Defects In Multicrystalline Silicon Research Articles

Direct observation of hydrogen at defects in multicrystalline silicon

AbstractHydrogen passivation is a key industrial technique used to reduce the recombination activity of defects in multicrystalline silicon (mc‐Si). However, not all dislocations and grain boundaries respond well to traditional hydrogen passivation techniques. In order to understand the reasons for these different behaviours, and how superior passivation might be achieved, a method is required for the direct observation of hydrogen at these defects. Here, we present a novel characterisation technique based on a combination of transmission Kikuchi diffraction (TKD), atom probe tomography (APT), and isotopic substitution that enables unambiguous detection and quantification of hydrogen atoms present at crystallographic defects in mc‐Si.

Learning Quality Rating of As-Cut mc-Si Wafers via Convolutional Regression Networks

This paper investigates deep convolutional neural networks (CNNs) for the assessment of defects in multicrystalline silicon (mc-Si) and high-performance mc-Si wafers for solar cell production based on photoluminescence (PL) images. We identify and train a CNN regression model to forecast the I-V parameters of passivated emitter and rear cells from given PL images of the as-cut wafers. The presented end-to-end model directly processes the PL image and does not rely on the human-designed image feature. Domain knowledge is replaced by a model based on a huge variety of empirical data. The comprehensive dataset allows for the evaluation of the generalizability of the model with test wafers from bricks and manufacturers not presented in the training set. We achieve mean absolute prediction errors as low as 0.11%abs in efficiency for test wafers from “unknown” bricks, which improves handcrafted feature-based methods by 35%rel at simultaneously lower computational costs for prediction. Samples with high prediction errors are investigated in detail showing an increased iron point defect concentration.

Identification of colloidal silica polishing induced contamination in silicon

This paper presents a multiscale characterisation approach, analysing the effect of colloidal silica polishing on crystallographic defects in multicrystalline silicon. Colloidal silica polishing for as little time as 30 min was found to significantly increase the recombination activity of all defects, as measured by electron beam induced current mapping. The impurities responsible for the room temperature contamination of defects due to colloidal silica polishing were Cu and Ni, as measured by atom probe tomography.

Dependence of the Bulk Electrical Properties of Multisilicon on the Grain Misorientation Parameters

The recombination activity of intragrain defects in multicrystalline silicon is investigated by the electron or laser beam induced current methods. The interrelation of the grain orientation with the character of the distribution of intragrain defects (dislocations and impurity inclusions) and their recombination activity is revealed. The defect grain structure is investigated using various etching procedures to reveal the defects. It is shown that the defect density and distribution in the grains depend on their orientation relative the growth axis. Therefore, it is intragrain defects and impurities that are to a large degree responsible for degradation of the nonequilibrium carrier lifetime when compared with grain boundaries.

Зависимость объемных электрофизических свойств мультикремния от параметров разориентации зерен

AbstractThe recombination activity of intragrain defects in multicrystalline silicon is investigated by the electron or laser beam induced current methods. The interrelation of the grain orientation with the character of the distribution of intragrain defects (dislocations and impurity inclusions) and their recombination activity is revealed. The defect grain structure is investigated using various etching procedures to reveal the defects. It is shown that the defect density and distribution in the grains depend on their orientation relative the growth axis. Therefore, it is intragrain defects and impurities that are to a large degree responsible for degradation of the nonequilibrium carrier lifetime when compared with grain boundaries.

Electrical Activity of Extended Defects in Multicrystalline Silicon

The excess carrier lifetime (τ) distribution in multicrystalline silicon grown by the Bridgman technique from high-purity metallurgical silicon (HPMG-Si) is studied. The features of the variation in τ, caused by the grain-boundary structure of ingots, are revealed. The grain boundaries, dislocations, and impurity microinclusions are studied by electron probe microanalysis (EPMA) and scanning electron microscopy (SEM) using selective acid etching. The electrical activity of extended defects is measured by the electronbeam- induced-current (EBIC) method.

Impact of annealing on the formation and mitigation of carrier-induced defects in multi-crystalline silicon

Carrier-induced degradation (CID) of multi-crystalline silicon (mc-Si) wafers is a major problem currently affecting the photovoltaic industry. A large number of studies investigating this phenomenon have provided important clues regarding the identification of the defect, however, as of yet, none have isolated a specific cause. In this work, we provide further insight into the kinetics of CID in mc-Si by presenting a detailed study of the impact of dark annealing on the formation and subsequent mitigation of the carrier-induced defect. Previous work has shown that such anneals can modulate the kinetics of the defect. Here, we extend that work and demonstrate that dark annealing can result in accelerated defect formation and extended degradation throughout a subsequent light soaking cycle, irrespective to when the dark annealing was applied. It is suggested that dark annealing could release extra defect precursors into mc-Si, which then become recombination active upon illumination. Therefore, the subsequent degradation after dark annealing might not necessary involve a reverse reaction. Through multiple dark anneal and light soak cycles, the extent of degradation in each cycle continues to reduce. A direct reverse reaction (de-stabilisation) alone does not explain this observation. We suggest that this effect could be explained by the presence of a reservoir of defect precursors, which are gradually depleted throughout the dark annealing processes. Finally, it is demonstrated that dark annealing alone could potentially cause a similar degradation to illumination at elevated temperature.

Recombination parameters of lifetime-limiting carrier-induced defects in multicrystalline silicon for solar cells

In p-type multicrystalline silicon solar cells, carrier-induced degradation (CID) can cause up to 10% relative reduction in conversion efficiency. Although, a great concern has been drawn on this degradation in the photovoltaic community, the nature of this degradation is still yet unknown. In this contribution, the recombination parameters of the responsible defect causing this degradation are extracted via temperature and injection dependent lifetime spectroscopy. Three wafers from three different ingots were processed into cell precursor and lifetime structures for the study. Similar defect recombination parameters were obtained for all samples. Two candidates for the defect energy level were identified: Et − Ei = −(0.32 ± 0.05) eV or Et − Ei = (0.21 ± 0.05) eV in the lower and upper bandgap halves, respectively. The capture cross section ratios were found to be k = 56 ± 23 or k = 49 ± 21 for the lower and upper bandgap halves, respectively. Contrary to previous studies, these parameters have been extracted for the responsible defect of CID, without making assumptions regarding the defect energy level. The result allows to model and to predict the impact of this defect on the solar cell efficiency.

Investigation of dislocation cluster evolution during directional solidification of multicrystalline silicon

Dislocation clusters are the main crystal defects in multicrystalline silicon and are detrimental for solar cell efficiency. They were formed during the silicon ingot casting due to the relaxation of strain energy. The evolution of the dislocation clusters was studied by means of automated analysing tools of the standard wafer and cell production giving information about the cluster development as a function of the ingot height. Due to the observation of the whole wafer surface the point of view is of macroscopic nature. It was found that the dislocations tend to build clusters of high density which usually expand in diameter as a function of ingot height. According to their structure the dislocation clusters can be divided into light and dense clusters. The appearance of both types shows a clear dependence on the orientation of the grain growth direction. Additionally, a process of annihilation of dislocation clusters during the crystallization has been observed. To complement the macroscopic description, the dislocation clusters were also investigates by TEM. It is shown that the dislocations within the subgrain boundaries are closely arranged. Distances of 40–30nm were found. These results lead to the conclusion that the dislocation density within the cluster structure is impossible to quantify by means of etch pit counting.

Effect of etching time on morphological, optical and structural properties of silicon nanowire arrays etched on multi-crystalline silicon wafer

In this work we report a rapid, cost effective and easy approach to prepare an antireflective surface by using silver assisted chemical etching method to form silicon nanowire arrays on multicrystalline silicon (mc-Si) wafer. We present a correlation between morphological, optical and structural properties of the obtained nanostructured mc-Si. We found that the length of silicon nanowires increases with increasing etching time. Beyond 30 min the wires are broken. This behavior is due to the existence of defects in multicrystalline silicon which facilitate the etching of the nanowires. AFM and SEM show the change of the morphology of the samples before and after the chemical etching. We investigate that the reflectivity of the nanowires etched for 30 min decreases to less than 3% which is beneficial for solar light trapping. Furthermore, we explore the role of etching time in the enhancement of Raman signal. We deduce that etching time plays a key role on the morphological, optical and structural properties of silicon nanowires.

Three-dimensional minority-carrier collection channels at shunt locations in silicon solar cells

In this contribution, we demonstrate the value of using a multiscale multi-technique characterization approach to study the performance-limiting defects in multi-crystalline silicon (mc-Si) photovoltaic devices. The combination of dark lock-in thermography (DLIT) imaging, electron beam induced current imaging, and both transmission and scanning transmission electron microscopy (TEM/STEM) on the same location revealed the nanoscale origin of the optoelectronic properties of shunts visible at the device scale. Our site-specific correlative approach identified the shunt behavior to be a result of three-dimensional inversion channels around structural defects decorated with oxide precipitates. These inversion channels facilitate enhanced minority-carrier transport that results in the increased heating observed through DLIT imaging. The definitive connection between the nanoscale structure and chemistry of the type of shunt investigated here allows photovoltaic device manufacturers to immediately address the oxygen content of their mc-Si absorber material when such features are present, instead of engaging in costly characterization.

Interaction of Iron with Extended Defects in Multicrystalline Silicon Studied by Local Gettering

In order to adjust temperature treatments during solar cell processing to multicrystalline silicon, the interactions of metal impurities in multicrystalline silicon with extended defects like grain boundaries have to be understood. For this purpose, we investigate neighboring wafers of block-cast multicrystalline silicon intentionally doped with iron during crystal growth. Samples are investigated with LBIC after aluminum gettering (AlG) experiments with an Al layer only on parts of the sample at different temperatures. LBIC data are quantitatively analyzed using two- dimensional gettering simulations and pattern recognition techniques combined with multivariate statistics. LBIC- images and simulations show pronounced regions of reduced excess carrier recombination around grain boundaries (denuded zone) in uncovered regions. This can be attributed to impurity accumulation at grain boundaries and their related depletion in the adjacent bulk. In covered regions, no denuded zones are observed. Temperature variation provides evidence that our experiments are mainly in the regime where gettering efficiency is limited by impurity precipitate dissolution, unless the temperature is chosen well above the solubility temperature corresponding to the average iron concentration.

Transmission electron microscopy analysis of extended defects in multicrystalline silicon using in‐situ EBIC/FIB sample preparation

AbstractThis paper reports results of microstructural investigations of block cast multicrystalline silicon materials deliberately contaminated with iron and copper impurities from the melt. Electron microscopy techniques have been used to study interactions of grain boundaries, light element impurities such as nitrogen and oxygen, and metal impurities. The key to get access to extended defects typically present in a small density is the recently developed in‐situ EBIC/FIB approach to sample preparation.Special attention is drawn to oxygen and nitrogen containing precipitates at grain boundaries and resulting secondary defects. Strong accumulation of copper impurities is observed which is related to reduced excess carrier lifetimes in these regions. It is shown that copper silicide precipitation mainly accounts for local dislocation networks with a high recombination activity. These observations provide evidence that decoration of grain boundaries with light element contaminants leads to efficient nucleation of metal impurity precipitates. (© 2013 WILEY‐VCH Verlag GmbH & Co. KGaA, Weinheim)

Effect of contamination with iron on the electron-beam-induced current contrast of extended defects in multicrystalline silicon

The effect of contamination with iron on the recombination activity of extended defects in multicrystalline silicon has been studied by the electron-beam-induced current (EBIC) technique. It has been shown that this process does not lead to the appearance of EBIC contrast of the Σ3 and Σ9 grain boundaries. It has been revealed that iron diffusion results in a significant increase in the contrast of dislocations introduced by plastic deformation and of traces behind the dislocations in single-crystal silicon, while the dislocation contrast in multicrystalline silicon remains practically unchanged.

Structural Study of Small Angle Grain Boundaries in Multicrystalline Si

Small angle grain boundaries (SA-GBs) are known as the most electrically active defects in multicrystalline silicon. These SA-GBs are classified in “general” and “special” by the normal and strong electrical activity at 300K, respectively. In this study, the origins of these electrical activities of SA-GBs were elucidated by using electron beam induced current (EBIC) and transmission electron microscopy (TEM). It was found that both general and special SA-GBs were composed of edge-type and 60 deg / screw dislocations. The fraction of edge dislocation in special SA-GB was higher than that of general one, which suggests that strong electrical activity is mainly originated in edge dislocations.

Properties of a-Si:N:H films beneficial for silicon solar cells applications

AbstractAmorphous silicon-nitride thin films a-Si:N:H were obtained by plasma enhanced chemical vapour deposition (PECVD) method from SiH4+NH3 at 13.56 MHz. The process parameters were chosen to obtain the films of properties suitable for optoelectronic and mechanical applications. FTIR analysis of a-Si:N:H films indicated the presence of numerous hydrogen bonds (Si-H and N-H) which passivate structural defects in multicrystalline silicon and react with impurities. The morpho-logical investigations show that the films are homogeneous. The deposition of a-Si:N:H layers leads to the decrease in friction coefficient of used substrates. Optical properties were optimised to obtain the films of low effective reflectivity, large energy gap Eg from 2.4 to 2.9 eV and refractive index in the range of 1.9 to 2.2. Reduction of friction coefficient for monocrystalline silicon after covering with a-Si:N:H films was observed: from 0.25 to 0.18 for 500 cycles.

Scanning X-Ray Excited Optical Luminescence Microscopy as a New Tool for the Analysis of Recombination Active Defects in Multi-Crystalline Silicon

The results of investigations of solar grade mc-Si by means of combination of scanning X-ray beam excited optical luminescence microscopy (SXEOL), X-ray beam induced current (XBIC) and X-ray fluorescence (XRF) are presented. It was found, that for relatively clean sample SXEOL and XBIC provide similar information about the recombination activity of defects while for the samples with a high transition metal content there are significant differences in the provided information. The reasons of the revealed XBIC - SXEOL differences are discussed.

Transmission Electron Microscopy Investigations of Metal-Impurity-Related Defects in Crystalline Silicon

This contribution summarizes recent efforts to apply transmission electron microscopy (TEM) techniques to recombination-active extended defects present in a low density. In order to locate individual defects, electron beam induced current (EBIC) is applied in situ in a focused ion beam (FIB) machine combined with a scanning electron microscope. Using this approach defect densities down to about 10cm-2 are accessible while a target accuracy of better than 50nm is achieved. First applications described here include metal impurity related defects in multicrystalline silicon, recombination and charge collection at NiSi2 platelets, internal gettering of copper by NiSi2 precipitates and site-determination of copper atoms in NiSi2.

Micro-spectroscopy on silicon wafers and solar cells

Micro-Raman (μRS) and micro-photoluminescence spectroscopy (μPLS) are demonstrated as valuable characterization techniques for fundamental research on silicon as well as for technological issues in the photovoltaic production. We measure the quantitative carrier recombination lifetime and the doping density with submicron resolution by μPLS and μRS. μPLS utilizes the carrier diffusion from a point excitation source and μRS the hole density-dependent Fano resonances of the first order Raman peak. This is demonstrated on micro defects in multicrystalline silicon. In comparison with the stress measurement by μRS, these measurements reveal the influence of stress on the recombination activity of metal precipitates. This can be attributed to the strong stress dependence of the carrier mobility (piezoresistance) of silicon. With the aim of evaluating technological process steps, Fano resonances in μRS measurements are analyzed for the determination of the doping density and the carrier lifetime in selective emitters, laser fired doping structures, and back surface fields, while μPLS can show the micron-sized damage induced by the respective processes.

Infrared birefringence imaging of residual stress and bulk defects in multicrystalline silicon

This manuscript concerns the application of infrared birefringence imaging (IBI) to quantify macroscopic and microscopic internal stresses in multicrystalline silicon (mc-Si) solar cell materials. We review progress to date, and advance four closely related topics. (1) We present a method to decouple macroscopic thermally-induced residual stresses and microscopic bulk defect related stresses. In contrast to previous reports, thermally-induced residual stresses in wafer-sized samples are generally found to be less than 5 MPa, while defect-related stresses can be several times larger. (2) We describe the unique IR birefringence signatures, including stress magnitudes and directions, of common microdefects in mc-Si solar cell materials including: β-SiC and β-Si3N4 microdefects, twin bands, nontwin grain boundaries, and dislocation bands. In certain defects, local stresses up to 40 MPa can be present. (3) We relate observed stresses to other topics of interest in solar cell manufacturing, including transition metal precipitation, wafer mechanical strength, and minority carrier lifetime. (4) We discuss the potential of IBI as a quality-control technique in industrial solar cell manufacturing.