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.