Searching for Fullerene Electron Conductors for Perovskite Solar Cells

Abstract

Fullerene derivatives are useful as electron-transport materials (ETMs) in planar heterojunction perovskite solar cells, but the guideline for an efficient fullerene electron conductor is still unclear. Recently we have been able to evaluate photovoltaic performances of a series of typical fullerene derivatives, such as indene-C60 bisadduct (ICBA), 2-(3-ethoxycarbonylpropyloxy)-(5,8)-dihydronaphthyl-(6,7)-[6,6]-C60 (EDNC), and 2- benzyloxy-(5,8)-dihydronaphthyl-(6,7)-[6,6]-C60 (BDNC) as well as [6,6]-phenyl-C61-butyric acid methyl ester (PCBM), in inverted planar heterojunction perovskite solar cells with the cell structure of ITO/PEDOT:PSS/CH3NH3PbI3/fullerenes/Al. The LUMO energy level of PCBM matches well with that of CH3NH3PbI3, but ICBA has ~0.2 eV higher LUMO energy level than that of PCBM. The devices using PCBM and ICBA as ETMs exhibited power conversion efficiency (PCE) of 15.4% and 1.45%, respectively, with implication about LUMO energy level of fullerene derivative is critical to influence photovoltaic behaviors of perovskite solar cells. Although EDNC has the similar LUMO with PCBM, lower electron mobility of EDNC resulted in slightly lower PCE (12.64%). Both EDNC and BDNC have almost the same LUMO energy level and electron mobility, on the other hand, better surface morphology of EDNC contributes to better photovoltaic performance for EDNC [than that of BDNC (7.36%)]. This work demonstrated that guidelines such as LUMO energy level, electron mobility, and surface morphology should be envisaged in searching for fullerene derivatives as efficient ETM in perovskite solar cells.