Bathocuproine (BCP) buffer layer has been commonly used in inverted p–i–n perovskite solar cells (PSCs) for high performance, but its working mechanism has not been thoroughly elucidated. Here, a series of devices have been fabricated with controlled thicknesses of BCP layers deposited by thermal evaporation. With the aid of J–V data fitting by a single-diode model, the effect of BCP layer thickness on the device performance has been identified. An optimal power conversion efficiency (PCE) up to 17.9% has been obtained for the device with a critical BCP thickness of 5 nm, thanks to the formation of ohmic contact and reduction of interfacial charge recombination. While if the BCP layer is too thin or too thick, charge accumulation will emerge due to different mechanisms and lead to device performance degradation, which have been clearly confirmed by capacitance–voltage (C–V) characteristic displaying a peak capacitance at a certain bias region under illumination. The insertion of such a critical thin BCP layer between PCBM and Ag also resulted in long-term stability improvement of the devices, of which the T80 lifetime increased from ∼6 hours to ∼50 hours in ambient air. This work gives insight into improving PSCs' performance through buffer layer's optimization, and the measurement of C–V characteristic provides a new clue to detect the defection of photovoltaic device associated with charge accumulation.
