Chlorine activated stacking fault removal mechanism in thin film CdTe solar cells: the whole story

The efficiency of as-deposited CdTe solar cells is typically <5 %. An activation process involving the post-treatment of the CdTe surface with cadmium chloride at approximately 400 deg C improves the absorber microstructure by removing stacking faults, fills the grain boundaries with chlorine and leads to efficiencies of up to ~22 %. Stacking fault removal and improvement in device efficiency are thus correlated but the question of whether this correlation is direct or indirect has to date not been established. Although some types of stacking fault could be hole traps, those most commonly observed are tetrahedral in nature which recent work has shown to be electrically benign. For the first time, in this paper, we not only explain the passivation responsible for this efficiency increase but crucially elucidate the associated stacking fault removal mechanism. Experimental work shows that as chlorine is added to the system, the stacking faults gradually disappear from the absorber layer upwards to the surface with some grains being fault-free in the intermediate state before full saturation. At saturation chlorine decorates all the grain boundaries and so the effect of chlorine on a model system of a tetrahedral stacking fault bounded by two grain boundaries is investigated using density functional theory (DFT). We find that if the stacking faults between the grain boundaries were to be removed without Cl, this would result in an energy increase for the system. Increasing the chlorine concentration in the grain boundaries decreases the energy difference between the faulted and unfaulted system until a cross-over occurs close to the point at which chlorine saturates the grain boundaries. The atomic mechanisms and energy profile for the stacking fault removal is presented. The removal process is via a cascade effect whereby the system energy gradually increases as the fault is sequentially removed until the layers snap into place. The energy barrier for this to occur is easily overcome with the 400 deg C temperature treatment. Density of States (DOS) plots of the chlorine saturated structures show that defect passivation occurs in the highly reconstructed grain boundaries due to both interstitial and substitutional chorine. Chlorine saturation is shown to disconnect the two sides of the grain boundary avoiding electronic defects which arise from the interaction of dangling bonds across this region. It is thus concluded that the cell efficiency increase observed is due to the electronic effects of chlorine in the grain boundaries and that the observed stacking fault removal is a bi-product of the chlorine grain boundary saturation.

Fault Prediction Modelling in Open Source Software Under Imperfect Debugging and Change-Point

Instant demand of products and services by technologically active users has increased the demand for open source software (OSS)-based applications. Unfortunately, with the complexity and lack of understanding of OSS-based systems, it becomes difficult for a testing team to remove the faults and the fault removal rate becomes low in comparison to what it should be. This also results in generating new faults during removal. Also, the rate at which the testing team detects/corrects fault need not be same during the entire process of testing due to various reasons viz. change in testing strategy, understanding of code, change in resources, etc. In the existing literature on OSS, authors have developed many models considering the above aspects separately. In this article, all of the above aspects have been combined to develop a general framework for predicting the number of faults in OSS. The comparison of eight models on the basis of their prediction capability on two well-known Open Source Software datasets is created and then ranked using normalized criteria distance approach.

Zero-Sequence Differential Current Protection Scheme for Converter Transformer Based on Waveform Correlation Analysis

Through the analysis of the recovery inrush current generated by the external fault removal of the converter transformer, it is pointed out that the zero-sequence current caused by the recovery inrush may result in the saturation of the neutral current transformer (CT), whose measurement distortion contributes to the mis-operation of zero-sequence differential current protection. In this paper, a new scheme of zero-sequence differential current protection based on waveform correlation is proposed. By analyzing the characteristics of zero-sequence current under internal fault, external fault and external fault removal, the waveform correlation of the zero-sequence current measured at the terminal of the transformer and the zero-sequence current measured at the neutral point of the transformer is used for identification. The polarity of the CT is selected to guarantee the zero-sequence currents at the terminal and neutral point of the transformer exhibit a "ride through" characteristic under external fault, then the waveform similarity is high, and the correlation coefficient is positive. On the other hand, when internal fault occurs, zero-sequence current waveforms on both sides differ from each other largely, and the correlation coefficient is negative. Through a large number of simulations verified by PSCAD/EMTDC, this criterion can accurately identify internal and external faults, exempt from effects of the recovery inrush. Moreover, it presents certain ability for CT anti-saturation.

Testing Resource based Optimal Release Policy for Software System Incorporating Fault Reduction Factor and Change point

In today’s competitive scenario, it is essential for software developers to perform rigorous testing. It helps them to satisfy the demand for reliable software systems. Various external and internal factors like human expertise, fault dependency, code complexity, dynamic approaches etc. affect the process of fault removal. Hence, along the time-line fault removal rate may change. The point on time-line beyond which rates are altered is termed as the change point. Also in many practical situations, the number of failures experienced may not coincide with the number of faults removed from the system. This ratio is computed by Fault Reduction Factor (FRF). Here, we have proposed testing effort based model considering effort-dependent FRF with and without change point for gauging the failure pattern of a software system. The FRF has been modelled by logistic curve. The developed models have been verified using real-life software fault datasets. Model parameters are estimated and various performance criteria are employed to check the goodness of fit. Later, we have developed a software cost model to determine the optimal Release testing effort that minimizes total expected cost of fault removal during testing phase and operational phase of software life cycle subject to a reliability constraint. A numerical study has been also taken to demonstrate the results. Cost sensitivity analysis has been carried out to identify the crucial cost component and role of each cost component on optimal testing effort and overall cost of development.

Modeling and analysis of software fault detectability and removability with time variant fault exposure ratio, fault removal efficiency, and change point

Software reliability growth models have been proposed to assess and predict the reliability growth of software, remaining number of faults, and failure rate. In previous studies, software faults have been mainly categorized into two categories based on its severity in removal process: simple faults and hard faults. In reality, fault detectability is one of the crucial factors which can influence the reliability growth of software. The detectability of a software fault depends on how frequently the instructions containing faults are executed. However, fault removability of a software fault depends on fault removal efficiency of debugging team. The main motive of this article is to incorporate the fault detectability in software reliability assessment. Fault exposure ratio is an essential factor for software reliability modeling that controls the per-fault hazard rate. It is strongly dependent on fault detectability. In this article, the effect of fault detectability, fault removability, fault exposure ratio, and fault removal efficiency has been considered simultaneously in software reliability growth modeling. Moreover, a logistic fault exposure ratio has been introduced. The effect of change point is incorporated in the proposed software reliability growth model. Two illustrative examples with software testing data have been presented.