Localization and imaging of local shunts in solar cells using polymer‐dispersed liquid crystals

An easy‐to‐use technique for the localization and imaging of local shunts in solar cells is introduced. The method is based on temperature‐sensitive polymer‐dispersed cholesteric liquid crystal foils, covering the reverse‐biased solar cell. The unique optical properties of the cholesteric liquid crystal, known as selective reflection, render the local shunts of a solar cell visible as a color distribution in the foil, which is directly correlated with the spatial shunt distribution. The novel method is applied to several laboratory and commercial silicon solar cells and its high sensitivity is demonstrated. Copyright © 2001 John Wiley & Sons, Ltd.     

On the detection of shunts in silicon solar cells by photo‐ and electroluminescence imaging

Recently electroluminescence (EL) and photoluminescence (PL) imaging were reported to allow detection of strong ohmic shunts in silicon solar cells. Comparing lock‐in thermography (LIT) images with luminescence images of various shunted cells, measured under different conditions, the ability of luminescence techniques for shunt detection is investigated. Luminescence imaging allows identifying ohmic shunts only if they reach a certain strength. The detection limit for PL measurements of linear shunts was estimated to be in the order of 15 mA at 0·5 V bias for a point‐like shunt in multicrystalline (mc) cells. Pre‐breakdown sites can also be detected by electroluminescence under reverse bias. Copyright © 2007 John Wiley & Sons, Ltd.     

Quantitative evaluation of shunts in solar cells by lock‐in thermography

Infrared lock‐in thermography allows to image shunts very sensitively in all kinds of solar cells and also to measure dark currents flowing in certain regions of the cell quantitatively. After a summary of the physical basis of lock‐in thermography and its practical realization, four types of quantitative measurements are described: local I–V characteristics measured thermally up to a constant factor (LIVT); the quantitative measurement of the current through a local shunt; the evaluation of the influence of shunts on the efficiency of a cell as a function of the illumination intensity; and the mapping of the ideality factor n and the saturation current density J₀ over the whole cell. The investigation of a typical multicrystalline solar cell shows that the shunts are predominantly responsible for deterioration of the low‐light‐level performance of the cell, and that variations of the injection current density related to crystal defects are predominantly determined by variation of J₀ rather than of n. Copyright © 2003 John Wiley & Sons, Ltd.     

Calculation of quantitative shunt values using photoluminescence imaging

A proof of concept study for a method of determining quantitative shunt values in silicon solar cells from photoluminescence images is presented. The method is based on interpretation of the luminescence intensity around a local shunt or recombination‐active defect in terms of the extracted current. The theoretical relationship between the photoluminescence signal and the shunt current is derived. Experimental results on specifically prepared test structures show good agreement with known shunt resistance values. Experimental data on diffused wafers are presented. The effect of the front metallisation in complete cells on the appearance and interpretation of shunts in photoluminescence images is investigated experimentally. The limitations of the method are discussed. Copyright © 2012 John Wiley & Sons, Ltd.     

Degradation of individual cells in a module measured with differential IV analysis

A methodology is developed for the extraction of cell‐level properties from the analysis of differential IV response in a solar module with series connected cells. Through a combination of simulation and experimental verification we show that the shunt resistance and short circuit current of individual cells can be determined from a peak in the module differential resistance with cells that are partially shaded. The magnitude of the peak is equal to the shunt resistance of the cell for small values of shunt resistance. The current at which the peak occurs is proportional to the product of the short circuit current and the shading factor of the particular cell. With this methodology, we are able to measure degradation of 72 individual cells in a single commercial module after a high temperature/high humidity/high voltage stress test. Therefore, the statistics of degradation in this test were improved 72‐fold. Copyright © 2010 John Wiley & Sons, Ltd.     

Analyses and Simulation of V-I Characteristics for Solar Cells Based on P-N Junction

Through theoretical analyses of the Shockley equation and the difference between a practical P-N junction and its ideal model, the mathematical models of P-N junction and solar cells had been obtained. With Matlab software, the V-I characteristics of diodes and solar cells were simulated, and a computer simulation model of the solar cells based on P-N junction was also established. Based on the simulation model, the influences of solar cell＇s internal resistances on open-circuit voltage and short-circuit current under certain illumination were numerically analyzed and solved. The simulation results showed that the equivalent series resistance and shunt resistance could strongly affect the V-I characteristics of solar cell, but their influence styles were different.     

Accurate explicit equations for the fill factor of real solar cells—Applications to thin‐film solar cells

Even within the simplest real solar cell model, the exact value of the fill factor (FF) is only computable by numerical calculations. Here, we perform approximations to the power–voltage curve given by the one‐diode model with series and shunt resistance losses, obtaining explicit expressions for the voltage and current at the maximum power point, and thus an explicit approach for the FF. Over a broad range of possible solar cell parameters, including cells where the impact of shunt losses on the fill factor is not negligible, the approximate equations yield relative errors typically around 1%. The equations are applied to explore the dependence of FF on alternative buffer material thickness of organic solar cells, and to investigate the incidence of shunt and series resistance losses on the FF of Cu(In,Ga)Se₂ solar cells under indoor illumination conditions. Copyright © 2012 John Wiley & Sons, Ltd.     

Identifying parasitic current pathways in CIGS solar cells by modelling dark J–V response

The non‐uniform presence of shunting defects is a significant cause of poor reproducibility across large‐area solar cells, or from batch‐to‐batch for small area cells, but the most commonly used value for shunt parameterisation (the shunt resistance) fails to identify the cause for shunting. Here, the use of equivalent circuit models to describe dark current–voltage characteristics of ZnO:Al/i‐ZnO/CdS/CIGS/Mo devices in order to understand shunting behaviour is evaluated. Simple models, with a single shunt pathway, were tested but failed to fit experimental data, whereas a more sophisticated model developed here, which includes three shunting pathways, yielded excellent agreement throughout the temperature range of 183–323 K. The temperature dependence of fitting parameters is consistent with known physical models. Activation energies and contact barriers are determined from the model, and extracted diode factors are unique across the voltage range. A case study is presented whereby the model is used to diagnose poor reproducibility for CIGS devices (efficiency ~3–14% across a 100 cm² plate). It's shown that lower efficiencies correlated with greater prevalence of Ohmic and non‐Ohmic shunt currents, which may form due to pinholes in absorber and buffer layers respectively, whereas the quality of the main junction was constant for all cells (diode factor ~1.5–2). Electron microscopy confirmed the presence of ZnO:Al/i‐ZnO/Mo and ZnO:Al/CIGS/Mo regions, supporting the multi‐shunt pathway scheme disclosed by modelling. While the model is tested with CIGS cells here, this general model is a powerful diagnostic tool for process development for any type of thin‐film device. Copyright © 2015 John Wiley & Sons, Ltd.     

Photoluminescence and electroluminescence imaging of perovskite solar cells

Fast camera‐based luminescence‐imaging measurements on perovskite solar cells are presented. The fundamental correlation between the luminescence intensity and the open circuit voltage predicted by the generalised Planck law is confirmed, enabling various quantitative methods for the detection of efficiency‐limiting defects to be applied to this new cell structure. Interstinegly, it is found that this fundamental correlation is valid only for light‐soaked devices. Copyright © 2015 John Wiley & Sons, Ltd.     

Simulating single‐junction GaAs solar cells including photon recycling

To improve and accelerate further developments of III‐V solar cells an accurate and reliable modelling is necessary. In this work we present a comprehensive two‐ dimensional numerical model of single‐junction GaAs solar cells. Using this numerical model we achieved good correlation between measurements and simulations of single‐junction GaAs solar cells with different internal structures. We introduced the concept of optical coupling matrix in the numerical model to account for photon recycling effects. In addition, it is shown that thermionic currents have to be considered at the III‐V hetero‐interfaces. With this numerical model a powerful and flexible tool for solar cell simulation of III‐V compound semiconductor materials is now available. Copyright © 2006 John Wiley & Sons, Ltd.     

Contact fault characterisation of complex silicon solar cells: a guideline based on current voltage characteristics and luminescence imaging

Highest efficiency solar cells in industrial and R&D environments are increasingly sensitive to local performance limiting processing faults, which are best characterised by spatially resolved characterisation techniques. This work contains a discussion on the processing faults related to contact resistance and finger interruptions in interdigitated back contact silicon solar cells, which are prime example for a complex cell structure. Using experimental and simulated current–voltage measurements and luminescence images, we explore the strongly non‐linear effect of poor local contact resistances on the global series resistance, fill factor, short circuit current density and efficiency. A good agreement between global and spatially resolved characterisation of faults is found, and potential artefacts are discussed. In conclusion, we present seven cases of contacting faults in interdigitated back contact cells with distinct characteristics that can be identified using a flow chart of experiments. The resulting guideline should assist silicon solar cell manufacturers in localising and quantifying local contacting faults that reduce the cells efficiency in manufacturing of complex solar cells. Copyright © 2015 John Wiley & Sons, Ltd.     

Reduction of the apparent shunt resistance effect in silicon concentrator solar cells

Silicon solar cells of lowly doped base, under concentration, show a slope in the low voltage region. the slope varies with the irradiance and is similar in some aspects to that caused by a shunt resistance. This apparent shunt resistance reduces the fill factor and the efficiency. In consequence, such cells cannot operate effectively at concentrations above 20 suns. Actually, this apparent shunt resistance is due to the transition from low to high injection in the cell base. This is confirmed by an accurate simulation using the PC1D code. A semi-quantitative model of the mechanism is developed. It is shown that by thinning the cells the mechanism is strongly reduced. Thin cells are fabricated virtually free of low voltage slope at concentrations of 80 suns. Also, the existing short-circuit current sub-linearity is avoided in the thinned cells.     

Impact of light‐induced recombination centres on the current–voltage characteristic of czochralski silicon solar cells

We have investigated the effect of the light‐induced deep‐level recombination centre specific to boron‐doped, oxygen‐contaminated Czochralski (Cz) silicon on the current–voltage characteristic of Cz silicon solar cells by means of numerical simulation and experiment. The device simulation predicts the occurrence of a shoulder in the current–voltage curve after activating the characteristic recombination centre. The physical reason for the non‐ideal diode behaviour, characterised by a local ideality factor greater unity, is the strongly injection‐level‐dependent bulk lifetime produced by the deep‐level centre. The increased ideality factor causes a degradation in fill factor with the magnitude of degradation depending on the doping concentration of the Cz silicon base. In order to verify the theoretical predictions experimentally, we have performed measurements on high‐efficiency Cz silicon solar cells. Current–voltage curves recorded before and after light degradation clearly show the theoretically predicted change in shape and the reduction in fill factor. An excellent quantitative agreement between calculation and experiment is obtained for the subtracted current–voltage curves measured after and before illumination. Copyright © 2001 John Wiley & Sons, Ltd.     

Shunt types in crystalline silicon solar cells

Nine different types of shunt have been found in state‐of‐the‐art mono‐ and multicrystalline solar cells by lock‐in thermography and identified by SEM investigation (including EBIC), TEM and EDX. These shunts differ by the type of their I–V characteristics (linear or nonlinear) and by their physical origin. Six shunt types are process‐induced, and three are caused by grown‐in defects of the material. The most important process‐induced shunts are residues of the emitter at the edge of the cells, cracks, recombination sites at the cell edge, Schottky‐type shunts below grid lines, scratches, and aluminum particles at the surface. The material‐induced shunts are strong recombination sites at grown‐in defects (e.g., metal‐decorated small‐angle grain boundaries), grown‐in macroscopic Si₃N₄ inclusions, and inversion layers caused by microscopic SiC precipitates on grain boundaries crossing the wafer. Copyright © 2004 John Wiley & Sons, Ltd.     

On the Origin of Hysteresis in Perovskite Solar Cells

The origin of hysteresis behavior is probed in perovskite solar cells (PSCs) with simultaneous measurements of cell open circuit voltage (V_oc) and photoluminescence intensity over time following illumination of the cell. It is shown, for the first time, that the transient changes in terminal voltage and luminescent intensity do not follow the relationship that would be predicted by the generalized Plank radiation law. A mechanism is proposed based on the presence of a resistive barrier to majority carrier flow at the interface between the perovskite film and the electron or hole transport layer, in combination with significant interface recombination. This results in a decoupling of the internal quasi‐Fermi level separation and the externally measured voltage. A simple numerical model is used to provide in‐principle validation for the proposed mechanism and it is confirmed that mobile ionic species are a likely candidate for creating the time‐varying majority carrier bottleneck by its reduced conductivity. The findings show that the V_oc of PSCs may be lower than the limit imposed by the cell luminescence efficiency, even under steady‐state conditions.     

A new 2D model for the electrical potential in a cell stripe in thin‐film solar modules including local defects

The performance of solar modules is strongly influenced by the presence of local defects (shunts) in the module. Modeling cell stripes with local defects requires at least a 2D model. Most works on such 2D models are based on the numerical solution of the involved differential equations. These numerical models are quite computationally intensive and hence tedious for applications that require many evaluations of the model, for example, fitting experiments, computing accurate current/voltage characteristics, and finding a maximum power point. In this work, we present a fast 2D model for a cell stripe based on the superposition of several analytical expressions. This model uses a linearization of the solar cell current/voltage characteristics and takes the sheet resistance of one electrode into account (i.e., the other electrode is assumed to be a perfect conductor). With our model, the potential distribution in a cell stripe in the presence of local shunts can be computed in a matter of seconds. The model has been made freely available. Copyright © 2013 John Wiley & Sons, Ltd.     

Reciprocity between electroluminescence and quantum efficiency used for the characterization of silicon solar cells

Spectrally and spatially resolved electroluminescence emission of crystalline silicon solar cells is interpreted in terms of two electro‐optical reciprocity relations. The first relation links the photovoltaic quantum efficiency to the electroluminescence spectrum. Both methods contain information on recombination and the optical pathlength of the incident light, simultaneously. From the electroluminescence spectrum, we derive the pathlength enhancement factor of textured and untextured crystalline silicon solar cells. Further, we use local quantum efficiency measurements to quantitatively explain light induced current as well as panchromatic electroluminescence images. A second reciprocity relation connects open circuit voltage of a solar cell with the light emitting diode quantum efficiency of the same device. For a given quality of light trapping and a given open circuit voltage, we predict the attainable LED quantum efficiency and verify our results experimentally. Copyright © 2009 John Wiley & Sons, Ltd.     

The effect of grain boundary shunt currents on mis solar cell performance

The degradation of solar cells by grain boundaries can take any of three forms: recombination of minority carriers, forward current due to recombination in the space charge region of the junction or Schottky barrier, or forward currents due to shunting. There is no doubt that minority carrier recombination occurs and degrades the short circuit current. There seems little doubt that grain boundaries also degrade the solar cell open circuit voltage, but whether the degradation is due to recombination in the space charge region or due to shunting is not clear. To date most attention has been paid to space charge recombination. There is data, however, that shunt currents can flow, especially in doped regions of the grain boundaries with the conductivity along the grain boundary estimated from that data to be 10⁻¹² to 10⁻⁵ mhos/square. We will present an analysis assuming such grain boundary conductivities and show that the predictions of such a shunt model are in agreement with experiment. Specifically the shunt current is predicted to increase exponentially as qV/2kT where V is the forward bias, the shunt current significantly lowers the open circuit voltage, and it has negligible effect on the short circuit current.     

Analysis of the EWT‐DGB solar cell at low and medium concentration and comparison with a PESC architecture

Silicon represents an interesting material to fabricate low‐cost and relatively simple and high‐efficient solar cells in the low and medium concentration range. In this paper, we discuss a novel cell scheme conceived for concentrating photovoltaic, named emitter wrap through with deep grooved base (EWT‐DGB), and compare it with the simpler passivated emitter solar cell. Both cells have been fabricated by means of a complementary metal–oxide–semiconductor‐compatible process in our laboratory. The experimental characterization of both cells is reported in the range 1–200 suns in terms of conversion efficiency, open circuit voltage, short circuit current density and fill factor. In particular, for the EWT‐DGB solar cells, we obtain an encouraging 21.4% maximum conversion efficiency at 44 suns. By using a calibrated finite‐element numerical electro‐optical simulation tool, validated by a comparison with experimental data, we study the potentials of the two architectures for concentrated light conditions considering possible realistic improvements with respect to the fabricated devices. We compare the solar cell figures of merit with those of the state‐of‐the‐art silicon back‐contact back‐junction solar cell holding the conversion efficiency record for concentrator photovoltaic silicon. Simulation results predict a 24.8% efficiency at 50 suns for the EWT‐DGB cell and up to 23.9% at 100 suns for the passivated emitter solar cell, thus confirming the good potential of the proposed architectures for low to medium light concentration. Finally, simulations are exploited to provide additional analysis of the EWT‐DGB scheme under concentrated light. Copyright © 2017 John Wiley & Sons, Ltd.     

一种用红外热像监测硅太阳能光伏电池生产工艺的新方法

为了监测硅太阳电池生产工艺,设计了一套红外热像系统对单晶硅太阳电池的漏电情况进行研究。外加直流偏压时,太阳电池的漏电区域会明显发热。红外热像仪可以发现这些发热区域,从而确定漏电的位置。结合金相显微镜、扫描电子显微镜和能量色散谱等分析手段,总结了刻边、镀膜、丝网印刷和烧结工艺所造成的七种常见漏电形式。本文还提出了解决各类型漏电的可能方案,为进一步优化太阳电池生产工艺提供指导。