Revealing the Impact of Phase Transition on n = 1 2D Perovskite Photodetectors With Intrinsically Tunable Narrowband Detection

2D perovskites featuring a single layer of perovskite octahedra sandwiched between organic cations (n = 1) display narrow absorption due to their quantum-confined structure. They offer a compelling route to filter-free, narrowband photodetection compared with broadband 3D counterparts. While halide mixing provides spectral tunability, it introduces severe phase segregation and energetic disorder. Current understanding of this phenomenon is derived mainly from static material characterisation, leaving its dynamic impact in operational devices unexplored. Herein, we bridge this gap by integrating n = 1 (PEA)₂PbBr_xI_4-x into photoconductors, achieving tunable response from 400 to 520 nm, and a maximum specific detectivity (D^*) of 2.11 × 10¹¹ Jones at an applied bias of 20 V. We reveal the existence of two different phases and the gradual transition based on halide compositions, studied how mixing halide drives phase segregation with density functional theory calculations. Although the chloride additive PEACl suppressed phase segregation in mixed halides, it introduced additional traps that reduced photocurrent. The additive enhanced the out-of-plane orientation, disrupting in-plane transport in photoconductors, but significantly improved performance in photodiodes. This work provides the first device-level insight into halide immiscibility in n = 1 2D-perovskites, revealing that overcoming the performance limitations in these systems requires balancing long-range structural order with short-range electronic disorder. © 2026 The Author(s). Small published by Wiley-VCH GmbH.