The Role of Alkyl Chain Length and Halide Counter Ion in Layered Dion-Jacobson Perovskites with Aromatic Spacers

Mixed ionic-electronic conduction in Ruddlesden-Popper and Dion-Jacobson layered hybrid perovskites with aromatic organic spacers

The understanding of mixed ionic-electronic conductivity in hybrid perovskites has enabled major advances in the development of optoelectronic devices based on this class of materials. While recent investigations revealed the potential of using dimensionality effects for various applications, the implication of this strategy on mixed conductivity is yet to be established. Here, we present a systematic analysis of mixed conduction in layered (2D) hybrid halide perovskite films based on 1,4-phenylenedimethylammonium (PDMA) and benzylammonium (BzA) organic spacers in (PDMA)PbI4 and (BzA)2PbI4 compositions, forming representative Dion-Jacobson (DJ) and Ruddleson-Popper (RP) phases, respectively. Electrochemical measurements of charge transport parallel to the layered structure reveal mixed ionic-electronic conduction with electronic transport mediated by electron holes in both DJ and RP phases. In comparison to the 3D perovskites, larger activation energies for both ionic and electronic conductivities are observed which result in lower absolute values. While the layered perovskites still allow for a relatively efficient exchange of iodine with the gas phase, the lower change of conductivity on the variation of the iodine partial pressure compared with 3D perovskites is consistent with the exchange affecting only a fraction of the film, with implications for the encapsulating efficacy of these materials. We complement the analysis with a demonstration of the superior thermal stability of DJ structures compared to their RP counterparts. This can guide future explorations of dimensionality and composition to control the transport and stabilization properties of 2D perovskite films.

Manipulation of Crystal Orientation and Phase Distribution of Quasi-2D Perovskite through Synergistic Effect of Additive Doping and Spacer Engineering

The diammonium precursor 1,4-phenylenedimethanammonium (PDMA) was used as a large organic spacer for the preparation of Dion-Jacobson-type quasi-2D perovskites (PDMA)(MA)n-1PbnI3n+1 (MA = methylammonium). Films with composition ⟨n⟩ = 5 comprised randomly orientated grains and multiple microstructural domains with locally differing n values. However, by mixing the Dion-Jacobson-type spacer PDMA and the Ruddlesden-Popper-type spacer propylammonium (PA), the crystal orientation in both the vertical and the horizonal directions became regulated. High crystallinity owing to well-matched interlayer distances was observed. Combining this spacer-engineering approach with the addition of methylammonium chloride (MACl) led to full vertical alignment of the crystal orientation. Moreover, the microstructural domains at the substrate interface changed from low-n (n = 1, 2, 3) to high-n (n = 4, 5), which may be beneficial for hole extraction at the interface between perovskite and hole transport layer due to a more finely tuned band alignment. Our work sheds light on manipulating the crystallization behavior of quasi-2D perovskite and further paves the way for highly stable and efficient perovskite devices.

Comparative Analysis of Thiophene-Based Interlayer Cations for Enhanced Performance in 2D Ruddlesden-Popper Perovskite Solar Cells

2D Ruddlesden-Popper (RP) perovskites have appeared as a promising prospective material owing to their tunable optoelectronic peculiarities and structural stability. The choice of interlayer cations greatly influences the performance of the 2D RP perovskites. In this study, through theoretical calculations and experimental investigation, we demonstrate the intrinsic and device performance differences between two perovskites based on cations of thiophenemethylamine (TMA) and thiopheneethylamine (TEA). Using density functional theory (DFT) calculations, it exposes that as compared to (TMA)2PbI4, (TEA)2PbI4 exhibits more pronounced distortion of [PbI6]4- units and possesses a wider band gap and larger effective mass. The experimental results on the TMA- and TEA-based 2D perovskites further show that when TEA is used as the interlayer cation, the crystallization process tends to form more low-n phases, which hinder charge transfer and decrease light harvesting. On the other hand, when TMA is used as the interlayer cation, excessive low-n phases are not observed, and the thin film exhibits excellent quality with significantly improved electron mobility. The (TMA)2(FA)n-1PbnI3n+1 (n = 5) perovskite device shows a remarkable conversion efficiency of 16.56%, much higher than that of TEA-based devices (PCE = 2.58%). Moreover, the unencapsulated devices based on TMA were able to maintain 88% of their initial efficiency even after being exposed to the environment (RT, RH = 30 ± 5%) for a duration of 1080 h. These findings provide important insights into the differences between thiophene-based cations and the selection of organic interlayer cations for 2D RP perovskite solar cells.

Impacts of Exciton Binding Energy and Dielectric Confinement of Layered Lead Halide Perovskites on Carrier Relaxation and Exciton Phonon Interactions

This work highlights the significance of dielectric confinements and exciton binding energy of hybrid layered perovskites (LPs) in controlling the carrier relaxation dynamics of LPs for designing efficient optoelectronic devices. The polarizability of organic spacer cations in LPs modulates the carrier-phonon and carrier-carrier interactions, which eventually control the carrier relaxation dynamics. Here, we have varied the alkyl-ammonium chain length in the LPs to change the dielectric confinement, and the first-principles calculations reveal that the long-chain organic spacer experiences stronger dielectric confinement in comparison to short-chain organic spacer cation-based LPs. Transient absorption spectroscopic analysis suggests that the larger dielectric confinement and higher exciton binding energy exhibit faster carrier relaxation dynamics. The enhanced exciton-phonon interaction leads to faster carrier relaxation dynamics. The much softer phonon modes are responsible for the higher up-conversion of acoustic modes to optical modes, which leads to slower carrier relaxation dynamics in n-butylamine (BA) based LPs.

The rise of quasi-2D Dion-Jacobson perovskites for photovoltaics

With the advance of nanotechnology, the past couple of years have witnessed the fast development of quasi two-dimensional (2D) halide perovskites, which exhibit outstanding long-term stability against moisture and heat, compared with their three-dimensional (3D) counterparts. As one of the most common structures in 2D halide perovskites, quasi-2D Dion-Jacobson (DJ) perovskites show multiple-quantum-well structures with n layers of [BX6]4- octahedral inorganic sheets sandwiched by two layers of diammonium spacers, thus exhibiting superior structural stability due to the elimination of van der Waals gaps. Thanks to the achievement of high power conversion efficiency accompanied by impressive stability, quasi-2D DJ perovskite solar cells (PSCs) have recently drawn extensive attention in the field. This review first introduces the fundamental understanding of quasi-2D DJ halide perovskites, including their superior stability, high exciton binding energy, and compositional flexibility and tunable properties. We then summarize detailed strategies to prepare high-quality quasi-2D DJ perovskites for PSCs, encompassing compositional engineering, solvent engineering, additive addition, and annealing processes. Moreover, the surface/interface modification and 2D-3D hybrid perovskite heterojunction are also discussed, for providing strategies to optimize the fabrication of quasi-2D DJ PSCs. Lastly, current challenges and perspectives toward the future development of quasi-2D DJ perovskites for photovoltaics are outlined.

Stacking Arrangement and Orientation of Aromatic Cations Tune Bandgap and Charge Transport of 2D Organic-Inorganic Hybrid Perovskites

Chemical modifications on aromatic spacers of 2D perovskites have been demonstrated to be an effective strategy to simultaneously improve optoelectronic properties and stability. However, its underlying mechanism is poorly understood. By using 2D phenyl-based perovskites ([C6 H5 (CH2 )m NH3 ]2 PbI4 ) as models, the authors have revealed how the chemical nature of aromatic cations tunes the bandgap and charge transport of 2D perovskites by utilizing sum-frequency generation vibrational spectroscopy to determine the stacking arrangement and orientation of aromatic cations. It is found that the antiparallel slip-stack arrangement of phenyl rings between adjacent layers induces an indirect band gap, resulting in anomalous carrier dynamics. Incorporation of the CH2 moiety causes stacking rearrangement of the phenyl ring and thus promotes an indirect to direct bandgap transition. In direct-bandgap perovskites, higher carrier mobility correlates with a larger orientation angle of the phenyl ring. Further optimizing the orientation angle by introducing a para-substituted element in a phenyl ring, higher carrier mobility is obtained. This work highlights the importance of leveraging stacking arrangement and orientation of the aromatic cations to tune the photophysical properties, which opens up an avenue for advancing high-performance 2D perovskites optoelectronics via molecular engineering.

Spin Texture Sensitive Photodetection by Dion-Jacobson Tin Halide Perovskites

The organic spacer molecule is known to regulate the optoelectronic properties of two-dimensional (2D) perovskites. We show that the spacer layer thickness determines the nature of optical transitions, direct or indirect, by controlling the structural properties of the inorganic layer. The spin-orbit interactions lead to different electron spin orientations for the states associated with the conduction band minimum (CBM) and the valence band maximum (VBM). This leads to a direct as well as an indirect component of the transitions, despite them being direct in momentum space. The shorter chains have a larger direct component, leading to a better optoelectronic performance. The mixed halide Sn2+ Dion-Jacobson (DJ) perovskite with the shortest 4-C diammonium spacer outshines the photodetection parameters of those having longer (6-C and 8-C) spacers and the corresponding Ruddlesden-Popper (RP) phases. The DJ system with a 4-C spacer and equimolar Br/I embodies an unprecedentedly high responsivity of 78.1 A W-1 under 3 V potential bias at 485 nm wavelength, among the DJ perovskites. Without any potential bias, this phase manifests the self-powered photodetection parameters of 0.085 A W-1 and 9.9 × 1010 jones. The unusual role of electron spin texture in these high-performance photodetectors of the lead-free DJ perovskites provides an avenue to exploit the information coded in spins for semiconductor devices without any ferromagnetic supplement or magnetic field.

The Effect of the Alkyl Chains of the Alkylammonium Pesudohalide Additives on the Performance of Dion Jacobson Perovskite Solar Cells

Dion-Jacobson perovskite (DJP) films suffer from the high structural disorder and non-compact morphology, leading to inefficient and unstable solar cells (SCs). Here, how the alkyl chains of alkylammonium pseudohalide additives including methylammonium thiocyanate (MASCN) and ethylammonium thiocyanate (EASCN), and propylammonium thiocyanate (PASCN), impact the microstructures, optoelectronic properties and the performance of the solar cells is investigated. These additives substantially improve the structural order and the morphology of the DJP films, yielding more efficient and stable solar cells than the control device. They behave quite differently in modifying the morphological features. Particularly, EASCN outstands the additives in terms of the superior morphology, which is compact and uniform and consists of the largest flaky grains. Consequently, the corresponding device delivers a power conversion efficiency (PCE) of 15.27% and maintains ≈86% of the initial PCE after aging in the air for 182 h. Conversely, MASCN as an additive produces uneven DJP film and the device maintains only 46% of the initial PCE. PASCN as an additive produces the finest grains in the DJP film, and the corresponding device yields a PCE of 11.95%. From the economical point of view, it costs 0.0025 yuan per device for the EASCN additive, allowing for cost-effective perovskite solar cells.

The Impact of Spacer Size on Charge Transfer Excitons in Dion-Jacobson and Ruddlesden-Popper Layered Hybrid Perovskites

Organic materials can tune the optical properties in layered (2D) hybrid perovskites, although their impact on photophysics is often overlooked. Here, we use transient absorption spectroscopy to probe the Dion-Jacobson (DJ) and Ruddlesden-Popper (RP) 2D perovskite phases. We show the formation of charge transfer excitons in DJ phases, resulting in a photoinduced Stark effect which is shown to be dependent on the spacer size. By using electroabsorption spectroscopy, we quantify the strength of the photoinduced electric field, while temperature-dependent measurements demonstrate new features in the transient spectra of RP phases at low temperatures resulting from the quantum-confined Stark effect. This study reveals the impact of spacer size and perovskite phase configuration on charge transfer excitons in 2D perovskites of interest to their advanced material design.

Recent progress in layered metal halide perovskites for solar cells, photodetectors, and field-effect transistors

Metal halide perovskite materials demonstrate immense potential for photovoltaic and electronic applications. In particular, two-dimensional (2D) layered metal halide perovskites have advantages over their 3D counterparts in optoelectronic applications due to their outstanding stability, structural flexibility with a tunable bandgap, and electronic confinement effect. This review article first analyzes the crystallography of different 2D perovskite phases [the Ruddlesden-Popper (RP) phase, the Dion-Jacobson (DJ) phase, and the alternating cations in the interlayer space (ACI) phase] at the molecular level and compares their common electronic properties, such as out-of-plane conductivity, crucial to vertical devices. This paper then critically reviews the recent development of optoelectronic devices, namely solar cells, photodetectors and field effect transistors, based on layered 2D perovskite materials and points out their limitations and potential compared to their 3D counterparts. It also identifies the important application-specific future research directions for different optoelectronic devices providing a comprehensive view guiding new research directions in this field.

Perfluoroarenes: A Versatile Platform for Hybrid Perovskite Photovoltaics

The instability of hybrid organic-inorganic halide perovskites presents one of the pressing challenges for their application. This is associated with the sensitivity to moisture as well as mixed ionic-electronic conductivity that leads to enhanced ion migration under conditions of voltage and light bias. Some of the most effective strategies to stabilize hybrid perovskite materials during operation involve the use of interfacial molecular assemblies and low-dimensional perovskite architectures based on hydrophobic organic moieties that could suppress the effects of moisture or ion migration. For this purpose, perfluoroarenes have provided a versatile platform due to their enhanced hydrophobicity as well as the capacity to engage in various noncovalent interactions that affect the characteristics of the resulting assemblies as well as ion migration. This Perspective discusses the emerging role of perfluoroarenes in stabilizing hybrid perovskite materials and their photovoltaic devices through different modes of action, offering insights for the design of advanced materials.

Nanosegregation in arene-perfluoroarene π-systems for hybrid layered Dion-Jacobson perovskites

Layered hybrid perovskites are based on organic spacers separating hybrid perovskite slabs. We employ arene and perfluoroarene moieties based on 1,4-phenylenedimethylammonium (PDMA) and its perfluorinated analogue (F-PDMA) in the assembly of hybrid layered Dion-Jacobson perovskite phases. The resulting materials are investigated by X-ray diffraction, UV-vis absorption, photoluminescence, and solid-state NMR spectroscopy to demonstrate the formation of layered perovskite phases. Moreover, their behaviour was probed in humid environments to reveal nanoscale segregation of layered perovskite species based on PDMA and F-PDMA components, along with enhanced stabilities of perfluoroarene systems, which is relevant to their application.