Identifying inverted-hysteresis behavior of CH3NH3PbI3-xClx planar hybrid perovskite solar cells based on external bias precondition

Solution-processable hybrid perovskite solar cells offer potential in the photovoltaic field due to their low-cost fabrication and high efficiency. However, an undesirable current-voltage (J-V) hysteresis hampers the applications of perovskite solar cells. In particular, for the inverted device, the understandings for J-V hysteresis origination are not uniform, and the inverted hysteresis phenomenon has been further complicated the hysteresis behavior. In this report, an external bias precondition method is adopted to unveil the origin of the inverted hysteresis. The results indicate that the extents of inverted hysteresis are very much dependent on the bias direction of the precondition. To further unveil the effect of the precondition on inverted hysteresis, the microscopic J-V hysteresis was also observed by using conductive atomic force microscopy (c-AFM) measurements. The results indicate that ion migration and accumulation slowly built up at the grain boundaries of the perovskite film when repeating the scan using c-AFM. Furthermore, the transient characteristics based on capacity-frequency plots and open-circuit voltage decay further identify the presence of difference for the different bias preconditions on the device, because the different bias precondition could induce different directions of ions migration and accumulation. These observations are surprising; it can be further identified that the inverted hysteresis originates from the ionic migration and accumulation, and the grain boundary is as the channel of ionic migration and accumulation. Therefore, the grain boundary plays an important role on the hysteresis effect, and preparation of large-grain or single-crystal perovskite films is the way to reduce the hysteresis effect.