Quantum Well Energetics of an n = 2 Ruddlesden–Popper Phase Perovskite

Abstract

the metal-halide active layer from the degradation pathways that plague 3D MHPs. RPP devices have so far achieved efficiencies greater than 16% as the active layer of a solar cell with improved stability over their 3D counterparts. [6,7] Additionally, RPPs have served in conjunction with 3D perovskites as capping layers, improving the ambient stability of the system. [8,9] MHPs exhibit a great deal of tunability, with multiple options for cations (e.g., methylammonium (MA), formamidinium (FA), Cs, Rb, metals (Pb, Sn), and halides (I, Br, Cl), allowing for optimization of the band gap, charge carrier lifetimes, and structural stability. The quantum well structure of the RPPs adds several additional degrees of tunability to the energetics, including the well thickness, barrier thickness, and the nature of the organic ligand. The well thickness, controlled by the number of consecutive layers of Pb-halide octahedra, modulates the band gap and the exciton binding energy (E B ) by defining the quantum confinement of the charge carriers in the material. [10,11] The barrier thickness controls the level of electronic communication between various quantum wells. [12] The organic ligand, in addition to determining the barrier thickness, controls the height of the energetic barrier of the quantum wells, and out-ofplane mobility. [13] In order to harness the tunability and design appropriate RPPs for each application, a deeper understanding of how tuning these quantum wells affects their energetics is needed.The direct experimental determination of ionization energy [10] (IE) and electron affinity (EA) [14] has been reported for two 2D n = 1 RPPs, but not for higher order (n > 1) materials. These RPPs present a challenge in the fabrication of phase pure films for surface sensitive measurements, as the materials often comprise phases with different n's. Electron spectroscopy investigations of 2D RPPs that focus on single particle transport levels, and thus IE and EA, are necessary, as the relatively large exciton binding energy (E B ≈ 100s meV) in these materials precludes the derivation of a transport gap from optical absorption measurements. The present study focuses on the energetics of high purity films of the n = 2 RPP, butylammonium cesium lead iodide, (BA 2 CsPb 2 I 7 ). [15] Solar cell devices based on this material have previously been shown to maintain 92% of their PCE after 30 days of continuous operation in ambient conditions. [16] Using cesium instead of methylammonium increases the thermal stability [17] and charge carrier lifetimes. [18] A comprehensive investigation of the electronic energy levels of an n = 2 Ruddlesden-Popper phase perovskite is presented. Ultraviolet and inverse photoemission spectroscopies are used to probe the density of states in the valence and conduction bands, respectively, of the quasi-2D perovskite, butylammonium cesium lead iodide (BA 2 CsPb 2 I 7 ). By comparing experimental spectra with calculated projected density of states, the contributions from Cs, Pb, and I to the quantum well...

Structural and Electronic Impact of an Asymmetric Organic Ligand in Diammonium Lead Iodide Perovskites

“…The solution was then spin‐coated at 4000 RPM (4000 RPM/s acceleration) for 60 s onto glass substrates for optical and XRD characterization, and on ITO/glass for photoemission spectroscopy. A constant downward nitrogen pressure was applied to the samples after 5 s of spinning, and toluene was dropped after 20 s. [ 28 ] The samples were then annealed at several temperatures between 80 and 220 °C for 10 min. See the Supporting Information for the characterization of the resulting films.…”

Charge‐Carrier Transport in Quasi‐2D Ruddlesden–Popper Perovskite Solar Cells

“…The dashed lines corresponding to the ground state of the QWs, represent the IE and EA of the n = 1 and n = 2 perovskites. Reproduced with permission [32]. Copyright 2019, Wiley-VCH.…”

Controlling the Growth Kinetics and Optoelectronic Properties of 2D/3D Lead–Tin Perovskite Heterojunctions

“…These are perovskite‐like structures of the R 2 A n −1 B n I 3 n +1 form, where R is a large organic molecular cation spacer that forms planes periodically interrupting the 3D corner‐sharing octahedra perovskite framework and protects the structure against oxygen and moisture penetration . The physical confinement of the perovskite framework, however, leads to the formation of natural excitonic quantum wells, thus allowing one to modulate the bandgap of the material via the thickness of the well ( n ‐number) and the bulkiness of the molecule R . Most of the work on these 2D materials has been carried out on Pb‐based compositions but only a handful of studies have been performed with Sn‐based perovskites .…”