Reverse-bias Electroluminescence Research Articles

Diagnosing breakdown mechanisms in monocrystalline silicon solar cells via electroluminescence imaging

The local breakdown behavior may be harmful to solar cells and could possibly permanently damage the cell. Therefore, understanding the breakdown mechanisms in commercially competitive photovoltaic devices such as monocrystalline silicon (Si) solar cells is of great importance. Here, by using the reverse-biased electroluminescence (ReBEL) imaging technique, we observed three types of breakdown phenomena in monocrystalline Si solar cells: defect-induced breakdown, avalanche breakdown, and early breakdown. We have applied a variety of methods to diagnose each breakdown mechanism. The positions of defect-induced breakdown were first determined by combining EL and ReBEL imaging. An innovation method, the distributed circuit modeling was further introduced to trace the formation of different defect-induced breakdown sites. It is firstly applied this approach in the analysis of the breakdown mechanism. Then, avalanche breakdown was demonstrated by the temperature coefficient. The origin of its emission spectra was analyzed by the Si energy band structure combined with Baraff theory. Moreover, the characteristic of early breakdown was found to be consistent with the Zener effect, which may be caused by the metal stains such as aluminum (Al) during the manufacturing process.

Temperature Hypersensitive Organic Electroluminescence in a Reverse Biased, Frozen Polymer P–I–N Junction

AbstractSemiconductor junctions are among the most important material interfaces in modern technology. The solid polymer light‐emitting electrochemical cell (PLEC) possesses a semiconducting polymer homojunction that is reminiscent of a conventional p–n junction but also exhibits distinct features that are profoundly intriguing. The PLEC junction is formed under bias when the propagating p‐ and n‐doping fronts make contact. The PLEC junction can be immobilized by cooling after the initial junction formation. Further, the resulting frozen junction can be relaxed, or partially de‐doped into a p–i–n junction by controlled heating/cooling cycles. It is on such a frozen polymer p–i–n junction that the authors observed one of the most puzzling phenomena in organic electroluminescence (EL). The frozen p–i–n junction displays the brightest reverse bias (RB) EL ever imaged in a polymer junction and a large photovoltage. Moreover, the RB EL exhibits hypersensitivity to temperature, increasing in intensity by 22.6% K−1 when the de‐doped cell is cooled from 250 to 200 K. A model based on the tunnel injection of charge carriers with fast transit time across the intrinsic region explains the RB EL's hypersensitivity to temperature and super‐linear dependence on cell current.

Decoding the Polymer p–n Junction: Controlled Dedoping and Reverse Bias Electroluminescence

AbstractThe polymer light‐emitting electrochemical cell (PLEC) is a unique solid‐state device possessing attractive attributes for low‐cost applications, but also a junction structure that is still poorly understood. In a PLEC, the applied voltage causes in situ electrochemical p‐ and n‐doping of the semiconducting polymer and the formation of a dynamic light‐emitting p–n junction. Once the junction is fixed by cooling or chemical manipulation, the “frozen‐junction” PLEC exhibits a unipolar electroluminescence (EL) and photovoltaic response. Repeated thermal cycling, however, can cause the frozen‐junction PLEC to experience drastically enhanced EL under forward bias and the emergence of reverse bias EL. In this study, a combination of transport measurements and direct imaging is used to elucidate the origin of the mysterious reverse bias EL. A model is developed that explains the reverse bias EL as caused by the tunnel injection of electrons and holes from bandgap states into a dedoped “intrinsic” region between the p‐ and n‐doped regions. The model explains the location, relative intensity, and evolution of EL under both forward and reverse bias. The results hint at a junction that is much narrower than previously resolved.

Two‐Dimensional Hot Spot Temperature Simulation for c‐Si Photovoltaic Modules

A two‐step method to simulate the spatially resolved temperature of a partially shaded cell in a crystalline silicon photovoltaic (PV) module is presented and tested. First, an efficient module electronic simulation tool computes the operating conditions of a module's constituent cells. Second, a two‐dimensional finite‐element analysis simulation, utilizing forward‐, and revers‐bias electroluminescence measurements, is performed to spatially resolved cell temperature. With an outdoor experiment, un‐encapsulated cell temperatures are directly measured under controlled heat transfer conditions. A peak local cell temperature of 144 °C is observed on a multi crystalline silicon cell dissipating 54 W heat in the experiment, 57 °C higher than the result from a non‐spatially resolved simulation. Experimental results indicate cells without significant Ohmic shunts are suitable for temperature simulation with the aid of reverse bias electroluminescence imaging, which yields a maximum temperature prediction error of less than 15 °C. However, simulation for cells with significant Ohmic shunts is prone to underestimate the cell temperature by up to 45 °C. Multiple shading fractions from 12 to 80% lead to severe heating scenarios in such case.

Injection intensity-dependent recombination at various grain boundary types in multicrystalline silicon solar cells

If the ratio of two open circuit photoluminescence (Voc-PL) images taken at two different light intensities is displayed, some grain boundaries (GBs) may show up as bright lines. This indicates that these special GBs show distinct injection intensity-dependent recombination properties. It will be shown here that this results in an apparent ideality factor of the light emission smaller than unity. The effect is reproduced with numerical device simulations using a usual distribution of defects in the band gap along grain boundaries. Quantitative imaging of this apparent luminescence ideality factor by PL imaging is complicated by the lateral horizontal balancing currents flowing at open circuit. The local voltage response of an inhomogeneous solar cell at different injection levels under open circuit is modelled by Griddler simulations, based on PL investigations of this cell. The evaluation of Voc-PL images at different illumination intensities allows us to conclude that the apparent luminescence ideality factor at the special GBs is about 0.89, whereas in the other regions it is between 0.94 and 0.95. Reverse bias electroluminescence showed no pre-breakdown sites, and hyperspectral PL imaging showed only in one of the investigated GBs particular defect luminescence. TEM investigations of two GBs, one showing distinct injection intensity-dependent recombination and the other one showing none, revealed that the investigated special GB is a large-angle GB whereas the GB not showing this effect is a small-angle GB.

Breakdown voltage mapping through voltage dependent ReBEL intensity imaging of multi-crystalline Si solar cells

Device material quality affects both the efficiency and the longevity of photovoltaic (PV) cells. Therefore, identifying these defects can be beneficial in the development of more efficient and longer lasting PV cells. In this study, a combination of spatially-resolved, electroluminescence (EL), and light beam induced current (LBIC) measurements, were used to identify specific defects and features of a multi-crystalline Si PV cells. In this study, a novel approach is used to map the breakdown voltage of a PV cell through voltage dependent Reverse Bias EL (ReBEL) intensity imaging.

Study of Low Voltage Prebreakdown Sites in Multicrystalline Si Based Cells by the LBIC, EL, and EDS Methods

Breakdown sites in multicrystalline Si solar cells have been studied by reverse-bias electroluminescence, electron beam induced current (EBIC) and laser beam induced current (LBIC), and Energy Dispersive X-Ray Spectroscopy methods. In the breakdown sites revealed by EL at small reverse bias (~5 V), the enhanced aluminum and oxygen concentration is revealed. Such breakdowns can be located inside the depletion region because they are not revealed by the EBIC or LBIC methods. Breakdowns revealed by EL at larger bias correlate well with extended defects in the EBIC and LBIC images.

Experimental observation of TDDB-like behavior in reverse-biased green InGaN LEDs

This paper reports the outcome of a series of reverse-bias experiments performed on commercial GaN-based green LEDs. The experimental results showed that green LEDs submitted to reverse bias i) show a time-dependent failure when they are submitted to constant (reverse) voltage stress, at a bias point smaller than the BDV; ii) experience an increase of the reverse-bias electro-luminescence signal, well-correlated with the increase of the reverse leakage current; iii) the TTF (Time-To-Failure) related to the time-dependent breakdown process has an exponential dependence on stress voltage; iv) the TTF is Weibull distributed. This work provides the first experimental demonstration of time-dependent failure of GaN LEDs.

Classification of crystal defects in multicrystalline silicon solar cells and wafer using spectrally and spatially resolved photoluminescence

In this contribution, spectral photoluminescence (SPL) imaging detecting both the spectral distribution and the lateral position is applied on recombination active defects in multicrystalline silicon solar cells and wafers. The result is analysed by a Multivariate Curve Resolution (MCR) algorithm using the spectral photoluminescence response and their positions. (i) Without any pre-assumptions made, the algorithm distinguishes four different recombination active defect types. Looking at the spatial distribution, it is shown that two of these defect types coincide with two defect types that have been distinguished on solar cell level using an analysis of forward and reverse biased electroluminescence (denoted as Type-A and -B) previously. (ii) Using SPL, all previously classified defects can also be distinguished at the wafer level. Therefore, the defects limiting the solar cell efficiency are already present in the wafer material and not introduced by the solar cell process. This is of particular interest for the question of how to predict the solar cell efficiency based on the PL measurements at the wafer level. The SPL is able to distinguish between the recombination activity of the dominant Type-A and -B defects that cannot be distinguished by classical PL measurements of the band-to-band recombination at the wafer level. The technique also highlights the changes in recombination activity of the given defects throughout the fabrication process. (iii) Additionally, it is shown that the spectral peak positions of Type-A defects coincide with the known D3 and D4 lines and of Type-B defects with the D1 line on both solar cell and wafer level. Two further defects are captured by the MCR algorithm denoted as Type-VID3 and Type-D07 defects occurring as spot-like defects in isolated positions. Their spectral PL response is analysed as well.

Exploring failure mechanisms of near ultraviolet AlGaN/GaN light-emitting diodes by reverse-bias stress in water vapour

Reliability tests of GaN near UV light–emitting diodes (LED) were investigated by reverse–bias stress in water vapour. Various electrical, optical and material characterisations were performed to examine degradation trend and failure mechanism. On the basis of multiple material analyses, including scanning electron microscopy (SEM) with focus ion beam (FIB), energy dispersive X–ray spectrometry (EDX) and secondary ion mass spectroscopy (SIMS), deformation of the metal contact and the electromigration of Au could cause the increase of the leakage and reverse–bias electroluminescence, indicating some damaged high electric areas might be generated, which lead to LED failures. The characterisation techniques might resolve the long–term LED failure reasons. Furthermore, the reverse–bias LED operations are promising for short–time screening of the Near UV LED quality for future industrial applications.

Degradation of AlGaInP light emitting diodes under reverse-bias operations in salty water vapor

GaP-based light emitting diodes (LEDs) reliability tests are crucial to further development for high-performance LED lighting technology. In this study, we perform reverse-bias stress tests for LEDs in water vapor and salty water vapor ambient, respectively. The results indicate that salty water vapor can quickly degrade LEDs. To investigate the failure mechanisms, electrical measurements, forward-bias and reverse-bias electroluminescence images, and multiple material analyses have been taken to study the failure mechanism. Results indicate that NaCl clustering solids and atom diffusion may cause deformation of the electrode and enhance the degradation of the LEDs.

Localization and characterization of annealing‐induced shunts in Ni‐plated monocrystalline silicon solar cells

AbstractThe adhesion of Ni–Cu‐plated contacts requires annealing, which can lead to an electrical degradation of the solar cell. A combination of optical imaging methods and electron microscopical cross‐section analysis was used to investigate the annealing‐induced shunts on monocrystalline solar cells. The results show that Ni spikes cause the lowering of the pseudo fill factor and reveal that these nonlinear shunts show the same behavior as recombination active breakdown sites on multicrystalline silicon solar cells, including radiation of visible light while breakdown. Using reverse biased electroluminescence set‐up the shunts could be localized in top view with µm size lateral resolution. Subsequently, a cross‐section was prepared on a radiative breakdown spot. Advanced electron microscopic investigations reveal defect structures featuring nickel precipitates on the position of the light source. (© 2014 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

基于不同电压下的电致发光和红外成像的太阳能电池缺陷检测

The electroluminescence and infrared thermography were used to detect the defects of the solar cells by applying different forward bias and reverse bias voltage. The solar cell under forward bias mainly emits infrared light from 850 to 1 200 nm due to the recombination of electron and hole.The intensity of the emitted light depends on the series resistance and minority carrier diffusion length of the solar cell,so it can be used to detect the defects of finger disturbing,crack,and electrode etching,et al. The emission of light under reverse bias is intra-band emission which covers a wide spectrum from visible light to IR. The breakdown can be detected by reverse bias electroluminescence( RBEL). Only under high bias,weak Ohmic shunt appears in RBEL. Linear and nonlinear shunt can be detected using IR thermography under different bias. Strong Ohimc shunt can be detected in forward and reverse bias. The nonlinear shunt depends on the type.

Micro Characterization and Imaging of Spikes in Nickel Plated Solar Cells

Annealing induced silicidation of plated nickel contacts can severly lower the solar cell performance due to deep nickel silicide spikes penetrating the space charge region. This work summarizes several attempts to characterize performance limiting deep silicide structures and determines the influence of different passivation layer structuring technologies on the silicide growth. Reverse biased electroluminescence measurements revealed that the deep nickel silicides occur after an anneal and arelocated along the structured and plated passivation layer openings. Cross-section studies of the plated contacts demonstrated that deep silicide growth is present independently of the applied passivation layer structuring technology. While for laser-ablation the critical silicide structures could be mostly attributed to accelerated silicide growth at laser induced defects, there are also deep silicide structures that appear without any obvious correlation even in the case of defect free wet chemical passivation structuring of the contact openings. By varying the emitter doping profile and the annealing temperature after plating the distance of the space charge region and the surface was found to influence the annealing-induced solar cell performance decrease. Considering the presented results there are three ways to overcome annealing induced degradation of Ni plated solar cells. Either optimized process designs with sufficient contact adhesion and contact resistivity without the need of thermal silicide formation, improved silicide depth control (e.g. Ni source limited growth) or the implementation of a selective emitter design with pn-junction depths well above 1 μm within the contacted area.

Electromigration‐induced degradation around electrode of GaN light‐emitting diode in water vapour

Reverse-bias operations for a light-emitting diode can quickly screen the weakness of the device. Electromigration around the electrode on the lateral and vertical directions can be observed by optical microscope images, scanning electron microscope images and energy dispersive spectroscopy. The failure process caused by the electromigration can be revealed by the changes of forward-bias and reverse-bias electroluminescence images and the emergence of the reverse-bias EL may be attributed to the diffusion of gold. Furthermore, the increase of the leakage current may supress the forward-bias band to band recombination current especially near the metal contact.

Unusual electroluminescence from n-ZnO@i-MgO core–shell nanowire color-tunable light-emitting diode at reverse bias

Light-emitting diodes (LEDs) based on n-ZnO@i-MgO core-shell (CS) nanowires (NWs) are herein demonstrated and characterized. MgO insulating layers were rationally introduced as shells to modify/passivate the surface defects of ZnO NWs. A high-quality ZnO/MgO interface was attained and the optically pumped near-band-edge emission of the bare ZnO NWs was greatly enhanced after cladding i-MgO shells. Electroluminescence (EL) spectra measured in the whole UV-visible range revealed that light emission can only be detected when LEDs were applied with reverse bias. Moreover, the emission color can be tuned from orange to bright white with increasing reverse bias. We explored these interesting results tentatively in terms of the energy-band diagram of the heterojunction and it was found that the interfacial i-MgO shells not only acted as an insulator to prevent a short circuit between the two electrodes, but also offered a potential energy difference so that electron tunneling was energetically possible, both of which were essential to generate the reverse-bias EL. The dipole-forbidden d-d transitions by the Laporte selection rule in the p-NiO might be the reason to why there is no light being detected from the CS NW LED under forward bias. It is hoped that this simple and facile route may provide an effective approach in designing low-cost CS NW LEDs.

Breakdown characteristics of flexible Cu(In,Ga)Se2 solar cells

The behavior of solar cells under reverse bias conditions is of crucial importance for solar module design and stability. For flexible, low temperature deposited Cu(In,Ga)Se2 (CIGSe) solar cells indications for tunnel breakdown at operation temperatures around 300K are found using temperature and net carrier concentration dependent measurements of the breakdown voltage VBR. For temperatures below 200K, the temperature coefficient of the breakdown voltage becomes positive, indicating a change of the breakdown mechanism to avalanche breakdown. A reduction of VBR caused by the illumination with blue photons is demonstrated, which coincides well with results and models for high temperature deposited CIGSe on glass. Furthermore, spectrally resolved measurements of light emission during breakdown indicate a first ReBEL (reverse bias electroluminescence) signature for CIGSe solar cells.

Hot carrier-induced emission from the InGaN/GaN light-emitting diode by characterizing reverse-bias electroluminescence

Reverse-bias operations can test the robustness and screen the weaknesses of the light emitting diode device for a shorter period of several hours. The mechanism of the hot carrier-induced emission, which is distinct from the yellow luminescence and the band-to-band recombination, has been confirmed by electroluminescence observations and hot carrier-induced equation. Furthermore, the degradation of electrical properties and failure of the device have been confirmed by the reverse-bias constant-current and constant-voltage stress. Focus ion beam, scanning electron microscopy, and energy dispersive X-ray spectroscopy reveal that deformed metal line might be a possible origin of reverse-bias luminescence because of high electric field.

Electron microscope verification of prebreakdown-inducing α-FeSi2 needles in multicrystalline silicon solar cells

It had been shown already earlier by X-ray microanalysis that, in positions of defect-induced junction breakdown in industrial multicrystalline (mc) silicon solar cells, iron-containing precipitates may exist. However, the nature of these precipitates was unknown so far. Here, in such positions, scanning transmission electron microscopy was performed after defect-controlled focused ion beam preparation. First of all, the defect site was localized by microscopic reverse-bias electroluminescence imaging. The high accuracy of following FIB target preparation (<0.1 μm necessary) was obtained by both, electron beam-induced current imaging and secondary electron material contrast observation during the slice-by-slice milling of the TEM specimen. By nano-beam electron diffraction (NBED) and energy dispersive spectroscopy, the iron-containing precipitates were identified as α-type FeSi2 needles, about 30 nm in diameter and several μm in length. The FeSi2 needles show preferential orientation relationships to the silicon matrix and are located in terraced large-angle grain boundaries. Elaborate nano-beam electron diffraction investigation of the FeSi2 revealed orientation relationships of the precipitate to the silicon, which confirm earlier investigations on monocrystalline material. A model explaining the defect-induced breakdown mechanism due to rod-like α-FeSi2 precipitates is presented.

Optical characterizations and reverse-bias electroluminescence observation for reliability investigations of the InGaN light emitting diode

The reverse-bias operation of the InGaN light-emitting diode (LED) device can reveal device-reliability problems. This study uses optical characterization techniques, including surface temperature measurements, two-dimensional (2D) X-ray fluorescent (XRF) element analysis, 2D electroluminescence (EL) images processed by Matlab, and electrical measurements to visualize the current leakages around the metal contact of the device. Connections between the device performance and the reverse-bias EL current distribution have been established. This paper attributes the origin of the reverse-bias emission to a high electric field caused by weak structures during process variations. Hot electron-induced emissions due to a leakage current may be a mechanism of the reverse-bias emission. Furthermore, reverse-bias stress on the devices is performed on the LED devices to investigate reliability issues. The reverse-bias light emission is relevant to reliability problems because of its combination of optical characterization and electrical performance. These techniques provide a screening tool that will correlate device failures with the fabrication process for future industrial applications.