Defects play important roles in semiconductors (SCs). Unlike those in bulk SCs, defects in two-dimensional (2D) SCs are exposed to the surrounding environment, which can potentially modify their properties/functions. Air is a common environment, yet its impact on the defects in 2D SCs still remains elusive. Here we study the interaction between air and chalcogen vacancies (V(X)), the most typical defects in 2D SCs. Although the interaction is weak for most molecules in air, O2 can be chemisorbed at V(X) with a barrier that correlates with the SC cohesive energy and can be overcome even at room temperature for certain SCs. Importantly, the chemisorbed O2 changes the V(X) from commonly believed harmful carrier-traps to electronically benign sites. This unusual behavior originates from the isovalence between O2 and X when bonded with metal. Based on these findings, a facile approach is proposed to improve the performance of 2D SCs by using air/O2 to passivate the defects.

Defects play important roles in semiconductors (SCs). Unlike those in bulk SCs,d efects in two-dimensional (2D) SCs are exposed to the surrounding environment, which can potentially modify their properties/functions.A ir is ac ommon environment, yet its impact on the defects in 2D SCs still remains elusive.Here we study the interaction between air and chalcogen vacancies (V X ), the most typical defects in 2D SCs.Although the interaction is weak for most molecules in air,O 2 can be chemisorbed at V X with abarrier that correlates with the SC cohesive energy and can be overcome even at room temperature for certain SCs.I mportantly,t he chemisorbed O 2 changes the V X from commonly believed harmful carrier-traps to electronically benign sites.This unusual behavior originates from the isovalence between O 2 and Xw hen bonded with metal. Based on these findings,afacile approach is proposed to improve the performance of 2D SCs by using air/O 2 to passivate the defects.

Multi‐junction cells can significantly improve the energy yield of photovoltaic systems over a single‐junction cell. The internal interconnection scheme of the subcells is an important aspect in determining the resulting levelized cost of electricity. For a dual‐junction cell, two approaches are commonly discussed: series‐connected tandem cells with two terminals or independently working subcells in a four‐terminal (4T) tandem device. In this paper, we explore the working principle and the operation modes of a third, rarely discussed option: a three‐terminal (3T) tandem cell using a back‐contacted bottom cell with 3Ts. We use current–voltage measurements of illuminated 3T interdigitated back contact cells and confirm that the front and rear base contacts are at similar quasi‐Fermi level positions, which enables the bottom cell to either efficiently collect surplus carriers, in the case of a current‐limiting or carrier injecting top cell, or inject majority carriers, in the case of a current‐limiting bottom cell. As a result, no current matching is needed. The power output of an idealized 3T bottom cell without resistive effects is independent of the current density applied from the top cell. These characteristics of the 3T bottom cells enable a 3T tandem to operate as efficiently as a 4T tandem, while being compatible with monolithic design and not requiring intermediate grids. We propose a simple equivalent circuit model including additional resistive effects, which describes a real 3T bottom cell and achieves excellent agreement to the experiment. We deduce design guidelines for a 3T bottom cell in different operation regimes.

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