Decreasing Exciton Binding Energy in Two-Dimensional Halide Perovskites by Lead Vacancies

Abstract

perovskites, such as (BA) 2 (MA) n−1 Pb n X 3n+1 (X = Cl, Br, or I), have shown great promise in optoelectronic applications because of their stability, tunability, and unique electronic properties. However, their photovoltaic applications are hindered by high exciton binding energies in phases when n is small. On the basis of first-principles calculations, we predict that the exciton binding energy in the atomically thin 2D perovskites (n = 1 and 2) can be decreased significantly by neutral lead vacancies so that free charge carriers can be readily generated at room temperature. Moreover, the photogenerated electrons and holes are delocalized and spatially separated in the presence of lead vacancies; hence, radiative charge recombination can be suppressed. The lead vacancies are predicted to be formed in the 2D perovskites in an I-rich synthetic environment. Some intriguing experimental observations may be related to the presence of lead vacancies in the 2D perovskites.

Shedding light on moiré excitons: A first-principles perspective

“…However, the GW-BSE approach is prohibitively expensive for moiré excitons owing to the large number of atoms (~1600) in the unit cell. To circumvent the problem, we have recently developed an alternative first-principles method that can provide a reliable description of excitonic effect with much less computational cost (36)(37)(38)(39)(40). This method is based on time-dependent density functional theory (TDDFT) (41,42), with optimally tuned and range-separated hybrid (RSH) exchange-correlation (XC) functionals (details are in Materials and Methods) (43)(44)(45)(46).…”

“…To determine the energies and the many-body wave functions of excitons in MoS 2 /WS 2 heterostructures, we use a recently developed first-principles approach based on the linear-response TDDFT (LR-TDDFT) (41,42), with an optimally tuned, screened, and range-separated hybrid XC functional (OT-SRSH) (43)(44)(45)(46). The method has been implemented in conjunction with plane waves and pseudopotentials to study excitonic properties in semiconductors, including graphene fluoride, phosphorene, and 2D perovskites, and TMD heterostructures (36)(37)(38)(39)(40)64). The OT-SRSH involves the partition of the Coulombic interaction into a short-range and a long-range contribution based on the following expression (65)…”

Acetamidinium Cation to Confer Ion Immobilization and Structure Stabilization of Organometal Halide Perovskite Toward Long Life and High‐Efficiency p‐i‐n Planar Solar Cell via Air‐Processable Method

“…The organometal halide perovskite solar cell (OHPSC) holds many advantages over other types of solar cells, such as high absorption coefficient in visible light, low binding energy of its excitons, long diffusion length of carriers, tunable bandgap from various composition, and solution processing. Researchers are continuously working to improve the power conversion efficiency (PCE) of OHPSCs since it first debuted in 2009 .…”

Structurally Tunable Two-Dimensional Layered Perovskites: From Confinement and Enhanced Charge Transport to Prolonged Hot Carrier Cooling Dynamics

“… 97 (f) Band diagrams of (BA) 2 PbI 4 with and without lead vacancy V Pb , whose modification affects E b . 98 Images are reproduced with permission ( ) from refs (a) ( 84 ), copyright 2017 American Chemical Society; (b and c) ( 65 ) with slight modification, copyright 2019 Proceedings of the National Academy of Sciences; (d) ( 95 ), copyright 2018 Springer Nature; and (e) ( 97 ), copyright 2017 Royal Society of Chemistry. Panel f is reproduced with slight modifications from ref ( 98 ), copyright 2019 American Chemical Society.…”

“…Through first-principles calculations, Lu et al predicted the influence of many defects, among which only neutral Pb vacancies (V Pb ) in I-rich environments resulted in a drastic decrease in the E b of atomically thin 2D perovskites. 98 In particular, when a Pb atom is removed from the lattice of (BA) 2 (MA) n −1 Pb n I 3 n +1 ( n = 1 or 2), nearby I atoms become electron deficient, which leaves the highest occupied I 5p level vacant, as shown in Figure 3 f. 98 Thus, excitation no longer occurs from the I 5p to Pb 6p orbitals but rather between the two I 5p orbitals themselves, which results in a reduction in E b . 98 For instance, (BA) 2 PbI 4 revealed a decrease in its E b from 340 to 110 meV upon introducing V Pb at a concentration of ∼1.3 × 10 20 cm –3 .…”