Perovskite/perovskite/silicon triple-junction cells can deliver higher efficiencies than single- or dual-junction solar cells, but achieving this is not without its challenges. In a recent study, researchers from King Abdullah University of Science and Technology (KAUST), Northwestern University, University of Toledo and University of Toronto combined with a synergistic additive strategy (using potassium thiocyanate and methylammonium iodide), to stabilize the top perovskite, thus addressing a major hurdle. The team fabricated a perovskite-perovskite-silicon triple-junction solar cell with a new passivation strategy based on the utilization of potassium and thiocyanate.
This approach leads to an efficiency of over 26% for 1 cm2 triple-junction solar cells. The triple-junction solar cell is based on a 15.0%-efficient top perovskite solar cell modified with potassium thiocyanate (KSCN) and methylammonium iodide (MAI). According to the team, the triple-junction device displays a remarkable power conversion efficiency improvement compared to state-of-the-art devices.
Unlocking the potential of perovskite/perovskite/silicon triple-junction solar cells requires monolithic integration of a ∼2.0 eV band-gap perovskite subcell, characterized by a high bromide:iodide ratio (>7:3), and with low-temperature processability and high optoelectronic quality. However, light-induced phase segregation in such perovskites remains a challenge.
To address this, the team proposed modifying the wide-band-gap perovskite with potassium thiocyanate (KSCN) and methylammonium iodide (MAI) co-additives, where SCN− increases the perovskite grain size, reducing the grain boundary defect density; K+ immobilizes the halide, preventing the formation of halide vacancies; and MA+ eliminates the residual light-destabilizing SCN− in the perovskite films via double displacement reactions.
This co-additive strategy enables enhanced photostability, whereas individual usage of MAI and KSCN would result in adverse effects. Triple-junction tandem solar cells, incorporating co-additive-modified 2.0 eV perovskites as top cell absorbers, reach a 3.04 V open-circuit voltage and a PCE of 26.4% over a 1 cm2 area. The triple junction device achieved a short-circuit current density of 11.9mA/cm2 and a fill factor of 72.9%. The team claims this remarkable performance is mainly due to the high efficiency of the perovskite bottom cell, which is one of the highest ever reported for a cell with such a high bandgap.
