Dye-sensitized solar cells (DSCs) for outdoor applications must show both high efficiency and long-term stability. Here we introduce a co-sensitized, ionic liquid electrolyte-based DSC meeting these requirements. A key feature of our embodiment is the concerted action of two judiciously designed organic dyes, whose co-adsorption at the surface of a mesoscopic TiO(2 )scaffold results in the formation of a compact and highly robust self-assembled monolayer, harvesting sunlight across the whole visible region and converting the photons into charges with near-unity quantum efficiency. Apart from producing a high photocurrent, the dense dye layer blocks the back electron transfer from TiO2 to the redox electrolyte, increasing the photovoltage. This allows for the first time to attain a solar to electric power-conversion efficiency of 10% with an ionic liquid-based DSC Remarkably, the co-sensitized cell is stable under both full-sunlight soaking at 60 degrees C and heat stress at 85 degrees C for 1,000 hr.