There is a variety of possible ways to tune the optical properties of 2D perovskites, though the mutual dependence between different tuning parameters hinders our fundamental understanding of their properties. In this work we attempt to address this issue for (C n H 2n+1 NH 3 ) 2 PbI 4 (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 op-eration, 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 magnetic field providing strong evidence for the phonon related nature of the observed side bands.The inherent sensitivity of lead-halide perovkites to ambient conditions 1 is the Achilles heel which currently prevents the deployment of their superior properties 2-8 in real world applications. 9 The last few years have witnessed rapid development of numerous perovskite derivatives, [10][11][12][13][14][15][16][17] in an attempt to overcome the environmental stability issue. For example, 2D Ruddlesden-Popper perovskites have already demonstrated power conversion efficiency greater than 10%, with significantly improved stability. 15,18-20 Ruddlesden-Popper 1 arXiv:1909.06061v1 [cond-mat.mes-hall] 13 Sep 2019 halide perovskites are natural type I quantum wells formed by thin layers of halide perovskite separated by organic spacers layers, which act as barriers. 14,21,22 They are described by general formula A' 2 A m−1 M m X m+1 where A' is a monovalent organic cation acting as a spacer, A is a small monovalent cation (MA, FA, Cs), and M a divalent cation that can be Pb 2+ , Sn 2+ , Ge 2+ , Cu 2+ , Cd 2+ , etc., X an anion (Cl − , Br − , I − ) and m=1,2,3... is the number of octahedra layers in the perovskite slab. The hydrophobic nature of the organic spacers significantly increases the stability of these compounds promising an excellent long term performance in photovoltaic and light-emitting applications. 15,[18][19][20]23,24 A unique feature of 2D perovskites is that their properties can be tuned in far more ways than in the case of 3D perovskite semiconductors. The band gap can be tailored by varying chemical composition of the inorganic part 25,26 or by changing the thickness of the octahedral slabs, which significantly impacts the optoelectronic properties. 19,27,28 In addition, many of the 2D perovskite properties can be tuned by an appropriate choice of the building blocks with a plethora of different organic spacers to choose from. 15,20 Varying the organic spacer modifies the dielectric environment of the inorganic slab, affecting the exciton binding energy and the band gap. 14,28,...
