Lab team turns end-of-life solar silicon into silicon nitride

The aluminum frame and junction box came off a single Suntech STP200-18/Ub solar module before the researchers cut its laminate into cell-sized pieces. From that end-of-life hardware, a South Korean team produced silicon nitride (Si₃N₄), a high-value ceramic, after recovering and purifying the silicon.

The route depends on separating several kinds of waste without trapping them in the silicon powder. After using a hot knife to separate the glass, the team milled the ethylene-vinyl acetate (EVA) laminate at 400 rpm, 600 rpm or 800 rpm. Sieves removed backsheet fragments and residual EVA, while combustion at 600 C for one hour removed remaining organic material. Hydrochloric acid targeted aluminum, copper, tin and lead; nitric acid dissolved silver. A water-settling step then removed titanium dioxide (TiO₂) from the backsheet.

More force did not mean cleaner silicon. At 800 rpm, severe particle agglomeration left 4,290 ppm of aluminum after hydrochloric-acid etching; at 400 rpm, the residue was 189 ppm. The researchers selected the 400 rpm powder for the next stage, showing why recycling cannot be optimized simply by milling more intensely.

In 5 minutes, the sedimentation step removed 71.4% of the TiO₂ while retaining a silicon recovery rate of 92.3%. After the impurities were controlled, the recycled silicon reached 99.95% purity. The powder was then heated under 95% nitrogen and 5% hydrogen, first at 1,350 C for one hour and then at 1,450 C for 10 minutes. The resulting silicon nitride contained 93.1% α-Si₃N₄, compared with 54.7% when the extra purification was skipped. That matters because the ceramic is valued for its strength, thermal stability, wear resistance and electrical insulation.

Concretely, the process could give solar-panel recyclers a higher-value use for recovered silicon instead of treating it only as a secondary raw material. The technology remains a proof of concept based on a single module, not yet a practical scalable process. Jin-Seok Lee said the team is working with Korean recycler Wonkwang S&T on mobile recycling closer to where modules are discarded, aiming to reduce transportation costs by approximately 30% and carbon emissions by more than 10% compared with conventional centralized recycling.