Defects in Hybrid Perovskites: The Secret of Efficient Charge Transport

Abstract

The interaction of free carriers with defects and some critical defect properties are still unclear in methylammonium lead halide perovskites (MHPs). Here, a multi-method approach is used to quantify and characterize defects in single crystal MAPbI 3 , giving a cross-checked overview of their properties. Time of flight current waveform spectroscopy reveals the interaction of carriers with five shallow and deep defects. Photo-Hall and thermoelectric effect spectroscopy assess the defect density, cross-section, and relative (to the valence band) energy. The detailed reconstruction of free carrier relaxation through Monte Carlo simulation allows for quantifying the lifetime, mobility, and diffusion length of holes and electrons separately. Here, it is demonstrated that the dominant part of defects releases free carriers after trapping; this happens without non-radiative recombination with consequent positive effects on the photoconversion and charge transport properties. On the other hand, shallow traps decrease drift mobility sensibly. The results are the key for the optimization of the charge transport properties and defects in MHP and contribute to the research aiming to improve perovskite stability. This study paves the way for doping and defect control, enhancing the scalability of perovskite devices with large diffusion lengths and lifetimes.

Molecular Doping Enabling Mobility Boosting of 2D Sn2+‐Based Perovskites

“…[ 56 ] In Sn‐based perovskites, V Sn is the main source of shallow traps, which are negatively charged for free holes to fall into and get “trapped.” Although trapping and de‐trapping take only several microseconds, this phenomenon delays the drift transport and reduces the overall carrier mobility. [ 20 ] Trapped carriers require additional activation energy through optical excitations or thermal energy to “de‐trap” back into the transport band and contribute to the net charge transport. This process is also known as the “trap and de‐trap” or “trap and release” model, as illustrated in Figure 2f.…”

“…For (PEA) 2 SnI 4 , hole transport is dominated by the thermal activation mode, where the free holes experience multiple trapping at shallow V Sn sites and releasing process, which slows down the movement. [19,20] To improve the charge transport properties of 2D halide perovskites, significant efforts have focused on synthesizing novel A-cation ligands, [15,21,22] blending with 3D perovskites, [23,24] device/interface engineering, [25][26][27][28] additive engineering, [29][30][31][32] and heterovalent atomic doping. [33][34][35] However, these approaches have the disadvantages of complexity, non-uniformity, and indeterminacy.…”

Self‐Healing Ability of Perovskites Observed via Photoluminescence Response on Nanoscale Local Forces and Mechanical Damage

“…Electrical experiments also give quite different picture of the defect types and concentrations in comparison with pure optical measurements. [ 87 , 88 ] Numerous theoretical calculations suggest that iodide interstitials and atomic lead (Pb 0 ) are deep defect states, which are expected to provide high charge trapping and recombination rates. [ 88 , 89 ] In addition, defect complexes (in analogy to donor–acceptor pairs) whose association and dissociation could provide switching have been suggested.…”

Self‐Healing and Light‐Soaking in MAPbI3: The Effect of H2O

“…Thus, they may have different doping densities, impacting [10] the crystal's Fermi level, especially given their relatively low doping. While normally such differences should not matter, for samples with very low doping densities as the MAPbI 3 crystals, [33,34] this can be an issue. If the photodamage and self-healing kinetics are influenced by the densities of free electrons and/or holes in the crystals, then this will affect the crystals' behavior after bleaching.…”