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Transition metal dichalcogenides (TMDs) exist in two distinct phases: the thermodynamically stable trigonal prismatic (2H) and the metastable octahedral (1T) phase. Phase engineering has emerged as a potent technique for enhancing the performance of TMDs in optoelectronics applications. Nevertheless, understanding the mechanism of phase transition in TMDs and achieving large-area synthesis of phase-controlled TMDs continue to pose significant challenges. This study presents the synthesis of large-area monolayered 2H-MoS 2 and mixed-phase 1T/2H-MoS 2 by controlling the growth temperature in the chemical vapor deposition (CVD) method without use of a catalyst. The field-effect transistors (FETs) devices fabricated with 1T/2H-MoS 2 mixed-phase show an on/off ratio of 10 7 . Photo response devices fabricated with 1T/2H-MoS 2 mixed-phase show ≈55 times enhancement in responsivity (from 0.32 to 17.4 A W -1 ) and 10 2 times increase in the detectivity (from 4.1 × 10 10 to 2.48 × 10 12  cm Hz W -1 ) compare to 2H-MoS 2 . Introducing the metallic 1T phase within the 2H phase contributes additional carriers to the material, which prevents the electron-hole recombination and thereby increases the carrier density in the 1T/2H-MoS 2 mixed-phase in comparison to 2H-MoS 2. This work provides insights into the self-doping effects of 1T phase in 2H MoS 2 , enabling the tuning of 2D TMDs properties for optoelectronic applications., (© 2024 Wiley‐VCH GmbH.)