Structural Configuration Effects of Freestanding TiO₂ Nanotube Arrays on Power Conversion Efficiency in Dye-Sensitized Solar Cells

Highlights: What are the main findings? Four f-TNA structural configurations (closed/open, up/down) were compared to optimize DSSC performance. The open f-TNA structure, with the barrier layer removed, showed the highest power conversion efficiency (PCE ≈ 7.73%). The best PCE (7.73%) was achieved using the open-up f-TNA photoanode configuration. Barrier layer removal in f-TNA effectively minimized charge transfer resistances (R_ct1 and R_ct2). The closed-up configuration yielded the lowest PCE (5.52%) due to poor electrolyte diffusion kinetics. What are the implications of the main findings? The structural configuration and orientation of f-TNA are critical design parameters for high-performance DSSC. Structural optimization alone is a highly effective, additive-free strategy for maxim-izing TiO₂ nanotube DSSC efficiency. Opening the f-TNA bottom is essential for enhancing charge collection and improv-ing electrolyte penetration. The study presents fundamental guidelines for engineering f-TNA electrodes in next-generation photoelectrochemical devices. Dye-sensitized solar cells (DSSCs) are known for their excellent low-light performance, cost-effectiveness, and flexibility. The photoanode has a crucial role in enhancing the overall performance of DSSCs and can be modified with different nanostructures. This study explores the impact of photoanode structure on the power conversion efficiency (PCE) of DSSCs, where four configurations of freestanding TiO₂ nanotube arrays (f-TNAs), closed-up, closed-down, open-up, and open-down, were employed as photoanodes. Performance was evaluated based on current density (J_sc), open-circuit voltage (V_oc), fill factor (FF), and PCE concerning dye adsorption, electrolyte diffusion, electron transport, and barrier layer. DSSCs based on open configurations, open-up and open-down f-TNAs, demonstrated superior performance, achieving PCE of 7.73% and 7.71%, respectively. The primary distinction between the DSSCs based on open-up f-TNAs and those based on open-down f-TNAs lies in the dye adsorption time and electron diffusion characteristics. The PCE for DSSCs with closed-down f-TNAs was measured at 6.78%, while DSSCs with closed-up f-TNAs showed a lower PCE of 5.52%. The presence of a barrier layer under the bottom of f-TNAs impacted the PCE for DSSCs with closed-down f-TNAs, whereas for DSSCs with closed-up f-TNAs, insufficient dye loading, poor electrolyte diffusion and barrier layer reduced the performance.