Ohmic Shunts Research Articles

Changes in ohmic shunts in Cu(In,Ga)Se2 devices during damp heat

To increase the bankability of photovoltaics (PV), a more accurate prediction of the lifetime and degradation-rate of PV modules is required. In past studies, average degradation rates for different technologies have been derived from field data. However, in Cu(In,Ga)Se2 (CIGS) based PV, localized shunts are commonly found in modules coming out of the factory. Although the impact of such localized defects on initial module performance has been studied in the past, their evolution and impact on module performance upon aging remain mostly unknown. In this paper, it is demonstrated that exposure of CIGS devices to elevated temperatures and humidity can result in the partial or complete recovery of localized ohmic shunts. This was observed for three types of shunting defects, each on a different CIGS device characterized before and after damp heat exposure. In each device, the reduction in shunting after damp heat could be observed in illuminated lock-in thermography, photoluminescence and current-voltage measurements. For two of the devices, the positive effect of shunt reduction was counteracted by other degradation mechanisms, such as increase in series resistance of TCO or back contact, while a 4% absolute efficiency increase was observed in the third device. Our data suggests that the shunting is mainly reduced due to oxidation of the materials initially forming the shunting path. The combined effects of moisture degradation and shunt reduction may limit detectability of moisture ingress in electrical performance data. These results illustrate the added value of regular visual inspection and luminescence in quality control.

Disentangling the effect of the hole-transporting layer, the bottom, and the top device on the fill factor in monolithic CIGSe-perovskite tandem solar cells by using spectroscopic and imaging tools

We present monolithic copper–indium–gallium–diselenide (Cu(In,Ga)Se2, CIGSe)-perovskite tandem solar cells with air- or N2-transferred NiO x :Cu with or without self-assembled monolayer (SAM) as a hole-transporting layer (HTL). A champion efficiency of 23.2%, open-circuit voltage (V) of 1.69 V, and a fill factor (FF) of 78.3% are achieved for the tandem with N2-transferred NiO x :Cu + SAM. The samples with air-transferred NiO x :Cu + SAM have V and FF losses, while those without SAM are heavily shunted. We find via x-ray and UV photoelectron spectroscopy that the air exposure leads to non-negligible loss in the Ni2+ species and changes in the NiO x :Cu’s work function and valence band maxima, both of which can negatively impact the V and the FF of the tandems. Furthermore, by performing dark lock-in thermography, photoluminescence (PL), and scanning electron microscopy studies, we are able to detect various morphological defects in the tandems with poor performance, such as ohmic shunts originating from defects in the bottom CIGSe cell, or from cracking/delaminating of the perovskite top cell. Finally, by correlating the detected shunts in the tandems with PL-probed bottom device, we can conclude that not all defects in the bottom device induce ohmic shunts in the tandems since the NiO x :Cu + SAM HTL bi-layer can decouple the growth of the top device from the rough, defect-rich and defect-tolerant bottom device and enable high-performing devices.

Investigation of the Temperature Sensitivity of 20-Years Old Field-Aged Photovoltaic Panels Affected by Potential Induced Degradation

One effect of moisture ingress on solar panels is potential induced degradation (PID). Solar panels affected by PID experience large leakage currents between the solar cells and the module’s frame, which leads to substantial power degradation. In the present work, the temperature coefficients of 3 old PV panels affected by PID were investigated. In the electroluminescence images, solar cells nearer to the edge of the modules appear darker due to ohmic shunting. IR thermal images acquired under clear sky outdoor conditions show that the majority of the warmer cells (hotspots) were located closer to the edge of the modules. The difference in cell temperature (∆T) due to PID effect ranges from 7 °C to 15 °C for the 3 field-aged modules. The average temperature coefficient of efficiency (βηm) was found to be −0.5%/°C. Also, it was observed that the temperature coefficients of open circuit voltage (βVoc = −0.4%/°C), maximum power point voltage (βVmpp = −0.5%/°C), and fill factor (βFF = −0.2%/°C), were the underlying factors for the degradation in the Pmax of the old solar panels affected by PID. This accounted for an average 1.2%/year overall degradation in the efficiency of these modules. Most notably, it was discovered that the PV modules affected by PID show negative temperature coefficients of maximum power point current (βJmpp) due to large leakage currents. This observed negative βJmpp we believe is characteristic of PV panels affected by PID.

Loosening effect of perovskite intermolecular exchanger with strong steric hindrance for highly sensitive photodetector

In this study, chloronaphthalene (CN) with unique physicochemical properties is incorporated in perovskite nucleation environment to fabricate high-quality photosensitive layer. Specifically, CN has a high boiling point and a bulky naphthalene ring, which induces a steric hindrance in the perovskite adduct phase. Thus, CN delays intermolecular interactions and promotes a stable nucleation environment, which provides sufficient time for the perovskite grains to grow. This is the loosening effect by CN. In this context, this results in the formation of enlarged grains with higher crystallinity for the CN-treated film, based on the LaMer model. According to the increased grain size, the length of grain boundaries which are the pathways to cause ohmic shunt get longer, resulting higher shunt resistance. As a result of the suppressed ohmic shunt and lower dark current density, the outstanding photodetector performance, especially detectivity was confirmed. Through LDR analysis, the possibility of applications under various light conditions was demonstrated. Moreover, better charge transfer and recombination properties were confirmed by PL and impedance analysis. As a result of observing the photodetector parameters for about 1000 h, improved device stability was proved.

Study of the physical and chemical origin of features observed in luminescence and thermography images of Cu(In,Ga)Se2

Luminescence and thermography have proven to be effective tools for the detection of failure modes and defects in Cu(In,Ga)Se2 (CIGS)-based photovoltaic (PV) devices. However, the chemical and physical origin of many of the features observed with these techniques are still unclear. This makes it difficult to asses their impact on device performance and lifetime. Here, features were identified in CIGS cells using spatial photoluminescence (PL), electroluminescence (EL), illuminated lock-in thermography (ILIT) and dark lock-in thermography (DLIT). Localized features were studied using optical microscopy, scanning electron microscopy, electron dispersive X-ray spectroscopy and confocal microscopy. The most commonly observed features could be associated with three different origins. Firstly, TCO sheet resistance resulting in a gradient in the direction of current flow was visible in each sample. For samples with high TCO sheet resistance this compromised detection of other features in EL. Secondly, about half of the detected features corresponded to areas of exposed molybdenum resulting in dark spots in PL and EL. These occurred in shunted and non-shunted form, with only the former causing hotspots in thermography. Thirdly, ohmic shunts induced by current-injection (current-induced shunts) were found to form large hotspots in ILIT and DLIT and a significant drop in luminescence intensity in EL and PL. Localized features in luminescence were only observed for the largest current-induced shunts as clear bright spots in PL and clear dark spots in EL. The results from this study contribute to the distinction of different defect types in modules using exclusively luminescence and thermography imaging techniques.

Modelization of electrical and optical characteristics of short-wave infrared type I InGaAsBi/InGaAs/InP quantum wells p-i-n detector

In the present work, we will exhibit a theoretical analysis and optimization of electrical and optical characteristics of a short-wave infrared p-i-n detector closely lattice matched to conventional (001) InP substrate by the use of quaternary dilute bismide alloy InxGa1−xAs1−yBiy/InxGa1−xAs quantum wells as an active layer. The content of about 6% of Bismuth has been responsible of red-shift of the 50% cut-off wavelength from 2.2 towards 2.8 μm at room temperature, resulting in a band gap reduction of nearly 305 meV caused by the bismuth incorporation. The temperature dependence of zero-bias resistance area product (R 0 A) and bias dependent dynamic resistance of the designed structure have been investigated thoroughly to analyses the dark current contributions mechanisms that might limit the electrical performance of the considered structure. It was revealed that the R 0 A product of the detector is limited by thermal diffusion currents when temperatures are elevated whereas the ohmic shunt resistance contribution limits it when temperatures are low. The modeled heterostructure, reveals a comforting dark current of 1.25 × 10−8 A at bias voltage of −10 mV at 300 K. The present work demonstrates that the p-i-n detector based on compressively strained InxGa1−xAs1−yBiy quantum well is a potential candidate for achieving a short-wave infrared detection.

Double-Mesoscopic Hole-Transport-Material-Free Perovskite Solar Cells: Overcoming Charge-Transport Limitation by Sputtered Ultrathin Al2O3 Isolating Layer.

The electrically insulating space layer takes a fundamental role in monolithic carbon-graphite based perovskite solar cells (PSCs) and it has been established to prevent the charge recombination of electrons at the mp-TiO2/carbon-graphite (CG) interface. Thick 1 μm printed layers are commonly used for this purpose in the established triple-mesoscopic structures to avoid ohmic shunts and to achieve a high open circuit voltage. In this work, we have developed a reproducible large-area procedure to replace this thick space layer with an ultra-thin dense 40 nm sputtered Al2O3 which acts as a highly electrically insulating layer preventing ohmic shunts. Herewith, transport limitations related so far to the hole diffusion path length inside the thick mesoporous space layer have been omitted by concept. This will pave the way toward the development of next generation double-mesoscopic carbon-graphite-based PSCs with highest efficiencies. Scanning electron microscope, energy dispersive X-ray analysis, and atomic force microscopy measurements show the presence of a fully oxidized sputtered Al2O3 layer forming a pseudo-porous covering of the underlying mesoporous layer. The thickness has been finely tuned to achieve both electrical isolation and optimal infiltration of the perovskite solution allowing full percolation and crystallization. Photo voltage decay, light-dependent, and time-dependent photoluminescence measurements showed that the optimal 40 nm thick Al2O3 not only prevents ohmic shunts but also efficiently reduces the charge recombination at the mp-TiO2/CG interface and, at the same time, allows efficient hole diffusion through the perovskite crystals embedded in its pseudo-pores. Thus, a stable VOC of 1 V using CH3NH3PbI3 perovskite has been achieved under full sun AM 1.5 G with a stabilized device performance of 12.1%.

Quantitative Estimation of Shunt Resistance in Crystalline Silicon Photovoltaic Modules by Electroluminescence Imaging

Ohmic shunts in crystalline silicon (c-Si) solar cells and modules are of critical concern as these affect the open-circuit voltage, fill factor, and the net output power. In modules, such shunts create a mismatch and can lead to hotspot formation that can thermally destroy the module. It is essential to estimate the shunt resistance value in a nondestructive manner in order to prioritize the removal of shunts on the basis of severity. In this paper, an approach to estimate the quantitative shunt resistance values in crystalline silicon photovoltaic (PV) modules by electroluminescence (EL) imaging has been presented. The quantitative properties of EL emission from the solar cell has been correlated with shunt resistive effects, which has led to the estimation of the voltage and current across the shunts, and hence the shunt resistance values. The approach has been experimentally supported by externally introducing shunt resistances of different values in a solar cell. The estimated shunt resistances have been found to be in a good agreement with the actual shunt values. This work can be helpful in detailed quantitative analysis of ohmic shunts, which can be further used to improve the performance and reliability of PV modules.

Potential-induced degradation: Recombination behavior, temperature coefficients and mismatch losses in crystalline silicon photovoltaic power plant

In recent years, potential-induced degradation (PID) has gained considerable attention due to the significant negative impact on output power of photovoltaic power plant. In this paper, the recombination behavior of PID-affected solar modules dismounted from the photovoltaic power plant was investigated by the two-diode model of photovoltaic module. The mechanism of PID shunting, which is caused by sodium decorated stacking faults across n+-p junction, was further verified by analyzing changes in the quasi-neutral region and the space-charge region for PID-affected p-type crystalline silicon solar modules. It was found that the depletion region recombination increases with the extent of PID. The electroluminescence images also revealed that there exist strong ohmic shunts for PID-affected modules. Then the fundamental temperature coefficients of PID-affected solar modules were investigated. It was found that the increased depletion region recombination leads to a larger temperature coefficient of maximum power for PID-affected solar modules. The result was also verified by the thermography images of PID-affected modules. Finally, the relative mismatch losses (RML) of PID-affected module strings were analyzed. The results revealed that the RML of PID-affected module strings increase as the root-mean-square error (RMSE) of current at maximum power point progressed for the first time.

Spatially Resolved Identification of Shunt Defects in Thin Film Solar Cells via Current Transport Efficiency Imaging Combined with 3D Finite Element Modeling

Inhomogeneously distributed shunt defects significantly reduce the performance of thin film solar cells. The existing uses of simple equivalent circuit models and lumped cell‐level parameters are insufficient to understand the behaviors of different shunting types. This study demonstrates how the reciprocity theorem, which bridges the relationship between the terminal differential changes and local responses of a solar cell, for the current transport efficiency can be used to spatially identify ohmic and nonohmic shunt defects exemplified in CdTe solar cells. Differential electroluminescence imaging and three‐dimensional finite element modeling are used to determine the current transport efficiency images. Due to the specific local differential conductance behaviors, the application of current transport efficiency imaging for the detailed analysis of different shunt defects is successfully verified in both experimental and simulation methods. In addition, the influences of two types of shunt defects on the electrical potentials and current flow of a cell are discussed. The modeling results indicate that a nonohmic shunt defect with a larger junction voltage dip and leakage current is more detrimental than an ohmic shunt defect within the cell.

Ultrathin CdTe solar cells with absorber layer thinner than 0.2 microns

In this study, ultrathin Cadmium telluride (CdTe) solar cells with absorber thickness from 50 to 200 nm were fabricated. The short-circuit current (JSC) and open-circuit voltage (VOC) were found to decrease significantly with the thickness of absorber layer decreasing. The decrease of the JSC was mainly because of the insufficient light absorption. Even so, the JSC was still found to be 8.2 mA/cm2, which was about 32% of that of a normal CdTe solar cell when the thickness of absorber layer was reduced to ∼1% of that of a normal CdS/CdTe solar cell, i.e. 50 nm. The reasons, which caused the decrease of VOC, were also discussed in this study. The dark current–voltage characteristics were analyzed and the contribution of ohmic shunting current to the total leakage current was found to increase with the thickness of CdTe absorber layer decreasing. The device characteristics of the ultrathin CdTe solar cells under weak light irradiance and at different temperatures were also investigated. This study provides a guideline for the fabrication of ultrathin CdTe solar cells in the future.

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.

Advanced Local Characterization of Silicon Solar Cells

Solar cells made from multicrystalline silicon material, which still represent the majority of solar cells produced today, are by nature inhomogeneous devices. Their bulk excess carrier lifetime, which decisively influences the short circuit current density, the saturation current density, and the effective bulk diffusion length, may vary from position to position by an order of magnitude or more. Moreover, such cells may contain ohmic shunts and, in particular in the edge regions, positions of significant depletion region recombination current. The classical local characterization method for solar cells is light beam‐induced current (LBIC) mapping. This method does not give direct information on factors influencing the dark current density. A reliable quantitative local characterization of inhomogeneous cells can be performed by combining dark lock‐in thermography (DLIT) with electro‐ and photoluminescence (EL and PL) imaging. In this contribution, the well‐established and the most recent findings on the application of these methods are reviewed and typical application examples are presented. It is shown that luminescence imaging is most appropriate for local series resistance imaging and for qualitatively imaging of all kinds of recombination‐active defects. DLIT, on the other hand, shows a lower spatial resolution, but is most appropriate for imaging local two‐diode parameters like depletion region‐ and bulk saturation current densities, the latter allowing to draw conclusions also to the local short circuit current density. A comprehensive local characterization of silicon solar cells is possible by applying both DLIT and luminescence imaging, which complement each other.

Ohmic shunts in two-terminal dual-junction solar cells with current mismatch

The impact of ohmic shunts on the current–voltage characteristics of single junction solar cells is well understood. Yet, for monolithic dual-junction solar cells, the effects of shunts have far less been investigated, especially if there is a current mismatch between the two sub cells. In this work, we investigate theoretically and experimentally how current–voltage characteristics of a monolithic GaAs/GaAs tandem solar cell depend on shunts in the top or the bottom sub cell, when either of this sub cells is current limiting. The open-circuit voltage of the device is observed to transition to the open-circuit voltage of the non-shunted sub cell as the shunting increases. In the same way, the fill factor is found to be more significantly affected when ohmic shunts occur in the current-limiting sub cell. Finally, as the current-limiting sub cell is shunted, the short-circuit current of the device is observed to transition to the short-circuit current of the non-current-limiting sub cell. These results allow identifying the shunted sub cell and enable characterization of ohmic shunts under current limiting conditions in dual-junction solar cells.

DLIT- versus ILIT-based efficiency imaging of solar cells

Efficiency imaging of solar cells means to know which region of an inhomogeneous cell contributes by which degree to the efficiency at maximum power point of the cell. This knowledge allows us to judge how strong certain defect regions influence the efficiency of the whole cell. Efficiency imaging can be performed based on dark lock-in thermography (DLIT) imaging within the model of independent diodes, or based on illuminated lock-in thermography (ILIT), which does not assume any cell model. Moreover, by 2-dimensional finite element simulation of the cell based on DLIT results, an efficiency image can be obtained, which takes into account the distributed nature of the series resistance. In this contribution these three methods are applied to one and the same multicrystalline cell containing ohmic shunts and the results are compared to each other. Conclusions to the accuracy of solely DLIT-based efficiency imaging are drawn.

Growth of carbon and nitrogen containing precipitates in crystalline solar silicon and their influence on solar cells

magnified imageThe growth mechanisms, structural, mechanical, and electrical properties of silicon carbide and silicon nitride precipitates in solar silicon are reviewed and some new aspects about the distribution of trace elements in these precipitates are reported in this review. SiC and Si3N4 precipitates may have detrimental impact on the quality of solar silicon material. Therefore intensive research has been done at these precipitates in the past, which is summarized first. The properties of the different types of precipitates reported in literature are then described in detail. Especially SiC precipitates may have detrimental impact on the electrical behavior of solar cells by causing severe ohmic shunting. Furthermore SiC precipitates are very hard and may harm the wafering process. An outlook of the impact of such precipitates on new cell concepts and new solar silicon materials, such as n‐type, is given.

Influence of local shunting on the electrical performance in industrial Silicon solar cells

The paper presents the results of investigation on the influence of an ohmic shunt located at various spatial positions on the solar cell performance by distributed diode model based simulations. By systematically varying the parameters such as shunt resistance, proximity to metallization, shunt area, and irradiance a deep insight about the shunt impact on the solar cell performance have been obtained. Further, effect of spatial positioning of shunts has been investigated by considering shunted region of same area and severity at various locations of the solar cell, via the simulation approach. The presented simulation approach has been experimentally validated. The influence of shunt on the relative power and relative open circuit voltage has been studied considering different irradiance levels. The study revealed new insights about significance of spatial positions of the shunts and the proximity of finger and busbar metallization. A dramatic improvement in the solar cell’s electrical performance can be gained by either preventing the occurrence of shunts at the identified critical locations or isolating them by laser or chemical technique or removing them.

Shading-induced failure in thin-film photovoltaic modules: Electrothermal simulation with nonuniformities

Finite element electrothermal modeling is employed to study shading-induced failure in monolithically integrated thin-film photovoltaic modules. A key element is spatial nonuniformity in current-voltage characteristics, which causes inhomogeneous current flow when part of a module is under reverse bias due to shading. Time-dependent calculations show that spots with lower reverse breakdown voltage experience greater current density and localized thermal runaway that can cause permanent damage, resulting in ohmic shunts. Such failure events lead to performance loss, especially when the module is in normal operating conditions and shunted currents lead to abnormally high module temperatures. Our simulations of temperature distributions, current-voltage characteristics, and electroluminescence are compared to data from the literature for copper indium gallium diselenide devices.

Current transport mechanisms in mercury cadmium telluride diode

This paper reports the results of modelling of the current-voltage characteristics (I-V) of a planar mid-wave Mercury Cadmium Telluride photodiode in a gate controlled diode experiment. It is reported that the diode exhibits nearly ideal I-V characteristics under the optimum surface potential leading to the minimal surface leakage current. Deviations from the optimum surface potential lead to non ideal I–V characteristics, indicating a strong relationship between the ideality factor of the diode with its surface leakage current. Diode's I–V characteristics have been modelled over a range of gate voltages from −9 V to −2 V. This range of gate voltages includes accumulation, flat band, and depletion and inversion conditions below the gate structure of the diode. It is shown that the I–V characteristics of the diode can be very well described by (i) thermal diffusion current, (ii) ohmic shunt current, (iii) photo-current due to background illumination, and (iv) excess current that grows by the process of avalanche multiplication in the gate voltage range from −3 V to −5 V that corresponds to the optimum surface potential. Outside the optimum gate voltage range, the origin of the excess current of the diode is associated with its high surface leakage currents. It is reported that the ohmic shunt current model applies to small surface leakage currents. The higher surface leakage currents exhibit a nonlinear shunt behaviour. It is also shown that the observed zero-bias dynamic resistance of the diode over the entire gate voltage range is the sum of ohmic shunt resistance and estimated zero-bias dynamic resistance of the diode from its thermal saturation current.

Local Solar Cell Efficiency Analysis Performed by Injection-dependent PL Imaging (ELBA) and Voltage-dependent Lock-in Thermography (Local I-V)

In this contribution two methods for performing local efficiency analysis of solar cells are compared with each other by applying them to a solar cell and a neighboring wafer. The first method called “ELBA” is based on injection-dependent photoluminescence (PL) imaging of a passivated wafer. The second method called “Local I-V” is based on voltage-dependent dark lock-in thermography (DLIT) on a solar cell. The results of both methods with respect to the influence of the bulk on the local solar cell parameters are comparable with each other. However, since only “Local I-V” is investigating a finished solar cell, it may image also local ohmic shunts, inhomogeneous front- and backside and depletion region recombination, and Rs effects, whereas “ELBA” is suited for assessing bulk-related efficiency losses in detail, which are not concealed by the aforementioned cell-related loss mechanisms in this method.