Chlorinated conjugated copolymer interface modification enables record-efficiency of all-PbS quantum dot tandem solar cells

All-lead sulfide (PbS) quantum dot (QD) tandem solar cells (TSCs) offer a promising pathway to surpass single-junction efficiency limits. However, their performance is hindered by the vulnerability of the wide-bandgap (WBG) top cell, which suffers chemical and physical damage during the deposition of 1,2-ethanedithiol (EDT)-capped hole-transport layers (HTLs) and interconnecting layers. This results in morphological voids and interfacial trap states. Here, we introduce a chlorinated benzodithiophene-based conjugated copolymer (D18-Cl) as a multifunctional interfacial passivator at the WBG/HTL interface. D18-Cl could bind to unsaturated Pb sites via chlorine and passivate sulfur vacancies through thiophene ring coordination. This strategy eliminates pinholes and shields the WBG absorber from subsequent processing damage. Furthermore, D18-Cl modification aligns the energy levels by elevating the highest occupied molecular orbital (HOMO) level of the absorber, creating a graded pathway that reduces the hole-extraction barrier and suppresses non-radiative recombination. Consequently, the D18-Cl-modified semitransparent WBG cell achieves a power conversion efficiency (PCE) of 10.36%. When integrated into a monolithic TSC, the device yields a record PCE of 13.148% with an unprecedented fill factor of 73.14%. The encapsulated TSC retains over 90% of its initial PCE after 500 h of MPP tracking, demonstrating exceptional stability.