Achieving high efficiency and long-term stability in tandem solar cells remains a challenging issue, primarily due
to the critical selection of appropriate material systems and the mitigation of interfacial recombination losses.
This simulation study presents a detailed design and optimization process for a Se/Sb2(S, Se)3 tandem solar cell
in a 2-T monolithic hole-transport layer (HTL)-free configuration. Selenium (Se) serves as the wide bandgap
(WBG), with Eg = 1.9 eV, absorber in the front sub-cell, paired with Sb2(S, Se)3 as the narrow bandgap (NBG),
with Eg = 1.32 eV, absorber in the rear sub-cell, creating a complementary absorption profile across the solar
spectrum. Both baseline solar cell structures were chosen based on experimental cells to ensure the credibility of
the TCAD simulations. Initially, a 2-T Se/Sb2(S, Se)3 tandem was designed, achieving a simulated power conversion
efficiency (PCE) of 10.64 %. To further simplify the possible fabrication process, reduce interfacial
recombination, and enhance structural stability while maintaining competitive performance, an HTL-free tandem
structure was introduced. This design removes the HTLs from both sub-cells. Using comprehensive Silvaco TCAD
simulations, a systematic optimization process was performed, including fine-tuning the electron-transporting
layer (ETL) parameters, specifically focusing on conduction band offset (CBO) and doping concentrations to
facilitate effective charge transport. Our analysis also considers the impact of bulk defect density in both absorber
layers, which has shown a significant role in improving carrier lifetimes. Furthermore, we explored the role of
absorber layer thicknesses and their effects on current matching conditions, achieving a final configuration that
balances light absorption between the top and rear cells. With the proposed configurations, the optimized tandem
reached a simulated PCE of 22 % and an open-circuit voltage of 1.95 V. These findings provide a pathway
toward scalable, high-efficiency eco-friendly tandems suitable for a variety of photovoltaic applications, highlighting
the potential of Se/Sb2(S, Se)3 tandem structures in advancing next-generation solar technologies.