Self-Assembly of Delta-Formamidinium Lead Iodide Nanoparticles to Nanorods: Study of Memristor Properties and Resistive Switching Mechanism.

In the quest for advanced memristor technologies, this study introduces the synthesis of delta-formamidinium lead iodide (δ-FAPbI3) nanoparticles (NPs) and their self-assembly into nanorods (NRs). The formation of these NRs is facilitated by iodide vacancies, promoting the fusion of individual NPs at higher concentrations. Notably, these NRs exhibit robust stability under ambient conditions, a distinctive advantage attributed to the presence of capping ligands and a crystal lattice structured around face-sharing octahedra. When employed as the active layer in resistive random-access memory devices, these NRs demonstrate exceptional bipolar switching properties. A remarkable on/off ratio (105) is achieved, surpassing the performances of previously reported low-dimensional perovskite derivatives and α-FAPbI3 NP-based devices. This enhanced performance is attributed to the low off-state current owing to the reduced number of halide vacancies, intrinsic low dimensionality, and the parallel alignment of NRs on the FTO substrate. This study not only provides significant insights into the development of superior materials for memristor applications but also opens new avenues for exploring low-dimensional perovskite derivatives in advanced electronic devices.

Dual-color photoluminescence modulation of zero-dimensional hybrid copper halide microcrystals.

Zero-dimensional hybrid copper(I) halides (HCHs) are attractive due to their interesting photoluminescence (PL) properties and the high abundance and low toxicity of copper. In this study, we report green-red dual emission from rhombic 1-butyl-1-methyl piperidinium copper bromide [(Bmpip)2Cu2Br4] microcrystals (MCs) prepared on borosilicate glass. The structure and elemental composition of the MCs are characterized by single crystal X-ray diffraction analysis, scanning electron microscopy, and energy-dispersive X-ray spectroscopy. Interestingly, MCs prepared on an ITO-coated glass plate show an intense green emission compared to the dual emission on a bare glass or plastic substrate. Furthermore, the intensity of the green emission from the MC is enormously increased by powdering using a conductive material, suggesting the deactivation of the red-emitting state by a charge transfer interaction with the conductor. These findings open a new strategy to suppress the self-trapping of excitons by longitudinal optical phonons and control the dual emitting states in HCHs.

Influence of crystal structure and composition on optical and electronic properties of pyridinium-based bismuth iodide complexes.

This study investigates the impacts of structure and composition on the optical and electronic properties of a series of pyridinium-based bismuth iodide complexes. Organic substrates with various functional groups, such as 4-aminopyridine (4-Ampy), 4-methylpyridine (4-Mepy), 4-dimethylaminopyridine (4-Dmapy), and 4-pyridinecarbonitrile (4-CNpy) with different electron-donating and electron-withdrawing groups at the para position of the pyridine ring were employed. Crystallographic analysis reveals various bismuth iodide structures, including 1D chains and discrete 0D motifs. The optical band gap of these materials, identified via diffuse reflectance spectroscopy (DRS) and verified with density functional theory (DFT) calculations, is influenced by the crystal packing and stabilising interactions. Through a comprehensive analysis, including Hirshfeld surface (HS) and void assessment, the study underscores the influence of noncovalent intermolecular interactions on crystal packing. Spectroscopic evaluations provide insights into electronic interactions, elucidating the role of electron donor and acceptor substituents within the lattice. Thermogravimetric differential thermal analysis (TG-DTA) indicates structural stability up to 250 °C. Linear sweep voltammetry (LSV) reveals significant conductivity in the range of 10-20 mS per pixel at 298.15 K. X-ray absorption spectroscopy (XAS) at the Bi L3 edge indicates a similar oxidation state and electronic environment across all samples, underscoring the role of bismuth centres surrounded by iodides.

Two-Dimensional Halide Perovskites: Approaches to Improve Optoelectronic Properties.

Three-dimensional (3D) halide perovskites (HPs) are in the spotlight of materials science research due to their excellent photonic and electronic properties suitable for functional device applications. However, the intrinsic instability of these materials stands as a hurdle in the way to their commercialization. Recently, two-dimensional (2D) HPs have emerged as an alternative to 3D perovskites, thanks to their excellent stability and tunable optoelectronic properties. Unlike 3D HPs, a library of 2D perovskites could be prepared by utilizing the unlimited number of organic cations since their formation is not within the boundary of the Goldschmidt tolerance factor. These materials have already proved their potential for applications such as solar cells, light-emitting diodes, transistors, photodetectors, photocatalysis, etc. However, poor charge carrier separation and transport efficiencies of 2D HPs are the bottlenecks resulting in inferior device performances compared to their 3D analogs. This minireview focuses on how to address these issues through the adoption of different strategies and improve the optoelectronic properties of 2D perovskites.

Organic Spacer Cation Assisted Modulation of the Structure and Properties of Bismuth Halide Perovskites.

ConspectusLead halide perovskites are under the spotlight of current research due to their potential for efficient and cost-effective next-generation optoelectronic devices. The unique photonic and electronic properties of these solution-processable materials brought them to the forefront of materials science. However, the toxicity and instability of lead-based perovskites are the major hurdles for their commercialization. These issues initiated an effort towards the development of environmentally friendly, lead-free perovskites. In this context, bismuth halide perovskites (BHPs) were ideal rivals for lead-based congeners due to their excellent chemical stability, lower toxicity, and structural versatility. Understanding the crystal structure and optoelectronic properties of BHPs is crucial for designing them for specific, tailor-made applications. This Account aims to review our recent research progress on the role of functional organic spacer cations in modulating the electronic confinements, optical properties, and photoconductivity of BHPs. We have employed a comprehensive experimental and theoretical investigation to probe the intriguing optical and electronic properties of these materials. Our findings on the structure-optoelectronic property correlations will be valuable guidelines for the rational selection of organic spacer cations in designing BHPs featuring low exciton binding energy, narrow optical bandgap, enhanced visible light absorption, and high photoconductivity. One of our key findings is that by increasing the electron affinity of the organic spacer ligands, photoconductivity and visible light absorption of BHPs could be significantly enhanced. We hope that the fundamental level understanding of the photophysical properties discussed in this Account will lead to new design rules for developing high-performance BHP materials.

Self-Assembled Organic Cations-Assisted Band-Edge Tailoring in Bismuth-Based Perovskites for Enhanced Visible Light Absorption and Photoconductivity.

Bismuth-based zero-dimensional perovskites garner high research interest because of their advantages, such as excellent moisture stability and lower toxicity in comparison to lead-based congeners. However, the wide optical bandgap (>2 eV) and poor photoconductivity of these materials are the bottlenecks for their optoelectronic applications. Herein, we report a combined experimental and theoretical study of the structural features and optoelectronic properties of two novel and stable zero-dimensional bismuth perovskites: (biphenyl bis(methylammonium))1.5BiI6·2H2O (BPBI) and (naphthalene diimide bis(ethylammonium))1.5BiI6·2H2O (NDBI). NDBI features a remarkably narrower bandgap (1.82 eV) than BPBI (2.06 eV) because of the significant orbital contribution of self-assembled naphthalene diimide cations at the band edges of NDBI. Further, the FP-TRMC analysis revealed that the photoconductivity of NDBI is about 3.7-fold greater than that of BPBI. DFT calculations showed that the enhanced photoconductivity in NDBI arises from its type-IIa band alignment, whereas type-Ib alignment was seen in BPBI.

Anisotropic Photoconductivity and Long-Lived Charge Carriers in Bismuth-Based One-Dimensional Perovskite with Type-IIa Band Alignment.

Bismuth-based perovskites are attracting intense scientific interest due to low toxicity and excellent moisture stability compared to lead-based analogues. However, high exciton binding energy, poor charge carrier separation, and transport efficiencies lower their optoelectronic performances. To address these issues, we have integrated an electronically active organic cation, naphthalimide ethylammonium, between the [BiI52-]n chains via crystal engineering to form a novel perovskite-like material (naphthalimide ethylammonium)2BiI5 (NBI). Single crystal analysis revealed a one-dimensional quantum-well structure for NBI in which inter-inorganic well electronic coupling is screened by organic layers. It exhibited anisotropic photoconductivity and long-lived charge carriers with milliseconds lifetime, which is higher than that of CH3NH3PbI3. Density functional theory calculations confirmed type-IIa band alignment between organic cations and inorganic chains, allowing the former to electronically contribute to the overall charge transport properties of the material.

Following the TRMC Trail: Optimization of Photovoltaic Efficiency and Structure-Property Correlation of Thiophene Oligomers.

Semiconducting conjugated oligomers having same end group (N-ethylrhodanine) but different central core (thiophene: OT-T, bithiophene: OT-BT, thienothiophene: OT-TT) connected through thiophene pi-linker (alkylated terthiophene) were synthesized for solution processable bulk-heterojunction solar cells. The effect of the incorporation of an extra thiophene to the central thiophene unit either through C-C bond linkage to form bithiophene or by fusing two thiophenes together to form thienothiophene on the optoelectronic properties and photovoltaic performances of the oligomers were studied in detail. Flash photolysis time-resolved microwave conductivity (FP-TRMC) technique shows OT-TT has significantly higher photoconductivity than OT-T and OT-BT implying that the former can outperform the latter two derivatives by a wide margin under identical conditions in a bulk-heterojunction solar cell device. However, the initial photovoltaic devices fabricated from all three oligomers (with PC71BM as the acceptor) gave power conversion efficiencies (PCEs) of about 0.7%, which was counterintuitive to the TRMC observation. By using TRMC results as a guiding tool, solution engineering was carried out; no remarkable changes were seen in the PCE of OT-T and OT-BT. On the other hand, 5-fold enhancement in the device efficiency was achieved in OT-TT (PCE: 3.52%, VOC: 0.80 V, JSC: 8.74 mA cm(-2), FF: 0.50), which was in correlation with the TRMC results. The structure-property correlation and the fundamental reasons for the improvement in device performance upon solvent engineering were deduced through UV-vis absorption, atomic force microscopy, bright-field transmission electron microscopy, photoluminescence quenching analysis and two-dimensional grazing incidence X-ray diffraction studies.

A versatile approach to organic photovoltaics evaluation using white light pulse and microwave conductivity.

State-of-the-art low band gap conjugated polymers have been investigated for application in organic photovoltaic cells (OPVs) to achieve efficient conversion of the wide spectrum of sunlight into electricity. A remarkable improvement in power conversion efficiency (PCE) has been achieved through the use of innovative materials and device structures. However, a reliable technique for the rapid screening of the materials and processes is a prerequisite toward faster development in this area. Here we report the realization of such a versatile evaluation technique for bulk heterojunction OPVs by the combination of time-resolved microwave conductivity (TRMC) and submicrosecond white light pulse from a Xe-flash lamp. Xe-flash TRMC allows examination of the OPV active layer without requiring fabrication of the actual device. The transient photoconductivity maxima, involving information on generation efficiency, mobility, and lifetime of charge carriers in four well-known low band gap polymers blended with phenyl-C(61)-butyric acid methyl ester (PCBM), were confirmed to universally correlate with the PCE divided by the open circuit voltage (PCE/V(oc)), offering a facile way to predict photovoltaic performance without device fabrication.

Semiconducting cross-linked polymer nanowires prepared by high-energy single-particle track reactions.

High-energy charged particle irradiation of cross-linking polymers gives nanowires formed by cross-linking reactions along the ion track trajectories. Here, the direct formation of nanowires consisting of a conjugated polymer by single-particle nanofabrication technique (SPNT) is investigated. Poly(9,9'-di-n-octylfluorene) (PFO), regioregular poly(3-hexylthiophene) (rrP3HT), and poly[2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) underwent an efficient cross-linking reaction upon irradiation, resulting in the formation of 1-dimensional nanostructures with high and desired aspect ratio reaching up to ∼200. The size of nanowires was perfectly interpreted by well-sophisticated theoretical aspects based on the statistical theory of polymer backbone configurations, suggesting that simple cross-linking reactions of the polymers determine the size and structure of nanowires. PFO based nanostructures exhibited sharp and intense emission with high fluorescence quantum yield indicating the absence of any significant inter/intra polymer chromophore interactions in the nanowires assemblies.

Detection and distinction of DNT and TNT with a fluorescent conjugated polymer using the microwave conductivity technique.

We report the detection and distinction of dinitrotoluene (DNT) and trinitrotoluene (TNT) by the microwave conductivity technique using a cyclopentadithiophene-bithiazole-based polymer (CPDT-BT) as sensor. Although the conventional fluorescence quenching experiments showed just "turn OFF" of the polymer fluorescence for both DNT and TNT, time-resolved microwave conductivity (TRMC) revealed that the photoconductivity of the polymer, which is "turned OFF" in the pristine state became "ON" in the presence of DNT but remained "OFF" with TNT, allowing easy distinction between them. Moreover, the decay rate of the transient kinetics was found to be sensitive to the DNT concentration, implementing a unique method for the determination of unknown DNT concentration. The observations are discussed in viewpoint of charge separation (CS) and formation of charge transfer (CT) complex by considering deeper LUMO of TNT than DNT calculated from the DFT method. This study brings out a novel technique of speedy detection and distinction of environmentally important analytes, an alternative to the fluorescence quenching.

Optoelectronic properties of dicyanofluorene-based n-type polymers.

Three new donor-acceptor-type copolymers (P1-P3) consisting of dicyanofluorene as acceptor and various donor moieties were designed and synthesized. Optoelectronic properties were studied in detail by means of UV-visible absorption and fluorescence spectroscopy, cyclic voltammetry, space-charge-limited current (SCLC), flash-photolysis time-resolved microwave conductivity (FP-TRMC), and density functional theory (DFT). All polymers showed strong absorption in the UV-visible region and the absorption maximum undergoes redshift with an increasing number of thiophene units in the polymer backbone. SCLC analysis showed that the electron mobilities of the polymers in the bulk state were 1 to 2 orders higher than that of the corresponding hole mobilities, which indicated the n-type nature of the materials. By using FP-TRMC, the intrapolymer charge-carrier mobility was assessed and compared with the interpolymer mobility obtained by SCLC. The polymers exhibited good electron-accepting properties sufficiently high enough to oxidize the excited states of regioregular poly(3-hexylthiophene) (P3HT (donor)), as evident from the FP-TRMC analysis. The P3 polymer exhibited the highest FP-TRMC transients in the pristine form as well as when blended with P3HT. Use of these polymers as n-type materials in all-polymer organic solar cells was also explored in combination with P3HT. In accordance with the TRMC results, P3 exhibited superior electron-transport and photovoltaic properties to the other two polymers, which is explained by the distribution of the energy levels of the polymers by using DFT calculations.

Excitation energy migration in oligo(p-phenylenevinylene) based organogels: structure-property relationship and FRET efficiency.

Excitation energy migration (EM) and assisted energy transfer (ET) properties of a few oligo(p-phenylenevinylene) (OPV) based organogelators with different end functional groups have been studied using picosecond time-resolved emission spectroscopy (TRES). EM was found to be more efficient in OPV gelators with small end functional groups (OPV3-4) when compared to that of the gelators with bulky end groups (OPV1-2) in the gel state. TRES studies at elevated temperature and in chloroform solution highlight the role of the self-assembled scaffolds in assisting the EM and ET processes. Increase in temperature and solvent polarity leads to the aggregate breaking and hence adversely affects the EM and ET efficiencies. The effect of EM efficiency on the fluorescence resonance energy transfer (FRET) properties of the OPV gels was studied by using OPV1 and OPV3 as the donors and OPV5 as the acceptor. Better transfer of excitation energy was observed in the donor system (OPV3) having higher EM efficiency even at very low concentration (3.1 mol%) of the acceptor molecules, whereas ET efficiency was lower in the donor system (OPV1) with low EM efficiency.

Detection of explosive vapors with a charge transfer molecule: self-assembly assisted morphology tuning and enhancement in sensing efficiency.

Use of a fluorescent organic molecule consisting of binaphthyl functionalized with donor-acceptor substituted stilbenes for the detection of dinitrotoluene (DNT) and trinitrotoluene (TNT) vapors and enhancement in its sensing efficiency via self-assembly assisted morphology tuning are described.

Organogels as scaffolds for excitation energy transfer and light harvesting.

The elegance and efficiency by which Nature harvests solar energy has been a source of inspiration for chemists to mimic such process with synthetic molecular and supramolecular systems. The insights gained over the years from these studies have contributed immensely to the development of advanced materials useful for organic based electronic and photonic devices. Energy transfer, being a key process in many of these devices, has been extensively studied in recent years. A major requirement for efficient energy transfer process is the proper arrangement of donors and acceptors in a few nanometers in length scale. A practical approach to this is the controlled self-assembly and gelation of chromophore based molecular systems. The present tutorial review describes the recent developments in the design of chromophore based organogels and their use as supramolecular scaffolds for excitation energy transfer studies.

Self-assembled pi-nanotapes as donor scaffolds for selective and thermally gated fluorescence resonance energy transfer (FRET).

Self-assembled nanotapes of a few tailor-made oligo(p-phenylenevinylene)s (OPVs) have been prepared and used as supramolecular donor scaffold to study the fluorescence resonance energy transfer (FRET) to a suitable acceptor. In nonpolar solvents, FRET occurs with nearly 63-81% efficiency, exclusively from the self-assembled OPVs to entrapped Rhodamine B, resulting in the quenching of the donor emission with concomitant formation of the acceptor emission at 625 nm. The efficiency of FRET is considerably influenced by the ability of the OPVs to form the self-assembled aggregates and hence could be controlled by structural variation of the molecules, and polarity of the solvent. Most importantly, FRET could be controlled by temperature as a result of the thermally reversible self-assembly process. The FRET efficiency was significantly enhanced (ca. 90%) in a xerogel film of the OPV1 which is dispersed with relatively less amount of the acceptor (33 mol %), when compared to that of the aggregates in dodecane gel. FRET is not efficient in polar solvents due to weak self-organization of the chromophores. These results indicate that energy transfer occurs exclusively from the self-assembled donor and not directly from the individual donor molecules. The present study illustrates that the self-assembly of chromophores facilitates temperature and solvent controlled FRET within pi-conjugated nanostructures.

Self-location of acceptors as "isolated" or "stacked" energy traps in a supramolecular donor self-assembly: a strategy to wavelength tunable FRET emission.

Control over supramolecular assemblies of donor and acceptor arrays in nanoscale dimension that facilitate efficient energy transfer resulting in tunable emission is an outstanding challenge. In pursuit of this goal, we have designed a supramolecular donor-acceptor organogel with tunable emission from green to red through controlled energy transfer by simply varying the acceptor concentration. Temperature-dependent UV/vis absorption, XRD, and AFM studies of the coassembly of 1 (donor) and 2 (acceptor) revealed the intercalation of 2 within the self-assembly of 1. Upon excitation of the decane gels of 1 with 0-2 mol % of 2, quenching of the emission of the former at 509 nm with the formation of the monomer emission of the latter at 555 nm is observed. Upon further addition of 2 (2-20 mol %), the emission was continuously red-shifted to 610 nm, which corresponds to the aggregate emission of 2. Consequently, a 98% quenching of the donor emission was observed at 509 nm. Fluorescence microscopic studies provided visual evidence for the color tuning of the FRET emission. Thus efficient trapping of excitons by "isolated" or "aggregated" acceptors through a subtle control of the self-assembly and the photophysical properties of the donor-acceptor building blocks allowed a continuous shifting of the emission color anywhere between green and red (lambdamax, 509-610 nm) in a supramolecular light harvesting system.