| Authors |
Tierno, M; Ruiz, JH; Taboada, S; Díez, E; Rodríguez, A; Caballero, LJ; Villanueva, ND; Marrón, DF; Abrosimov, NV; del Cañizo, C |
| Abstract |
Due to the exponential increase in installed photovoltaic (PV) capacity, a large amount of waste is expected, once the PV modules reach the end of their useful life, typically in the range of 25-30 years. This reality urgently calls for the design and adoption of affordable, effective, and massive strategies of recycling and reutilisation of all PV components, and particularly of those most energy -demanding in their manufacturing and thus responsible for the largest quote of environmental impact: the solar cells. This work explores two key processes for the recycling of silicon solar cells, namely the recovery of the metal contacts and the recrystallisation of the silicon substrate. On the one hand, the demetallisation of first -generation solar cells in KOH-ethanol-water solutions is explored. A 2 3 factorial design of experiments was developed to optimize the process, indicating that the temperature of the bath is the most influential variable. With 10 % KOH, 5 % ethanol and 60 degrees C for 105 min, or alternatively, 15 % KOH, 5 % ethanol and 65 degrees C for 60 min, it was possible to completely detach the metal front (mainly Ag) and back contacts (mainly Ag and Al) with an overall silicon weight loss of only 13 and 15 %, respectively, with resulting carrier lifetimes in the remaining silicon matrix above 100 mu s, high enough for new solar cell fabrication targeting efficiencies exceeding 20 %. Additionally, Czochralski recrystallisation of old multicrystalline wafers has been tested to evaluate the possibility of reinjecting recovered silicon at an earlier stage of the PV value chain. After successful growth and slicing in wafers, a thorough optoelectronic characterisation program has been implemented, demonstrating the fulfilment of the basic requirements needed to make new solar cells, in terms of wafer resistivity (-1 Omega cm), oxygen content (-10 18 cm -3 ), mobility (-1000 cm 2 /V & sdot; s) and carrier lifetimes (above 100 mu s threshold in all cases and reaching maximum values of 350 mu s). Demetallised samples and recrystallised wafers are subsequently used to manufacture P/Al solar cells, and results show that the solar cell performance is not limited by the material quality. The recovery and reutilisation of the two most valuable elements of Si -based PV -modules, namely the precious metals and the ultrapurified silicon, key for the establishment of PV as an example of circular economy, are thus demonstrated. |