Effect of Anchoring\nUnit, <i>N</i>‑Alkyl\nChain Length, and Thickness of Titanium Dioxide Layer on the Efficiency\nof Dye-Sensitized Solar Cells (DSSCs) and Tandem DSSCs

Dye-sensitized solar cells (DSSCs) and tandem dye-sensitized\nsolar\ncells employing metal-free organic dyes are an efficient substitute\nfor expensive nonrenewable silicon-based solar cells. Phenothiazine-based\nmetal-free organic dyes having high molar extinction coefficient values\nand compatible HOMO–LUMO levels have the potential to replace\ndyes based on rare earth metals. A series of simple donor–acceptor\nphenothiazine dyes, differing in anchoring unit structure and <i>N</i>-alkyl chain length, were synthesized, characterized, and\ntested in dye-sensitized solar cells. Additionally, the effect of\nTiO<sub>2</sub> layer thickness on the photovoltaic response of fabricated\ncells was analyzed. The thermal, optical, and electrochemical properties\nof the synthesized dyes with cyanoacrylic acid or ethyl 2-(1H-tetrazol-5-yl)\nacetate were investigated using DSC, UV–vis, photoluminescence\nspectroscopy, and cyclic and differential pulse voltammetry, respectively.\nBased on density functional theory, the frontier molecular orbital\nstructures and energy levels as well as the adsorption energy of the\ndyes on the TiO<sub>2</sub> surface and its size were estimated. The\nselected dye was applied with N719 for the preparation of tandem dye-sensitized\nsolar cells. The effects of both anchoring unit and <i>N</i>-alkyl chain length as well as TiO<sub>2</sub> thickness on device\nperformance were demonstrated. The devices with TiO<sub>2</sub> layer\nthickness of about 9 μm, sensitized with the PTZ dyes bearing\ncyanoacryclic acid anchoring unit, showed better photovoltaic performance.\nThe modification of device structure involved utilization of 2-cyano-3-(10-ethyl-10H-phenothiazin-3-yl)acrylic\nacid (PEC) dye, blocking layer, and coadsorbent (CDCA), and the fabrication\nof tandem DSSCs resulted in an increase of photovoltaic efficiency\nto 7.34% as compared to simple (before modification) DSSCs with power\nconversion value of 3.73%.