Efficient, stable silicon tandem cells enabled by anion-engineered wide-bandgap perovskites

Abstract

There has been growing interests in perovskite-silicon tandem solar cells to reach ultrahigh efficiency beyond the Shockley-Queisser limit. The ideal band gap for the tandem configuration is ~1.67 to 1.75 eV for the top cell and 1.12 eV for the bottom cell. The band gap of perovskites can be tuned by (partial) replacement of iodine anions with bromine or chlorine. However, the replacement of I with Br by more than 20%, which is necessary to enlarge the band gap to ~1.7 eV, leads to stability issues under illumination through phase separation. One approach to stabilize the perovskite is to introduce a two-dimensional (2D) phase in which sheets of [PbX6]−2 octahedra are separated by an excess number of long-chain molecules. Most of the recent studies have focused on the cation components of the 2D additives rather than the anions. We developed a 2D-3D mixed wide band gap (1.68 eV) perovskite using a mixture of thiocyanate with the more conventional choice, iodine. Through a careful application of atomic resolution transmission electron microscopy, we demonstrated that electrical and charge transport properties as well as the physical location of 2D passivation layers can be controlled with anion engineering of the 2D additives. Moreover, we can use this approach to extend light stability and to improve device performance. For a perovskite device, we achieved a PCE of 20.7% that retained > 80% of its initial efficiency after 1000 hours of continuous illumination in working conditions. For a monolithic 2T perovskite/Si tandem solar cells, the champion 2T tandem device achieved a PCE of 26.7%.