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

The variety of possible ways to tune the optical properties of 2D perovskites is their huge advantage, while at the same time, the mutual dependence between different tuning parameters hinder our fundamental understanding of their properties. In this work, we attempt to address this issue for (CnH2n+1NH3)2PbI4 (with n=4,6,8,10,12) using optical spectroscopy in high magnetic fields up to 67T. Our experimental results, supported by DFT calculations, clearly demonstrate that the reduced mass of the exciton increases by around 30% in the low temperature phase. This is reflected by a 2-3 fold decrease of the diamagnetic coefficient. Our studies shows that the effective mass which is essential parameter for optolectronic 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 specta track each other in magnetic field providing strong evidence for the phonon related nature of the observed side bands.