Optimizing the Back Contact of Kesterites and Perovskites: Band Edge Design and Defect Engineering in Molybdenum Chalcogenides

Abstract Back contacts, as an important part of solar cell devices, play a critical role in efficient separation of carriers and reduction of interfacial recombination. Here, by analyzing interfacial band alignment, thickness and defect properties of back contacts, a design principle for back contacts is proposed to properly accommodate the interrelated factors of short‐circuit current ( J sc ), open‐circuit voltage ( V oc ), series resistance ( R s ), and nonradiative recombination centers (NRCs) at the same time. A preferred back contact should have (i) a high valance band maximum (VBM) and (ii) a low Fermi level relative to absorber to drive hole extraction and block electron leakage, (iii) a thin design under the premise of p + ‐type characteristic to effectively reduce R s , which also ensures a wide depletion region on the absorber side to promote hole collection, and (iv) no additional deep‐level defects as interfacial NRCs. Taking molybdenum chalcogenide as example, p + ‐type MoSe 2 /MoS 2 realized by group VB element doping satisfies the design principles and is promising as an excellent back contact for kesterite/perovskite solar cells. This study gives theoretical guidance for designing an ideal back contact and shows how to improve the photovoltaic performance of solar cells by interfacial optimization.