Charge transfer rates at the interfaces of the perovskite absorber also strongly influence hysteresis. If unbalanced charge collection exists, i.e., if the charge transfer rates between perovskite and the n-/p-type selective contacts are quite different, charges will accumulate on the interface with a lower charge collection rate and build up a transient capacitance. Evidence of trapped charges was found at two interfaces in the conventional n-i-p structure,147 where the electron and hole mobilities in the ETM and HTM differ, respectively.104 Interestingly, the n-i-p device employing a thin mesoporous ETM and an HTM with desired hole mobility typically exhibits negligible hysteresis, which is likely due to the enhanced surface area for electron injection and improved hole transport, respectively. In contrast, the inverted p-i-n cells exhibit much less hysteresis, presumably due to a balanced charge carrier transport and surface passivation on the perovskite/fullerene interface.104 However, it was demonstrated that the so-called hysteresis-free p-i-n devices exhibit substantial hysteresis when the temperature is reduced to 175 K [Fig. 8(d)].153 Thus, changing the device architecture may not address the underlying mechanism of hysteresis in the perovskite materials themselves. Moreover, as the devices aged, the hysteresis was aggravated due to the degraded electronic quality of perovskite, especially at interfaces.138 This shows the importance of improving the stability of the perovskite and the engineering at the interfaces to prevent materials degradation. Furthermore, compositional engineering may also reduce the hysteresis. Unlike , possesses an asymmetric charge transfer rate, which balances the charge extraction at either side of the perovskite and alleviates the hysteresis.56 Further development may show that the hysteresis could be reduced or eliminated.