Double fullerene cathode buffer layers afford highly efficient and stable inverted planar perovskite solar cells

Fullerene derivatives especially [6,6]-phenyl-C61-butyric acid methyl ester (PCBM) with strong electron-accepting abilities have been commonly implemented as indispensable cathode buffer layers (CBLs) of inverted (p-i-n) planar perovskite solar cells (iPSCs) to facilitate electron transport. However, only a single fullerene CBL is typically used in iPSC devices, resulting in interfacial energy offset between fullerene CBL and metal cathode and consequently insufficient electron transport. Herein, we synthesized a novel bis-dimethylamino-functionalized fullerene derivative (abbreviated as PCBDMAM) and applied it as an auxiliary fullerene interlayer atop of PCBM to form a PCBM/PCBDMAM double fullerene CBL, leading to dramatic enhancement of both efficiency and ambient stability of iPSC devices. Incorporation of PCBDMAM interlayer facilitates the formation of interfacial dipole layer between PCBM and Ag cathode, resulting in decrease of the work function of the Ag cathode. As a result, the CH3NH3PbI3 (MAPbI3) iPSC devices based on PCBM/PCBDMAM double fullerene CBL exhibit the highest power conversion efficiency (PCE) of 18.11%, which is drastically higher than that of the control device based on single PCBM CBL (14.21%) and represents the highest value reported for double fullerene CBL-based iPSC devices. Moreover, due to the higher hydrophobicity of PCBDMAM than PCBM, iPSC devices based on PCBM/PCBDMAM double fullerene CBL shows an enhanced ambient stability, retaining 67% of the initial PCE after storage 1440 h exposure under the ambient atmosphere without any encapsulation, whereas only 43% retaining was achieved for the control device based on single PCBM CBL.