Charge transfer and surface morphology analysis of heteroatom-doped activated carbon for dye-sensitized solar cells

This study explores the feasibility of heteroatom-doped activated carbon (Ac) as a low-cost substitute for platinum (Pt) counter electrode (CE) in dye-sensitized solar cells (DSSCs). Ac was doped with nitrogen (N), sulfur (S), and phosphorus (P), both individually and in combination (S-N-P), to examine their effects on Ac structure, electrochemical behavior, and DSSC performance. Scanning electron microscopy (SEM) demonstrated notable micromorphological changes due to doping, influencing porous structure and surface uniformity. While undoped Ac exhibited a rough and irregular microstructure, N-doping reduced surface roughness and irregularity, leading to a more ordered porous structure. However, it also increased charge transfer resistance due to the formation of larger pores, ultimately resulting in lower efficiency. P-doping introduced structural disorder, further elevating charge transfer resistance and shortening electron lifetime, resulting in the lowest DSSC power conversion efficiency (PCE) of 0.9 %. In contrast, S-doping produced a more compact structure with enhanced electrocatalytic activity, improving PCE (3.2 %). The best performance was observed in S-N-P co-doped Ac, which achieved a PCE of 5.0 %, approaching that of Pt (6.6 %), due to reduced charge transfer resistance (R1 = 6.5 Omega). Electrochemical impedance spectroscopy (EIS) confirmed that lower charge transfer resistance correlates with higher DSSC performance. Raman and XPS analyses further supported this result by confirming balanced defect density and rich surface functionalization in the S-N-P co-doped Ac. The research demonstrates the potential of multiheteroatom doping of Ac for developing scalable, sustainable, and environmentally friendly alternatives to platinum for DSSC applications.