Silicon Solar Cell Process Research Articles

A Definition Rule for Defect Classification and Grading of Solar Cells Photoluminescence Feature Images and Estimation of CNN-Based Automatic Defect Detection Method

A nondestructive detection method that combines convolutional neural network (CNN) and photoluminescence (PL) imaging was proposed for the multi-classification and multi-grading of defects during the fabrication process of silicon solar cells. In this paper, the PL was applied to collect the images of the defects of solar cells, and an image pre-processing method was introduced for enhancing the features of the defect images. Simultaneously, the defects were defined by 13 categories and three divided grades of each under the definition rules of defects that were proposed in accordance with distribution and characteristics of each defect category, and expand data were processed by various data augmentation. The model was therefore improved and optimized based on the YOLOv5 as the feature extractor and classifier. The capability of the model on distinguishing categories and grades of solar cell defects was improved via parameter tuning and image pre-processing. Through experimental analysis, the optimal combination of hyperparameters and the actual effect of data sample pre-processing on the training results of the neural network were determined. Conclusively, the reasons for the poor recognition results of the small target defects and complex feature defects by the current model were found and further work was confirmed under the foundation of the differences in recognition results between different categories and grades.

Characterization of tunnel oxide passivated contact fabricated by sputtering and ion implantation technique

Tunnel oxide passivated contact (TOPCon) structures using highly doped n-type polycrystalline silicon were fabricated using facing target sputtering and ion implantation techniques for a SiH4-free fabrication process of high-efficiency silicon solar cells. We investigated the structural and electrical properties of the highly doped n-type poly-Si layers to optimize the ion implantation process. We also investigated the surface passivation quality of our TOPCon structure. An effective carrier lifetime of 2.01 ms and an implied open circuit voltage of 704 mV were obtained for our sample annealed at 950 °C. The sample also exhibits a low contact resistance of 3.22 × 10−3 Ω cm−2. Our results open the way for SiH4-free fabrication of silicon solar cells with a TOPCon structure.

P‐9.2: ESL products with amorphous silicon solar cells integrated into the backplane

This paper presents a product combining amorphous silicon solar cells and ESL. This product combines the amorphous silicon solar cell process with the TFT backplane of ESL products. At the same time, this product uses solar cells as the device for collecting light energy, and the new energy conversion equipment collects the electric energy generated by solar energy and generates stable output electric energy to provide ESL devices. This scheme is not only environmentally friendly, but also reduces the thickness of products using external solar cells.

Photovoltaics literature survey (No. 181)

Photovoltaics literature survey (No. 181)

Copper in compensated p- and n-type Czochralski silicon: Diffusivity, influence on the majority charge carrier density and mobility

Copper contamination risks are present throughout the whole crystalline silicon solar cell process flow, namely during ingot growth processes using recycled feedstock, wafer sawing and metallisation. Copper is a fast-diffusing impurity which can modify the carrier transport and recombination properties. The aim of this study is to present new insights into the properties of copper-contaminated highly doped and compensated silicon, with a focus on the copper diffusivity and the influence of this impurity on the majority carrier mobility and density. For this, compensated Czochralski silicon ingots were grown from electronic grade feedstock doped with boron and phosphorus, and the wafers intentionally contaminated with copper. Its concentrations were measured by Glow Discharge Mass Spectrometry, while the electrical properties were extracted from Hall Effect measurements. The dopant concentrations were used to compute the effective diffusivity of interstitial copper and important changes in the copper diffusivity are expected in the vicinity of the p-type/n-type transition. For the n-type compensated wafers, a strong migration of copper atoms towards the surface and a reduced copper bulk content were highlighted. Furthermore, the presence of precipitates was experimentally revealed. Both the equilibrium electron density and electron mobility were not influenced by the contamination. In p-type compensated material, copper atoms did not accumulate at the surface, probably due to both the limited copper precipitation in p-type silicon and the lower copper diffusivity. In addition, the equilibrium hole density was not influenced. However, the hole carrier mobility was affected, likely associated to the higher bulk copper content. • Highly doped compensated silicon was intentionally contaminated with copper. • Electrical properties were obtained from Hall Effect measurements. • Large differences in copper diffusivity close to the p-to n-type transition. • Strong migration of copper towards the surface in n-type compensated wafers. • Equilibrium electron density and mobility are not influenced by the contamination.

Past, Present, and Future Outlook for Edge Isolation Processes in Highly Efficient Silicon Solar Cell Manufacturing

This work highlights present research and mass production results of wet‐chemical solutions for industrial edge isolation of silicon solar cells, aiming for a reduction of nitric acid consumption and production costs as well as a simultaneous increase in efficiency. All processes are applied to either industrially passivated emitter and rear contact (PERC) or tunnel oxide‐passivated contact (TOPCon) solar cells. Herein, a review of different edge isolation techniques in the history of silicon solar cell processing is presented. Subsequently, novel wet‐chemical approaches are focused on, namely 1) HNO3‐reduced edge isolation (InOxSide Fusion), 2) HNO3‐free edge isolation (InOxSide Blue), and 3) batch cluster solution—a combination of an acidic inline and an alkaline batch tool for emitter edge isolation of PERC and TOPCon solar cells. For each of the approaches, cell results and total cost of ownership estimations are presented. Based on all findings, a comprehensive discussion between inline versus batch‐cluster processing is presented. All investigations are performed on industrial equipment, wafer sizes, and a solar cell efficiency level of above 23%.

Attenuation of short‐circuit effect along screen‐printed silver gridlines during the contact firing process of crystalline silicon solar cells

AbstractThe “short‐circuit effect” in the fire‐through Ag metallization of crystalline Si (c‐Si) solar cells refers to the poor contact formation caused by an electrical short between the Ag gridline and underlying Si emitter during contact firing. This study employs two different Ag pastes containing PbO‐ and TeO2‐based glass frits to investigate the dependence of the short‐circuit effect on the length and pattern of the Ag finger lines. The results show that regardless of the employed glass frits, the short‐circuit effect is mitigated even near the short spot and gradually attenuated along the finger line away from the spot as the Ag finger line extends radially longer than the critical length (>35 mm). We demonstrate that this attenuation is independent of the finger line width and predominantly attributed to the ohmic drop of the electrode potential along the Si substrate. The results also show that, regardless of the glass frits, the contact quality is strongly correlated with the density of the Ag crystallites that are formed on the Si emitter surface. The 6‐in full‐cell tests indicate that the isolation of the short spots by segmenting the Ag finger lines does not necessarily result in the mitigation of the short‐circuit effect. We suggest that the reduction of Ag+ on the Si emitter surface should be the key process that must be controlled to achieve high‐quality contacts during contact firing, providing further insights into the electrochemical characteristics of contact firing reactions for the future development of Ag pastes.

Enhancement of the conversion efficiency of selective emitter PERC solar cells by post-oxidation

Thermal oxidation has been widely adopted in the fabrication process of silicon solar cells. In this paper, we investigate the effect of thermal oxidation on the performance of passivated emitter and rear cells (PERC) after the diffusion process. It is found that the application of a diffusion furnace for post-oxidation results in the re-doping of phosphorus, which affects the minority carrier lifetime and implied Voc. In comparison, the use of an annealing furnace for post-oxidation brings an improvement of electrical properties of solar cells because of the gettering effect. Furthermore, the increase in thickness of the oxide layer mitigates the laser-induced damage during the laser doping process, which is beneficial for the enhancement of cell efficiency. Batch data shows that post-oxidation using an annealing furnace leads to an absolute efficiency improvement by 0.1%.

Process–Structure–Properties Relationships of Passivating, Electron‐Selective Contacts Formed by Atmospheric Pressure Chemical Vapor Deposition of Phosphorus‐Doped Polysilicon

Herein, we investigate the process–structure–properties relationships of in situ phosphorus (P)‐doped polycrystalline silicon (poly‐Si) films by atmospheric pressure chemical vapor deposition (APCVD) for fabricating poly‐Si passivating, electron selective contacts. This high‐throughput in‐line APCVD technique enables to achieve a low‐cost, simple manufacturing process for crystalline silicon (c‐Si) solar cells featuring poly‐Si passivating contact by excluding the need for vacuum/plasma environment, and additional post‐deposition doping steps. A thin layer of this P‐doped poly‐Si is deposited on an ultrathin (1.5 nm) silicon oxide (SiO x ) coated c‐Si substrate to fabricate the passivating contact. This is followed by various post‐deposition treatments, including a high‐temperature annealing step and hydrogenation process. The poly‐Si films are characterized to achieve a better understanding of the impacts of deposition process conditions and post‐deposition treatments on the microstructure, electrical conductivity, passivation quality, and carrier selectivity of the contacts which assists to identify the optimal process conditions. In this work, the optimized annealing process with post‐hydrogenation yields passivating contact with a saturation current density (J 0) of 3 fA cm−2 and an implied open‐circuit voltage (iV OC) of 712 mV on planar c‐Si wafer. Junction resistivity values ranging from 50 to 260 mΩ cm2 are realized for the poly‐Si contacts processed in the optimal annealing condition.

Damage and residual layer analysis of reactive ion etching textured multi-crystalline silicon wafer for application to solar cells

As part of the surface texturing process of multi-crystalline silicon solar cells, isotropic etching is performed by using an acidic solution. Furthermore, metal catalyst chemical etching (MCCE), reactive ion etching (RIE), and laser etching are used to further decrease surface reflectance. This study aimed to increase the power conversion efficiency of the solar cell by improving the short-circuit current density (Jsc) using MCCE and RIE. During RIE, a byproduct and a plasma damage layer are formed on the silicon surface, which decrease the efficiency of the solar cell and therefore need to be identified and effectively removed. Transmission electron spectroscopy, energy dispersive X-ray spectroscopy and X-ray photoelectron spectroscopy analyses were performed to identify the byproducts formed during RIE to confirm that it was amorphous silicon oxide. Furthermore, an etching process using acidic and alkaline base solutions was used to remove the plasma damage layer and the Jsc loss was calculated using the reflectance. As a results, confirm a difference of up to ∼ 1.6 mA/cm2, and an improvement of approximately 0.6% was observed in the solar cell efficiency. These results show a method of minimizing Jsc loss and removing surface damage in a silicon solar cells fabricated using the RIE method.

A comprehensive hydrometallurgical recycling approach for the environmental impact mitigation of EoL solar cells

With the rapid increase of the installed capacity of crystalline silicon photovoltaic (PV) modules, the number of used modules that reach their End of Life (EoL) is accumulating dramatically and it is expected to pose significant environmental hazards. To solve this upcoming issue, this research investigated a comprehensive hydrometallurgical recycling process for crystalline silicon (C-Si) solar cells with regard to the problems of low element separation efficiency, inadequate component recovery rate, and deficient environmental considerations in the current recycling processes. Appropriate hydrometallurgical recovery processes were designed to recover valuable elements and entrap hazardous material lead (Pb) while properly treating wastewater from the recycling process. Nitric acid (HNO3) is an effective choice for elemental leaching from the waste, with 98.12% and 99.57% simultaneous leaching rates of silver (Ag) and aluminum (Al), respectively. The overall recovery rate of Ag was 96.13% using the hydrochloric acid (HCl) precipitation-ammonia solubilization-hydrazine process route. The purity of Ag after reduction was 99.8%. The silicon nitride (SiNx) and silicon phosphide (Si3P4) layers on the surface of the silicon wafer can be completely etched and removed by low-concentration HCl, and the product obtained is pure silicon. Most importantly, heavy metal elements such as Pb were captured to prevent potential damage to the environment and human health.

Study on the surface cleaning process of polysilicon fleece after diamond wire cutting

In the texturing process of diamond wire polycrystalline silicon solar cells optimizing the preparation of the light-trapping texture structure needs to add texturing additives. In this thesis, the phenomenon which additives remain on the surface of the silicon wafer was found through the contact angle test. The texturing silicon wafer are cleared by ultrasound with water and absolute ethanol respectively, and the change of contact angle was used to verify the cleaning effect of the two cleaning processes. The research results show that additives remain on the surface of the texturing silicon wafer, and ultrasonic cleaning with absolute ethanol can significantly improve the cleanliness of the flock sheet.

Morphology evolution of the light trapping structure using atmospheric plasma textured c-Si wafer for silicon solar cells

Applying atmospheric plasma etching to the surface texturing process of silicon solar cells is a promising strategy for the current photovoltaic manufacturing industry due to its low equipment cost and good fabrication flexibility. This paper investigates the morphology evolution of the silicon surface etched by an Ar/CF4/O2 plasma and the associated optical properties. Results show that the generation of the light trapping structure on the polished silicon surface can be divided into two stages on the basis of the multi-scale morphological images and the quantitative evaluation of roughness parameters. The initial roughening stage mainly involves the formation of high-frequency nanoroughness that can act as an effective medium layer with a gradual refractive index. The resulting optical medium effect can reduce the surface reflectance within a broad range of wavelengths. At the next texturing stage, the low-frequency and high-amplitude microroughness dominates the morphology of the etched silicon surface. It features inverted parabolic structures with a high aspect ratio, which can cause multiple reflections of the incident light. The optical medium effect resulting from the nanoroughness is also inherited. Thus, the anti-reflectance property of the etched silicon surface is greatly improved. This work demonstrates that the light trapping properties of silicon surface etched by atmospheric plasma jet are a synergy of the optical medium effect and geometrical optics. Insights into the morphology evolution and optical properties of the textured surfaces are important for developing a new surface texturing process of silicon solar cells.

Adapting M2 silicon half-wafers processing on industrial-scale equipment dedicated to 4″ solar technology

PurposeThe purpose of this study is to address the issue of technology equipment formerly dedicated to the process of 4- and even 5-inch photovoltaic cells and whose use has become critical with the evolution of silicon wafer size standards (M2–M10). Fortunately, the recent concept of 6'' half-cut cell with its many advantages appears promising insofar as it offers the possibility of further extend the use of costly, still operational process equipment, but doomed to obsolescence.Design/methodology/approachIn the background of a detailed Al-BSF process, the authors show how to experimentally adapt specific accessories and arrange 6” half-wafers to enable the upgrade of a complete industrial process of silicon solar cells at a lower cost. Step by step, the implementation of the processes for the two wafer sizes (4” wafers and 6” half wafers) is compared and analyzed in terms of performance and throughput.FindingsGlobally, the same process effectiveness is observed for both types of wafers with slightly better sheet resistance uniformity for the thermal diffusion carried out on the half wafers; however, the horizontal arrangement of the wafer carriers in the diffusion and the plasma-enhanced chemical vapor deposition tubes limits the thermal balance regarding the total number of cells processed per batch.Originality/valueIn terms of the development of prototypes on a preindustrial scale, this paves the way to further continue operating outdated equipment for high-performance processes (passivated emitter and rear contact, Tunnel oxide passivated contact (TOPCon)), while complying with current standards for silicon wafers up to M10 format.

The enhanced light trapping nature of NiOx thin films deposited by magnetron sputtering onto silicon solar cells at room temperature

Nickel oxide (NiOx) thin films were successfully deposited onto silicon solar cells (SSCs) using dc magnetron sputtering. The effect of the optical absorption–emission process of light with thickness of NiOx thin films (NiOx-TFs) and its influence on the electrical process of SSCs modified were investigated. Atomic force microscopy (AFM) showed that the particle size of NiOx-TFs increased with an increasing thickness, x-ray diffraction (XRD) indicated that the samples only presented the NiO phase, x-ray photoelectron spectroscopy (XPS) studies showed that the films clearly exhibited the presence of an oxidation state of Ni2+.The reflectance and transmission were measured with thicknesses of 1, 2 and 3 nm and the absorbance edges were obtained which overlapped the pseudo edge of the glass substrate at 315 nm. While the thickness of 20 nm NiOx-TFs showed edge at 330 nm, with an optical band gap of 3.75 eV. Furthermore, electrical properties depend on the films thickness, exhibited PCE around 1.06, 2.27 and 0.77% (fixed holder) and 1.54, 1.38 and −0.65% (moveable holder) for nominal thicknesses of 1, 2 and 3 nm respectively.

Dry Phosphorus silicate glass etching and surface conditioning and cleaning for multi-crystalline silicon solar cell processing

As an alternative to the wet chemical etching method, dry chemical etching processes for Phosphorus silicate glass [PSG} layer rel11ova] using Tri?uormethane/Sulfur Hexafluoride (CHF3/ SF6) gas mixture in commercial silicon-nitride plasma enhanced chemical vapour deposition (SiN-PECVD) system is applied. The dependence of the solar cell performance on the etching temperature is investigated and optimized. It is found that the SiN-PECVD system temperature variation has a signi?cant impact on the whole solar cell characteristics. A dry plasma cleaning treatment of the Si wafer surface after the PSG removal step is also investigated and developed. The cleaning step is used to remove the polymer ?lm which is formed during the PSG etching using both oxygen and hydrogen gases. By applying an additional cleaning step, the polymer ?lm deposited on the silicon wafer surface after PSG etching is eliminated. The effect of different plasma cleaning conditions on solar cell performance is investigated. After optimization of the plasma operating conditions, the performance of the solar cell is improved and the overall gain in ef?ciency of 0.6 % absolute is yielded compared to a cell without any further cleaning step. On the other hand, the best solar cell characteristics can reach values close to that achieved by the conventional wet chemical etching processes demonstrating the effectiveness of the additional O2/H2 post cleaning treatment.

Intense pulsed light in back end processing of solar cells with passivating contacts based on amorphous or polycrystalline silicon layers

Intense pulsed light (IPL) is capable of entirely replacing thermal annealing (curing and contact formation) within back end processing of silicon solar cells with passivating contacts. In order to demonstrate this, full-size silicon heterojunction (SHJ) cells with IPL-processed screen-printed metal contacts are fabricated. The device with the highest conversion efficiency reaches 23.0%, which is confirmed by Fraunhofer ISE CalLab. On average, IPL-annealed SHJ cells outperform their thermally treated pendants by 0.3–0.4%abs, in particular due to higher open-circuit voltages and fill factors. To further exploit the potential of IPL, it is applied to tunnel oxide passivating contacts (TOPCon). 2 cm × 2 cm-sized p-type solar cells with TOPCon layers on both sides are fabricated. On the front they exhibit indium tin oxide (ITO) layers as well as screen-printed metal contacts which are either thermally or IPL-annealed. The trade-off between low contact resistivity at the TOPCon/ITO interface and high-quality surface passivation (open-circuit voltages of up to 709.4 mV) is balanced, which is a current challenge for the optimization of such devices. The IPL-processed cells’ series resistances do not differ significantly from those of thermally treated ones. Due to the pulse durations of several milliseconds only, IPL is very fast and offers a high throughput and, therefore, cost saving potential.

Optimization of Ni/Cu Plating Process for Silicon Solar Cells through Understanding of Underlying Electrochemical Reaction Mechanisms

To meet future energy demand and limit global warming, researchers and industrials aim to make solar technologies more efficient and cost effective. Si-wafer based photovoltaic technology accounted for about 95 % of the total production in 2017 [1] and one of the key processing step allowing to reduce cost and increase efficiency of these solar cells is the metallization step [2]. Nowadays, screen printing is the leading metallization technology as it is simple, fast and suitable for mass production. However, this method relies on silver containing pastes, an expensive metal with a fluctuating cost [3]. Moreover, screen-printing has almost reached its limitations in term of contact quality and different ways to apply metallic contacts are required for high efficiency cell architectures and advanced cells concepts. In this context, nickel copper (Ni/Cu) electrochemical metallization, also called “plating”, is under investigation as it not only gives a solution to metal cost reduction by using cheaper metals but also has the potential to overcome screen-printing limitations [4]. Indeed, Ni/Cu plating is suitable for advanced silicon solar cell architectures and, among others interests, form very thin and non-porous metallic contacts with low contact resistivity and better aspect ratio. Despite this, Ni/Cu plating requires more improvements, especially in terms of reliability and adhesion before being implemented in mass production. Plating process optimization is only achievable through a complete understanding of electrochemical reactions mechanisms involved which is the purpose of this work. We have investigated the Ni/Cu electrochemical metallization for n-PERT (Passivated Emitter Rear Totally diffused) bifacial silicon solar cells having a front textured side and rear polished side. The different steps of the Ni/Cu plating process are the following: 1) localized laser contact opening of dielectric layers using a UV picosecond laser, 2) silicon surface deoxydation to remove silicon oxide, 3) palladium activation of the silicon surface through galvanic displacement, 4) electroless nickel deposition, 5) electrolytic copper deposition, 6) final annealing to form NiSi metallic contact. Firstly, we have demonstrated that the silicon surface morphology and composition were non-homogenous after localized laser ablation over all the laser ablation conditions. Indeed, after laser ablation, silicon oxide is still present on the silicon surface with different thicknesses over the laser opened area geometry. Consequently, palladium activation appears to be non-homogeneous with a higher number of palladium particles on the border of laser opened area. Since nickel deposition rate depends on the number of palladium particles deposited, border effects of the nickel layer have been observed. Adherence and quality of the metallic contacts are thus adversely affected. Accordingly, we have investigated in details the roles of the different parameters involved in surface deoxydation and palladium galvanic displacement reactions in order to obtain a more homogeneous activated silicon surface before nickel deposition. This allowed us to control palladium deposition process by promoting palladium nucleation over growth, enabling to have more homogeneous silicon activated surface. A changer: “”is currently being done for nickel electroless deposition and copper electrolytic deposition to increase the quality of metallic contact and silicon metal interface through a better control of deposition conditions. Moreover, a direct plating route without palladium activation, which is a challenge for metallization of bifacial solar cells, will be presented.

Effect of Oxygen Precipitation in Silicon Wafer on Electrical Characteristics of Fully Ion-Implanted n-Type PERT Solar Cells

Fully ion-implanted n-type PERT solar cells with boron-implanted emitter and phosphorus-implanted back surface field were fabricated on the n-type silicon wafers obtained from the top part of the magnetic field-applied Czochralski ingot. It was observed that the electrical parameters are widely dispersed; among 11 solar cells, the best performing achieved 20.4% front side conversion efficiency, whereas the worst achieved 19.0%. Although the silicon wafers had low oxygen concentrations of 3–4 × 1017 atoms/cm3, the density of oxygen precipitates in the silicon wafers was on the order of 109 /cm3 as a consequence of the fully ion-implanted n-type PERT silicon solar cell processes. In addition, it was observed that the front side conversion efficiencies of the solar cells depended on the density of oxygen precipitates. Furthermore, the behavior of the oxygen precipitation during the fabrication processes of the solar cell is discussed.

A study of laser-induced surface defects in silicon and impact on electrical properties

Laser processing of silicon solar cells has unique advantages that offer the potential for low-cost high-efficiency photovoltaic devices. The understanding, monitoring, and control of laser-induced defects in silicon become important challenges that limit photovoltaic efficiency. In this work, the fundamental investigation of laser-induced defects was achieved by identifying defect types and origins at different laser-fluence regimes, assessing defect concentrations, and evaluating their impact on surface electrical properties and photovoltaic device performance. Studies showed that below laser melting, little degradation of electrical properties is observed, but no defects are identified; between laser melting and ablation, point defects and oxygen incorporation mainly occur; above laser ablation, dislocations and strain are primarily generated. Laser-induced dislocation density and strain are found likely to increase exponentially with laser fluence, and laser-induced strain is identified to be a possible major source of dislocation generation. In order to understand carrier recombination and charge transport in laser-processed silicon surface, we quantified the drift mobility, conductivity, carrier lifetime, and leakage current at various laser fluences and dislocation levels. The laser-defect induced degradation of surface electrical properties is governed by a probable exponential relationship with laser fluence, suggesting that laser-processing fluences near silicon melting should be carefully chosen for minimizing induced defects and electrical property degradation. Finally, the control of laser-induced defects was demonstrated through laser or furnace post annealing of laser-processed solar cell devices. After two laser-annealing steps, the open circuit voltage, fill factor, series resistance, and shunt resistance were significantly improved, resulting in an increased photovoltaic efficiency.