Internal Quantum Efficiency Measurements Research Articles

A 3D simulation model to study all-inorganic CsPbX3 (X = Br and I) perovskites-based light-emitting diodes with different hole-transporting layers

The development of numerical models is essential for optimizing perovskite light-emitting diodes (PeLEDs) and explaining their physical mechanism for further efficiency improvement. This study reports, for the first time, on a detailed device modelling of an all-inorganic perovskite LED consisting of CsPbX3 (X = Br and I) as light emitting layer (LEL) with different hole transporting layers (HTLs), employing COMSOL Multiphysics simulation package. Therefore, a 3D simulation model is served to investigate the appropriate HTLs that meet the design requirements of a PeLED in terms of band off-set engineering. For this purpose, a series of all-inorganic halide perovskites with different HTLs such as PEDOT: PSS, CuSCN and MoO3 are simulated under the same theoretical settings, and the performances of LEDs are compared with each other. This is done through studying their electronic properties using current density–voltage (J-V) curves and internal quantum efficiency (IQE) measurements. The results obtained from the J-V curves reveal that all the CsPbBr3-based samples with different HTLs exhibit the same turn-on voltage (V on) of approximately 4.2 V, while this value increases to 5.8 V for the CsPbI3-based samples. Compared with the PeLEDs based on CsPbI3, the PeLEDs based on CsPbBr3 indicate lower V on due to the formation of shorter charge carrier injection barriers at their interfaces. Furthermore, among the various simulated structures, the highest IQE is obtained for perovskite CsPbI3-based LED with MoO3 HTL (5.21%). The effect of different parameters on the performance of the proposed configurations are also investigated, and it turns out that the thickness of LELs and lifetime of charge carriers have a decisive role to play in the efficiency of PeLEDs. This theoretical study not only successfully explains the working principle of PeLEDs but also clearly shows researchers how to produce high-performance LEDs in the laboratory by knowing the physical properties of materials and accurately adjusting energy band alignments.

Advanced analysis of internal quantum efficiency measurements using machine learning

AbstractThe internal quantum efficiency (IQE) is given as the ratio between the externally collected electron current and the photon current absorbed by the device. Spectral analysis of IQE measurements is a powerful method to identify performance‐limiting mechanisms in solar cells. It also enables the extraction of key electrical and optical parameters. However, the potential of IQE measurements is only rarely fully utilized, presumably due to the significant complexity associated with the fitting process and its sensitivity to noise. In this study, machine learning is proposed as an efficient method to extract quantitative information from IQE measurements. The extraction method is automated and easy to use, providing an array of specific device parameters. By simplifying the analytical process, the developed machine learning algorithms also extract the parasitic absorption of the antireflection coating, a key parameter that is difficult to obtain by traditional methods. Although the method has been developed for and tested on silicon solar cells, it can be adapted and applied to other types of solar cells.

High quantum efficiency and thermal stability Sr3LiNbO6:Mn4+, Zn2+ phosphors for application in indoor plant growth lighting

New double perovskite Sr3LiNbO6:x%Mn4+ and Sr3LiNbO6:x%Mn4+, 1.25x%Zn2+ far-red-emitting phosphors were reported for the first time. The phosphors were characterized by XRD, SEM, PL, PLE, UV–vis spectroscopy, CIE color coordinate and internal quantum efficiency (IQE) measurements. Under the excitation of 365 nm light, Sr3LiNbO6:x%Mn4+ phosphors show a strong far-red emission peak at 695 nm, which is suitable for use in indoor plant application. Furthermore, by co-doping of Zn2+ ions, a more homogeneous particle size distribution was obtained and the emission intensity can raise up to 1.29 times that of Mn4+ single-doped samples. The Sr3LiNbO6:0.6%Mn4+, 0.75%Zn2+ phosphor can obtain an IQE of 78.4% and an emission intensity of 80.6% at 423 K from that at 284 K, showing a high luminous efficiency and thermal stability. Finally, a LED lamp was fabricated with the use of far-red-emitting optimized phosphor, which shows an effective promotion on growth rate and content of photosynthesis pigments of wheat. The results show that Sr3LiNbO6:Mn4+, Zn2+ phosphors may have a promising future in the field of agriculture cultivations.

Absolute Measurement of Emission Internal Quantum Efficiency in III-Nitride Semiconductors by Simultaneous Photo-Acoustic and Photoluminescence Spectroscopy

Absolute Measurement of Emission Internal Quantum Efficiency in III-Nitride Semiconductors by Simultaneous Photo-Acoustic and Photoluminescence Spectroscopy

Unity quantum efficiency in III-nitride quantum wells at low temperature: Experimental verification by time-resolved photoluminescence

Using time-resolved photoluminescence (PL) measurements, we present an experimental verification for 100% internal quantum efficiency (IQE) of III-N quantum wells at low temperatures. Conventional IQE measurements, such as temperature- and power-dependent PL, require a low-temperature normalization, where usually an IQE of 100% is assumed. This assumption neglects remaining nonradiative recombination that may be present even at cryogenic temperatures. From time-resolved PL measurements, the effective charge carrier decay time and the lifetime of radiative recombination can be evaluated separately. We state that the low-temperature IQE of a quantum well corresponds to 100%, whenever the effective charge carrier decay is dominated only by a radiative recombination. In this case, the temperature-dependent measurements show a synchronous rise of the effective lifetimes together with the radiative lifetimes, since only the radiative lifetime increases with temperature in a 2D system. At the same time, nonradiative processes are thermally activated, which results in a decreasing lifetime with temperature. Thereby, absolute IQE measurements become possible, since we provide a robust indicator for the absence of nonradiative recombination in quantum wells at low temperature.

Novel non-rare-earth red-emitting phosphor Li4AlSbO6:Mn4+ for plant growth: Crystal structure, luminescence properties and its LED device

Bright red emission of Mn4+ ions was obtained in a novel non-rare-earth host of Li4AlSbO6. Luminescence properties as well as crystal structure and particle morphology of Li4AlSbO6:Mn4+ phosphors were studied. The phosphors can effectively produce intensive red emission at 673 nm with the excitation of UV or blue light. The optimum doping concentration of Mn4+ is obtained at 0.15 mol%. The internal quantum efficiency measurement shows that Li4AlSbO6:Mn4+ is an efficient phosphor with the best quantum yield of 54.8%. The electroluminescence of phosphor-converted LED device based on Li4AlSbO6:Mn4+ is well coupled with the absorption of several important plant pigments, which suggests that the phosphor is a promising candidate for indoor plant lighting.

Optimized amorphous silicon nitride layers for the front side passivation of c-Si PERC solar cells

Plasma-enhanced chemical vapour deposition (PECVD) SiNx is the typical choice as anti-reflection coating (ARC) for Silicon based solar cells. However, there still exists a room for improvement in passivation quality of SiNx while maintaining good optics for the front side of a solar cell. In this paper, we studied in detail the optical and electrical properties of SiNx layers by varying the chamber pressure and substrate temperature in an industrially used inline PECVD tool. Both the optical as well as electrical properties of SiNx layers were found to be significantly influenced by the chamber pressure and substrate temperature. A trade-off between excellent optics and low surface recombination is observed with an increase in chamber pressure, whereas higher substrate temperature generally led to better passivation quality. The Si-H bond density, which is expected to directly influence the quality of surface passivation, increased at high pressure and at low substrate temperature. Based on our investigations, a good compromise between optics and surface passivation is struck to prepare optimized SiNx layers and apply them as passivation layers for the front side of passivated emitter and rear cell (PERC) solar cells. The best solar cells show high short-circuit current density (jSC) of 39.9 mA/cm2 corresponding to the SiNx layers with low parasitic absorption, good antireflection property, and excellent passivation of the surface and bulk silicon. The current-voltage (I-V) results are found to be in agreement with internal quantum efficiency (IQE) measurements of the solar cells.

Internal quantum efficiency measurements in InAs/GaSb superlattices for midinfrared emitters

The internal quantum efficiency of a series of InAs/GaSb superlattices was investigated as a function of carrier generation rate through variable excitation, quasicontinuous-wave photoluminescence measurements. GaSb thicknesses were varied to maximize the internal quantum efficiency for midwave infrared emission. Internal quantum efficiencies were determined from measurements of the photoluminescence radiance and extraction efficiencies computed within a two-slab model. The peak internal quantum efficiencies varied from 15% to 29% at 77 K, which is in good agreement with expectations from InAs/GaSb superlattice light-emitting diode performance.

High-efficiency multi-crystalline black silicon solar cells achieved by additive assisted Ag-MACE

It is important for uniform texture on crystallites with different orientation of diamond wire sawn multi-crystalline silicon solar cells by Ag metal-assisted chemical etching method (Ag-MACE). In this paper, an additive was adopted to make the nanostructures on crystallites with different orientation of multi-crystalline silicon wafers with roughly the same depth and diameter. This additive contains biological enzymes, which are macromolecular proteins with specific catalytic functions. The uniform texture produced by the additive is demonstrated to be helpful in improving the cell performance in terms of surface morphology, reflectivity, effective minority carrier lifetime, auger recombination rate, internal quantum efficiency and external quantum efficiency measurements as well as electroluminescence spectra characterizations. The chemical mechanism of additive assisted etching reaction during the texturing process was discussed. Finally, mass-produced multi-crystalline silicon solar cells made by Ag-MACE with the additive achieve a maximum efficiency and average efficiency of 19.56%, and 19.24%, respectively, which were 0.65% and 0.64% absolutely higher than that without the additive. Moreover, the effective minority carrier lifetime and auger recombination rate of the former was nearly twice and a third of that of the latter respectively.

Study on Optical Properties and Internal Quantum Efficiency Measurement of GaN-based Green LEDs

In this paper, the optical properties of GaN-based green light emitting diode (LED) are investigated and the internal quantum efficiency (IQE) values are measured by temperature dependent photoluminescence (TDPL) and power dependent photoluminescence (PDPL) methods. The "S-shaped” shift of peak wavelength measured at different temperature disappears gradually and the spectra broadening can be observed with increasing excitation power. The IQE calculation results of TDPL, which use the integrated intensity measured at low temperature as unity, can be modified by PDPL in order to acquire more accurate IQE values.

The impact of diffusion gettering on solar cell efficiency and light induced degradation

In this paper, the influence of an annealing process at 500 °C for 30 min. after emitter diffusion of multi-crystalline solar cells was investigated. Neighboring sister wafers from a silicon ingot were processed using light doped emitter (LDE) and low temperature annealing (LTA). Emitter profiles measured by Electrochemical Capacitance Voltage (ECV) showed that there is no obvious difference between LDE and LTA due to the lower annealing temperature. The lifetime and iron concentration measurement indicated that the gettering of LTA diffusion is much better than LDE. Therefore, the solar cell efficiency of LTA was up to 19.01%, which is 0.28%abs higher than LDE due to the higher open circuit voltage (Voc) and short-circuit current (Isc). The Internal Quantum Efficiency (IQE) measurement with better long wavelength response showed the wafer quality was much improved after LTA diffusion. To further investigate diffusion gettering performance, light-induced-degradation (LID) was analyzed both for LDE and LTA diffusion. The LID of LDE diffusion with 1.15%rel. is obviously higher than LTA diffusion of 0.46%rel., attributing to the higher iron concentration of LDE diffusion emitter to result in higher iron-boron pair dissociation.

Implementation of full-area-deposited electron-selective TiOx layers into silicon solar cells

We examine two different silicon solar cell designs featuring full-area electron-selective contacts based on ultrathin (2–3 nm) titanium oxide (TiOx) films deposited by atomic layer deposition. The first cell design applies a layer stack to the cell front, which is composed of an ultrathin intrinsic amorphous silicon (i-a-Si:H) layer for interface passivation, the TiOx film and an indium tin oxide (ITO) layer to provide a good lateral conductance for electrons to the metal fingers. Whereas carrier lifetime measurements on test structures promise high implied open-circuit voltages Voc up to 726 mV, the realized solar cells achieve disappointingly low Voc values <400 mV. The J-V parameters of this cell type are negatively affected by a reverse diode occurring due to the contacting of the TiOx by the high-work function ITO layer. In the second cell type, we implement a layer stack to the cell rear, which is composed of an ultrathin silicon oxide (SiOy) layer, the TiOx film and a full-area-deposited aluminum (Al) layer. Initial Voc values of these cells are relatively low (<600 mV), but improve significantly after annealing at 350°C. The best cell featuring a SiOy/TiOx/Al rear contact achieves an open-circuit voltage of 661 mV and an efficiency of 20.3%. No reverse diode is observed, which is attributed to the lower work function of the Al compared to ITO in the first cell design. From internal quantum efficiency measurements, we extract a rear surface recombination velocity Srear of (52±20) cm/s for our best cell, which is well compatible with efficiencies exceeding 23%.

Measuring the internal quantum efficiency of light-emitting diodes: towards accurate and reliable room-temperature characterization

Abstract For accurate and reliable measurement of the internal quantum efficiency (IQE) of light-emitting diodes (LEDs), the method should be theoretically solid and experimentally simple to use without any prior assumption of physical parameters or complicated equipment. In this paper, we critically review the conventional characterization techniques for measuring the IQE of LEDs, including the methods based on temperature-dependent electroluminescence and constant AB(C) models. After reviewing the limitations of the existing IQE measurement techniques, we present the recently proposed method based on the improved AB model, called room-temperature reference-point method (RTRM). The RTRM is then applied to various LED devices to show how the IQE measurement techniques can be utilized to analyze their optoelectronic performances quantitatively.

Efficiency enhancement of light color conversion through surface plasmon coupling.

The efficiency enhancement of light color conversion from blue quantum well (QW) emission into red quantum dot (QD) emission through surface plasmon (SP) coupling by coating CdSe/ZnS QDs on the top of an InGaN/GaN QW light-emitting diode (LED) is demonstrated. Ag nanoparticles (NPs) are fabricated within a transparent conductive Ga-doped ZnO interlayer to induce localized surface plasmon (LSP) resonance for simultaneously coupling with the QWs and QDs. Such a coupling process generates three enhancement effects, including QW emission, QD absorption at the QW emission wavelength, and QD emission, leading to an overall enhancement effect of QD emission intensity. An Ag NP geometry for inducing an LSP resonance peak around the middle between the QW and QD emission wavelengths results in the optimized condition for maximizing QD emission enhancement. Internal quantum efficiency and photoluminescence (PL) decay time measurements are performed to show consistent results with LED performance characterizations, even though the QD absorption of PL excitation laser may mix with the SP-induced QD absorption enhancement effect in PL measurement.

Auger recombination in AlGaN quantum wells for UV light-emitting diodes

We show that the often observed efficiency droop in AlGaN quantum well heterostructures is an internal carrier loss process, analogous to the InGaN system. We attribute this loss process to Auger recombination, with C = 2.3 × 10−30 cm6 s−1; a similar value found commonly in InGaN-based devices. As a result, the peak internal quantum efficiency (IQE) of our structures is limited to 66%. These values were obtained by resonant excitation (time-resolved) photoluminescence (PL), avoiding common error sources in IQE measurements. The existence of strong Auger recombination implies that simple methods employed for IQE determination, such as temperature-dependent PL, may lead to erroneous values. Auger losses will have to be considered once the challenges regarding carrier injection are solved.

Hybrid perovskite films approaching the radiative limit with over 90% photoluminescence quantum efficiency

Reducing non-radiative recombination in semiconducting materials is a prerequisite for achieving the highest performance in light-emitting and photovoltaic applications. Here, we characterize both external and internal photoluminescence quantum efficiency and quasi-Fermi-level splitting of surface-treated hybrid perovskite (CH3NH3PbI3) thin films. With respect to the material bandgap, these passivated films exhibit the highest quasi-Fermi-level splitting measured to date, reaching 97.1 ± 0.7% of the radiative limit, approaching that of the highest performing GaAs solar cells. We confirm these values with independent measurements of internal photoluminescence quantum efficiency of 91.9 ± 2.7% under 1 Sun illumination intensity, setting a new benchmark for these materials. These results suggest hybrid perovskite solar cells are inherently capable of further increases in power conversion efficiency if surface passivation can be combined with optimized charge carrier selective interfaces. Surface treatment is shown to yield passivated perovskite films with very high quasi-Fermi level splitting and internal photoluminescence quantum efficiency, indicating that further improvements in the performance of perovskite optoelectronics should be feasible.

18.88%-efficient multi-crystalline silicon solar cells by combining Cu-catalyzed chemical etching and post-treatment process

Mass production of diamond-wire-sawn (DWS) multi-crystalline silicon (mc-Si) solar cells reached a significant point of maturity through utilization of metal-catalyzed chemical etching (MCCE). However, the reported studies always focus on how to optimize the MCCE process but there are few studies concentrating on the post-treatment techniques to improve the cell efficiency. In this paper, we use a combination of Cu-MCCE method and a HF/HNO3/H3PO4 post-processing treatment process to decorate the surface of Si textures for the first time. The submicron polygonal chamfered cone structure produced by the post-processing treatment is demonstrated to be helpful in reducing surface recombination and improving the cell performance in terms of surface morphology, reflectivity, internal quantum efficiency (IQE) and external quantum efficiency (EQE) measurements as well as Electroluminescence (EL) spectra characterizations. The highest efficiency of Cu-MCCE mc-Si solar cells subjected to such post-processing treatment process is 18.88% with short circuit current (Isc), open circuit voltage (Voc) and fill factor (FF) of 36.67 mA/cm2, 638.6 mV and 80.64%, respectively, in great contrast to that (16.81%) of Cu-MCCE mc-Si solar cells without any post treatment. The post-treatment process is, therefore, of great potential for the Si photovoltaic industry, especially for the Cu-MCCE mc-Si solar cells.

Spectrally-resolved internal quantum efficiency and carrier dynamics of semipolar core-shell triangular nanostripe GaN/InGaN LEDs

We investigate the spectrally resolved internal quantum efficiency (IQE) and carrier dynamics in semipolar core–shell triangular nanostripe light-emitting diodes (TLEDs) using temperature-dependent photoluminescence (TDPL) and time-resolved photoluminescence (TRPL) at various excitation energy densities. Using electroluminescence, photoluminescence, and cathodoluminescence measurements, we verify the origins of the broad emission spectra from the nanostructures and confirm that localized regions of high-indium-content InGaN exist along the apex of the nanostructures. Spectrally resolved IQE measurements are then performed, with the spectra integrated from 400–450 nm and 450–500 nm to obtain the IQE of the QWs mainly near the sidewalls and apex of the TLEDs, respectively. TDPL and TRPL are used to decouple the radiative and non-radiative carrier lifetimes for different regions of the emission spectra. We observe that the IQE is higher for the spectral region between 450 nm and 500 nm compared to the IQE between 400 and 450 nm. This result is in contrast to the typical observation that the IQE of planar GaN-based LEDs is lower for longer wavelengths (i.e., higher indium contents). We also observe a longer non-radiative recombination lifetime for the longer wavelength portion of the spectrum. Several explanations are proposed for the improved IQE and longer non-radiative lifetime observed near the apex of the nanostructures. The results show that nanostructures may be leveraged to design more efficient green LEDs, potentially addressing a long-standing challenge in GaN-based materials.

ZnO thin film and porous silicon co-gettering of impurities in multicrystalline silicon through a VTP process

In this study, a novel zinc oxide and porous silicon co-gettering experiment has been applied as a beneficial approach to improve the electrical properties of low quality multicrystalline silicon substrates (mc-Si). We investigated the combined effect of ZnO and porous silicon (ZnO/PS) to enhance the gettering process efficiency. A systematic comparison has been carried out between two different routes of annealing after coating the front surface with zinc oxide thin film. The first one consists of a one step annealing (classic annealing) and the second consists in subjecting the samples to a variable annealing temperature process (VTP) using two steps, starting by the precipitate temperature at Tp = 1000 °C followed consecutively by a gettering temperature at Tg = 850 °C for different durations. Obtained results show that the Zn-gettering processes via the classic annealing process allow a significant improvement of the substrate quality. Nonetheless, highest values of the effective minority carrier lifetime exceeding 508 µs were obtained after using VTP annealing. Zn-gettering was investigated and found to have a crucial effect on the recombination activities mainly into the grain boundaries. Obtained results showed a significant enhancement of the gettered multicrystalline silicon electrical properties, resulting after the high increase of the effective minority carrier lifetime from 5 to 508 µs and the increase of the Hall mobility from 72 to 193 cm2 V− 1 s− 1. These enhancements have been further confirmed with the internal quantum efficiency (IQE) measurements. Thus, it is inferred that the applied co-gettering experiment has a synergetic effect to improve the electrical properties of the mc-Si substrates. The morphological structure of silicon was signed out by the Field emission scanning electron microscopy (FESEM).

Yttrium oxide passivation of porous silicon nanostructures for improved photoluminescence and optoelectronic properties

This paper reports on the effect of yttrium oxide as a novel treatment to improve the photoluminescence (PL) intensity and stability of porous silicon (PS). Yttrium oxide (Y2O3) was incorporated into the PS layers by impregnation method using a saturated aqueous solution. The penetration of yttrium into the PS microstructure was examined using the Energy dispersive X-ray spectrometry (EDS) and the backscattered electron detector (BED-C) for composition imaging and analysis. The morphology of the front surface was studied using a field emission scanning electron microscope. The deposited yttrium oxide onto the PS layers was thermally activated to passivate efficiently the silicon dangling bonds, and prevent the PS from huge oxidation. The PL peak intensity of impregnated PS was increased noticeably compared to the as-prepared untreated PS. Unlike the as-prepared PS photoluminescence dependence with aging, the yttrium-passivated PS layers PL peak shows no shifts during aging allowing a high stability. Furthermore, we obtained a significant improvement of the effective minority carrier lifetime (τeff) after a short anneal at 600 °C, while increasing the temperature reduces noticeably the passivation properties. The improved surface passivation experienced after the thermal annealing can be ascribed to yttrium diffusion into the PS layer, with a resulting redistribution of yttrium oxide and subsequent passivation of silicon dangling bonds in the sub-interface region, this was confirmed by EDS analysis. The internal quantum efficiency measurements were performed to study the optoelectronic properties of the processed monocrystalline silicon substrates.