Charge Carrier Dynamics in Co-evaporated MAPbI₃ with a Gradient in Composition

Abstract

Co-evaporation of metal halide perovskites by thermal evaporation is an attractive method since it does not require harmful solvents and enables precise control of the film thickness. Furthermore, the ability to manipulate the Fermi level allows the formation of a graded homojunction, providing interesting opportunities to improve the charge carrier collection efficiency. However, little is known about how these properties affect the charge carrier dynamics. In this work, the structural and optoelectronic properties of co-evaporated MAPbI 3 films varying in thickness (100, 400, and 750 nm) with a gradient in composition are analyzed. The X-ray diffraction patterns show that excess PbI 2 is only present in the thick layers. From X-ray photoelectron spectroscopy depth analysis, the I/Pb atomic ratio indicates methylammonium iodide deficiencies that become more prominent with thicker films, resulting in differently ndoped regions across the thick MAPbI 3 films. We suggest that due to these differently n-doped regimes, an internal electric field is formed. Side-selective time-resolved microwave photo conductivity measurements show an elongation of the charge carrier lifetimes on increasing thickness. These observations can be explained by the fact that excess carriers separate under the influence of the electric field, preventing rapid decay in the thick films.

Efficient and Stable Self-Passivation Perovskite Solar Cells Prepared in Ambient Air Based on an Antisolvent-Free Method

“…Then, the α for the 1 times perovskite film is 1.398 × 10 5 , and 5.0205 × 10 4 cm −1 for the 2 times perovskite film at 750 nm, which means most of incident light can be absorbed by both perovskite films, this is consistent with the literature report. 54,55 We also measured UV−vis spectra and XRD for wet perovskite films prepared by different concentrations of precursor solutions (Figure S6a,b). The wet and annealed perovskite films showed similar features in our system, which is also shown in 2D GIWAXS patterns of 1 and 2 times DMAc-based wet films (Figure S6c,d).…”

Direct In Situ Conversion of Lead Iodide to a Highly Oriented and Crystallized Perovskite Thin Film via Sequential Deposition for 23.48% Efficient and Stable Photovoltaic Devices

“…Organic–inorganic hybrid perovskites have become promising optoelectronic materials due to their high carrier mobility, tunable direct band gap, low exciton binding energy, and long carrier diffusion length. − The power conversion efficiency (PCE) of perovskite solar cells (PSC) has rapidly climbed from 3.8 to 25.7% in the past decade. − The preparation methods of PSC mainly include spin-coating, ,,− vapor deposition, and thermal evaporation . Among them, the spin-coating method is the most widely used, and it can be divided into a one-step solution method , and a sequential deposition method. − In the sequential deposition method, perovskite films are obtained by the reaction of lead halide films (such as PbI 2 ) with organic halide solutions (such as formamidine iodide and methylammonium iodide).…”

“…6−12 The preparation methods of PSC mainly include spin-coating, 10,11,13−15 vapor deposition, 16 and thermal evaporation. 17 Among them, the spin-coating method is the most widely used, and it can be divided into a one-step solution method 4,18 and a sequential deposition method. 19−25 In the sequential deposition method, perovskite films are obtained by the reaction of lead halide films (such as PbI 2 ) with organic halide solutions (such as formamidine iodide and methylammonium iodide).…”