Edge States Drive Exciton Dissociation in Ruddlesden–Popper Lead Halide Perovskite Thin Films

Abstract

Efficient photovoltaic cells based on thin films of solution-processed 2D Ruddlesden–Popper hybrid perovskites (RPPs) represent an exciting breakthrough due to their enhanced tunability and chemical stability relative to those fabricated from 3D phases. However, reports of efficient charge separation and current collection are in apparent contradiction with the well-known enhancement of the exciton binding energy in multilayered halide perovskites, which should lower the device’s internal quantum efficiency and voltage. This controversy has led to various proposals for the electronic and physical structure of RPP thin films, including phase inhomogeneity as the driving force for exciton dissociation and transport. We address this apparent paradox in high-quality hot-cast RPP films by correlating ultrafast transient absorption spectroscopy with X-ray scattering measurements. We show that a hot-casting fabrication method produces highly phase pure n = 3 (BA)2(MA) n−1Pb n I3n+1 RPP structures. The high-phase purity and large grain sizes allow us to observe vertical transport of excitons via a diffusive process and allow us to determine that charge separation is primarily driven by dissociation at surface localized subgap electronic states. We analyze the differential absorption kinetics in films of varying thickness to directly determine that the excitonic diffusion constant is ∼0.18 cm2 s–1. We propose that a surface localized structural distortion, observed using surface selective grazing incidence X-ray scattering measurements, is responsible for the creation of the surface localized defect states. We find that the density and spatial distribution of these defect states is a function of preparation conditions.

Origin and physical effects of edge states in two-dimensional Ruddlesden-Popper perovskites

“…The edge state emission was also observed in 2D RP perovskite films ( Blancon et al., 2017 ; Kinigstein et al., 2020 ). But different from the exfoliated crystal which exhibits two PL peaks, the PL spectra of 2D RP perovskite films were dominated by the low energy emission of the edge states and exhibited only one PL peak at low energy region, implying the edge states are strongly located at the surface of films ( Blancon et al., 2017 ; Kinigstein et al., 2020 ). This observation can be further confirmed by ultrafast transient absorption (TA) spectroscopy.…”

“…Copyright © 2019, American Chemical Society. (D–F) Schematic of the front/back pumping geometry transient absorption characterization for perovskite films and the 5 ps transient spectra obtained in different geometries with (BA) 2 (MA) 2 Pb 3 I 10 hot casting (E) and post annealed (F) film, reprinted from ( Kinigstein et al., 2020 ). Copyright © 2020, American Chemical Society.…”

“…This observation can be further confirmed by ultrafast transient absorption (TA) spectroscopy. The films exhibited different TA spectra when the pump was incident from the front and rear sides, indicating the asymmetric edge state concentration distribution was along with the depth of the films ( Figures 1 D and 1E) ( Kinigstein et al., 2020 ). It was also found that the fabricated method will affect the distribution of the edge states; the TA spectra of post annealed films exhibit less asymmetry in front/rear side incidents compared with that of hot casting films ( Figure 1 F) ( Kinigstein et al., 2020 ).…”

“…But for LED, excitons are desirable than free carriers because LED requires efficient radiative electron-hole recombination and the free carriers will reduce the quantum efficiency of LED. Thus, it is essential to consider exciton dissociation and free carrier property of 2D RP perovskites when designing optoelectronic devices ( Baranowski and Plochocka, 2020 ; Baranowski et al., 2019 ; Blancon et al., 2017 ; Buizza et al., 2019 ; Cho et al., 2020 ; Fujiwara et al., 2020 ; Gan et al., 2021 ; He et al., 2020 ; Kinigstein et al., 2020 ; Li et al., 2020a , 2020b ; Lin et al., 2020 ; Magdaleno et al., 2021 ; Milot et al., 2016 ; Motti et al., 2019 ; Proppe et al., 2018 ; Zhang et al., 2019b , 2020 ; Zhou et al., 2019 , 2020 ).…”

“…One considers that the edge states are the intrinsic electronic states at the edge that makes the bandgap of the edge area narrower than that of the bulk area ( Hong et al., 2021 ; Kepenekian et al., 2018 ; Wang et al., 2020 ; Zhang et al., 2019b ). Another is that the extrinsic structure distortion at the edge leads to lower energy emissions ( Kinigstein et al., 2020 ; Qin et al., 2020 ; Shi et al., 2019 ; Zhao et al., 2019 ). Supported by significant theoretical and experimental evidence for each of the viewpoints, however, each theory cannot fully interpret all aspects of the observations.…”