NDI-based small molecules as electron transporting layers in solution-processed planar perovskite solar cells

High-temperature preparation of metal oxide-based electron transporting materials is considered to be a potential obstacle toward the commercialization of perovskite solar cells. Inverted perovskite solar cells can overcome this problem by employing metal-oxide free, low-temperature-fabricated, and solution-processed electron transporting materials. However, the conventionally-used electron transporting materials (e.g. phenyl-C61-butyric acid methyl ester (PCBM)) has several drawbacks including poor morphology control and high cost, which make its application impractical. Thus, scientists are actively searching novel organic small molecules to replace PCBM because these small compounds have tunable frontier molecular orbitals as well as good film morphology control. More importantly, these molecules can be prepared through inexpensive synthesis routes. Herein, we report the synthesis of two novel naphthalenediimide (NDI)-based electron transporting materials (4,4′-(piperazine-1,4-diyl)bis(2,7-dioctylbenzo[lmn]-[3,8]phenanthroline-1,3,6,8(2 H,7 H)-tetraone) (PDPT) and 9,9′-(piperazine-1,4-diyl) bis(4-(4-methylpiperidin-1-yl)-2,7 dioctylbenzo [lmn]-[3,8]phenanthroline-1,3,6,8(2 H,7 H)-tetraone) (PMDPT)), and found that the inverted perovskite solar cells with PMDPT as an electron transporting layer can reach a power conversion efficiency up to 9.2% while the efficiency of PSCs based on PDPT can only approach 7.6%. We believe that this improvement in the efficiency of PMDPT-based PSCs ascribes to the increased number of nitrogen atoms in the framework of PMDPT, which passivates the electron trap centers on the surface of the perovskite layer. This passivation results in less charge recombination, therefore delivering a higher Voc and PCE.