Stable and Efficient Organic Dye-Sensitized Solar Cell Based on Ionic Liquid Electrolyte

Abstract

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 TiO2 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°C and heat stress at 85°C for 1,000 hr. The two key challenges for organic dyes in DSCs consist in device thermal stability and net efficiency gain with a lower-energy-gap dye. It is imperative to implement the synthetic control of the excited state of a low-energy-gap, high-absorption-coefficient organic dye because the reduction of energy gap will always lead to an extremely fast deactivation of excited state via conical intersection, which can intercept the channel of electron injection. To break though the energy-gap law restriction, we employ an aromatic electron acceptor, 4-(benzo[c][1,2,5]thiadiazol-4-ylethynyl)benzoic acid, to afford a near-infrared-absorbing organic dye. This new dye is coupled with a wide-energy-gap organic dye for a stable and efficient co-sensitized solar cell based on an ionic liquid electrolyte. Our findings bode well for the forthcoming mass production of efficient and stable DSCs. The authors demonstrate the co-grafting of two energy-gap complementary organic dyes onto TiO2 nanocrystals to afford a compact and robust self-assembled monolayer, resulting in a reduction of interfacial charge recombination in an ionic liquid-based solar cell. The co-sensitized panchromatic solar cell with 10% efficiency presents excellent efficiency stability under the thermal stress of 85°C for 1,000 hr.