Enhancement Of Quantum Efficiency Research Articles

Large quantum efficiency enhancements of pristine conjugated polymer MEH-PPV by interlayer polymer diffusion

Photoluminescence (PL) efficiency of a neat conjugated polymer (CP) increases substantially when the CP molecular segments are subjected to increased tensions resulting from tight packaging of neighboring chains established by interdiffusion under solvent annealing. The enhancements become even greater on low-bandgap substrate, such as Si-wafer, due to reduced heterojunction quenching (HJQ) by the increased stress. After infusing polystyrene (PS) into a MEH-PPV film a depth of ~3.5 nm by solvent annealing, we found that the PL emissions at λmax0 ~580 nm of the pristine non-permeated portion increased ~6 folds on Si-wafer. For the same diffusion depth, the enhancement reduced to ~50% on a non-quenching substrate (glass cover slip). The observation was in agreement with previous findings that PL efficiencies of CPs can be enhanced dramatically by molecular stretching, which can also produce an effect of reduced HJQ. Owing to PS/MEH-PPV intermixing at the interface, a new peak also emerged, but blue shifted substantially to ~520 nm owing to chain separation and conformation distortions of the molecular segments confined within the tiny mixing zone. After extended annealing, the intermixing zone in the bilayer system may undergo dewetting, which creates further increased PL to the sheared pristine MEH-PPV molecules. The results suggest a close relationship between segmental stress states and the optoelectronic behavior of conjugated polymers.

Si-embedded metal oxide transparent solar cells

Abstract Transparent photovoltaic (TPV) is the technology of solar cells to convert light to electric energy. Different from the typically dark or opaque solar cells, TPV is transparent by passing the visible range lights. Metal-oxide semiconductor TPV was developed for short ultraviolet (UV) utilization. The solid-phase sputtering method was applied to sequentially deposit p-type NiO onto n-type ZnO forming the electric field. The pristine metal-oxide TPV has strong absorption of UV but bears the inevitable limit of carrier collection. Extension of light-utilization is a pivot to enhance TPV performances. And thus, a thin Si film was embedded between p-NiO and n-ZnO to improve TPV performances according to the enhanced quantum efficiency as well as the extended absorbing light. The Si-embedded TPV has 40% transmittance to achieve the large open circuit voltage (0.6 V) and short circuit current density (2.8 mA/cm2) values for high light-conversion efficiency of 4.8%. This may be the first report for the hybrid of Si-embedded metal-oxide heterojunction for TPVs. Si film would enhance the metal-oxide TPV performances due to the improved carrier collection efficiency and the extended light utilization for longer wavelengths. The inorganic TPV may be applied for the invisible power generator.

Influence of gold ions on visible and NIR luminescence features of Er3+ ions in lead boroselenate glass ceramics

This work is focused on the amplification of green (G), orange (O) and NIR (1.5 μm) emissions of Er3+ ions by co-doping with varying contents (traces ≤ 0.1 mol%) of Au2O3 in an exotic PbO-B2O3-SeO2 glasses ceramic. The glasses were synthesized by conventional melt quenching method and were crystallized by heat-treating them at crystallization temperature for a period of 24 h. Initial characterization of the samples by means of SEM, XRD, XPS, IR and Raman studies suggested that the post-heated samples are entrenched with poly-crystal grains (with sizes varying from 100 to 500 nm). These studies have also revealed that the concentration of such nano-crystallites is increased with increased content of Au2O3 up to 0.075 mol%. The XRD studies, in particular, have indicated that the glass ceramics are composed of Au2(SeO3)2(SeO4) (crystal phase containing Se ions of mixed valence states) and Au0 nano particles. The XPS studies have also indicated the presence of Se and gold ions in different valence states in these samples. IR and Raman spectra have suggested increased degree of internal chaos in the glass ceramic with increasing concentration of Au2O3. Optical absorption (OA) spectra of Au2O3 doped glass ceramic samples (without Er3+ ions) exhibited a broad band in the visible region identified as surface plasmon resonance (SPR) band of gold metallic particles. The photoluminescence (PL) spectra of these samples exhibited an emission peak due to inter-band transition from sp to d energy bands of Au0 metallic particles. This emission is identified to play a crucial role in enhancing PL emission of Er3+ ions in the visible and NIR regions. Finally, we have recorded optical absorption, photoluminescence spectra and decay profiles of the Er3+ doped (with Au2O3 as co-dopant) glass ceramics. The spectra were characterized using J–O theory and different radiative parameters (viz., transition probabilities, branching ratios, radiative lifetime) of green, orange and NIR emission transitions of Er3+ ions were evaluated. The results indicated a significant reinforcement of green (4S3/2 → 4I15/2), orange (4F9/2 → 4I15/2) and NIR (4I13/2 → 4I15/2) emissions of Er3+ ions due to co-doping with Au2O3. Quantitative analysis of the results of PL emission indicated that 0.075 mol% of Au2O3 is the optimal concentration for achieving the highest quantum yield of these three emissions in this glass ceramic. Additional population of 4S3/2, 4F9/2 and 4I13/2 energy levels of Er3+ ions from the excited state of Au0 metallic particles is found to be the reason for such enhancement of quantum efficiency.

Internal quantum efficiency enhancement employing composition-graded last quantum barrier and electron blocking layer

We reduce the problem of asymmetrical distribution of carriers (electrons and holes) by engineering the last barrier and electron blocking layer (EBL) of green indium gallium nitride (InGaN)-based multiquantum well light-emitting diodes. We employ stair-engineered EBL with a graded InGaN last quantum barrier to enhance the device performance. The efficiency droop ratio of the proposed device is ∼15 % at 100 A / cm2. Similarly, the light output power is also enhanced by about three times, as compared to the reference structure. In addition, a carrier transport issue across the active region is also mitigated in our design.

Voltage preserved GaInP single junction solar cell using type-A InP multiple quantum well structure with enhanced efficiency

Two photon absorption phenomenons is used to enhance the short circuit current density (Jsc) in most of the intermediate band solar cell (SC). But, higher recombination in the IB region results in degradation of the open circuit voltage (Voc). Therefore limits the performance of IB cell. A GaInP single junction (SJ) cell is designed with type-A multiple InP quantum well (QW) with proper dimensions in the intrinsic region. The objective of the study is to enhance the Jsc, while preserving the Voc. The type-A QW structure was formed in the quantum region of the cell. The insertion of QWs increases the Jsc, while the proper dimension of the QW layers, helps in voltage preservation. The recombination in the mini-bands of the sequential quantum region exhibits radiative recombination. The long wavelength absorption indicates enhanced quantum efficiency (QE). The designed structure of GaInP based multi QW solar cell shows superior efficiency of 22.41 % and QEs well above 80 %.

Controlled bunching approach for achieving high efficiency active region in AlGaN-based deep ultraviolet light-emitting devices with dual-band emission

Developing efficient active region structures with low sensitivity to threading dislocation density (TDD) is of great technical significance for AlGaN-based deep ultraviolet (DUV) light-emitting devices. Here, we propose an active region strategy by introducing bunching effect to form self-assembled sidewall quantum wells (SQWs) with much stronger carrier confinement, resulting in a significant enhancement of internal quantum efficiency (from 46% to 59%) compared to the commonly adopted multiple quantum wells (MQWs) due to the lower sensitivity to TDD. As a demo, an AlGaN-based DUV light-emitting diode (LED) with the proposed active region involving both SQWs and MQWs presents dual-band emission and a consequent 68% enhancement in light output power compared to the DUV-LED with only MQWs as the active region, suggesting that the proposed architecture is fully suitable for the development of high performance DUV light-emitting devices even based on poor or medium quality materials.

Enhanced quantum efficiency and thermal stability in CaWO4:Eu3+ phosphor based on structural modification induced by co-doping Al3+

The thermally stable phosphors with excellent luminous properties are of great significance for their practical applications in the white light emitting diodes (wLEDs). Herein, a variety of CaWO4:xEu3+, yAl3+ (0 ≤ x ≤ 0.24; 0 ≤ y ≤ 0.10) red phosphors could be efficiently pumped by near-UV LED chips were successfully fabricated through the solid-state method. The phase composition, microstructure, luminescent behaviors, quantum efficiency as well as thermal stability are fully studied. For the CaWO4:Eu3+, Al3+ system, it is confirmed that the change of [Ca/Eu/Al-O] polyhedral volume caused by cation-size mismatch is not a decisive factor for the change in the overall unit cell volume, and the additional [W-O] tetrahedral volume changes should also be considered. Moreover, it is also found that, as the co-doping of Al3+, the polyhedron distortion resulting from the cation-size mismatch and the induced oxygen deficiency causes the loss of local symmetry around Eu3+, resulting in the improved chromaticity feature and the excellent luminescence properties. Impressively, the prepared CaWO4:0.20Eu3+, 0.08Al3+ phosphor possesses a higher quantum efficiency of 84.2% and a superior thermal stability retaining 83% at 423 K with respect to that at room temperature. These results show that the CaWO4:0.20Eu3+, 0.08Al3+ red phosphor can be served as an excellent candidate for wLEDs.

A discrete core-shell-like micro-light-emitting diode array grown on sapphire nano-membranes

A discrete core-shell-like micro-light-emitting diode (micro-LED) array was grown on a 100 nm-thick sapphire nano-membrane array without harmful plasma etching for chip singulation. Due to proper design for the sapphire nano-membrane array, an array of multi-faceted micro-LEDs with size of 4 μm × 16 μm was grown. Threading dislocation density in the micro-LED formed on sapphire nano-membrane was reduced by 59.6% due to the sapphire nano-membranes, which serve as compliant substrates, compared to GaN formed on a planar substrate. Enhancements in internal quantum efficiency by 44% and 3.3 times higher photoluminescence intensity were also observed from it. Cathodoluminescence emission at 435 nm was measured from c-plane multiple quantum wells (MQWs), whereas negligible emissions were detected from semi-polar sidewall facets. A core-shell-like MQWs were formed on all facets, hopefully lowering concentration of non-radiative surface recombination centers and reducing leakage current paths. This study provides an attractive platform for micro-LEDs by using sapphire nano-membrane.

Simultaneously Enhancing Photoluminescence Quantum Efficiency and Optical Gain of Polyfluorene via Backbone Intercalation of 2,5‐Dimethyl‐1,4‐Phenylene

AbstractSimultaneous enhancement of photoluminescence quantum efficiency (PLQE) and optical gain in semiconducting polymer films is desirable for optically‐ or electrically‐pumped organic solid‐state lasers. In this work, a simple self‐dilution effect is achieved by introducing a small amount (≈10% by weight) of 2,5‐dimethyl‐1,4‐phenylene (DP) units in the backbone of poly(9,9‐dioctylfluorene) (PFO). The resulting copolymers, compared with PFO (PLQE 39%), exhibit a significantly increased PLQE (66%) while keeping similar absorption and photoluminescence profile, concomitant with an enhancement in optical gain properties. The radiative decay rate increases sharply along with a sustaining reduction in the non‐radiative decay rate in these copolymers, following similar principle of physical dilution of a luminescent compound in solution or in a polymer matrix. Among all the copolymers, the one containing 10% DP units exhibits the lowest amplified spontaneous emission/distributed feedback laser threshold (10.9 nJ/1.4 nJ, eightfold reduction), and the highest gain coefficient (54.4 cm−1). The results demonstrate that a moderate DP/fluorene ratio can maximize the beneficial self‐dilution effects. These investigations shed light on new design strategies to achieve conjugated polymers with concurrent high PLQE and large optical gain properties.

Manipulation of [Ga]/([Ga] + [In]) profile in 1.4-μm-thick Cu(In,Ga)Se2 thin film on flexible stainless steel substrate for enhancing short-circuit current density and conversion efficiency of its solar cell

Flexible 1.4-μm-thick Cu(In,Ga)Se2 (CIGS) films on stainless steel (SUS) substrates are fabricated by the process named “multi-layer precursor method” as the absorbers of the thin-film solar cells. It is shown that the [Ga]/([Ga] + [In]) (GGI) or bandgap (Eg) profiles were easily controlled to enhance photovoltaic performances, especially short-circuit current density (JSC) for the increased conversion efficiency (η). It is determined that the double-grading GGI profile is most pronounced with the presence of the CuInSe2 (CIS) phase in the CIGS film, when annealing time and temperature during the fabrication are reduced to 1 min and 610 °C, respectively. Consequently, the minimum (mini) Eg value in the Eg profile is reduced to approximately 1.05 eV close to the Eg of the CIS material, resulting in the enhancement of external quantum efficiency in the long wavelength region. The JSC of the flexible CIGS solar cell is ultimately increased up to 40 mA/cm2, thus enhancing the η to 17.5% despite the reduction of CIGS thickness to 1.4 μm. It is disclosed that the reduced CIGS thickness to 1.4 µm with very short annealing time (1 min) leads to the possible reduction of the production cost of the flexible thin-film solar cell with the high η.

Giant enhancement of silicon plasmonic shortwave infrared photodetection using nanoscale self-organized metallic films

Many consumer technologies rely on photodetection of infrared light, such as lidar, low visibility imaging, proximity sensors/range finders, etc. However, silicon, the standard material of the semiconductor industry, becomes transparent for wavelengths above 1.1 µm, as the photons no longer have sufficient energy to stimulate direct band-to-band absorption. We report here a Schottky photodetector design that extends silicon’s optical detection range beyond this 1.1 µm limit, by utilizing internal photoemission of hot carriers. Our design relies on an ultra-thin fractally nanostructured aluminum optical absorber and yet remarkably achieves over 50% absorption of incident light. We demonstrate 2 orders of magnitude improvements of responsivity, noise-equivalent power, and specific detectivity as compared to a reference Schottky photodetector made of bulk metal films. We attribute this to the combination of superior transport and momentum relaxation processes from the nanoscale fractal geometries. Specifically, we show a direct link between internal quantum efficiency enhancement and structural parameters such as perimeter-to-surface ratio. Finally, our devices also function as bulk refractive index sensors. Our approach uses an exceedingly simple complementary metal-oxide-semiconductor (CMOS)-compatible “bottom up” fabrication that is cheap and scalable and is a promising candidate for future cost-effective and robust shortwave infrared photodetection and sensing applications.

Enhanced quantum efficiency and thermal stability in tunable yellow-emitting Sr Ca1-AlSiN3:Ce3+ phosphor

Abstract Ce3+ doped CaAlSiN3 is a promising broadband orange phosphor that can be applied in white light-emitting diodes (wLEDs) solid state lighting for general illumination. However, its emission color and quantum efficiency are not good enough to achieve high color rendering properties and luminous efficacy of wLEDs. In this work, we proposed to realize the spectral tuning and property enhancement of CaAlSiN3:Ce3+ by bandgap engineering via the cationic substitution of Ca by Sr. The relationship between photoluminescence and local coordination structure as well as electronic structure were revealed. The substitution led to a blue-shift in emission maximum by 13 nm (from 595 to 582 nm) and a highest external quantum efficiency of 55% (enhanced by 10%). This broadband yellow phosphor enabled to produce wLEDs with a color rendering index of 83.3, luminous efficacy of 114.6 l m/W and a color temperature of 4604 K.

Giant defect emission enhancement from ZnO nanowires through desulfurization process

Zinc oxide (ZnO) is a stable, direct bandgap semiconductor emitting in the UV with a multitude of technical applications. It is well known that ZnO emission can be shifted into the green for visible light applications through the introduction of defects. However, generating consistent and efficient green emission through this process is challenging, particularly given that the chemical or atomic origin of the green emission in ZnO is still under debate. In this work we present a new method, for which we coin term desulfurization, for creating green emitting ZnO with significantly enhanced quantum efficiency. Solution grown ZnO nanowires are partially converted to ZnS, then desulfurized back to ZnO, resulting in a highly controlled concentration of oxygen defects as determined by X-ray photoelectron spectroscopy and electron paramagnetic resonance. Using this controlled placement of oxygen vacancies we observe a greater than 40-fold enhancement of integrated emission intensity and explore the nature of this enhancement through low temperature photoluminescence experiments.

1T semi-floating gate image sensor with enhanced quantum efficiency and high response speed

The active pixel image sensor based on a semi-floating gate transistor (SFGT-APS) has been proposed and investigated; however, the quantum efficiency (QE) and the sensitivity are too poor to meet low illumination intensity and high-speed application due to the shallow junction of photodiodes. In this work, we demonstrate a new structure, called buried photodiode semi-floating gate transistor active pixel image sensor (BSFGT-APS), which possesses enhanced QE, high sensitivity, and fast response speed. Moreover, BSFGT-APS has the same fill factor with SFGT-APS, which is 55% with a 1 μm2 photodiode in 0.13 μm process. The basic device characteristics were investigated by Sentaurus TCAD simulation, including potential of photodiode, transient response, dark current, conversion gain, and full well capacity.

Quantum efficiency of plasmonic-coupled quantum dot infrared photodetectors for single- color detection: the upper limit of plasmonic enhancement

We report a measurement of the quantum efficiency for a surface plasma wave (SPW)-coupled InAs/In0.15Ga0.85As/GaAs dots-in-a-well (Dwell) quantum dot infrared photodetector (QDIP) having a single-color response at ∼10 µm. A gold film perforated with a square array of complex, non-circular apertures is employed to manipulate the near-fields of the fundamental SPW. The quantum efficiency is quantitatively divided into absorption efficiency strongly enhanced by the SPW, and collection efficiency mostly independent of it. In the absorption efficiency, the evanescent near-fields of the fundamental SPW critically enhances QDIP performance but undergoes the attenuation by the absorption in the Dwell that ultimately limits the quantum efficiency. For the highest quantum efficiency available with plasmonic coupling, an optimal overlap between Dwell and SPW near-fields is required. Based on experiment and simulation, the upper limit of the plasmonic enhancement in quantum efficiency for the present device is addressed.

Color tuning of β-Ca3(PO4)2-type phosphor with enhanced quantum efficiency via self-charge compensation for healthy and warm solid state lighting application

Phosphor with high efficiency and tunable photoluminescence is critical to solid state lighting device. Herein, three β-Ca3(PO4)2-type phosphors with improvement of quantum efficiency and tunable emission color were synthesized through the self-charge compensation method via the incorporation of Na+ or K+. The quantum efficiency and emission intensity were increased by 2.81 and 1.99 times at most due to the reduction of Eu3+ to Eu2+ in Ca10Na(PO4)7:0.01Eu2+, which was ascribed to the achievement of charge balance and cavity neutralization in Na(4) site after the incorporation of Na+ in the neighboring Ca(1), Ca(2) and Ca(3) sites. Tunable emission colors from near white to green were realized owing to the change of occupation ratio of Eu2+ at Ca(1), Ca(2) and Ca(3) sites relative to Na(4) site. Moreover, this method was applied into the Ca10-yK1+y(PO4)7:0.01Eu2+ and 0.1Ca9Y(PO4)7–0.9Ca10-zNa1+z(PO4)7:0.01Eu2+, 0.22Mn2+ systems to verify the practicability of self-charge compensation. The quantum efficiencies of the two kinds of systems were increased by 1.66 and 1.28 times, respectively, along with the decrease of blue light ratio significantly. Additionally, no blue light hazard and warm WLEDs was fabricated via the modified single-phase full visible emission 0.1Ca9Y(PO4)7–0.9Ca9.9Na1.1(PO4)7:0.01Eu2+, 0.22Mn2+ phosphor, which exhibited an excellent color rendering index of 92.9 at a correlated color temperature of 3887 K. This work illustrated that the universal self-charge compensation method could efficiently improve the multifaceted properties of phosphor to meet the needs of modern lighting.

Pyrophosphate Phosphor Solid Solution with High Quantum Efficiency and Thermal Stability for Efficient LED Lighting.

SummaryPhosphors with high quantum efficiency and thermal stability are greatly desired for lighting industries. Based on the design strategy of solid solution, a series of deep-blue-emitting phosphors (Sr0.99-xBax)2P2O7:0.02Eu2+ (SBxPE x = 0–0.5) are developed. Upon excitation at 350 nm, the optimized SB0.3PE phosphor shows a relatively narrow full width at half maximum (FWHM = 32.7 nm) peaking at 420 nm, which matches well with the plant absorption in blue region. Moreover, this phosphor exhibits obvious enhancement of internal quantum efficiency (IQE) (from 74% to 100%) and thermal stability (from 88% to 108% of peak intensity and from 99% to 124% of integrated area intensity at 150°C) compared with the pristine one. The white LED devices using SB0.3PE as deep-blue-emitting component show good electronic properties, indicating that SB0.3PE is promising to be used in plant growth lighting, white LEDs, and other photoelectric applications.

Material-to-device performance correlation for AlGaN-based solar-blind p–i–n photodiodes

We report on crystalline quality-to-device performance correlation for self-powered Al0.40Ga0.60N, p–i–n ultraviolet (UV) photodetectors on c-plane sapphire. The active p–i–n detector stack was grown over an AlN buffer. Careful optimization of the nucleation density on growth surface helped achieve a two-orders and one-order of magnitude reduction in the screw and edge dislocation density in the buffer layer, respectively. This resulted in a nine-orders of magnitude reduction in the reverse leakage current from 4.3 mA to 4.2 pA (at 10 V). Correspondingly, a thirteen-fold enhancement in the zero-bias external quantum efficiency (EQE) from 3.4% to 45.5%, when measured under 289 nm front-illumination was also observed. The detector epi-stack grown over the optimal AlN buffer layers led to the realization of high-performance p–i–n detectors with a dark current density below 4 nA cm−2 at 10 V and a zero-bias EQE of 74.7% under back-illumination. This is one of the highest zero-bias EQE reported for solar-blind detectors realized on template-free and mask-free III-nitrides grown using metal organic chemical vapor deposition on any substrate. The deep-UV-to-visible rejection ratio exceeded 106 while the deep-UV-to-near UV rejection exceeded 103. The thermal-noise limited detectivity was estimated to be 4 × 1014 cm Hz1/2 W−1. Hopping conduction along screw dislocation-mediated localized trap states was found to be the dominant carrier transport mechanism in the samples exhibiting high reverse leakage. For these samples, the responsivity (photocurrent) exhibited an exponential variation with reverse bias and a nonlinear variation with input optical power. This is explained using a hole-trapping associated gain mechanism and its impact on the transient characteristics of the detectors is investigated. A 6 × 1 linear array of the highest EQE detectors was realized and detector performance parameters were found to be comparable before and after wire bonding. This study is expected to enhance the understanding of III-nitrides-based vertical, self-powered detectors and benefit the development of high-performance, focal plane arrays using less complicated growth techniques.

Quantum efficiency enhancement by employing specially designed AlGaN electron blocking layer

Abstract Green light-emitting diodes light-emitting diodes with specially designed AlGaN electron blocking layer are numerically investigated. Examination of simulation results shows that applying proposed electron blocking layers, the injection of holes inside the active region can be improved. The improved performance for LEDs with specially designed AlGaN electron blocking layer is illustrated through the simulated results i.e. energy band diagrams, distribution of hole concentration and rate of radiative recombination inside the active region. The LEDs with graded AlGaN EBL have considerably enhanced light output power (LOP) and internal quantum efficiency.

Излучательная рекомбинация и ударная ионизация в полупроводниковых наноструктурах (О б з о р)

In this review we discuss processes of radiative recombination and impact ionization in light emitting structures based on bulk semiconductors, heterostructures with high potential barriers, nanostructures with deep quantum wells, as well as in nanocrystals with quantum dots. It is demonstrated that enhancement of quantum efficiency and optical power of luminescence in all of the structures under study is determined by the common physical mechanism. This mechanism is creation of additional electron-hole pairs during impact ionization process by hot carriers heated at high band offset on the heterointerface at current pumping or due to carrier multiplication in the process of multiexciton generation in nanocrystals at high-energy photons illumination.