Excited-State and Charge Carrier Dynamics in a High-Photovoltage and Thermostable Dye-Sensitized Solar Cell
Apart from the harvesting of more infrared solar photons for a higher photocurrent, improving the photovoltage and thermal stability of a dye-sensitized solar cell are the other two key challenging issues for its performance enhancement. Herein we report a metal-free donor–acceptor dye (SC-4) characteristic of a triphenylamine-dihexylbithiophene electron donor and a benzothiadiazole-benzoic acid electron acceptor. This organic dye can be utilized for the fabrication of sensitized titania solar cells exhibiting excellent photovoltages of 1005 and 825 mV, respectively, when a volatile tris(1,10-phenanthroline)cobalt-based electrolyte and a nonvolatile iodine-based ionic liquid composite electrolyte are applied, respectively. With respect to the control dye C239 using the traditional electron acceptor cyanoacrylic acid, dye SC-4 displays not only an enhanced photovoltage owing to slower interfacial charge recombination but also an improved stability of photocurrent and efficiency even under a long-term thermal aging at 85 °C, because of negligible desorption of dye molecules from the surface of titania. The photovoltage drop of dye-sensitized solar cells under the thermal stress is identified for the first time as the intrinsic instability of the interface between titania and electrolyte, which needs to be judiciously passivated in a future study. Ultrafast PL measurements and theoretical calculations have unveiled that torsional energy relaxation and electron injection occur from the multiple nonequilibrium excited states of organic dyes, resulting in a highly distributive kinetics of electron injection. Graphic Abstract ACS Electronic Supporting Info
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