π-Conjugated Indole-Based 2D Perovskite Microcavities for Self-Powered Ultraviolet Photodetection

We report the synthesis and optoelectronic characterization of a two-dimensional layered halide perovskite (2DLP), TPA₂PbBr₄, incorporating tryptammonium (TPA) as a π-conjugated organic spacer molecule. TPA-Br, derived from indole-containing tryptamine, enables CH-π and NH-π interactions, enhancing structural rigidity and electronic coupling with the layered framework. TPA₂PbBr₄ microcrystals grown via a slow vapor diffusion method exhibit a highly oriented Ruddlesden-Popper-type structure, confirmed by powder and single-crystal X-ray diffraction (XRD) analyses. In situ temperature-dependent XRD confirms high thermal stability without phase transition from 183 to 433 K. Optical measurements reveal a sharp excitonic absorption peak near 398 nm and a narrow photoluminescence peak at 407 nm. Uniform spin-coated thin films display an excitonic double peak and pronounced Fabry-Pérot microcavity modes in their reflectance spectra, indicating strong optical confinement. The Fabry-Pérot microcavity sustained by the TPA₂PbBr₄ thin films facilitates such strong light-matter coupling, manifested by the formation of polariton modes. Planar photodetectors (PDs) with TPA₂PbBr₄ films exhibit stable ultraviolet photoresponse, low dark current, responsivity (R) of 1 mA/W, and detectivity (D*) of 2.2 × 10¹⁰ Jones. Vertically configured PDs based on TPA₂PbBr₄ thin films exploit their built-in electric field, enabling self-powered operation. The combination of strong excitonic features, microcavity-enhanced light absorption, and self-biased device operation highlights TPA₂PbBr₄ as a promising material for UV-selective PDs and low-power optoelectronic applications, benefiting from its strong excitonic features and π-conjugated organic spacer chemistry.