Defect Diagnosis And Formation Mechanisms Of GaAs Solar Cells Via Absolute Electroluminescence

As a typical representative of solar cells,Gallium Arsenide(GaAs)solar cells are widely used in aerospace,national defense,flexible wear,and future driving thanks to their benefits like high conversion efficiency,high light absorption coefficient,and strong anti-radiation performance.With the increasing complexity of solar cell materials,structures,and processes,defects become a critical challenge that hinders conversion efficiency and shortens the service life of GaAs solar cells.On the one hand,defects can introduce non-radiative recombination mechanisms into solar cells,inhibit the carrier collection ability of devices,and lead to the spatially non-uniformity of cell performance,which seriously affect the conversion efficiency of the solar cell.On the other hand,during the long-term deployment of solar cells,the absorber layer may be subject to harsh environmental factors,leading to changes in material properties and deterioration of defects.These phenomena can accelerate the performance degradation rate of solar cells and severely endanger device conversion efficiency and service life.Therefore,it is of great scientific value and application value to develop defect diagnosis techniques and conduct in-depth research on GaAs solar cell defects.Absolute electroluminescence(EL)spectroscopy and imaging techniques can quantitatively obtain the spatial distribution of solar cell radiative recombination intensity,and are widely used in solar cell performance diagnosis and defect visualization.However,the current researches on GaAs solar cell defects have problems such as the lack of reliable evaluation models for defect formation mechanisms and not considering the defect aging of solar cells under long-term storage.In terms of defect diagnosis technology,manual visual inspection methods have problems such as time-consuming and high randomness.Besides,current automatic defect detection methods have problems of low interpretability,resource-intensive,and data-sensitive,etc.,making it difficult to achieve in-depth diagnosis and impact evaluation for solar cell defects.Therefore,to address the deficiencies of existing methods,our work starts from the aspects of theoretical and technical researches.We utilize absolute EL to study the defect formation and defect aging mechanisms of GaAs solar cells.We further design automatic defect diagnosis schemes for GaAs solar cells.To be specific,the contributions of this paper are described as follows.1.This paper explores the mechanism of defect-induced current coupling in GaAs multijunction solar cells(MJSC).Through the self-built absolute EL measurement system,a unique defect-induced current coupling phenomenon is observed in GaAs MJSC.To reveal the defect-induced current coupling phenomenon,a carrier-balance model of MJSC including defects is first proposed to analyze the physical formation mechanism of the phenomenon,which considers the local carrier generation and loss processes,and combines the photoelectric coupling characteristics between subcells.The proposed model can quantitatively characterize the defect-induced absolute EL distribution of subcells,and reveal the physical origin of defects.Then,a 3-dimensional(3D)equivalent electrical model of the MJSC is established to quantitatively simulate the absolute EL characteristics around the defect,and the key electrical parameters are extracted to explore the electrical formation mechanism of the defect.Based on 3D numerical simulation technology,we further predict the absolute EL characteristics of the MJSC under defect-free conditions,and evaluate the impact of defects on the conversion efficiency of MJSC.Extensive experimental and simulation results conducted on an In Ga P/GaAs MJSC reveal that the defect-induced current coupling phenomenon origins from the increase of non-radiative recombination mechanisms and the generation of shunt paths in the absorber layer of GaAs bottom-cell.Affected by this defect,the conversion efficiency of the GaAs MJSC is predicted to be reduced by 0.408%.2.This paper studies the defect aging mechanism of GaAs solar cells under long-term storage and non-working conditions.We conduct 6 years of absolute EL tracking and monitoring on GaAs solar cells which have been stored in a dry box,and observe the slow defect aging phenomenon over time.Based on the monitoring results,we first analyze the aging phenomenon of absolute EL characteristics,spectral response,and defect characteristics for GaAs solar cells.Then,the 3D equivalent electrical models of GaAs solar cells at different aging times are established,and the formation mechanisms of aging defects are studied from the point of view of electrical origins.The proposed electrical models can quantitatively assess the effect of defects on absolute EL distributions,and reflect the defect aging process over time.Furthermore,an evaluation model for GaAs solar cell performance degradation is proposed to quantitatively evaluate the effect of the defect aging on essential photovoltaic parameters(such as the conversion efficiency)of solar cells.Extensive experimental and simulation results show that defects introduced during the cell fabrication are more likely to originate from the joint effect of decreasing local shunt resistance and increasing local series resistance.Besides,defects originating from increasing series resistance are more likely to gradually form and manifest in the EL image during long-term storage.Affected by the defect aging,the conversion efficiency of GaAs single junction solar cells decreased to 90% of the initial efficiency after 67 months of storage,and the conversion efficiency of GaAs MJSC decreased to 97.91% of the initial efficiency after26 months of storage.3.This paper proposes a set of automatic defect diagnosis schemes for GaAs solar cells.We first model the defect features in GaAs solar cell absolute EL images,and propose a threshold-selection-based automatic defect detection technique to identify the specific locations of defects in solar cells.Then,an electrical-model-based defect classification method is designed.By iteratively fitting the parasitic resistance parameters of the defect in the electrical model,the statistical analysis of the defect formation mechanism is achieved under the constraint of the deviation value.Furthermore,we describe the correlation between defect and neighborhood absolute EL intensity,and propose a datacorrelation-based absolute EL imaging recovery technique to simulate solar cell EL distributions under the defect-free condition.Finally,a performance prediction mechanism is further designed to evaluate the defect-induced performance degradation of GaAs solar cells.This paper conducts extensive experiments on multiple GaAs solar cell samples and discusses the current labor-intensive methods and automatic detection methods,verifying the effectiveness of the proposed automatic defect diagnosis schemes in terms of defect detection,classification,and impact evaluation.The theoretical research in this paper constitutes a comprehensive understanding of the GaAs solar cell defect formation mechanism and long-term defect aging mechanism from both physical and electrical origin.The automatic defect diagnosis schemes proposed in terms of technical research can better achieve automatic defect detection,classification,and impact evaluation for GaAs solar cells.The results of our work have guiding significance for material selection,structural design,and process improvement of efficient GaAs solar cells,and can help provide an effective reference for improving the conversion efficiency and service life of GaAs solar cells.