Monolithic Thin-Film Chalcogenide-Silicon Tandem Solar Cells Enabled by a Diffusion Barrier

Following the recent success of monolithically integrated Perovskite/Si tandem solar cells, there has been a surge of interest in alternative wide bandgap top-cell materials with prospects of a fully earth-abundant, stable and efficient tandem solar cell. Thin film chalcogenides such as Cu₂ZnSnS₄ (CZTS, 1.6
eV band gap) or Cu2BaSnS4 (CBTS, 2.0 eV band gap) could be suitable candidates. However, this class of materials has the disadvantage that generally at least one high temperature step (> 500 C) is needed during the synthesis, which could contaminate the Si bottom cell. Here, we systematically investigate the monolithic integration of CZTS and CBTS on a Si bottom solar cell. A simple double-sided Tunnel Oxide Passivated Contact (TOPCon) structure is used as bottom cell, and a thin TiN layer is selected as both a diffusion barrier and a recombination layer between the two sub-cells. We show that TiN successfully mitigates in-diffusion of CZTS elements into the c-Si bulk during the high temperature sulfurization process, and no evidence of electrically active deep Si bulk defects in samples protected by just 10 nm TiN. Post-process minority carrier lifetime in Si exceeded 1.5 ms, i.e., a promising implied open-circuit voltage (i-Voc) of 715 mV after the high temperature sulfurization. Based on these results, we demonstrate the first proof-of-concept twoterminal CZTS/Si and CBTS/Si tandem devices with a efficiencies up to 3.3%. A general implication of this study is that the growth of complex semiconductors on Si using high temperature steps is technically feasible, and can potentially lead to efficient monolithically integrated two-terminal tandem solar cells.