LED Mode Research Articles

Improving photovoltaic performance of light-responsive double-heterojunction nanorod light-emitting diodes.

Double heterojunction nanorods enable both electroluminescence and light harvesting capabilities within the same device structure, providing a promising platform for energy-scavenging displays and related applications. However, the efficiency of the photovoltaic mode remains modest for useful power conversion and may be challenging to improve without sacrificing performance in electroluminescence. Through a facile on-film partial ligand exchange with benzenethiol integrated into the device fabrication step, we achieve an average of more than threefold increase in power conversion efficiency while maintaining the maximum external quantum efficiency and the maximum luminance in the LED mode. The improved photovoltaic performance is mainly due to the increase in the short circuit current, which we attribute to the enhanced charge separation afforded by the partial ligand exchange. The recovery of the photoluminescence lifetime under the forward bias suggests that the hole traps introduced by benzenethiols are filled prior to reaching the voltage at which light emission begins, allowing LED performance to be maintained and possibly improved.

The Effect of Different Immersion Media, Polymerization Modes, and Brushing on the Color Stability of Different Composite Resins.

This in vitro study investigated color stability of two photo-polymerized nano-filled and nano-hybrid composite resins in different polymerization modes immersed in different staining solutions before and after brushing. Disc-shaped specimens (n = 120) were prepared from two composite resins based on filler particle size: nano-filled (Filtek Z350, shade A1, 3M ESPE) and nano-hybrid (Spectra ST-HV, shade A1, Dentsply Sirona) composite resins (n = 60 per composite type). Specimens of each resin type were photopolymerized using the following polymerization modes: LED conventional, ramp, and pulse (n = 20 specimens per resin type and LED mode). After preparation, baseline color of the specimens was evaluated using a spectrophotometer (Easyshade V, VITA), and color change was evaluated using the CIE L*a*b* formula. Specimens were soaked in distilled water for 4 weeks in separate containers. The specimens from each polymerization-mode group were separated into two groups of 10 specimens; one group was stored in tea and the other group was stored in cola, each 1 hour daily for 4 weeks. After 4 weeks, the color was measured again. The specimens were brushed for 2 minutes under 200-g weight using an electronically powered toothbrush on the polymerized side of the specimen. The color was reevaluated immediately after brushing. Color-difference data (ΔE) between groups were analyzed by one-way analysis of variance for main comparison and by independent t test for color change after brushing. Nano-filled composite resin was more color-stable than nano-hybrid composite resin (P < .001), regardless of the staining media. For both composite resin types, the conventional polymerization method produced a more color-stable result (P < .0001). ΔE after brushing was reduced significantly (P < .0001). Both staining solutions have significant effects on color change, with tea staining more than cola (P < .0001). Nanofilled composite resin had greater color stability than nano-hybrid composite resin after immersion in staining solutions. Polymerization mode influences the color stability of both composite resin types. Int J Periodontics Restorative Dent 2023;43:247-255. doi: 10.11607/prd.6427.

Spectroscopy and microscopy analysis of semiconductor lasers in student laboratories

Light emitting diodes and semiconductor lasers are key elements of photonic systems, and every student enrolled in applied optics or photonics programs at universities learns about their fundamental physics and applications. We present part of a laboratory course for students of a photonics Master’s program which complements theoretical lectures on these fundamental photonic components. During this course, students investigate diode lasers using four different techniques with emphasis on the transition from spontaneous emission (LED mode) to lasing action. The first two parts—recording diode characteristics of current versus voltage and output power versus current, as well as determining the spatial characteristics of the commercial diode lasers—are common in most similar laboratory courses. Here we focus on the remaining two investigations. On the one hand, diode laser dimensions are quantitatively analyzed using visible microscopy and also electron microscopy. This includes images of the laser chip during operation in LED or laser mode which allows us to gain an understanding of light propagation in and around the active layer waveguide. On the other hand, changes of the emission spectra while increasing the injection current from LED to laser operation are measured with high accuracy using a Fourier transform spectrometer. This allows determination of the longitudinal mode spacing, which is in very good agreement with intuitive, theoretical predictions, though parts of the analysis are surprising for many students.

Dual-functional light-emitting perovskite solar cells enabled by soft-covered annealing process

Solution-processable perovskite semiconductors have demonstrated excellent performance in both perovskite solar cells (PeSCs) and perovskite light emitting diodes (PeLEDs). Their low exciton binding energy and excellent charge transport properties allow the conversion of photons into free electrons and holes and vice versa with minimal energy loss. In addition, planar PeSCs and PeLEDs have similar device architecture. These aspects inspired us to fabricate a dual-functional device with both SC and LED functions, here termed a light-emitting perovskite solar cells (LEPeSCs). However, the absence of an effective fabrication method to control grain size, uniformity, grain boundaries, and coverage of perovskite film inevitably limits device performance of dual-functional LEPeSCs. Here, we have demonstrated an efficient dual-functional LEPeSCs with planar device configuration of ITO/PEDOT:PSS/CH3NH3PbBrI2/PC61BM/Ca/Al, which is able to reversibly convert luminous energy to electric energy or electric energy to luminous energy in a single device. By developing a new soft-covered annealing method, we obtained optimized grain morphology with large grain size, high uniformity, and decreased grain boundaries, which is difficult to achieve with conventional fabrication methods. The LEPeSCs exhibits high performance in both PeSCs and PeLEDs modes, with power conversion efficiency of 14.02% in SC mode and bright red-light emission of 1710 cd/cm2 at 4 V in LED mode.

Portable Generator Based on Direct-Flame Metal-Supported Solid Oxide Fuel Cells (MS-SOFCs)

Metal-Supported solid oxide fuel cells (MS-SOFCs) tolerate rapid thermal cycling [1,2]. This makes them ideally suited for applications requiring rapid start-up time, or tolerance to thermal fluctuations arising from load-following or intermittent fuel flow. In direct flame mode, SOFCs are heated by a flame impinging on the anode side of the cell, and the flame also partially oxidizes the fuel to electrochemically- active species (such as hydrogen and carbon monoxide) [3–6]. In this work, we have developed a prototype consumer product by retrofitting MS-SOFCs to a commercial propane-fueled camping stove (Figure 1). The power produced by the fuel cells is conditioned by a simple microelectronic circuit to provide LED lighting and USB mobile phone charging. Previous efforts [7] to systematically characterize the behavior of MS-SOFCs in direct-flame mode in carefully controlled lab environment will be summarized, including the impact of: air-to-fuel ratio in the flame, fuel flow velocity, burner-to-MS-SOFC distance, catalyst composition, and application of thermal insulation to the cell. The insight from these experiments was used in the present work to guide the design of a free-standing prototype, including cell size and stacking arrangement, and interaction with the stove flame and microelectronics. The prototype performance is shown in Figure 2. The MS-SOFC stack can be heated up from room temperature to delivery of useful power in less than 10 seconds. Operation points in LED mode and USB-charger mode fall on the overall operating curve of the MS-SOFC stack, and successful charging of smartphones is demonstrated. [1] M.C. Tucker, Progress in metal-supported solid oxide fuel cells: A review, J. Power Sources. 195 (2010) 4570–4582. doi:10.1016/j.jpowsour.2010.02.035. [2] J. Mougin, A. Brevet, J.-C. Grenier, R. Laucournet, P.-O. Larsson, D. Montinaro, L.M. Rodriguez-Martinez, M.A. Alvarez, M. Stange, L. Bonneau, E. Concettoni, L. Stroppa, Metal Supported Solid Oxide Fuel Cells: From Materials Development to Single Cell Performance and Durability Tests, ECS Trans. 57 (2013) 481–490. doi:10.1149/05701.0481ecst. [3] M. Horiuchi, S. Suganuma, M. Watanabe, Electrochemical Power Generation Directly from Combustion Flame of Gases, Liquids, and Solids, J. Electrochem. Soc. 151 (2004) A1402. doi:10.1149/1.1778168. [4] M.M. Hossain, J. Myung, R. Lan, M. Cassidy, I. Burns, S. Tao, J.T.S. Irvine, Study on Direct Flame Solid Oxide Fuel Cell Using Flat Burner and Ethylene Flame, ECS Trans. 68 (2015) 1989–1999. doi:10.1149/06801.1989ecst. [5] M. Vogler, M. Horiuchi, W.G. Bessler, Modeling, simulation and optimization of a no-chamber solid oxide fuel cell operated with a flat-flame burner, J. Power Sources. 195 (2010) 7067–7077. doi:10.1016/j.jpowsour.2010.04.030. [6] K. Wang, R.J. Milcarek, P. Zeng, J. Ahn, Flame-assisted fuel cells running methane, Int. J. Hydrogen Energy. 40 (2014) 4659–4665. doi:10.1016/j.ijhydene.2015.01.128. [7] M.C. Tucker, A.S. Ying, Metal-Supported Solid Oxide Fuel Cells Operated in Direct-Flame Configuration, J. Power Sources. (2017). Figure 1

“Solar tree”: Exploring new form factors of organic solar cells

Organic solar cells have great potential as a clean and renewable solar energy conversion system, due to their low cost materials, ease of production, and lack of harmful emissions. The rapid improvement in organic solar cell performance in recent years has triggered significant interests in developing organic solar cells for commercial applications. Harnessing the unique set of characteristics of organic solar cells, here we demonstrate a new form factor for organic solar cells, a “solar tree” or an electricity-generating artificial tree with organic solar cells as leaves. We first fabricated polymer:fullerene based organic solar cells on flexible plastic substrates that show similar performance to devices on rigid glass substrates using the inverted device structure. Large-area flexible devices were fabricated and cut into palm leaf shapes with an active device area of 6.5 cm2 using a steel rule die. 12 leaf-shaped organic solar cells were then assembled to form a prototype “solar palm tree”. Two different wiring configurations among the devices provided different power delivery modes: a low-voltage, high-current “fan mode” and a high voltage, low-current “LED mode”.

ZnSe‐based laser diodes: New approaches

In this paper we report on recent results concerning conventional edge emitting laser structures containing either quantum wells or quantum dots as region and vertical-cavity surface-emitting lasers. In the first part a series of four similar laser structures containing quantum wells with an emission wavelength of 520 nm was grown by molecular beam epitaxy in order to perform a systematic study of lifetime improvement. They differed in the alternating implementation of an additional 5 nm thick ZnSSe layer with a high sulfur composition of 25% neighboring the quantum well. A high stability of the CdZnSSe active layer was observed by introducing such a kind of strain compensating layers. Lifetime measurements showed a significant improvement up to one order of magnitude using p- and n- & p-side layers. In the second part electro-optical characteristics of ridge and planar CdSe quantum dot laser diodes were compared. A reduction of the threshold current density by a factor of 4.7 for the ridge structure was obtained. This has to be associated to the reduction of current spreading inside the laser diodes. Furthermore, a significant slower degradation of CdSe quantum dot structures compared to common ZnSe-based QW structures was observed. An operating time over 2700 h in pulsed mode experiments at 50 A/cm2 in LED mode was achieved. In the third part we report on the realization of an optically pumped monolithic vertical-cavity surface-emitting laser operating at a wavelength of 511 nm. The microresonator has a quality factor of 3200 while the threshold excitation power density for the onset of lasing is 22 kW/cm2 at room temperature. Micropillars of different diameter fabricated out of this structure show discrete optical modes due to the three dimensional optical confinement of the optical wave. (© 2005 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim)

Current spreading in AlGaN:Mg cladding layers of laser structures

AbstractThe consequences of the anisotropic resistance in AlGaN/GaN strained layer superlattices for the operation of laser diodes were studied for structures driven in LED mode. A series of laser structures containing different Mg-doped AlGaN bulk and AlGaN/GaN strained layer superlattice cladding layers was compared to estimate the current spreading in the cladding due to the formation of 2D-hole gases. Current-voltage measurements revealed a significant spread of the current path in the superlattices, whereas no spreading was seen for the bulk cladding. In contrast to this, investigations of the electroluminescence showed no significant differences.

Size effect modulation light sources — Possibility of LED mode operation at room temperature

A new method is proposed to realize a constant charge operation in a size effect modulation LED (SEM/LED) with a high speed switching capability. The theoretical estimation shows a possibility of promising LED mode operation of the SEM/LED at 300K with the aid of the proposed circuit scheme.

Coherence of a room-temperature CW GaAs/GaAlAs injection laser

The temporal coherence of a stripe-geometry double-heterojunction GaAs/GaAlAs laser operating CW at room temperature was determined. A heterodyne detection scheme was used involving the mixing of the laser field with a frequency-shifted and time-delayed image of itself in an interferometer. Because the laser device oscillated in several longitudinal modes, the autocorrelation function of its output exhibited resonances for specific time delays. The rate at which the amplitude of these resonances decreased with increasing time delays provided a measure of an apparent coherence length associated with individual longitudinal modes. The coherence length, so defined, was found to increase linearly with drive current in excess of threshold. This observation is interpreted as evidence that the intrinsic linewidth of a longitudinal mode is inversely proportional to the coherent optical power in that mode. Apparent coherence lengths were a few centimeters for a few milliwatts of total optical power emitted per facet. For a perfectly balanced interferometer, a sharp heterodyne beat signal was also observed when the laser device was operated considerably below threshold, i.e., in the LED mode.

Recombination enhanced annealing effect in AlGaAs/GaAs remote junction heterostructure lasers

New structure lasers, the remote junction heterostructure (RJH) lasers, are made to obtain information about slow degradation of AlGaAs/GaAs DH lasers. The RJH laser is characterized by the presence of a thin clad layer between the active layer and the p-n junction. During the LD and LED mode aging process, the RJH lasers showed a marked reduction of threshold current. This reduction was accompanied by increased spontaneous lifetime and pileup of defects at the p-n junction. From these observations, a model was proposed in which point defect generation in the active layer and defect motion toward the p-n junction during the aging are assumed. The rate equation was derived for concentration of the point defect, and the solution of this equation was compared with the experimental results with reasonable agreement. The parameters relating to the slow degradation were determined, and the ultimate life of conventional DH lasers was discussed using these parameters.

Accelerated aging test of Ga1−xAlxAs DH lasers

An accelerated aging test is performed at temperatures of 70, 110, and 140 °C to estimate the activation energy. Samples are operated in the LED mode at high temperatures of 110 and 140 °C. At 70 °C samples are operated in lasing mode of optical output 0.27 mW/μm. The laser lifetime is defined to be a time at which the cw threshold current measured at a standard temperature 25 °C reaches 1.5 times the initial value. The activation energy 0.74 eV is obtained. It is shown that the degradation does not depend on the optical output power below the power level of 0.3 mW/μm and that the activation energy is indifferent to the operation mode, LED mode, or lasing mode, at a low power level. The use of the LED mode operation and the definition of the laser lifetime in terms of the characteristics at low standard temperature enable one to perform the accelerated aging test at high temperatures above 110 °C where cw operation is almost impossible. The reliability is examined at 70 °C using 50 samples and an average laser lifetime is obtained. The application of the activation energy gives the average laser lifetime at the junction temperature of 30 °C to be 3×105 h.

Accelerated aging and a uniform mode of degradation in (Al,Ga)As double-heterostructure lasers

An investigation of certain of the degradation and accelerated aging characteristics of currently fabricated (Al,Ga)As lasers is described. Our observations of the properties of devices aged at 70 °C, both as light-emitting diodes and as lasers, support two conclusions relevant to degradation: (1) The dominant degradation process in present devices is not lasing related in devices operated at power levels ⩽5 mW/face; (2) laser failure is strongly correlated with an approximately 50% reduction in the spontaneous light emission (at a given current below threshold) from these devices. These inferences are consistent with a spatially uniform mode of laser degradation not, for example, related to dark-line defects. They also imply that an LED mode of laser testing could be effective in identifying and eliminating presently dominating degradation mechanisms, i.e., those limiting continuously operating laser lifetimes to median values of about five years at room temperature. A different degradation mechanism which is operative at elevated temperatures even in unbiased devices has also been identified.