Thin-film solar cells with a CdS:Al/CdS/PbS heterojunction were fabricated using an ammonia-free chemical bath deposition method. XRD analysis revealed hexagonal CdS films with (002) preferred orientation and cubic PbS films with (111), (200), and (220) crystallographic planes. Al-doping reduced CdS crystallite sizes from 37 nm to 17 nm and increased its bandgap from 2.44 eV to 2.47 eV. AFM studies demonstrated enhanced surface uniformity with lower roughness (1.96 nm) for CdS:Al films compared to undoped CdS (2.80 nm). The integration of an ultrathin CdS:Al buffer layer (44 nm) significantly enhanced device performance, increasing the fill factor from 26 % to 87 %, open-circuit voltage from 0.0585 V to 0.2840 V, and short-circuit current density by an order of magnitude from 3.59 x 10-3 to 43.4 x 10-3 mA/cm2. This improvement is attributed to optimized band alignment, reduced interfacial recombination, and enhanced charge carrier separation efficiency. The approach offers a scalable and low-cost method for engineering interfaces and tailoring solar cells' behavior, with insights applicable to emerging photovoltaic technologies.