Device architecture for efficient, low-hysteresis flexible perovskite solar cells: Replacing TiO^sub 2^ with C^sub 60^ assisted by polyethylenimine ethoxylated interfacial layers

We here report methylammonium lead-iodide-based perovskite solar cells (psSCs) in which a TiO2 layer (a de-facto electron transport layer of psSCs) is replaced with a layer of C60 evaporated onto indium tin oxide layers covered with polyethylenimine ethoxylated (PEIE) layers. Unlike TiO2 layers requiring a high-temperature sintering, C60 is deposited while the substrate is held at room temperature, making it compatible with various plastic substrates for flexible psSCs. The PEIE layers are shown to play key roles, not only as an electron-collecting interfacial layer but also as a surface modifier that helps maintain the integrity of the C60 layers during spin-coating of the perovskite active layers. Using the proposed device architecture, we demonstrate flexible psSCs that exhibit power conversion efficiency as high as 13.3% with low hysteresis.