Glued III-V on Si tandem solar cells using hybrid transparent conductive layers

The photovoltaic market is dominated by c-Si solar cells, with a record efficiency of 26.7 ^% which approaches the detailed balance limit. Tandem solar cells combining a III-V semiconductor top cell with a Si bottom cell are one of the most studied routes to exceed 30% efficiency. Direct growth of III-V on Si has achieved 25.9% efficiency but remains a challenge. Direct wafer bonding has demonstrated state of the art efficiencies of up to 35.9%, but this technique is not suitable for industrial production as it requires extremely low roughness, low particle contamination, surface activation and bonding under vacuum. The integration of III-V on Si using conductive transparent glues may circumvent these constraints. In this contribution, we design and fabricate a 2-terminal AlGaAs/ARC/Glue/ARC/TOPCon tandem cell using sol-gel derived TiO2 ARCs and PEDOT:PSS-based glue. The ARCs ensure Ohmic contact with the sub-cells (replacing the tunnelling recombination junction) as well as transmission of red photons from the III-V to the Si cell. The gluing layer ensures the electrical interconnection between sub-cells and, thanks to its lower refractive index, an enhanced photon recycling in the top cell (up to 0.9% improvement of the tandem’ efficiency). We use electromagnetic simulations to optimize the ARC/Glue/ARC stack and achieve current matching; current losses from reflection (1.1 mA/cm2) remaining essentially equal to those of the direct bonding architecture (1.0 mA/cm2). TiO2 ARCs and PEDOT:PSS-based glue are synthesized and deposited by spin-coating. The lamination process is performed in air, using a simple hydraulic hot press at low curing temperature (120°C). The 1st-generation cells show a promising VOC (1.8 V), while the efficiency is currently limited by high series resistance. The 2nd generation is currently under fabrication using an improved lamination process to solve this problem.