Display omitted •Demonstration of simple in-situ synthesis of CsPbI3-xBrx using sodium dodecyl sulphate.•Dual-passivation of cationic and anionic vacancy by SDS is demonstrated.•Suppression of halide migration and segregation. The phenomenal growth of metal halide perovskites over the last decade makes them intriguing for next-generation optoelectronic materials. Despite numerous advantages over the traditional light absorbing materials, mixed-halide perovskites suffer from defects which enhances halide migration and segregation, leading to shorter lifetime of charge carriers. Hear in, we employ a simple defect passivation technique using sodium dodecyl sulphate (SDS) to leading to significantly higher stability and higher photoluminescence efficiency. The uncoordinated lead sites or halide interacts and Schottky defects or cation vacancy are dual passivated by both anion and cation of SDS resulting in suppression of halide migration and thus segregation. The passivated samples exhibit their ability to have a longer carrier lifetime ∼65 ns and also retain the longer lifetime (∼56 ns) post prolonged illumination unlike the non-passivated samples which show complete reduction in lifetime with illumination time.