A mere 2 nm conformal titanium dioxide overlayer coated by atomic layer deposition is shown to act as a blocking layer for high-efficiency solid-state perovskite (CH3NH3PbI3) absorber-based solar cells. Surpassing the existing multilayer passivation, this ultrathin sub-nanometer layer leads to a photovoltaic power conversion efficiency of 11.5%. As a service to our authors and readers, this journal provides supporting information supplied by the authors. Such materials are peer reviewed and may be re-organized for online delivery, but are not copy-edited or typeset. Technical support issues arising from supporting information (other than missing files) should be addressed to the authors. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.