Two-dimensional metal halide perovskites (2D-MHP), first synthesized in the 1990’s , have recently been the subject of considerable attention. In addition to interesting (opto)electronic properties linked to their reduced dimensionality, they appear to exhibit higher resistance to the environment, e.g., moisture, than their 3D-MHP counterparts, thereby offering great potential for various applications. The importance of these materials warrants in-depth investigations of their electronic properties.[2–4] Here we present recent electronic structure measurements of a series of solution-processed films of 2D butylammonium methylammonium lead iodide compounds, BA2MAn-1PbnI3n+1, n=1 - 4. XRD, AFM, UV-vis absorption, and ultra-violet and inverse photoemission spectroscopies are used to investigate these compounds. We measure valence and conduction band spectra, and determine ionization energy (IE), electron affinity (EA) and single particle gap as a function of n. We find that the single particle gap decreases from 2.77 eV for n=1 to 1.87 eV for n=4 (and 1.6 eV for the 3D-MHP MAPbI3), with IE decreasing and EA increasing in a nearly symmetric fashion, in contrast to previous results. We use the single particle gap and the onset of optical absorption at the exciton peak to calculate the exciton binding energy EB. In agreement with previous results, EB is found to be large for n=1 and 2 (390 and 110 meV, respectively). However, we find that the exciton binding energy decreases very rapidly thereafter, reaching below 50 meV by n=3. Finally, a simple model is presented to justify the electron and hole levels and the single particle gap in these quantum wells structures.
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