CAS invents photovoltaic silicon wafer technology

PV module recycling is gaining importance and the technology to reprocess and purify the silicon material from end-of-life modules for reuse in the PV chain to manufacture new cells has proven to be a major challenge. Recently, Xu Xinhai, Wang Yin and Lai Dengguo at the University of Chinese Academy of Sciences have invented a technology to recycle and upgrade scrap crystalline silicon PV modules, successfully obtaining desirable and high purity silicon wafers with an intact structure, minimised thickness and excellent light capture capability.

The results have been published in Resources, Conservation & Recycling and are included in sciencedirect.

1. The reuse value of recycled silicon

Recycling and reuse is one of the most attractive strategies to offset the environmental impact and transform waste PV modules into a sustainable resource for the PV industry. Therefore, many efforts are focused on recovering resources from waste PV modules, especially crystalline silicon (c-Si) from c-Si based PV modules, which have a major market share.

The valuable aluminium bezel is removed by chemical dissolution and thermal decomposition, and the residual encapsulation material EVA, which remains after separation, can be further recovered from tempered glass, Si cells and Cu solder tape. Solar grade silicon can be recovered and then subjected to a chemical etching purification process and re-injected as a raw material for solar cell manufacturing.

The power conversion efficiency of Si solar cells depends primarily on their electrical and optical properties, including the quality of the silicon wafers (e.g. . , intrinsic purity, thickness), metal electrodes, surface passivation and the light trapping ability of the surface structure (Ye et al., 2014). For non-destructively recycled silicon wafers, it is difficult to enhance the electrical properties by improving their intrinsic purity unless they are melted to reproduce the ingots.

Cost-effective, sustainable and benign options therefore need to be sought. Scientists have proposed an ideal recycling model for recovering intact silicon wafers with direct reuse characteristics in commercial PV modules for the remanufacture of new solar cells.

2. Skip the ingots and recycle the wafers directly

Scientists at the Key Laboratory of Urban Pollutant Transformation of the Chinese Academy of Sciences are hoping to adjust to this impossibility.

In the researchers' view, skipping the process of ingot production and wafer cutting could save around 40% of PV module production costs, but the current technology still faces significant challenges. For example, impurities on the surface of recycled silicon cells need to be removed by chemical etching in order to obtain pure silicon wafers. This process is traditionally drastic and uncontrollable, and tends to lead to a dramatic reduction in wafer thickness.

With this in mind, the group sought ways to recover silicon wafers suitable for the production of high-efficiency cells and modules. The researchers have newly developed a solvent thermal expansion combined with thermal decomposition (SSTD) method that integrates non-destructive silicon cell recovery by the SSTD process, sequential acid etching for silicon wafer pre-purification, a new extended MACE method for ultra-purification and simultaneous ultra-low reflectance surface texture fabrication, and in-system reuse of the recovered material.

The chemical treatment methods employed both purify the silicon wafers and improve their surface properties. After obtaining high purity and intact wafers, the researchers have tuned the surface texture of the recovered Si wafers by applying a single-step MACE process with Cu/Ag assisted chemical etching, while controllably constructing various anti-reflective textures including interesting dual-scale micro/nanostructures, resulting in a range of surface structures including DMNs, nanowires, nanopores and inverted rectangular cones that can significantly reduce surface reflectivity and produce 'black silicon' wafers.

3. Amazing research results

With this technique, the researchers have succeeded in obtaining ideal and high-purity silicon wafers with an intact structure, minimised thickness and excellent light capture capability. According to the paper, the recovered wafers have good thickness (165 μm), resistivity (1.02-2.28 Ω-cm), carrier lifetime (1.12-2.47 μs) and ultra-low reflectivity (5-15%) compared to commercial wafers, making it feasible to produce high-efficiency PV modules.

A rough economic assessment shows that the production cost of this integrated strategy is lower than the price of silicon wafers from conventional recycling processes or industrial production processes, and also allows for the complete recycling of aluminium frames, tempered glass, copper strip and high purity silver and aluminium powder, which can be reused within the system, resulting in economic viability and high resource sustainability.

The work is part of the National Natural Science Foundation of China (No. 52102120) and is supported by the Southeast Asian National Biomass Waste Resource-based Sustainable Development Technology R&D and Application Demonstration, and is listed as a "Strategic Pioneering Science and Technology Special Project (A)" of the Chinese Academy of Sciences (No. XDA23030301). XDA23030301), a key project for industrial pioneering in Fujian Province (No.2019H0056), and a key project for social development in Fujian Province (No.2021Y0069).