3D hybrid perovskites (APbX₃) have made a significant impact on the field of optoelectronic materials due to their excellent performance combined with facile solution deposition and up-scalable device fabrication. Nonetheless, these materials suffer from environmental instability. To increase material stability, the organic cation (A) is substituted by the non-volatile cesium cation. However, the desired photoactive cesium lead(II) iodide black phase is metastable in ambient conditions and spontaneously converts into the photo-inactive yellow δ-phase. In this work, the black phase is stabilized by the formation of a quasi-2D perovskite containing a benzothieno[3,2-b]benzothiophene (BTBT) large organic ammonium cation. Thermal analysis shows that degradation of the butylammonium (BA)-based quasi-2D perovskite (BA)₂CsPb₂I₇ sets in at ≈130 °C, while (BTBT)₂CsPb₂I₇ is phase-stable until ≈230 °C. Additionally, the (BTBT)₂CsPb₂I₇ film does not show any sign of degradation after exposure to 77% Relative Humidity in the dark for 152 days, while (BA)₂CsPb₂I₇ degrades in a single day. Photoconductor-type detectors based on (BTBT)₂CsPb₂I₇ demonstrate an increased external quantum efficiency and a similar specific detectivity compared to the BA-based reference detectors. The results demonstrate the utility of employing a BTBT cation within the organic layer of quasi-2D perovskites to significantly enhance the stability while maintaining the optoelectronic performance.