Stacking n-type layers: effective route towards stable, efficient and hysteresis-free planar perovskite solar cells

Perovskite solar cell (PSC) has drawn widespread concern for its high efficiency and facile low-temperature solution fabrication, promising for the alternative low-cost photovoltaic energy. However, before commercializing, the problems regarding to the environmental stability and electrical hysteresis of PSCs still need to be tackled. We present a simple route by stacking n-type materials TiO₂ (spray pyrolysis) and SnO₂ (aqueous colloidal) for planar devices, and achieve a stable (~91% efficiency >90 days at 30%RH without encapsulation), hysteresis-free (hysteresis index 0.004) and efficient (18.03%) perovskite solar cells. Moreover, we carry out a system stability evaluation for unsealed devices under harsh conditions with intense UV illumination, heat and damp in an environmental chamber. Both TiO₂ and SnO₂ devices demonstrate inferior performance compare to the stacked samples in the accelerated stability tests. Photoluminescence, phase and electrical characterizations suggest that the performance enhancement is ascribed to the synergetic optimization from concealing the defective interface and promotion of carriers transfer and blocking. With the optimized n-type layers, a high efficiency of 14.39% is obtained for the 5 × 5 cm² sub-module. These results suggest that by stacking n-type layers could enable superior overall performances to common electron transport layers (ETLs) (TiO₂, SnO₂ and etc.), providing facile routes for fabricating efficient PSCs with high stability.