Quasi‐2D Hybrid Perovskite Formation Using Benzothieno[3,2‐b]Benzothiophene (BTBT) Ammonium Cations: Substantial Cesium Lead(II) Iodide Black Phase Stabilization

Abstract 3D hybrid perovskites (APbX 3 ) 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) 2 CsPb 2 I 7 sets in at ≈130 °C, while (BTBT) 2 CsPb 2 I 7 is phase‐stable until ≈230 °C. Additionally, the (BTBT) 2 CsPb 2 I 7 film does not show any sign of degradation after exposure to 77% Relative Humidity in the dark for 152 days, while (BA) 2 CsPb 2 I 7 degrades in a single day. Photoconductor‐type detectors based on (BTBT) 2 CsPb 2 I 7 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.