Phase-Transition-Induced Carrier Mass Enhancement in 2D Ruddlesden-Popper Perovskites

There are a variety of possible ways to tune the optical properties of 2D perovskites, though the mutual dependence among different tuning parameters hinders our fundamental understanding of their properties. In this work, we attempt to address this issue for (CnH2n+1NH3)(2)PbI4 (with n = 4, 6, 8, 10, 12) using optical spectroscopy in high magnetic fields up to 67 T. Our experimental results, supported by DFT calculations, clearly demonstrate that the exciton reduced mass increases by around 30% in the low-temperature phase. This is reflected by a 2-3-fold decrease of the diamagnetic coefficient. Our studies show that the effective mass, which is an essential parameter for optoelectronic device operation, can be tuned by the variation of organic spacers and/or moderate cooling achievable using Peltier coolers. Moreover, we show that the complex absorption features visible in absorption/transmission spectra track each other in a magnetic field, providing strong evidence for the phonon-related nature of the observed side bands.