped AlInN LQB serves two purposes, i.e., as both LQB and effective EBL. [ABSTRACT FROM AUTHOR]

The lattice-matched AlInGaN/AlGaN quaternary superlattice electron blocking layer (EBL) has been applied in the a-plane non-polar AlGaN-based deep ultraviolet light-emitting diode (DUV-LED) to improve the DUV-LED performance. Compared to the a-plane DUV-LED with traditional AlGaN EBL, the internal quantum efficiency (IQE) of the DUV-LED with lattice-matched quaternary superlattice EBL can be enhanced by 57.1% and the optical output power is upgraded about 1.67 times at 200 mA. These results can be interpreted as the ameliorated effective barrier height for carriers, resulting in enhanced electron blocking capability and hole injection efficiency, leading to improved radiative recombination in the active region and eventually boosted optical and electrical performance. [ABSTRACT FROM AUTHOR]

Bulk heterojunction polymer solar cells (PSCs) blended with non-fullerene-type acceptors (NFAs) possess good solar power conversion efficiency and compatibility with flexible electronics, rendering them good candidates for mobile photovoltaic applications. However, their internal absorption performance and mechanism are yet to be fully elucidated because of their complicated interference effect caused by their multilayer device structure. The transfer matrix method (TMM) is ideal for analyzing complex optical electric fields by considering multilayer interference effects. In this study, an active layer (AL) thickness-dependent TMM is used to obtain accurate information on the photon-capturing mechanisms of NFA-based PSCs for comparison with experimental results. Devices with AL thicknesses of 40–350 nm were prepared, and the AL-thickness-dependent device parameters with incident photon-to-current efficiency spectra were compared with the calculated internal absorption spectra of the TMM. The spectrally and spatially resolved spectra as a function of the AL thickness and excitation wavelength revealed that the power conversion efficiency of the NFA-blended PSC decreased with the increasing AL thickness after reaching a maximum of ~100 nm; by contrast, the internal absorption efficiency showed the opposite trend. Furthermore, the TMM spectra indicated that the spatial distribution of the photogenerated charge carriers became significantly imbalanced as the AL thickness increased, implying that the AL-dependent loss stemmed from the discrepancy between the absorption and the extracted charge carriers.

The serious separation of electron–hole wavefunctions, which is caused by the built-in electric field, prevents electron–hole radiative recombination in quantum wells (QWs) in high-In-content InGaN-based red light-emitting diodes (LEDs). Here, we propose a staggered structure that inserts an ultrathin InN layer in the single quantum well (SQW) to reduce the piezoelectric polarization and suppress the quantum confined Stark effect (QCSE). We have numerically simulated the effects of SQW with the InN insertion layer (IL) on the energy band structure and electron–hole wavefunctions of the red LED. Owing to alleviated piezoelectric polarization and improved overlaps of electron–hole wavefunctions, the simulation results have revealed that the internal quantum well (IQE) of the red LED with InN IL exhibits 42% higher than that of the red LED with a square-shaped QW (SSQW) at 60 A/cm2, and the efficiency droop ratio of red LED with InN IL is 48% lower than that of red LEDs with SSQW. Furthermore, we have found that the position of InN IL can affect the energy states of carriers, which has a great influence on the IQE and peak emission wavelength of red LEDs.

Models describing the tunneling of electrons and holes through parabolic InxGa1−xN/GaN quantum well/LED structures with respect to strain were developed. The transmission coefficient, tunneling lifetime, and efficiency of LED structures were evaluated by solving the Schrödinger equation. The effects of the mole fraction on the structure strain, resonant tunneling and tunneling lifetime, and LH–HH splitting were characterized. The value of LH–HH splitting increased and remained higher than the Fermi energy; therefore, only the HH band was dominant in terms of the valence band properties. The results indicate that an increase in the mole fraction can lead to efficiency droop. [ABSTRACT FROM AUTHOR]

• AlGaN-based TJ is one of the most promising parts of optical devices and has been widely utilized as carrier tunnelling in DUV LEDs to mitigate both the contact resistance for Ohmic contact and low hole injection efficiency at 254 nm emission. • The internal-quantum efficiency (IQE) of approximately ∼88 % in the TJ-based LED with zero efficiency droop is achieved at 254 nm emission wavelength. • The operating voltages were reduced from 21 V to 5.4 V under 200 mA operation. The Minamata Convention of 2020 mandates the replacement of conventional mercury UV lamps with deep ultraviolet (DUV) light-emitting diodes (LEDs) emitting at 254 nm. But both the traditional DUV LEDs (C-LEDs) and tunnel junction (TJ)-based UVC LEDs face challenges such as high operating voltages and inefficient hole injection. Utilizing p-AlGaN and n-AlGaN layers-based TJ in DUV LEDs shows promise in addressing these issues, particularly in mitigating contact resistance and improving hole injection efficiency at the 254 nm emission wavelength. This study presents an approach to manipulate quantum tunnelling probability in n-AlGaN/p-AlGaN tunnel junctions by optimizing doping levels and thickness using APSYS Software simulations. The result is a suppression of Auger recombination and increased radiative recombination rates in 254 nm TJ-based DUV LEDs compared to C-LED. Theoretical modeling shows an internal quantum efficiency (IQE) of approximately 88 % with zero efficiency droop in TJ-based LEDs, a significant improvement over the approximately 66 % IQE with a 53 % efficiency droop in C-LEDs. This study reveals the highest theoretically possible IQE of 88 % at a 254 nm emission wavelength in TJ-based LED, with no efficiency droop. Moreover, TJ-LEDs show linear increases in light output powers (LOP) with varying current densities due to lower Auger recombination rates in their multi-quantum wells (MQWs). Notably, operating voltages reduce significantly from 21 V to 5.4 V under 200 A/cm2 operation, attributed to optimized TJ thickness and doping, along with a careful selection of lower Al-content in the contact layer. These findings pave the way for enhanced UV emitter growth using techniques like metal–organic chemical vapor deposition (MOCVD) and molecular beam epitaxy (MBE), promising advancements in biomedical applications. [ABSTRACT FROM AUTHOR]

AlGaN-based deep ultraviolet light-emitting diodes (DUV LEDs) have great application prospects in sterilization, UV phototherapy, biological monitoring and other aspects. Due to their advantages of energy conservation, environmental protection and easy miniaturization realization, they have garnered much interest and been widely researched. However, compared with InGaN-based blue LEDs, the efficiency of AlGaN-based DUV LEDs is still very low. This paper first introduces the research background of DUV LEDs. Then, various methods to improve the efficiency of DUV LED devices are summarized from three aspects: internal quantum efficiency (IQE), light extraction efficiency (LEE) and wall-plug efficiency (WPE). Finally, the future development of efficient AlGaN-based DUV LEDs is proposed.

Herein, how the macroscopic characterizations can be utilized to extract information on the defect level and crystal quality of the epitaxial layers of the light‐emitting diodes (LEDs) is presented. After review of the current–voltage (I–V) and light output power–current (L–I) characteristics, actual examples are utilized to show how different defect levels in the devices are reflected in macroscopic characteristics including the internal quantum efficiency (IQE). We show that the ideality factor from the I–V and the S‐parameter from the L–I can serve as useful guides to denote the dominance of the radiative recombination in the active region of the device. The minimum ideality factor is proposed as a possible figure of merit for the defect level of the epitaxial layers of the device and shown to be correlated with the maximum IQE value. [ABSTRACT FROM AUTHOR]

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The epitaxial growth of transparent p‐AlGaN‐based ultraviolet‐A (UVA), light‐emitting diodes (LEDs) may solve the problems of UVA light absorption through the GaN buffers and p‐GaN contact layers at (326–341 nm)‐band emission, respectively. Herein, first, an idea of conventional n‐AlGaN buffer layer (BL) and n‐AlGaN electron source layer (ESL) for the suppression of threading dislocation density (TDD) and enhancement of internal quantum efficiency (IQE) of UVA emitters, using low‐pressure metalorganic vapor‐phase epitaxy (LP‐MOVPE) is attempted. As a result, the total‐TDDs is reduced from ≈ 3 × 109 cm−2 to ≈ 1 × 109 cm−2 in the n‐AlGaN ESL of a 326 nm‐band UVA multiquantum‐wells (MQWs), and IQE is also improved from 30% to 52% at room temperature (RT). Second, an idea of Si‐doped n‐AlGaN Superlattices (SLs)‐based BL, using LP‐MOVPE is challenged. Subsequently, a record IQE of 56% at RT and high crystal quality in 341 nm‐band UVA MQWs are observed. Finally, using a well thickness ≈ 2 nm in SLs‐based UVA MQWs, the light power and external quantum efficiency (EQE), respectively, are remarkably enhanced from 3.5 mW and 0.5% to 7.5 mW and 1.4% on wafer in 341 nm‐Band UVA LED. The perspective for the improvements of UVA emitter's performances is also discussed. [ABSTRACT FROM AUTHOR]

This paper presents a novel device called Field Effect Photodiode (FEPD) to overcome the inherent drawbacks of PIN Photodiode (PIN-PD) and control the output photocurrent. Also, the proposed PIN-PD can be applied as a fast optical switch that provides a desired ION/IOFF ratio for optical applications in the nanoscale regime. The proposed device combines a Metal Semiconductor Field Effect Transistor (MESFET) and a regular PIN-PD device that can convert the incident light with photon energy greater than the semiconductor's bandgap to the regulated photocurrent by changing the gates bias which mounted over the absorption region. The significant parameters such: responsivity, output current in both the ON and OFF states, dark current, and ION/IOFF ratio that play key roles in the optical applications have been extracted. To extract and illustrate the electrical and optical results of both the regular PIN-PD and the proposed FEPD in this work, we have used TCAD tools as a semiconductor simulator. [ABSTRACT FROM AUTHOR]

A 365 nm AlxGa1-xN-based ultraviolet light-emitting diodes (LEDs) with double Al composition gradient last quantum barrier and hole supply layer structure has been studied. Experimental results show that the proposed structure enhances the carrier injection efficiency and suppresses the overflow of electrons. The introduction of three-dimensional hole gas further enhances the hole injection efficiency. As a result, the wall plug efficiency and electroluminescence intensity are significantly improved. In addition, the carrier concentration and the radiative recombination rate in the active region of the quantum well increases. Looking at the energy band diagram, one sees that the double Al composition gradient structure leads to higher electron blocking and alleviates the hole blocking effect. Overall, we conclude that the double Al composition gradient structure provides a pathway to achieve high-efficiency ultraviolet LEDs.

The optical properties of nonpolar AlGaN multiple quantum wells (MQWs) emitting at 280 nm were investigated intensively using temperature-dependent and time-resolved photoluminescence spectra associated with the characterization of structural properties. The densities of superficial pits and basal-plane stacking faults (BSFs) were reduced by 33.8% and 35.9%, respectively, for nonpolar AlGaN MQWs due to the carefully optimized dual nitridation. It was found that the nonpolar MQWs emission can be significantly improved by reducing the BSFs density as the BSFs emission was the main competing channels. Moreover, an internal quantum efficiency of 39% for nonpolar Al0.43Ga0.57N MQWs at emission wavelength of 279 nm was achieved even the full width at half maximum values of X-ray rocking curves were 0.565° for c-direction and 0.797° for m-direction. This fact means that a highly efficient deep ultraviolet light sources can be expected by means of nonpolar AlGaN due to the elimination of quantum confined Stark effect induced the decrease in radiative lifetime.

High-power, broad-area, semiconductor lasers are attractive sources for material processing, aerospace, and laser pumping. The design of the active region is crucial to achieve the required high power and electro-optical conversion efficiency, since the temperature significantly affects the performance of the quantum well, including the internal quantum efficiency and mode gain. In this work, the temperature effects on the active region of a 808 nm high-power semiconductor laser were investigated theoretically and experimentally. The simulations were performed with a Quasi-3D model, which involved complete steady-state semiconductor and carrier confinement efficiency combined with a new mathematical method. The critical aluminum content of the quantum barrier was proposed and the relationship between temperature and various loss sources was disclosed in the temperature range of 213 to 333 K, which provides a reliable reference for the design of epitaxial structures of high-power semiconductor lasers in different operating conditions. Subsequently, the optimized epitaxial structure was determined and used to fabricate standard laser bar chips with a cavity length of 2 mm. The experimental electro-optical conversion efficiency of 71% was demonstrated with a slope efficiency of 1.34 W/A and an injection current of 600 A at the heatsink temperature of 223 K. A record high electro-optical conversion efficiency of 73.5% was reached at the injection current of 400 A, while the carrier confinement efficiency was as high as 98%.

The advent of small molecule non-fullerene acceptor (NFA) materials for organic photovoltaic (OPV) devices has led to a series of breakthroughs in performance and device lifetime. The most efficient OPV devices have a combination of electron donor and acceptor materials that constitute the light absorbing layer in a bulk heterojunction (BHJ) structure. For many BHJ-based devices reported to date, the weight ratio of donor to acceptor is near equal. However, the morphology of such films can be difficult to reproduce and manufacture at scale. There would be an advantage in developing a light harvesting layer for efficient OPV devices that contains only a small amount of either the donor or acceptor. In this work we explore low donor content OPV devices composed of the polymeric donor PM6 blended with high performance NFA materials, Y6 or ITIC-4F . We found that even when the donor:acceptor weight ratio was only 1:10, the OPV devices still have good photoconversion efficiencies of around 6% and 5% for Y6 and ITIC-4F , respectively. It was found that neither charge mobility nor recombination rates had a strong effect on the efficiency of the devices. Rather, the overall efficiency was strongly related to the film absorption coefficient and maintaining adequate interfacial surface area between donor and acceptor molecules/phases for efficient exciton dissociation.

Threading dislocations in c‐plane (Al,Ga)N layers are surrounded by areas with reduced light generation efficiency, called "dark spots." These areas are observable in luminescence measurements with spatial resolution in the submicrometer range. Dark spots reduce the internal quantum efficiency in single layers and light‐emitting devices. In cathodoluminescence measurements, the diameter of dark spots (full width at half maximum [FWHM]) is observed to be 200–250 nm for GaN. It decreases by 30–60% for AlxGa1−xN with x ≈ 0.5. Furthermore, the dark spot diameter increases with increasing temperature from 83 to 300 K in AlGaN, whereas it decreases in GaN. Emission energy mappings around dark spots become less smooth and show sharper features on submicrometer scales at low temperature for AlGaN and, on the contrary, at high temperature for GaN. It is concluded that charge carrier localization dominates the temperature dependence of dark spot diameters and of the emission energy distribution around threading dislocations in AlGaN, whereas the temperature‐dependent excitation volume in cathodoluminescence and charge carrier diffusion limited by phonon scattering are the dominant effects in GaN. Consequently, with increasing temperature, nonradiative recombination related to threading dislocations extends to wider regions in AlGaN, whereas it becomes spatially limited in GaN. [ABSTRACT FROM AUTHOR]

We continue the comparative studies of high-power AlGaAs/InGaAs/GaAs semiconductor lasers with different waveguide designs. In this work, measurements were carried out for a continuous-wave mode of operation: lightâ€"current and currentâ€"voltage characteristics, threshold characteristics and their temperature dependences and crystal temperature depending on the pump current. The data obtained made it possible to reconstruct the dependences of internal optical losses and internal quantum efficiency using calculations. It was shown that the main factor affecting the power characteristics in the continuous mode of operation is the temperature stability of the laser and its relationship with internal optical losses and internal quantum efficiency. Thus, lasers based on a heterostructure with a doped GaAs waveguide with characteristic temperatures T 0 and T 1 of 120 K and 170 K, respectively, demonstrated an increase in internal optical losses to 2 cmâˆ'1 and a drop in the quantum efficiency to 90%, which led to saturation of the output power by high pump currents. Lasers based on the heterostructure with an AlGaAs waveguide, which are characterized by higher temperature stability (T 0 = 161 K and T 1 = 280 K), had lower internal optical losses and a higher quantum efficiency. This made it possible to obtain optical power 1 W higher than that for lasers with GaAs waveguides. [ABSTRACT FROM AUTHOR]

Modern light-emitting diodes (LEDs) require further improvements in the wall-plug efficiency (WPE). Thus, understanding and elucidating the physical factors limiting the WPE is of crucial importance. This study aims to understand and elucidate such factors via a comparative study of two different LED material systems, i.e. III-phosphide (AlGaInP) red and III-nitride (AlGaInN) blue LEDs. The WPE was decoupled into its component elements, which indicated that the dominant contribution to WPE reduction depends on the sample. To further investigate, thermodynamic and current component analyses were conducted. These analyses clearly demonstrated that the carrier dynamics is dependent of the material systems, i.e. internal quantum efficiency and voltage efficiency are simultaneously degraded due to the carrier accumulation in the blue sample. We found that the WPE can be improved using strategies dependent on the samples, such as improvement of the light extraction in the red sample and suppression of the carrier accumulation in the blue sample. [ABSTRACT FROM AUTHOR]