Superior photoelectrodes for solid-state dye-sensitized solar cells using amphiphilic TiO₂

Anatase amphiphilic TiO₂ (am-TiO₂) nanoparticles with an average diameter of ∼15 nm were synthesized using a sol-gel hydrothermal synthesis. The colloidal dispersion of am-TiO₂ in alcoholic solvents had outstanding stability for several months at room temperature (RT). Hierarchically structured TiO₂ nanospheres (TiO₂-NSs) were then successfully prepared by an electrostatic spray (e-spray) method using the am-TiO₂ dispersion at RT. The e-sprayed TiO₂-NSs were employed as photoelectrodes for solid-state dye-sensitized solar cells (ss-DSSCs), in which a novel plastic crystal/polymer blend based solid-state electrolyte (PC-SSE), iodine-doped succinonitrile/polyacrylonitrile, was used. A photovoltaic power conversion efficiency (PCE) of ∼7.6% was reached under simulated AM 1.5 G (100 mW cm^-2) illumination, while the incident photon-to-electrical current conversion was >60% upon excitation at 550 nm. The unique morphology of the TiO₂-NSs, in which primary anatase TiO₂ nanoparticles are interconnected within the submicron-scale spheres, exhibited superior characteristics as photoelectrodes for our ss-DSSCs compared to conventional TiO₂ nanoparticles (TiO₂-NPs). The nanoparticles-in-sphere shape offered enhanced surface area and pore volume for high dye-loading and beneficial solid-state electrolyte (SSE) infiltration, while the submicron-scale of the spheres induced efficient visible-light scattering. The efficient interconnection among the primary anatase TiO ₂ nanoparticles within the e-sprayed NSs offered enhanced charge collection efficiency and photoelectron density, which were confirmed by intensity-modulated photocurrent/photovoltage spectroscopy and electrochemical impedance spectroscopy. These superior properties of the TiO₂-NSs were manifested in the cell performance. The PCE of the TiO₂-NS-based ss-DSSCs was >2 times higher than that of TiO₂-NP-based ss-DSSCs, with notable concurrent improvement in the short-circuit current (J _SC = 13.9 mA cm^-2), open circuit voltage (V_OC = 765 mV), and fill factor (FF = 71.6%). The optimized cell efficiency of 7.6% is thus far the highest reported efficiency for PC-SSE-based DSSCs. Both the single-crystal-like anatase primary particles and the advantageous morphology effectively contributed to the high performance of the ss-DSSCs. Since the e-spray method is a high-throughput, scalable process, our hierarchically structured TiO₂-NSs are a promising material for high-efficiency ss-DSSCs, possessing a range of advantages over conventional TiO ₂-NPs.