Control of Molecular Orientation in Naphthalene Diimides

Abstract

Lead halide perovskite solar cells represent attractive cost-effective and solution-processable photovoltaics. At present, the record-high performance perovskite solar cells are based on an n-i-p device configuration, in which metal oxides, such as TiO2, are generally utilized as electron transporting layer. Deposition of metal oxides usually requires high temperature sintering process, which prevents mass production and application to flexible devices on plastic substrates. Here, we focused on the naphthalene diimide (NDI) skeleton as alternative transparent n-type materials. By introducing hydrogen atoms at the imide moiety (NDI-H), we envisioned the achievement of face-on orientation on the substrates, which is preferable for vertical charge transport, by the formation of intermolecular N–H···O hydrogen bondings. To realize both the solution-processability and the durability to the deposition of the upper layers, we designed NDI-Boc having tert-butoxycarbonyl (Boc) solubilizing groups at the imide moiety, which can be cleaved by simple thermal annealing. Infrared p-polarized multiple-angle incidence resolution spectroscopy (pMAIRS) not only confirmed the cleavage of the Boc group, but also revealed that the molecular orientation of NDI changed from edge-on to face-on during the cleavage process. Furthermore, NDI-H film works as electron transporting layer in n-i-p type planer perovskite solar cells with the power conversion efficiency of 10.7%.