Multicrystalline Solar Cells Research Articles

Efficiency improvement of B/Ga co-doped p-type multicrystalline silicon passivated emitter and rear cells induced by B/Al co-doping

In the present work, electrical performance of B/Ga co-doped p-type multicrystalline silicon passivated emitter and rear cell (PERC) was improved by B/Al co-doping. By combining the Al-Si alloying process with the relative high solid solubility of B in Si, high doping concentration in B/Al BSF was realized with the maximum doping concentration of 1.86E19 cm−3, which was almost one magnitude higher than Al doping. Benefiting from the relative high doping concentration, the saturation current density expelled the influence of emitter decreased from 135.1 fA cm−2 to 121.1 fA cm−2. The suppression of saturation current together with the J01 both indicated the electrical improvement of PERC solar cell, with the open-circuit voltage (VOC) enlargement of 1.5 mV and efficiency improvement of 0.02%. After body defect passivation, the electrical parameters of all PERC solar cells increased slightly, while the superiority of the B/Al paste aggravated with the efficiency enlargement of 0.05%. All these efforts will be helpful to deepen the understanding of B/Ga co-doped p-type multicrystalline PERC solar cell as well as B/Al co-doping.

Limiting Defects in n‐Type Multicrystalline Silicon Solar Cells

Multicrystalline (mc) solar cells made from n‐type silicon feedstock have shown record efficiencies of 22.3% in a tunnel‐oxide passivating contact (TOPCon) cell structure. Still, material‐related carrier recombination limits the attainable efficiency. Herein, the findings of metallic impurity and structural defect concentration present in n‐type mc silicon are summarized, and their limiting properties on carrier lifetime and cell performance are elaborated. Applying a dedicated model for carrier recombination at precipitate–silicon interfaces, it is demonstrated that carrier recombination at metallic precipitates may dominate in the analyzed material. Direct evidence of the recombination activity of iron precipitates in n‐type silicon is given by an analysis of intentionally grown iron precipitates: From a comparison of micro X‐ray fluorescence (μXRF) analyses of differently sized iron precipitates and local carrier recombination from micro‐photoluminescence (μPL), a direct correlation with enhanced carrier recombination is found. From these results, it is concluded that the high‐performance n‐type mc silicon is limited by recombination at (decorated) structural defects, namely, dislocation clusters and grain boundaries, whereas defects in the inner grains are not limiting the efficiency potential. Finally, cell degradation under illumination and elevated temperature for n‐type mc‐silicon solar cells are discussed.

The influence of emitter resistance on the electrical parameters of mono- and multicrystalline silicon solar cells

Purpose The purpose of this study was the comparison and analysis of the electrical parameters of two kinds of silicon solar cells (mono- and multicrystalline) of different emitter resistance. Design/methodology/approach By controlling of diffusion parameters, silicon mono- (Cz-Si) and multicrystalline (mc-Si) solar cells with different emitter resistance values were produced – 22 and 48 Ω/□. On the basis of current-voltage measurements of cells and contact resistance mapping, the properties of final solar cells based on two different materials were compared. Additionally, the influence of temperature on PV cells efficiency and open circuit voltage (Uoc) were investigated. The PC1D simulation was useful to determine spectral dependence of external quantum efficiency of solar cells with different emitter resistance. The silicon solar cells of 25 cm2 area and 240 µm thickness were investigated. Findings Considering the all stages of cell technology, the best structure is silicon solar cell with sheet resistance (Rsheet) of 45-48 Ω/□. Producing of an emitter with this resistance allowed to obtain cells with a fill factor between 0.725 and 0.758, Uoc between 585 and 612 mV, short circuit current (Isc) between 724 and 820 mA. Originality/value Measurements and analysis confirmed that mono- and multicrystalline silicon solar cells with 48 Ω/□ emitter resistance have better parameters than cells with Rsheet of 22 Ω/□. The contact resistance is the highest for mc-Si with Rsheet of 48 Ω/□ and reaches the value 3.8 Ωcm.

Application of Silicon Nanostructure Arrays for 6-inch Mono and Multi-Crystalline Solar Cell

In this study, we fabricate uniform silicon nanowire (SiNW) arrays on 6-inch mono- and multi-crystalline wafers by employing the improved solution-processed metal-assisted chemical etching (MacEtch) method. Furthermore, the improved MacEtch can be applied to various crystalline orientation wafers. The SiNW arrays are 470 nm in length with high density; they demonstrate a good optical trapping effect and reflectance well below 6% over a broad wavelength range from 300 to 1100 nm. The improved MacEtch shows no difference in reflectance for a pyramid/SiNW mono-crystalline wafer with appropriate uniformity; the average delta from the center to other positions is within 22%. The effective lifetime is lower for SiNW arrays because the higher surface state causes higher surface recombination.Finally, we make the multi-crystalline wafer into an Al-BSF solar cell device with MacEtch SiNW texture, resulting in an averaged power conversion efficiency of 17.83%, which is higher than that of standard acid-textured solar cell devices. Consequently, the improved MacEtch concept is suitable for commercial mass production in the photovoltaic industry.

Enhanced Crack Segmentation (eCS): A Reference Algorithm for Segmenting Cracks in Multicrystalline Silicon Solar Cells

The annually produced quantity of solar modules has steadily increased over the past decades. Rising production speeds and the associated high throughput of wafers, cells, and modules will make an automatized quality inspection mandatory. In the case of visual optical inspection, automatized quality control by using machine vision is already possible. To localize cracks in solar cells, luminescence imaging is used, where several approaches for an automatized inspection exist, but a standard solution for an automatized inspection algorithm is not yet available. This is, in particular, true for multicrystalline solar cells, where the grainy structures in the luminescence images are hard to distinguish from small cracks. Another obstacle in automatic crack analysis is that reference segmentation algorithms are generally not publicly available. Accordingly, a new algorithm can hardly be compared by ranking it to an existing standard. In this paper, we adapted the vesselness algorithm for automatic processing of electroluminescence images of multicrystalline silicon solar cells. Segmentation of cracks in multicrystalline solar cells with the proposed enhanced crack segmentation algorithm shows very promising results on the used database compared with three different commonly used approaches. Furthermore, the segmentation code is made publicly available, and we propose that this algorithm may serve as a reference algorithm, sparking further progress in automatized crack segmentation for multicrystalline silicon solar cells.

OPTIMIZATION OF POROUS SILICON CHARACTERISTICS FOR SOLAR BATTERIES

The aim is to form the anti-reflective layers of porous silicon on the solar cell surface in the presence of the current-collecting contact comb at the effective area, as well as the study of the state of front contact system of solar cells after etching on their main characteristics.The analysis of the influence of electrochemical etching on the state of the solar cell front contact system without antireflection coating was carried out on the basis of the dependences of the reflectance spectrum of porous silicon on its thickness and refractive index. In the range of wavelengths of 550-850 nm, a minimal reflection from the porous layer surface for the thickness of porous silicon from 70 to 100 nm was found, and its refractive index ranges from 1.35 to 1.9.The critical time of staying of the solar cell structure in the electrolyte, the excess of which leads to the damage of contact fingers of the front current-collecting contact comb was found. For anodization in an electrolyte C2 H5 OH:HF=1:1, the critical time was 45 s. The reduction of the anodizing time to 10 s at anodic charge density of 0.46 C/cm2 allowed to obtain porous silicon layers without significant damage to the current-collecting comb. By reducing the duration of electrochemical treatment to 3-6 s and increasing the anode current, it was possible to completely prevent the damage of the contact system.The results of study of the main solar cell parameters before and after formation of anti-reflective layer on the basis of porous silicon showed that for mono- and multicrystalline solar cells the increase in photocurrent exceeded 50%, the efficiency increased by 25% and 22% respectively for mono- and multicrystalline samples. At the same time, for all tested samples, the open-circuit voltage decreased by 2.5%, and the fill factor degraded by 10.7% and 18.8% for mono- and multicrystalline solar cells, respectively.

Geometrical Morphology of Inkjet-Printed Finger Electrodes on Untreated Multi-Crystalline Silicon Solar Cells

The geometrical morphologies fabricated by continuous and interlacing printing modes on untreated multi-crystalline solar cells are quantitively explored for uniform and high aspect-ratio finger electrodes. The voltage waveform of printhead is well modulated by in-house developed inkjet prototype printer for optimizing the droplet volume and velocity. As the fingers are fabricated with continuous printing mode by varying printing parameters, the bulging and coffee ring morphologies could not be eliminated resulting from the rough and anisotropic features. The interlacing printing mode is first introduced for finger electrodes fabrication. Results show that the width of the printed finger is not influenced by the printing layers and maintains the stable values of ∼60 μ m with pre-heating temperature 80 °C. Moreover, the interlacing printing mode has a high tolerance of droplet spacing variance (30–45 μ m) and suppresses the coffee ring morphology which obtains uniform finger electrodes. The study not only offers good guidance of finger electrodes fabrication for multi-crystalline solar cell manufacturing, but also proposes in-depth insights for the three-dimensional circuit structures fabrication of the emerging printed electronics.

Accurate and robust crack detection using steerable evidence filtering in electroluminescence images of solar cells

Solar cells are the key part of photovoltaic power system, and the online quality inspection of solar cells ensures conversion efficiency and usable lifetime of photovoltaic modules. However, automatic crack detection in EL images of solar cells has been a challenging task, owing to the heterogeneously textured background, low contrast between crack and surrounding background including randomly distributed crystal grains, the diversity of crack types, and so on. To address these challenges, this paper presents a new accurate and robust crack detection scheme for multicrystalline solar cells. Firstly, a novel steerable evidence filter is developed to generate the crack saliency map, which significantly enhances the contrast between crack and surrounding background. Secondly, a segmentation-based method including local threshold and minimum spanning tree is employed, which ensures the extraction of complete crack. Next, the crack can be accurately located in the inspection image by computing the crack skeleton. Finally, experimental results on defective and defect-free EL images show that the proposed scheme is accurate and robust, and various types of cracks can be correctly detected. The proposed scheme achieved average detection rate of 94.4% on the data set, which performs better than the previous methods.

The impact of diffusion gettering on solar cell efficiency and light induced degradation

In this paper, the influence of an annealing process at 500 °C for 30 min. after emitter diffusion of multi-crystalline solar cells was investigated. Neighboring sister wafers from a silicon ingot were processed using light doped emitter (LDE) and low temperature annealing (LTA). Emitter profiles measured by Electrochemical Capacitance Voltage (ECV) showed that there is no obvious difference between LDE and LTA due to the lower annealing temperature. The lifetime and iron concentration measurement indicated that the gettering of LTA diffusion is much better than LDE. Therefore, the solar cell efficiency of LTA was up to 19.01%, which is 0.28%abs higher than LDE due to the higher open circuit voltage (Voc) and short-circuit current (Isc). The Internal Quantum Efficiency (IQE) measurement with better long wavelength response showed the wafer quality was much improved after LTA diffusion. To further investigate diffusion gettering performance, light-induced-degradation (LID) was analyzed both for LDE and LTA diffusion. The LID of LDE diffusion with 1.15%rel. is obviously higher than LTA diffusion of 0.46%rel., attributing to the higher iron concentration of LDE diffusion emitter to result in higher iron-boron pair dissociation.

MACE nano-texture process applicable for both single- and multi-crystalline diamond-wire sawn Si solar cells

The photovoltaic (PV) industry requires efficient cutting of large single and multi-crystalline (sc- and mc-) silicon (Si) wafers. Historically multi-wire slurry sawing (MWSS) dominated, but the higher productivity of diamond-wire-sawing (DWS) holds promise for decreasing PV costs in the future. While surface texturing of DWS wafers is more complicated than of MWSS wafers, especially in mc-Si wafers, nanotexturing has been shown to overcome this challenge. While the benefit of nanotexturing is thus clearer in mc-Si, a universal nano-texture process that also works on sc-Si would simplify and reduce the investments costs of PV production-lines. In this paper, such a nano-texture process is developed using a metal-assisted chemical etch (MACE) technique. Step-by-step characterization of surface structure and reflectance of the MACE process is used after: 1) wafering, 2) standard acidic texturing etch, 3) silver nanoparticles deposition, and 4) MACE nanotexturing for both sc and mc-Si. The results show that the same MACE process works effectively for both sc-Si and mc-Si wafers. Finally, the nano-textured wafers are processed into PV cells in an industrial process line with conversion efficiencies of 19.4% and 18.7%, for sc-Si and mc-Si solar cells, respectively.

Luminescence based high resolution finite element simulation of inhomogeneous solar cells

The application of the model of independent diodes with an effective series resistance in units of Ωcm2 leads to wrong predictions of the local diode voltages in solar cells, if their dark current is strongly inhomogeneous. Therefore, multicrystalline solar cells should be modelled by finite element methods such as that implemented in Griddler. Until now, the local parameters for this modelling were obtained for inhomogeneous solar cells by evaluating luminescence and dark lock-in thermography (DLIT) images. The inhomogeneous local diode parameters, in particular the saturation current density J01, were obtained by the DLIT-based analysis (Local I-V), and the local emitter contact resistances were obtained by evaluating luminescence-based local diode voltages. This method, however, is experimentally demanding and shows a limited spatial resolution due to thermal blurring of the DLIT results. Previous luminescence-based methods for imaging J01, based on the model of independent diodes, have been found to be erroneous. In this model it is assumed that each elementary cell region (image pixel) is connected to the terminals of the cell by an independent series resistance (Trupke et al., 2007) [1]. Meanwhile alternative luminescence evaluation methods are available for reliably imaging J01 with high spatial resolution. This opens the way for a purely luminescence-based finite element simulation of inhomogeneous solar cells. In this contribution this method is described and applied to a multicrystalline PERC cell made from high performance silicon material. The results are compared to that of a luminescence plus DLIT based evaluation.

Analysis for efficiency potential of crystalline Si solar cells

Abstract

Device Parameter Extraction for Loss Analysis of Silicon Solar Cells Based on Intelligent Model Fitting

A fully automated and rigorous loss analysis routine that provides a breakdown of the loss components occurring in silicon solar cells is presented in this work. The routine combines large-area two-dimensional modeling and smart auto-fitting routines with luminescence imaging. This allows the spatially resolved information in luminescence images to be analyzed to extract recombination parameters partitioned by regions (e.g. wafer edge, under metal contacts, over passivated areas), as well as the spatial distribution of contact resistance. After these cell parameters have been extracted, a loss analysis of open-circuit voltage can be performed by simulating the open-circuit condition and examining the various recombination currents, and a loss analysis of fill factor can be performed by successively turning off the effects of factors that degrade it in simulation. The technique is demonstrated on a multicrystalline silicon PERC solar cell.

Plasma-enhanced chemical vapor-deposited SiN and liquid-phase-deposited SiO2 stack double-layer anti-reflection films for multi-crystalline solar cells

We successfully fabricated a double-layer anti-reflection film for multi-crystalline silicon solar cells through liquid-phase deposition to deposit silicon dioxide (SiO2) film on a multi-crystalline silicon surface and plasma-enhanced chemical vapor deposition to deposit silicon nitride (SiN) film on a SiO2 film surface. The SiO2 film thicknesses were 10, 15, and 20 nm, and the SiN film thickness was 60 nm. The reflectance of the substrate markedly decreased as the double-layer film was deposited. Moreover, with different film thicknesses, the minimal values of the reflectance were 4.91%, 2.75%, and 2.83% at the wavelengths of 609, 597, and 634 nm. The average reflection reached 6.92% when the thicknesses of SiO2 and SiN films were 20 and 60 nm in the wavelength range of 400–1100 nm. The minority carrier lifetimes of the multi-crystalline silicon substrates were 7.44, 7.21, and 7.27 μs at different film thicknesses. The short circuit current density and efficiency of the solar cell reached 34.52 mA/cm2 and 16.56% when the thicknesses of the SiO2 and SiN films were 20 and 60 nm, respectively. Low reflectance and good cell performance indicated that the double-layer film composed of SiO2 and SiN films were suitable for multi-crystalline silicon solar cells as the anti-reflection film.

Nickel assisted chemical etching for multi-crystalline Si solar cell texturing: a low cost single step alternative to existing methods

The texturing of silicon surfaces is a well-known method of reducing the reflection from the surface of crystalline Si solar cell devices. With the utilization of diamond wires in recent advances in wafer slicing technology, surface texturing for the multi-crystalline Si wafers by the traditional acid-based texturing technique has become difficult. Metal-Assisted Etching (MAE) has been shown to be a promising and low-cost alternative to the traditional acid-based isotropic texturing. This paper reports, for the first time, a new single-step Ni-assisted etching technique to obtain nano-scale porous structures, that is, black silicon on multi-crystalline wafers. We observed lower reflection results in comparison with standard isotropic texturing using a standard acid solution. The structural and optical properties of the surface were identified through reflection measurements and scanning electron microscopy imaging. As a final step, the optimized texturing process was applied to multi-crystalline solar cell devices and showed promising results regarding cell performance parameters.

Boosting spectral response of multi-crystalline Si solar cells with Mn2+ doped CsPbCl3 quantum dots downconverter

Conventional multi-crystalline Si solar cells present limited spectral response in the near ultraviolet region due to the parasitic absorption caused by passivation layer and heavily doped emitter layer. Converting ultraviolet light into visible light is a promising way to improve the performance of the devices. Herein, CsPbCl3: Mn2+ quantum dots are applied onto the front of the multi-crystalline Si solar cells as the luminescent downconverter to improve light harvesting in the short wavelength region. CsPbCl3: Mn2+ quantum dots with large Stokes shift (>1000 meV) and high quantum yield (62%) effectively convert the normally low responding spectral in the ultraviolet region into usable visible light at ∼600 nm for improving photoelectric conversion efficiency of the devices. Combining with the unique light-trapping architecture of multi-crystalline Si solar cells, external quantum efficiency in the ultraviolet region is evidently improved, leading to an increased conversion efficiency of 6.2%. Meanwhile, solar cells coated with CsPbCl3: Mn2+ quantum dots luminescent downconverter layer also exhibit favorable photostability under AM 1.5G illumination and reproducibility. Based on these characteristics, we deem this light-management strategy could be applied on mass product multi-crystalline Si solar cells, and easily extended to enhance the performance of other solar cells.

Determination of boron and hydrogen in materials for multicrystalline solar cell production with prompt gamma activation analysis

For the optimization of the manufacturing process of multicrystalline silicon (mc-Si) for solar cells in order to reduce energy consumption and costs, it is important to know the distribution of the impurities. In our project, we analyzed the raw materials, the crucibles, and the solidified blocks (ingots) with different methods based on neutron activation. In this paper, we present the technique and the results of the boron and hydrogen determination with prompt gamma activation analysis (PGAA). We show that the distribution of boron in the ingots can be successfully described with a Scheil curve. Concerning the raw material, the most relevant information is the average elemental mass fraction. For this purpose, PGAA is well suited since it provides a bulk analysis with detection limits down to the low ng/g range for boron and µg/g range for hydrogen. In this context, the hydrogen content of fluidized bed reactor (FBR) feedstock is discussed in detail. These were the first extensive PGAA studies of the boron and hydrogen content in mc-Si.

Lock-in and pulsed thermography for solar cell testing.

Inspection of solar cells is an important part of their production process because even small defects can cause a significant drop of a whole photovoltaic module performance. LED illuminated lock-in (LEDILIT) and flash-pulse thermographic techniques (FPT) were compared in this study. Lock-in methods are more commonly used for solar cell inspection. The aim of the study was to discover if the FPT is the appropriate method for inspection of defects of multicrystalline solar cells. Experimental setup, inspection results, and advantages/disadvantages of both methods are presented. It is demonstrated that the LEDILIT is the suitable technique for inspection of defects connected with a photovoltaic effect. Local shunts, cracks, and artificial laser-made defects were detected. Only some of the most significant shunts and laser-made defects were identified by the FPT. However, the FPT inspection is much faster than the LEDILIT. The FPT was also able to indicate an inhomogeneity at a bottom layer of a cell, which was not connected with a photovoltaic effect and not revealed by the LEDILIT.

Micro-cracks detection of multicrystalline solar cell surface based on self-learning features and low-rank matrix recovery

PurposeThis paper aims to improve the generalization capability of feature extraction scheme by introducing a micro-cracks detection method based on self-learning features. Micro-cracks detection of multicrystalline solar cell surface based on machine vision is fast, economical, intelligent and easier for on-line detection. However, the generalization capability of feature extraction scheme adopted by existed methods is limited, which has become an obstacle for further improving the detection accuracy.Design/methodology/approachA novel micro-cracks detection method based on self-learning features and low-rank matrix recovery is proposed in this paper. First, the input image is preprocessed to suppress the noises and remove the busbars and fingers. Second, a self-learning feature extraction scheme in which the feature extraction templates are changed along with the input image is introduced. Third, the low-rank matrix recovery is applied to the decomposition of self-learning feature matrix for obtaining the preliminary detection result. Fourth, the preliminary detection result is optimized by incorporating the superpixel segmentation. Finally, the optimized result is further fine-tuned by morphological postprocessing.FindingsComprehensive evaluations are implemented on a data set which includes 120 testing images and corresponding human-annotated ground truth. Specifically, subjective evaluations show that the shape of detected micro-cracks is similar to the ground truth, and objective evaluations demonstrate that the proposed method has a high detection accuracy.Originality/valueFirst, a self-learning feature extraction method which has good generalization capability is proposed. Second, the low-rank matrix recovery is combined with superpixel segmentation for locating the defective regions.

PERC Solar Cell Performance Predictions From Multicrystalline Silicon Ingot Metrology Data

The influence of the as-grown material quality on the performance of multicrystalline silicon PERC solar cells is investigated using recently developed spectral photoluminescence imaging techniques on ingot level, i.e., on bricks, and is examined in conjunction with photoluminescence measurements on as-cut wafers. The effect of material parameters, including bulk lifetime, dislocation density, and resistivity, is studied with regard to their effect on cell output across a sample set of three directionally solidified production bricks of widely varying bulk lifetime and dislocation density. The data are analyzed statistically using a linear mixed model. Bulk lifetime is found to be a statistically significant predictor of the cell performance across the studied sample set. The prediction accuracy is found greatest for material with low dislocation density where a linear correlation between cell performance and as-grown bulk lifetime is found. The dislocation densities measured on as-cut wafers remain a more accurate predictor for medium to highly dislocated material, but predictions are improved by adding bulk lifetime as an additional predictor in the model. Dislocation density measurements taken on the side facets of silicon bricks were identified as not being significant predictors for this dataset. However, the detection may enable the classification of bricks into broad dislocation defect classes, which is expected to further improve the overall prediction accuracy for models which use brick metrology only. With increasing cell efficiencies and an ongoing trend of reducing the dislocation density in industrial multicrystalline wafers, our findings suggest that the bulk lifetime, measurable on bricks, i.e., directly after ingot growth, becomes an increasingly relevant parameter.