Electroluminescence Techniques Research Articles

Catastrophic Optical Bulk Damage – A New Failure Mode in High-Power InGaAs-AlGaAs Strained Quantum Well Lasers

ABSTRACTHigh-power single-mode (SM) and multi-mode (MM) InGaAs-AlGaAs strained quantum well (QW) lasers with emission wavelengths of 910 − 980 nm are extensively used in various fiber lasers and amplifiers for both telecom and defense applications. In particular, underseas and satellite communication systems require stringent reliability from these lasers. Since these lasers predominantly fail by catastrophic and sudden degradation due to COD, it is crucial especially for space satellite applications to investigate reliability, failure modes, and degradation mechanisms of these lasers. Catastrophic optical mirror damage (COMD) was known to be the only failure mode until our group reported a new failure mode in MM and SM InGaAs-AlGaAs strained QW lasers in 2009 and 2016, respectively. Our group reported that bulk failure due to catastrophic optical bulk damage (COBD) has become the dominant failure mode of both SM and MM lasers. Since there have been limited reports on COBD compared to COMD, the intent of this paper is to introduce our studies on COBD that have spanned the last decade. We investigated reliability, failure modes, and degradation processes in SM and MM InGaAs-AlGaAs strained QW lasers by performing short-term step-stress tests and long-term accelerated life-tests as well as failure mode analyses using various nondestructive and destructive micro-analytical techniques including electron beam induced current (EBIC), time-resolved electroluminescence (EL), deep level transient spectroscopy (DLTS), focused ion beam (FIB), and high-resolution TEM. EBIC and EL techniques were employed to study dark line defects generated in degraded lasers stressed under different test conditions. Time-resolved EL techniques were employed to study initiation and progressions of dark spots and dark lines in real time as lasers were aged. DLTS techniques were employed to study electron traps in both pristine and degraded lasers. Lastly, FIB and high-resolution TEM were employed to prepare cross sectional and plan view TEM specimens to study DLD areas in post-aged lasers. We also report our current understanding on degradation mechanisms responsible for COBD in both SM and MM lasers.

Spin-orbit coupling effects on energy transfer channel in organic semiconductors

Spin-orbit coupling (SOC) effects on energy transfer channel in polymer-phosphorescent dopant system have been investigated. The blue-emitting host polymer, poly [9, 9-di-n-hexyl-fluorenyl-2, 7-diyl] (PFO), was doped into a variety of concentrations between 0.1 and 5% with a red emissive phosphorescent dye, bis (2-[2′-benzothienyl)-pyridinato-N, C3′] iridium (acetyl-acetonate) (BtpIr). The mechanism for the SOC effects on energy transfer channel between the host and the dopant was studied using both photoluminescence (PL) and electroluminescence (EL) techniques. The SOC effect enhances enormously the intersystem crossing (ISC) rate arising by the strong overlap of the delocalized π orbitals of the PFO and BtPIr. The ISC energy transfer channel competes effectively with Fӧrster energy transfer channel and Dexter energy transfer channel. The energy transfer from S1 (PFO) state to T1 (PFO and BtPIr) state is increased by SOC. Consequently, the population of the S1 (PFO) state decreased. So the Förster energy transfer intensity is deceased. SOC leads to such an indirect decrease for the Förster energy transfer intensity. In other hand, the intensity of T1 (PFO) state increased. So the Dexter energy transfer channel intensity is increased. It allows more PFO triplets to the Dexter energy transfer channel to BtPIr triplets. The increased population of the triplet state brings about a greater number of detectable photons that are emitted from the BtPIr molecules. EL spectra implied that only a few singlets formed on the PFO contribute to the emission, most of them are transformed into triplet state by ISC channel.

Measurement of Relaxation Time of Excess Carriers in Si and CIGS Solar Cells by Modulated Electroluminescence Technique

Excess carrier lifetime plays a crucial role in determining the efficiency of solar cells. In this paper, we use the frequency dependence of inphase and quadrature components of modulated electroluminescence (MEL) to measure the relaxation time (decay) of excess carriers. The advantage of the MEL technique is that the relaxation time is obtained directly from the angular frequency at which the quadrature component peaks. It does not need knowledge of the material parameters like mobility, etc., and can be used for any finished solar cells which have detectable light emission. The experiment is easy to perform with standard electrical equipment. For silicon solar cells, the relaxation time is dominated by recombination and hence, the relaxation time is indeed the excess carrier lifetime. In contrast, for the CIGS solar cells investigated here, the relaxation time is dominated by trapping and emission from shallow minority carrier traps.

Luminescent and electrical properties of oxygen‐implanted silicon

Light emitting diodes with an active defect‐rich region produced by oxygen implantation and a subsequent multistep annealing of silicon wafers were investigated by means of transmission electron microscopy, SIMS, capacitance voltage, deep level transient spectroscopy, electroluminescence (EL), and cathodoluminescence (CL) techniques. The properties of two groups of n‐based samples with and without thermal pre‐treatment at 1000 °C for 15 min were compared regarding their defect structure, defects electrical activity, luminescent spectra as well as the impact of prolonged intense electron irradiation. The observed difference in the properties of such groups was explained by a difference in the density and oxygen content of oxygen‐related defects. A significant distinction between EL and CL spectra at low excitation levels was found and interpreted to be due to particular defect kinds in near‐surface and the deepest layers of the implanted region. The blue shift of EL spectra upon excitation increase reported previously is explained by the increase of penetration depth of the holes and specific depth distribution of the defects of different kinds.

Analysis of degradation mechanisms in AlInN/GaN HEMTs by electroluminescence technique

In this paper, the impact of a severe on-state stress on the IDS (VDS, VGS) characteristics of AlInN/GaN devices is analyzed by electroluminescence technique performed at room temperature. In fact, the devices operate in bias conditions that allow measuring the bell- shaped gate current. To our knowledge, it is the first time that a bell-shaped gate current centered at a positive VGS and measured at room temperature has been shown for an AlInN/GaN transistor. We have also highlighted that electroluminescence spectra are related to the superposition of intraband radiative electron transitions, Fabry-Perot oscillations, and emission bands induced by recombination of electrons due to electron traps. In these conditions, it is not so easy to extract energies levels of electron traps existing in unstressed and stressed AlInN/GaN HEMTs from electroluminescence spectra. Thus, we have also shown that the electrical degradations induced by on-state stress are mainly related to the trapping of hot electron by deep pre-existing electron traps in the devices. Moreover, we have highlighted the existence of two electron traps activated at 1.6 and 1.8eV in the devices.

Cross-Characterization for Imaging Parasitic Resistive Losses in Thin-Film Photovoltaic Modules

Thin-film photovoltaic (PV) modules often suffer from a variety of parasitic resistive losses in transparent conductive oxide (TCO) and absorber layers that significantly affect the module electrical performance. This paper presents the holistic investigation of resistive effects due to TCO lateral sheet resistance and shunts in amorphous-silicon (a-Si) thin-film PV modules by simultaneous use of three different imaging techniques, electroluminescence (EL), lock-in thermography (LIT) and light beam induced current (LBIC), under different operating conditions. Results from individual techniques have been compared and analyzed for particular type of loss channel, and combination of these techniques has been used to obtain more detailed information for the identification and classification of these loss channels. EL and LIT techniques imaged the TCO lateral resistive effects with different spatial sensitivity across the cell width. For quantification purpose, a distributed diode modeling and simulation approach has been exploited to estimate TCO sheet resistance from EL intensity pattern and effect of cell width on module efficiency. For shunt investigation, LIT provided better localization of severe shunts, while EL and LBIC given good localization of weak shunts formed by the scratches. The impact of shunts on the photocurrent generation capability of individual cells has been assessed by li-LBIC technique. Results show that the cross-characterization by different imaging techniques provides additional information, which aids in identifying the nature and severity of loss channels with more certainty, along with their relative advantages and limitations in particular cases.

Electro- and photoluminescence imaging as fast screening technique of the layer uniformity and device degradation in planar perovskite solar cells

In this study, we provide insights into planar structure methylammonium lead triiodide (MAPbI3) perovskite solar cells (PSCs) using electroluminescence and photoluminescence imaging techniques. We demonstrate the strength of these techniques in screening relatively large area PSCs, correlating the solar cell electrical parameters to the images and visualizing the features which contribute to the variation of the parameters extracted from current density-voltage characterizations. It is further used to investigate one of the major concerns about perovskite solar cells, their long term stability and aging. Upon storage under dark in dry glovebox condition for more than two months, the major parameter found to have deteriorated in electrical performance measurements was the fill factor; this was elucidated via electroluminescence image comparisons which revealed that the contacts' quality degrades. Interestingly, by deploying electroluminescence imaging, the significance of having a pin-hole free active layer is demonstrated. Pin-holes can grow over time and can cause degradation of the active layer surrounding them.

Optical technique for photovoltaic spatial current response measurements using compressive sensing and random binary projections

Compressive sensing has been widely used in image compression and signal recovery techniques in recent years; however, it has received limited attention in the field of optical measurement. This paper describes the use of compressive sensing for measurements of photovoltaic (PV) solar cells, using fully random sensing matrices, rather than mapping an orthogonal basis set directly. Existing compressive sensing systems optically image the surface of the object under test, this contrasts with the method described, where illumination patterns defined by precalculated sensing matrices, probe PV devices. We discuss the use of spatially modulated light fields to probe a PV sample to produce a photocurrent map of the optical response. This allows for faster measurements than would be possible using traditional translational laser beam induced current techniques. Results produced to a 90% correlation to raster scanned measurements, which can be achieved with under 25% of the conventionally required number of data points. In addition, both crack and spot type defects are detected at resolutions comparable to electroluminescence techniques, with 50% of the number of measurements required for a conventional scan.

Power Degradation Caused by Snail Trails in Urban Photovoltaic Energy Systems

In recent years, a discoloration defect called as the snail trials emerged on crystalline silicon solar module in urban photovoltaic energy systems. It resulted in power degradation, and caused a serious concern about effects of this phenomenon on crystalline silicon solar modules, but very few publications have dealt with this phenomenon. In this paper, the crystalline silicon solar modules with snail trials are investigated by I-V and P-V characteristics, electroluminescence (EL) technique, thermography analysis, and energy production in photovoltaic power plant. The obtained results show that the snail trails may affect output of power for crystalline silicon solar modules compared with reference module, the energy production measured was about 9.1% lower than the normal array.

Optical characterization of voltage-accelerated degradation in CH3NH3PbI3 perovskite solar cells.

We investigate the performance degradation mechanism of CH3NH3PbI3 perovskite solar cells under bias voltage in air and nitrogen atmospheres using photoluminescence and electroluminescence techniques. When applying forward bias, the power conversion efficiency of the solar cells decreased significantly in air, but showed no degradation in nitrogen atmosphere. Time-resolved photoluminescence measurements on these devices revealed that the application of forward bias in air accelerates the generation of non-radiative recombination centers in the perovskite layer buried in the device. We found a negative correlation between the electroluminescence intensity and the injected current intensity in air. The irreversible change of the perovskite grain surface in air initiates the degradation of the perovskite solar cells.

Carrier dynamics analysis for efficiency droop in GaN-based light-emitting diodes with different defect densities using time-resolved electroluminescence

We developed a direct experimental approach for investigating the correlation between efficiency droop and recombination rate variation under current injection conditions by using time-resolved electroluminescence (EL) technique. We applied this approach to understand the droop phenomenon of GaN-based light-emitting diodes grown on patterned sapphire substrates (LED-on-PAT) and planar sapphire substrates (LED-on-PLA). Because of lower dislocation density and current leakage in LED-on-PAT compared to LED-on-PLA, it was found that the effective carrier density injected into quantum wells (QWs) in LED-on-PAT was higher than that of the LED-on-PLA under the same current injection conditions, based on the analysis of spectral broadening of EL spectra with varying current injection and photoluminescence experiments under resonant and non-resonant excitation conditions. The efficiency droop in LED-on-PAT was found to be much more severe than that of LED-on-PLA, despite the higher overall quantum efficiency of LED-on-PAT. From the time-resolved EL analysis, we could separate radiative and non-radiative recombination contributions and directly observe (i) the decrease and saturation of radiative recombination time and (ii) the increase and following decrease in behavior of non-radiative recombination time with increasing current injection level, showing a strong correlation between efficiency droop and recombination rate variation.

Degradation mechanism of perovskite CH3NH3PbI3 diode devices studied by electroluminescence and photoluminescence imaging spectroscopy

We investigate the degradation behavior of non-sealed perovskite CH3NH3PbI3 diode devices in air at room temperature by means of electroluminescence (EL) and photoluminescence (PL) imaging techniques. From the comparison of these images, we determine that the spatial fluctuation of the EL intensity is mainly due to fluctuations in the luminescence efficiency of the perovskite layer itself. By applying a constant voltage for tens of minutes, the EL intensity decreases gradually. It is observed that the temporal evolution of the EL intensity is governed by the degradation of the perovskite layer and the carrier injection at the interface.

Identification and characterisation of performance limiting defects and cell mismatch in photovoltaic modules

The performance and longevity of photovoltaic (PV) modules can be severely limited by cell mismatch occurring when a solar cell in a series-connected string produces a lower current than the other cells in that string. The current output of the entire string is limited by the weakest cell in the string so shading or damage to a single cell in a module can affect the entire module’s current output. Electrolumin-escence (EL) occurs when a positive current and voltage are applied to a solar cell and is used to identify damage and defects in the cell. In this study, the cell mismatch in three single crystalline silicon modules was investigated using EL and current-voltage (I-V) characterisation techniques. Two modules have a white discolouration that affects the majority of the cells in the module and also have signs of mechanical damage, while the third module acts as a reference as it has no discolouration and appears undamaged. The EL signal intensity is related to cell performance and identifies material defects, bad contacts and broken cells. Cell mismatch in a module results in a decrease in the performance parameters obtained from the I-V characteristic curve of the module. The I-V curves indicate the presence of current mismatch in the degraded modules, which is supported by the EL images of these modules. The use of EL images, in conjunction with the I-V curves, allows the degradation in the modules to be characterised.

Molybdenum oxide and molybdenum oxide-nitride back contacts for CdTe solar cells

Molybdenum oxide (MoOx) and molybdenum oxynitride (MoON) thin film back contacts were formed by a unique ion-beam sputtering and ion-beam-assisted deposition process onto CdTe solar cells and compared to back contacts made using carbon–nickel (C/Ni) paint. Glancing-incidence x-ray diffraction and x-ray photoelectron spectroscopy measurements show that partially crystalline MoOx films are created with a mixture of Mo, MoO2, and MoO3 components. Lower crystallinity content is observed in the MoON films, with an additional component of molybdenum nitride present. Three different film thicknesses of MoOx and MoON were investigated that were capped in situ in Ni. Small area devices were delineated and characterized using current–voltage (J-V), capacitance–frequency, capacitance–voltage, electroluminescence, and light beam-induced current techniques. In addition, J-V data measured as a function of temperature (JVT) were used to estimate back barrier heights for each thickness of MoOx and MoON and for the C/Ni paint. Characterization prior to stressing indicated the devices were similar in performance. Characterization after stress testing indicated little change to cells with 120 and 180-nm thick MoOx and MoON films. However, moderate-to-large cell degradation was observed for 60-nm thick MoOx and MoON films and for C/Ni painted back contacts.

Advanced large area characterization of thin-film solar modules by electroluminescence and thermography imaging techniques

This paper shows that the combination of both dark lock-in thermography (DLIT) and electroluminescence (EL) imaging techniques is especially suitable for in-depth shunt analysis of industrial Cu(In,Ga)Se2 (CIGS) thin-film modules and for the quantitative analysis of the local electrical cell properties, such as the internal junction voltage or the series resistances of the front and back contact. First results obtained for amorphous silicon (a-Si:H) based thin-film solar modules reveal that the quantitative EL analysis method is applicable to amorphous silicon technology as well. Using the consistent modeling of EL and DLIT images using a SPICE based model with input data generated from EL and DLIT measurements, we demonstrate that we can obtain properties of solar modules like the spatial voltage distribution, the resistances of the front and back contact, and the shunt resistance of local defects. We furthermore discuss two newly developed techniques, namely voltage-modulated lock-in thermography at the maximum power point (MPP-LIT) of an illuminated cell or module as well as differential EL analysis of illuminated cells and modules.

Fatigue degradation and electric recovery in Silicon solar cells embedded in photovoltaic modules

Cracking in Silicon solar cells is an important factor for the electrical power-loss of photovoltaic modules. Simple geometrical criteria identifying the amount of inactive cell areas depending on the position of cracks with respect to the main electric conductors have been proposed in the literature to predict worst case scenarios. Here we present an experimental study based on the electroluminescence (EL) technique showing that crack propagation in monocrystalline Silicon cells embedded in photovoltaic (PV) modules is a much more complex phenomenon. In spite of the very brittle nature of Silicon, due to the action of the encapsulating polymer and residual thermo-elastic stresses, cracked regions can recover the electric conductivity during mechanical unloading due to crack closure. During cyclic bending, fatigue degradation is reported. This pinpoints the importance of reducing cyclic stresses caused by vibrations due to transportation and use, in order to limit the effect of cracking in Silicon cells.

Surexpression de l’IL-31 dans une forme sporadique de lichen amyloïde : un lien avec le prurit ?

Radiative recombination of carriers in two kinds of GaxIn1−xP/GaAs double-junction tandem solar cell structures was investigated by using room-temperature electroluminescence (EL) and photoluminescence (PL) techniques. Efficient radiative recombination was observed simultaneously in the top and the bottom subcells in both the samples. By studying the behavior of EL and PL spectra, the radiative recombination intensity ΦEL was demonstrated to be reliant on the material-dependent radiative recombination coefficient, base layer doping concentration and thickness. Furthermore, dependence of ΦEL on substrate misorientation in both the subcells was also evidenced, which was explained in terms of the growth-induced variations in microstructure for the GaInP top cell and in potential barrier profile across the p–n junction for the GaAs bottom cell. Based on these observations, the radiative recombination in the two base layers of the subcells was demonstrated to be the major carrier loss mechanism in the GaxIn1−xP/GaAs double-junction tandem photovoltaic devices and should be suppressed.

Fabrication of ZnS:Cu/PVA Nanocomposite Electroluminescence Devices for Flat Panel Displays

The powder of ZnS nanoparticles were prepared by using chemical deposition technique and characterized by electroluminescence techniques are reported in this paper. The estimated size of ZnS:Cu nanocrystals with change in capping agent concentration and ZnS:Cu/PVA nanocomposites and no effect of doping has been observed on the absorption spectra. Electroluminescence (EL) investigations of nanocrystalline powder as well as nanocomposites, it is seen that Log B vs. 1/V curve is a straight line with negative slope. This indicates that EL is produced by acceleration-collision mechanism. The detail EL characterization and application in display devices of these materials are reported in this paper. Copyright © 2013 VBRI press.

Radiative recombination of carriers in the GaxIn1−xP/GaAs double-junction tandem solar cells

Radiative recombination of carriers in two kinds of GaxIn1−xP/GaAs double-junction tandem solar cell structures was investigated by using room-temperature electroluminescence (EL) and photoluminescence (PL) techniques. Efficient radiative recombination was observed simultaneously in the top and the bottom subcells in both the samples. By studying the behavior of EL and PL spectra, the radiative recombination intensity ΦEL was demonstrated to be reliant on the material-dependent radiative recombination coefficient, base layer doping concentration and thickness. Furthermore, dependence of ΦEL on substrate misorientation in both the subcells was also evidenced, which was explained in terms of the growth-induced variations in microstructure for the GaInP top cell and in potential barrier profile across the p–n junction for the GaAs bottom cell. Based on these observations, the radiative recombination in the two base layers of the subcells was demonstrated to be the major carrier loss mechanism in the GaxIn1−xP/GaAs double-junction tandem photovoltaic devices and should be suppressed.

A deep acceptor defect responsible for the yellow luminescence in GaN and AlGaN

In the present study, the electrical and optical properties of deep defects in p-i-n GaN junction and AlGaN/GaN heterojunction are investigated by means of the deep level transient spectroscopy (DLTS), Laplace DLTS, and electroluminescence (EL) techniques. We demonstrate that in both structures the yellow luminescence (YL) is a dominant band in the EL spectra recorded at room temperature. We correlate the YL band with the minority DLTS peaks observed at about 370 K. A gallium vacancy-related defect seems to be a probable candidate as to the origin of the defect. Another dominant majority peak observed in the DLTS studies was concluded to be linked with a donor-like defect in the upper half of the bandgap. The origin of the defect is discussed.