Highly conductive Na-embedded carbon nanowalls for hole-transport-material-free perovskite solar cells without metal electrodes

New generation solar cells are necessary to replace current silicon solar panels for low cost and high energy conversion efficiency. The perovskite photovoltaic device is a rising star among the 3rd generation solar cells due to its rapidly increased energy conversion efficiency. Recently invented Na-embedded carbon (Na@C) nanowalls, which possess ultrahigh conductivity and a large accessible surface area, are a very promising electrode material. Herein, the Na@C nanowall material was demonstrated as an excellent counter electrode for HTM-free perovskite solar cells (PSCs), leading to a high power conversion efficiency of 8.89% under the AM 1.5G simulated sunlight. This efficiency is almost two times as high as that (4.56%) of a PSC with regular graphene synthesized via chemical exfoliation of graphite.