Researchers at new energy technologies and nanomaterials (Liten) branch of the French Alternative Energies and Atomic Energy Commission (CEA) have developed a new technology to recover indium from discarded heterojunction (HJT) solar panels.

“Indium is a key material in HJT solar cells, where it is used in the transparent conductive oxide layer. Its recycling is essential for sustainable manufacturing and future waste management,” corresponding author Romain Duwald told pv magazine. “In our work, indium is directly recovered from solar cells through acid leaching under mild conditions. The indium tin oxide (ITO) layer is leached using diluted oxalic acid, which is less hazardous than conventional mineral acids. The indium is recovered at 4N purity in a single chemical step. The process also enables the separation of silver from the wafer, paving the way for the recovery of this other valuable metal.”

The researchers explained that conventional hydrometallurgical recycling approaches typically rely on acid leaching using hydrochloric acid, sulfuric acid, or nitric acid, often enhanced with oxidants such as hydrogen peroxide, although efficient separation of indium and tin remains a major challenge. To address this issue, they proposed a mild oxalic-acid-based one-pot, two-step process that enables indium tin oxide leaching, releases silver grids, and ultimately allows indium to be recovered as oxide for reuse.

For the experiments, the team used high-purity indium tin oxide powder, oxalic acid, sulfuric acid, and hydrogen peroxide, along with heterojunction solar cells provided by CEA INES. The cells consist of a silicon wafer coated on both sides with indium tin oxide and silver layers. In parallel, indium tin oxide powder was dispersed in acidic solutions, heated between 40 C and 70 C for up to 48 hours, then filtered after cooling to collect the leachate. The solar cells were crushed and treated with oxalic acid under controlled solid-to-liquid conditions to promote indium tin oxide leaching. Metal leaching yields were calculated from the concentrations of indium and tin measured in solution relative to the initial material composition.

Chemical analysis was performed using inductively coupled plasma optical emission spectroscopy to quantify dissolved metals. Solid phases were characterized by powder X-ray diffraction using copper K-alpha radiation to identify crystalline structures and reaction products. The resulting patterns were interpreted using reference databases and analysis software. Microstructural changes were observed using scanning electron microscopy, while energy-dispersive X-ray spectroscopy provided elemental mapping and compositional analysis. Together, these techniques enabled evaluation of dissolution efficiency, phase evolution, and metal recovery.

The results showed that at room temperature, sulfuric acid led to slow indium dissolution, while hydrogen peroxide significantly improved leaching by enhancing redox reactions that promote oxide breakdown. Oxalic acid also achieved moderate indium leaching and showed comparable performance to sulfuric acid under mildly reducing conditions. Increasing the temperature to 70 C strongly accelerated all systems, enabling near-complete indium recovery in sulfuric acid-based media, while oxalic acid exhibited high but less stable yields due to precipitation effects. In oxalic acid, indium rapidly formed insoluble indium oxalate, confirmed by X-ray diffraction and thermal analysis, explaining the decrease in dissolved indium over time.

Kinetic studies further showed that higher temperatures significantly improved leaching efficiency, with oxalic acid performing better at early stages, while sulfuric acid provided more stable final extraction. Activation energy calculations indicated that the process is chemically controlled rather than diffusion-limited, with both indium and tin dissolution governed by interfacial reactions. Oxalic acid was also found to act as both a reducing and complexing agent, influencing indium and tin dissolution behavior depending on the conditions. Finally, proof-of-concept tests on silicon heterojunction solar cells confirmed effective removal of the indium tin oxide layer, selective release of silver grids, and successful precipitation and calcination of high-purity indium oxide.

“After optimization of the indium leaching parameters, the best conditions were identified as a 0.2 M oxalic acid solution at 70 C for 4 h, achieving a 97% yield of indium in solution,” the researchers stated. “Dissolution of the indium tin oxide layer enabled detachment of the silver grids from the silicon wafer. Under these mild conditions, indium and tin were selectively leached, and subsequent complexation of indium cations by oxalate anions led to precipitation of indium oxalate. This enables separation of tin and indium in a single filtration step.”

Looking ahead, the academics aim to extend this recycling approach to other indium(III) oxide (In₂O₃)-based materials.

The proposed recycling tech was described in “Efficient recycling of indium and direct extraction of silver grids from heterojunction solar cells,” published in Solar Energy Materials and Solar Cells.