Ruthenium pentamethylcyclopentadienyl mesitylene dimer: a sublimable n-dopant and electron buffer layer for efficient n–i–p perovskite solar cells†
Electron-transport materials such as fullerenes are widely used in perovskite solar cells to selectively transfer the photogenerated electrons to the electrodes. In order to minimize losses at the interface between the fullerene and the electrode, it is important to reduce the energy difference between the transport level of the two materials. A common approach to reduce such energy mismatch is to increase the charge carrier density in the semiconductor through doping. A variety of molecular dopants have been reported to reduce (n-dope) fullerenes. However, most of them are either difficult to process or extremely air sensitive, with most n-dopants leading to the formation of undesirable side products. Dimers formed by 19-electron organometallic sandwich compounds combine strong reducing ability, clean reactivity, and moderate air stability, while being processable both from solution and in vacuum. In this work, we have investigated the use of pentamethylcyclopentadienyl mesitylene ruthenium dimer, (RuCp*mes)2, as a dopant for C60 in fully vacuum-deposited n–i–p perovskite solar cells. The (RuCp*mes)2 was either co-evaporated with the fullerene or deposited as a pure thin film on top of the transparent electrode prior to the deposition of the fullerene. It was found that both the co-evaporated blends and the bilayers are effective electron-transport layers, leading to solar cells with efficiencies up to 18%.