Insight into the Enhanced Charge Transport in Quasi-2D Perovskite via Fluorination of Ammonium Cations for Photovoltaic Applications

Preparation of high-performance quasi-two-dimensional (Q-2D) perovskite solar cells by fluorinated benzylamine groups at different substitution positions

Quasi-two-dimensional (Q-2D) perovskite solar cells have garnered significant attention due to their unique hydrophobic organic cations and commendable stability. However, there is currently no established set of criteria for selecting the appropriate organic cations to fabricate highly efficient and stable Q-2D perovskite solar cells. This work systematically examines the organic interstitial cations containing fluorine atoms at various substitution positions in phenylmethylamine, focusing on crystal orientation, film morphology, and the photoelectric conversion efficiency (PCE) of n = 5 perovskite films. Through density functional theory (DFT) calculations and crystal structure analysis, it is revealed that compared to PMA-F, oFPMA-F and mFPMA-F, pFPMA-F exhibits the largest dipole moment. Additionally, (pFPMA)2PbI4 demonstrates a larger effective mass and greater layer spacing compared to (PMA)2PbI4. The findings revealed that in comparison to PMA-F, oFPMA-F and mFPMA-F, the pFPMA-F 2D perovskite film exhibits a preferential in-plane orientation, superior crystallinity, and higher carrier mobility. Consequently, the Q-2D perovskite solar cell device utilizing pFPMA-F achieved a PCE of 15.88%, which markedly surpassed those of the PMA-F (9.15%), oFPMA-F (12.62%), and mFPMA-F (7.8%) counterparts. Additionally, the device architecture based on pFPMA-F demonstrated exceptional stability.

Progress of 2D Perovskite Solar Cells: Structure and Performance

2D perovskite has demonstrated great potential for application in photovoltaic devices due to the tunable energy bands, suppressed ion migration, and high stability. However, 2D perovskite solar cells (PSCs) display suboptimal efficiency in comparison to 3D perovskite solar cells, which can be attributed to the quantum confinement and dielectric confinement effects resulting from the intercalation of organic spacer cations into the perovskite lattice. This review starts with the fundamental structural characteristics, optoelectronic properties, and carrier transport dynamics of 2D PSCs, followed by the discussion of approaches to improve the photovoltaic performance of 2D PSCs, including the manipulation of crystal orientation, phase distribution, pure phase, organic layer, and device engineering. Then the advancements in the structural, humidity, thermal, and maximum power point tracking stability of 2D PSCs are summarized. Afterward, the applications of 2D perovskite in 2/3D PSCs to improve efficiency and stability are discussed. This review provides a comprehensive understanding of the relationship between 2D perovskite structure and the performance of the resulting 2D PSCs, as well as offers insights for constructing efficient and stable 2/3D PSCs by integrating 2D and 3D perovskites.

A Self-Powered Circularly Polarized Light Photodetector with High Responsivity Based on the Chiral Quasi-2D Perovskite Film

Low-dimensional hybrid organic-inorganic perovskites (HOIPs) containing chiral organic ligands have recently emerged as promising candidates for circularly polarized light (CPL) detection, which can distinguish left- and right-handed CPL directly. However, the increase in responsivity and realization of self-powered CPL photodetector remain a challenge. Meanwhile, there is a trade-off between the photocurrent responsivity and the ability to differentially absorb CPL in detectors based on these low-dimensional perovskites. Herein, we report the CPL photodetector based on chiral quasi-2D perovskite films (S/R-MBA)2MAPb2I7 and propose a crystallization regulation method using dimethyl sulfoxide (DMSO) and methylammonium thiocyanate (MASCN). We found that the photoelectric response capability and circular dichroism (CD) intensities of chiral quasi-2D perovskite can be enhanced simultaneously by the improved crystallinity and surface morphology of chiral films. Meanwhile, the formation of the tetragonal perovskite structure leads to symmetry-breaking distortion of the inorganic frameworks, further enhancing the chirality of the perovskite films. In addition, the distribution of n-phase can be tuned by DMSO and MASCN to form graded band alignment, effectively promoting the charge transfer in perovskite. As a result, a self-powered CPL photodetector with a high responsivity of 0.82 A/W and an anisotropy factor of 0.09 at 0 V bias is obtained. To the best of our knowledge, it is the first attempt to enhance the CD characteristics of chiral quasi-2D perovskite films. We believe our work further advances the research of low-dimensional chiral perovskite films in the field of CPL detection.

Fast, Highly Stable, and Low-Bandgap 2D Halide Perovskite Photodetectors Based on Short-Chained Fluorinated Piperidinium as a Spacer

Ruddlesden-Popper (RP) two-dimensional (2D) halide perovskite (HP), with attractive structural and optoelectronic properties, has shown great potential in optoelectrical devices. However, the relatively wide bandgap (Eg) and stability, which cause inferior efficiency, prevent its feasibility from further applications. To tackle these issues, for the first time, a novel fluorine-containing piperidinium spacer, (3-HCF2CF2CH2OCH2-PPH+), abbreviated as (4FH-PPH+), has been designed for the stable and efficient n = 1 2D HPs. Its fluorophobicity can significantly enhance the noncovalent interactions between cations and [PbI6]4- octahedra. The observed Eg of the fluorinated (4FH-PPH)2PbI4 perovskite is found to be 2.22 eV, which is the lowest value among all fluorinated perovskites reported so far. Interestingly, this fluorinated film-based 2D perovskite photodetector (PD) exhibits the outstanding responsivity of 502 mA W-1, photodetectivity of 5.73 × 1010 cm Hz1/2 W-1, and impressive response/recovery time of 42/46 ms under 450 nm at 20 V. To the best of our knowledge, it is found for the first time that the 2D HP, with the fluorinated short-chained segment included into the organic spacer, shows remarkable stability for up to 49 days. These results strongly demonstrate the potential of the 2D fluorinated short-chained (4FH-PPH)2PbI4 HP with a low Eg as a promising candidate for next-generation optoelectronic devices.

Enhancing Interlayer Charge Transport of Two-Dimensional Perovskites by Structural Stabilization via Fluorine Substitution

Two-dimensional lead-halide perovskites provide a more robust alternative to three-dimensional perovskites in solar energy and optoelectronic applications due to increased chemical stability afforded by interlayer ligands. At the same time, the ligands create barriers for interlayer charge transport, reducing device performance. Using a recently developed ab initio simulation methodology, we demonstrate that ligand fluorination can enhance both hole and electron mobility by 1-2 orders of magnitude. The simulations show that the enhancement arises primarily from improved structural order and reduced thermal atomic fluctuations in the system rather than increased interlayer electronic coupling. Arising from stronger hydrogen bonding and dipolar interactions, the higher structural stability decreases the reorganization energy that enters the Marcus formula and increases the charge transfer rate. The detailed atomistic insights into the electron and hole transfer in layered perovskites indicate that the use of interlayer ligands that make the overall structure more robust is beneficial simultaneously for chemical stability and charge transport, providing an important guideline for the design of new, efficient materials.

Molecular Structure Tailoring of Organic Spacers for High-Performance Ruddlesden-Popper Perovskite Solar Cells

Layer-structured Ruddlesden-Popper (RP) perovskites (RPPs) with decent stability have captured the imagination of the photovoltaic research community and bring hope for boosting the development of perovskite solar cell (PSC) technology. However, two-dimensional (2D) or quasi-2D RP PSCs are encountered with some challenges of the large exciton binding energy, blocked charge transport and poor film quality, which restrict their photovoltaic performance. Fortunately, these issues can be readily resolved by rationally designing spacer cations of RPPs. This review mainly focuses on how to design the molecular structures of organic spacers and aims to endow RPPs with outstanding photovoltaic applications. We firstly elucidated the important roles of organic spacers in impacting crystallization kinetics, charge transporting ability and stability of RPPs. Then we brought three aspects to attention for designing organic spacers. Finally, we presented the specific molecular structure design strategies for organic spacers of RPPs aiming to improve photovoltaic performance of RP PSCs. These proposed strategies in this review will provide new avenues to develop novel organic spacers for RPPs and advance the development of RPP photovoltaic technology for future applications.

What Matters for the Charge Transport of 2D Perovskites?

Compared to 3D perovskites, 2D perovskites exhibit excellent stability, structural diversity, and tunable bandgaps, making them highly promising for applications in solar cells, light-emitting diodes, and photodetectors. However, the trade-off for worse charge transport is a critical issue that needs to be addressed. This comprehensive review first discusses the structure of 3D and 2D metal halide perovskites, then summarizes the significant factors influencing charge transport in detail and provides a brief overview of the testing methods. Subsequently, various strategies to improve the charge transport are presented, including tuning A'-site organic spacer cations, A-site cations, B-site metal cations, and X-site halide ions. Finally, an outlook on the future development of improving the 2D perovskites' charge transport is discussed.

Additive Engineering for Stable and Efficient Dion-Jacobson Phase Perovskite Solar Cells

Because of their better chemical stability and fascinating anisotropic characteristics, Dion-Jacobson (DJ)-layered halide perovskites, which owe crystallographic two-dimensional structures, have fascinated growing attention for solar devices. DJ-layered halide perovskites have special structural and photoelectronic features that allow the van der Waals gap to be eliminated or reduced. DJ-layered halide perovskites have improved photophysical characteristics, resulting in improved photovoltaic performance. Nevertheless, owing to the nature of the solution procedure and the fast crystal development of DJ perovskite thin layers, the precursor compositions and processing circumstances can cause a variety of defects to occur. The application of additives can impact DJ perovskite crystallization and film generation, trap passivation in the bulk and/or at the surface, interface structure, and energetic tuning. This study discusses recent developments in additive engineering for DJ multilayer halide perovskite film production. Several additive-assisted bulk and interface optimization methodologies are summarized. Lastly, an overview of research developments in additive engineering in the production of DJ-layered halide perovskite solar cells is offered.