Controlling Phase Transitions in 2D Perovskites by Organic Cation Alloying

Two-dimensional perovskite quantum wells have emerged as promising materials for optoelectronics applications, including LEDs, nanoscale lasers, and phototransistors. Ruddlesden-Popper (RP) perovskites have monovalent cation spacers and often exhibit a solid-solid phase transition, in which the organic layer undergoes an order-to-disorder transition. This “melting” of the organic layers induces changes in the inorganic layers, impacting the emissive properties of the material. Here, we demonstrate control over the phase transition temperature of RP 2D perovskites by alloying two similar organic cations. Although halide alloying is used regularly to precisely tune electronic properties, cation alloying to control thermodynamic properties has not been explored. By blending hexylammonium (HA) and pentylammonium (PA) cations in different ratios, we can tune the phase transition temperature of 2D perovskites over a large temperature range. We demonstrate this tunability in single crystals and solution-processed thin films. By correlating temperature dependent grazing incidence wide angle X-ray scattering (GIWAXS) and photoluminescence (PL) measurements, we show that the phase transition has a direct impact on the PL intensity. We also directly image this phase transition with temperature-dependent PL imaging to show that the phase transition is highly heterogeneous at the microscale. Overall, our work provides the necessary design principles to control phase transitions in 2D perovskites with the potential to use these materials as dynamically switchable optoelectronic materials.