Carbazole-phenothiazine sensitizers boost tandem DSSC efficiency to 12.85 %

This study presents a significant advancement in tandem dye-sensitized solar cells (T-DSSCs) through the development of two carbazole-phenothiazine hybrid sensitizers, AEFH-1 and AEFH-2. AEFH-2, featuring a 4-carboxylcyanoacetamide acceptor group, thus achieving a remarkable power conversion efficiency (PCE) of 11.70 %, surpassing AEFH-1 (10.21 %) and the standard N719 dye (7.60 %). The superior performance of AEFH-2 is attributed to its optimized molecular design, which enhances the charge separation and electron injection efficiency. The incident photon-to-current efficiency (IPCE) of AEFH-2 reached 91.54 %, which was significantly higher than that of N719 (77.0 %) because of its strong electron-withdrawing groups and multiple anchoring functionalities, which improved TiO₂ binding and charge transfer. Furthermore, an innovative double-sided parallel tandem DSSC (PT-DSSC) architecture was developed by integrating (N719 (top) and AEFH-2 (bottom)), resulting in a PCE of 12.85 %. This configuration exhibited exceptional photovoltaic parameters, including a Voc of 0.900 V, Jsc of 21.01 mA/cm², and a fill factor of 67.95 %. The high IPCE of 97.50 % in the tandem setup was attributed to the complementary absorption profiles of N719 and AEFH-2, coupled with the light-trapping effect of the double-sided structure, enabling superior light harvesting and charge separation. Stability analysis further confirmed the durability of the tandem PT-DSSC, with performance maintained over 1000 h of continuous illumination, thus emphasizing its practical applicability. These findings underscore the potential of molecular engineering and architectural innovation to significantly enhance the efficiency and scalability of DSSCs, paving the way for high-performance, long-lasting solar energy devices.