Effect of Anchoring Unit, N-Alkyl Chain Length, and Thickness of Titanium Dioxide Layer on the Efficiency of Dye-Sensitized Solar Cells (DSSCs) and Tandem DSSCs

Dye-sensitized solar cells (DSSCs) and tandem dye-sensitized solar cells employing metal-free organic dyes are an efficient substitute for expensive nonrenewable silicon-based solar cells. Phenothiazine-based metal-free organic dyes having high molar extinction coefficient values and compatible HOMO–LUMO levels have the potential to replace dyes based on rare earth metals. A series of simple donor–acceptor phenothiazine dyes, differing in anchoring unit structure and N-alkyl chain length, were synthesized, characterized, and tested in dye-sensitized solar cells. Additionally, the effect of TiO₂ layer thickness on the photovoltaic response of fabricated cells was analyzed. The thermal, optical, and electrochemical properties of the synthesized dyes with cyanoacrylic acid or ethyl 2-(1H-tetrazol-5-yl) acetate were investigated using DSC, UV–vis, photoluminescence spectroscopy, and cyclic and differential pulse voltammetry, respectively. Based on density functional theory, the frontier molecular orbital structures and energy levels as well as the adsorption energy of the dyes on the TiO₂ surface and its size were estimated. The selected dye was applied with N719 for the preparation of tandem dye-sensitized solar cells. The effects of both anchoring unit and N-alkyl chain length as well as TiO₂ thickness on device performance were demonstrated. The devices with TiO₂ layer thickness of about 9 μm, sensitized with the PTZ dyes bearing cyanoacryclic acid anchoring unit, showed better photovoltaic performance. The modification of device structure involved utilization of 2-cyano-3-(10-ethyl-10H-phenothiazin-3-yl)acrylic acid (PEC) dye, blocking layer, and coadsorbent (CDCA), and the fabrication of tandem DSSCs resulted in an increase of photovoltaic efficiency to 7.34% as compared to simple (before modification) DSSCs with power conversion value of 3.73%.