Defect Engineering in CdS: Resolving <i>p</i> -Type Doping Controversies and Enabling Breakthrough <i>n</i> -Type Conductivity for Thin-Film Photovoltaics

Cadmium sulfide (CdS) serves as a vital wide-bandgap buffer/window layer in thin-film photovoltaics due to its superior optoelectronic properties. Despite its intrinsic weak <i>n</i>-type conductivity, persistent controversies surround the <i>p</i>-type doping feasibility. Concurrently, <i>n</i>-type enhancement in chemical bath deposition-grown CdS─where low carrier concentrations critically constrain device performance─faces persistent inconsistencies in dopant efficacy reports and insufficient exploration of alternatives. In this work, we resolve these challenges through systematic first-principles defect analysis, demonstrating that <i>p</i>-type doping remains fundamentally precluded by spontaneous V_S-mediated compensation and deep acceptor levels in Group IB (Cu_Cd, Ag_Cd, Au_Cd) and Group VA (N_S, P_S, As_S) substitutions, compounded by interstitial donor compensation (Li_i, Na_i, K_i) dominating over substitutional acceptors (Li_Cd, Na_Cd, K_Cd) in Group IA systems. For <i>n</i>-type enhancement, transformative breakthroughs emerge where counterintuitive Cd-rich conditions maximize the Group IIIA efficacy (Al_Cd, Ga_Cd, In_Cd), novel Group IIIB (Sc_Cd, Y_Cd, La_Cd) substitutions achieve 5-6 order enhancement, and halogen doping (Cl_S, Br_S, I_S) under S-poor conditions yields a breakthrough 6-7 order conductivity improvement─surpassing all cation-based approaches. These defect-engineered strategies establish new paradigms for high-performance CdS window layers.