Cove‐Edge Nanoribbon Materials for Efficient Inverted Halide Perovskite Solar Cells

Non-Fullerene Organic Electron Transport Materials toward Stable and Efficient Inverted Perovskite Photovoltaics

Inverted perovskite solar cells (PSCs) attract continuing interest due to their low processing temperature, suppressed hysteresis, and compatibility with tandem cells. Considerable progress has been made with reported power conversion efficiency (PCE) surpassing 26%. Electron transport Materials (ETMs) play a critical role in achieving high-performance PSCs because they not only govern electron extraction and transport from the perovskite layer to the cathode, but also protect the perovskite from contact with ambient environment. On the other hand, the non-radiative recombination losses at the perovskite/ETM interface also limits the future development of PSCs. Compared with fullerene derivatives, non-fullerene n-type organic semiconductors feature advantages like molecular structure diversity, adjustable energy level, and easy modification. Herein, the non-fullerene ETM is systematically summarized based on the molecular functionalization strategy. Various types of molecular design approaches for producing non-fullerene ETM are presented, and the insight on relationship of chemical structure and device performance is discussed. Meantime, the future trend of non-fullerene ETM is analyzed. It is hoped that this review provides insightful perspective for the innovation of new non-fullerene ETMs toward more efficient and stable PSCs.

Fusing perylene diimide with helicenes

Incorporating perylene diimide (PDI) units into helicene structures has become a useful strategy for giving access to non-planar electron acceptors as well as a method of creating molecules with unique and intriguing chiroptical properties. This minireview describes this fusion of PDIs with helicenes.

Molecular engineering of nitrogen-rich helicene based organic semiconductors for stable perovskite solar cells

Polycyclic heteroaromatics play a pivotal role in advancing the field of high-performance organic semiconductors. In this study, we report the synthesis of a pyrrole-bridged double azahelicene through intramolecular oxidative cyclization. By incorporating bis(4-methoxyphenyl)amine (OMeDPA) and ethylenedioxythiophene-phenyl-OMeDPA (EP-OMeDPA) into the sp3-nitrogen rich double helicene framework, we have successfully constructed two organic semiconductors with ionization potentials suitable for application in perovskite solar cells. The amorphous films of both organic semiconductors exhibit hole density-dependent mobility and conductivity. Notably, the organic semiconductor utilizing EP-OMeDPA as the electron donor demonstrates superior hole mobility at a given hole density, which is attributed to reduced reorganization energy and increased centroid distance. Moreover, this organic semiconductor exhibits a remarkably elevated glass transition temperature of up to 230 °C and lower diffusivity for external small molecules and ions. When employed as the p-doped hole transport layer in perovskite solar cells, TMDAP-EP-OMeDPA achieves an improved average efficiency of 21.7%. Importantly, the solar cell with TMDAP-EP-OMeDPA also demonstrates enhanced long-term operational stability and storage stability at 85 °C. These findings provide valuable insights into the development of high-performance organic semiconductors, contributing to the practical application of perovskite solar cells.

Electronic Materials: An Antiaromatic Propeller Made from the Four-Fold Fusion of Tetraoxa[8]circulene and Perylene Diimides

The synthesis of an antiaromatic tetraoxa[8]circulene annulated with four perylene diimides (PDI), giving a dynamic non-planar π-conjugated system, is described. The molecule contains 32 aromatic rings surrounding one formally antiaromatic planarized cyclooctatetraene (COT). The intense absorption (ϵ=3.35×10⁵  M^-1  cm^-1 in CH₂ Cl₂ ) and emission bands are assigned to internal charge-transfer transitions in the combined PDI-circulene π-system. The spectroscopic data is supported by density functional theory calculations, and nuclear independent chemical shift calculation indicate that the antiaromatic COT has increased aromaticity in the reduced state. Electrochemical studies show that the compound can reversibly reach the tetra- and octa-anionic states by reduction of the four PDI units, and the deca-anionic state by reduction of the central COT ring. The material functions effectively in bulk hetero junction solar cells as a non-fullerene acceptor, reaching a power conversion efficiency of 6.4 %.

Remote Control of Dynamic Twistacene Chirality

We report a reliable way to manipulate the dynamic, axial chirality in perylene diimide (PDI)-based twistacenes. Specifically, we reveal how chiral substituents on the imide position induce the helicity in a series of PDI-based twistacenes. We demonstrate that this remote chirality is able to control the helicity of flexible [4]helicene subunits by UV-vis, CD spectroscopy, X-ray crystallography, and TDDFT calculations. Furthermore, we have discovered that both the chiral substituent and the solvent each has a strong impact on the sign and intensity of the CD signals, highlighting the control of the dynamic helicity in this flexible system. DFT calculations suggest that the steric interaction of the chiral substituents is the important factor in how well a particular group is at inducing a preferred helicity.

Iterative Synthesis of Contorted Macromolecular Ladders for Fast-Charging and Long-Life Lithium Batteries

We report here an iterative synthesis of long helical perylene diimide (hPDI[n]) nanoribbons with a length up to 16 fused benzene rings. These contorted, ladder-type conjugated, and atomically precise nanoribbons show great potential as organic fast-charging and long-lifetime battery cathodes. By tuning the length of the hPDI[n] oligomers, we can simultaneously modulate the electrical conductivity and ionic diffusivity of the material. The length of the ladders adjusts both the conjugation for electron transport and the contortion for lithium-ion transport. The longest oligomer, hPDI[6], when fabricated as the cathode in lithium batteries, features both high electrical conductivity and high ionic diffusivity. This electrode material exhibits a high power density and can be charged in less than 1 min to 66% of its maximum capacity. Remarkably, this material also has exceptional cycling stability and can operate for up to 10,000 charging-discharging cycles without any appreciable capacity decay. The design principles described here chart a clear path for organic battery electrodes that are sustainable, fast-charging, and long lasting.

Small Size, Big Impact: Recent Progress in Bottom-Up Synthesized Nanographenes for Optoelectronic and Energy Applications

Bottom-up synthesized graphene nanostructures, including 0D graphene quantum dots and 1D graphene nanoribbons, have recently emerged as promising candidates for efficient, green optoelectronic, and energy storage applications. The versatility in their molecular structures offers a large and novel library of nanographenes with excellent and adjustable optical, electronic, and catalytic properties. In this minireview, recent progress on the fundamental understanding of the properties of different graphene nanostructures, and their state-of-the-art applications in optoelectronics and energy storage are summarized. The properties of pristine nanographenes, including high emissivity and intriguing blinking effect in graphene quantum dots, superior charge transport properties in graphene nanoribbons, and edge-specific electrochemistry in various graphene nanostructures, are highlighted. Furthermore, it is shown that emerging nanographene-2D material-based van der Waals heterostructures provide an exciting opportunity for efficient green optoelectronics with tunable characteristics. Finally, challenges and opportunities of the field are highlighted by offering guidelines for future combined efforts in the synthesis, assembly, spectroscopic, and electrical studies as well as (nano)fabrication to boost the progress toward advanced device applications.

Solution-processed two-dimensional materials for next-generation photovoltaics

In the ever-increasing energy demand scenario, the development of novel photovoltaic (PV) technologies is considered to be one of the key solutions to fulfil the energy request. In this context, graphene and related two-dimensional (2D) materials (GRMs), including nonlayered 2D materials and 2D perovskites, as well as their hybrid systems, are emerging as promising candidates to drive innovation in PV technologies. The mechanical, thermal, and optoelectronic properties of GRMs can be exploited in different active components of solar cells to design next-generation devices. These components include front (transparent) and back conductive electrodes, charge transporting layers, and interconnecting/recombination layers, as well as photoactive layers. The production and processing of GRMs in the liquid phase, coupled with the ability to "on-demand" tune their optoelectronic properties exploiting wet-chemical functionalization, enable their effective integration in advanced PV devices through scalable, reliable, and inexpensive printing/coating processes. Herein, we review the progresses in the use of solution-processed 2D materials in organic solar cells, dye-sensitized solar cells, perovskite solar cells, quantum dot solar cells, and organic-inorganic hybrid solar cells, as well as in tandem systems. We first provide a brief introduction on the properties of 2D materials and their production methods by solution-processing routes. Then, we discuss the functionality of 2D materials for electrodes, photoactive layer components/additives, charge transporting layers, and interconnecting layers through figures of merit, which allow the performance of solar cells to be determined and compared with the state-of-the-art values. We finally outline the roadmap for the further exploitation of solution-processed 2D materials to boost the performance of PV devices.

MOFs based on the application and challenges of perovskite solar cells

In recent years, perovskite solar cells (PSCs) have attracted much attention because of their high energy conversion efficiency, low cost, and simple preparation process. Up to now, the photoelectric conversion efficiency of solar cells has been increased from 3.8% to 25.5%. Metal–organic skeleton-derived metal oxides and their composites (MOFs) are widely considered for application in PSCs due to their low and flat charge/discharge potential plateau, high capacity, and stable cycling performance. By combining MOFs and PSCs, based on the composition materials of perovskite film, electron transport layer, hole transport layer, and interfacial interlayer of PSCs, this article discusses the photovoltaic performance or structure optimization effect of MOFs in each function layer, which is of great significance to improve the photovoltaic performance of the cell. The problems faced by MOFs on perovskite solar cells are summarized, the next research directions are discussed, and the development of this crossover area of MOFs–PSC is foreseen to accelerate the comprehensive research and popularization of MOFs on PSCs.

Highly efficient heterojunction solar cells enabled by edge-modified tellurene nanoribbons

Tellurene, a two-dimensional (2D) semiconductor, meets the requirements for optoelectronic applications with desirable properties, such as a suitable band gap, high carrier mobility, strong visible light absorption and high air stability. Here, we demonstrate that the band engineering of zigzag tellurene nanoribbons (ZTNRs) via edge-modification can be used to construct highly efficient heterojunction solar cells by using first-principles density functional theory (DFT) calculations. We find that edge-modification enhances the stability of ZTNRs and halogen-modified ZTNRs showing suitable band gaps (1.35-1.53 eV) for sunlight absorption. Furthermore, the band gaps of ZTNRs with tetragonal edges do not strongly depend on the edge-modification and ribbon width, which is conducive to experimental realization. The heterojunctions constructed by halogen-modified ZTNRs show desirable type 2 band alignments and small band offsets with reduced band gaps and enhanced sunlight absorption, resulting in high power conversion efficiency (PCE) in heterojunction solar cells. In particular, the calculated maximum PCE of designed heterojunction solar cells based on halogen-modified ZTNRs can reach as high as 22.6%.

Stringing the Perylene Diimide Bow

This study explores a new mode of contortion in perylene diimides where the molecule is bent, like a bow, along its long axis. These bowed PDIs were synthesized through a facile fourfold Suzuki macrocyclization with aromatic linkers and a tetraborylated perylene diimide that introduces strain and results in a bowed structure. By altering the strings of the bow, the degree of bending can be controlled from flat to highly bent. Through spectroscopy and quantum chemical calculations, it is demonstrated that the energy of the lowest unoccupied orbital can be controlled by the degree of bending in the structures and that the energy of the highest occupied orbital can be controlled to a large extent by the constitution of the aromatic linkers. The important finding is that the bowing results not only in red-shifted absorptions but also more facile reductions.

Recent Progress in Organic Electron Transport Materials in Inverted Perovskite Solar Cells

Organic n-type materials (e.g., fullerene derivatives, naphthalene diimides (NDIs), perylene diimides (PDIs), azaacene-based molecules, and n-type conjugated polymers) are demonstrated as promising electron transport layers (ETLs) in inverted perovskite solar cells (p-i-n PSCs), because these materials have several advantages such as easy synthesis and purification, tunable frontier molecular orbitals, decent electron mobility, low cost, good solubility in different organic solvents, and reasonable chemical/thermal stability. Considering these positive factors, approaches toward achieving effective p-i-n PSCs with these organic materials as ETLs are highlighted in this Review. Moreover, organic structures, electron transport properties, working function of electrodes caused by ETLs, and key relevant parameters (PCE and stability) of p-i-n PSCs are presented. Hopefully, this Review will provide fundamental guidance for future development of new organic n-type materials as ETLs for more efficient p-i-n PSCs.

Efficient Inverted Perovskite Solar Cells by Employing N-Type (D-A1 -D-A2 ) Polymers as Electron Transporting Layer

It is highly desirable to employ n-type polymers as electron transporting layers (ETLs) in inverted perovskite solar cells (PSCs) due to their good electron mobility, high hydrophobicity, and simplicity of film forming. In this research, the capability of three n-type donor-acceptor₁ -donor-acceptor₂ (D-A₁ -D-A₂ ) conjugated polymers (pBTT, pBTTz, and pSNT) is first explored as ETLs because these polymers possess electron mobilities as high as 0.92, 0.46, and 4.87 cm² (Vs)^-1 in n-channel organic transistors, respectively. The main structural difference among pBTT, pBTTz, and pSNT is the position of sp² -nitrogen atoms (sp² -N) in the polymer main chains. Therefore, the effect of different substitution positions on the PSC performances is comprehensively studied. The as-fabricated p-i-n PSCs with pBTT, pBTTz, and pSNT as ETLs show the maximum photoconversion efficiencies of 12.8%, 14.4%, and 12.0%, respectively. To be highlighted, pBTTz-based device can maintain 80% of its stability after ten days due to its good hydrophobicity, which is further confirmed by a contact angle technique. More importantly, the pBTTz-based device shows a neglected hysteresis. This study reveals that the n-type polymers can be promising candidates as ETLs to approach solution-processed highly-efficient inverted PSCs.

Enhanced Open-Circuit Voltage in Perovskite Solar Cells with Open-Cage [60]Fullerene Derivatives as Electron-Transporting Materials

The synthesis, characterization, and incorporation of open-cage [60]fullerene derivatives as electron-transporting materials (ETMs) in perovskite solar cells (PSCs) with an inverted planar (p-i-n) structure is reported. Following optical and electrochemical characterization of the open-cage fullerenes 2a–c, p-i-n PSCs with a indium tin oxide (ITO)/poly(3,4-ethylenedioxythiophene)-polystyrene sulfonate (PEDOT:PSS)/perovskite/fullerene/Ag structure were prepared. The devices obtained from 2a–b exhibit competitive power conversion efficiencies (PCEs) and improved open-circuit voltage (V_oc) values (>1.0 V) in comparison to a reference cell based on phenyl-C₆₁-butyric-acid methyl-ester (PC₆₁BM). These results are rationalized in terms of a) the higher passivation ability of the open-cage fullerenes with respect to the other fullerenes, and b) a good overlap between the highest occupied molecular orbital/lowest unoccupied molecular orbital (HOMO/LUMO) levels of 2a–b and the conduction band of the perovskite.

Conjugated Macrocycles in Organic Electronics

This Account describes a body of research on the design, synthesis, and application of a new class of electronic materials made from conjugated macrocycles. Our macrocyclic design takes into consideration the useful attributes of fullerenes and what properties make fullerenes efficient n-type materials. We identified four electronic and structural elements: (1) a three-dimensional shape; (2) a conjugated and delocalized π-space; (3) the presence of an interior and exterior to the π-surface; and (4) low-energy unoccupied molecular orbitals allowing them to accept electrons. The macrocyclic design incorporates some of these properties, including a three-dimensional shape, an interior/exterior to the π-surface, and low-lying LUMOs maintaining the n-type semiconducting behavior, yet we also install synthetic flexibility in our approach in order to tune the properties further. Each of the macrocycles comprises perylenediimide cores wound together with linkers. The perylenediimide building block endows each macrocycle with the ability to accept electrons, while the synthetic flexibility to install different linkers allows us to create macrocycles with different electronic properties and sizes. We have created three macrocycles that all absorb well into the visible range of the solar spectrum and possess different shapes and sizes. We then use these materials in an array of applications that take advantage of their ability to function as n-type semiconductors, absorb in the visible range of the solar spectrum, and possess intramolecular cavities. This Account will discuss our progress in incorporating these new macrocycles in organic solar cells, organic photodetectors, organic field effect transistors, and sensors. The macrocycles outperform acyclic controls in organic solar cells. We find the more rigid macrocyclic structure results in less intrinsic charges and lower dark current in organic photodetectors. Our macrocyclic-based photodetector has the highest detectivity of non-fullerene acceptors. The macrocycles also function as sensors and are able to recognize nuanced differences in analytes. Perylenediimide-based fused oligomers are efficient materials in both organic solar cells and field effect transistors. We will use the oligomers to construct macrocycles for use in solar energy conversion. In addition, we will incorporate different electron-rich linkers in our cycles in an attempt to engineer the HOMO/LUMO gap further. Looking further into the future, we envision opportunities in applying these conjugated macrocycles as electronic host/guest materials, as concatenated electronic materials by threading the macrocycles with electroactive oligomers, and as a locus for catalysis that is driven by light and electric fields.

Molecular Engineering of Triphenylamine-Based Non-Fullerene Electron-Transport Materials for Efficient Rigid and Flexible Perovskite Solar Cells

There has been a growing interest in the design and synthesis of non-fullerene electron transport materials (ETMs) for perovskite solar cells (PSCs), which may overcome the drawbacks of traditional fullerene derivatives. In this work, a novel donor-acceptor (D-A) structured ETM termed TPA-3CN is presented by molecular engineering of triphenylamine (TPA) as the donor group and (3-cyano-4,5,5-trimethyl-2(5 H)-furanylidene) malononitrile as the acceptor group. To further improve the electron mobility and conductivity and achieve excellent photovoltaic performance, a solution processable n-type dopant is introduced during the ETM spin-coating step. After device optimization, PSCs based on the doped TPA-3CN exhibit an impressive power conversion efficiency (PCE) of 19.2% with a negligible hysteresis. Benefitting from the low temperature and good solution processability of ETM TPA-3CN, it was further applied in flexible inverted PSCs and an impressive PCE of 13.2% was achieved, which is among the highest values reported for inverted flexible fullerene-free PSCs.

Naphthodiperylenetetraimide-Based Polymer as Electron-Transporting Material for Efficient Inverted Perovskite Solar Cells

An n-type conjugated polymer NDP-V [poly(naphthodiperylenetetraimide-vinylene)] with a backbone of alternating naphthodiperylenetetraimide and vinylene is successfully used as an efficient electron-transporting layer (ETL) material in inverted planar perovskite solar cells (PSCs). It was found that device based on NDP-V exhibits a maximum power conversion efficiency (PCE) of 16.54%, whereas a maximum PCE of 15.27% is obtained based on the fullerene derivative [6,6]-phenyl-C₆₁-butyric acid methyl ester (PC₆₁BM). The interfacial effect induced by NDP-V is studied using atomic force microscopy images, and NDP-V ensures good selective contact between the perovskite material and the metal electrode. Through steady-state and time-resolved photoluminescence, we find that NDP-V acts as an efficient electron extraction material. Additionally, compared with PC₆₁BM, NDP-V shows higher electron mobility, more hydrophobicity, and compatible energy levels with perovskite materials, thus providing higher device performance and better device stability. This work highlights the great potential of perylenediimide derivatives to replace the most popular PC₆₁BM ETL for inverted PSCs.

Toward Perovskite Solar Cell Commercialization: A Perspective and Research Roadmap Based on Interfacial Engineering

High-efficiency and low-cost perovskite solar cells (PVKSCs) are an ideal candidate for addressing the scalability challenge of solar-based renewable energy. The dynamically evolving research field of PVKSCs has made immense progress in solving inherent challenges and capitalizing on their unique structure-property-processing-performance traits. This review offers a unique outlook on the paths toward commercialization of PVKSCs from the interfacial engineering perspective, relevant to both specialists and nonspecialists in the field through a brief introduction of the background of the field, current state-of-the-art evolution, and future research prospects. The multifaceted role of interfaces in facilitating PVKSC development is explained. Beneficial impacts of diverse charge-transporting materials and interfacial modifications are summarized. In addition, the role of interfaces in improving efficiency and stability for all emerging areas of PVKSC design are also evaluated. The authors' integral contributions in this area are highlighted on all fronts. Finally, future research opportunities for interfacial material development and applications along with scalability-durability-sustainability considerations pivotal for facilitating laboratory to industry translation are presented.

Influence of Molecular Conformation on Electron Transport in Giant, Conjugated Macrocycles

We describe here the direct connection between the molecular conformation of a conjugated macrocycle and its macroscopic charge transport properties. We incorporate chiral, helical perylene diimide ribbons into the two separate macrocycles as the n-type, electron transporting material. As the macrocycles' films and electronic structures are analogous, the important finding is that the macrocycles' molecular structures and their associated dynamics determine device performance in organic field effect transistors. We show the more flexible macrocycle has a 4-fold increase in electron mobility in field effect transistor devices. Using a combination of spectroscopy and density functional theory calculations, we find that the origin of the difference in device performance is the ability of more flexible isomer to make intermolecular contacts relative to the more rigid counterpart.

A thiadiazole-capped nanoribbon with 18 linearly fused rings

Polycyclic aromatic hydrocarbons that extend over 2 nm in one dimension are seen as monodisperse graphene nanoribbons, which have attracted significant attention for a broad range of applications in organic electronics and photonics. Herein we report the synthesis of a stable bisthiadiazole-capped pyrene-containing nanoribbon with 18 linearly fused rings (NR-18-TD). Thanks to the presence of alternating tert-butyl and tri-iso-butylsilyl groups, NR-18-TD is highly soluble in organic solvents and therefore its structure and fundamental optoelectronic, redox and electrical properties could be unambiguously established. This work illustrates that NR-18-TD is a promising soluble NR-based n-type semiconductor for applications in organic electronics.

Functionalized Helical Building Blocks for Nanoelectronics

Molecular building blocks are designed and created for the cis- and trans-dibrominated perylenediimides. The syntheses are simple and provide these useful materials on the gram scale. To demonstrate their synthetic versatility, these building blocks were used to create new dimeric perylenediimide helixes. Two of these helical dimers are twistacenes, and one is a helicene. Crucially, each possesses regiochemically defined functionality that allows the dimer helix to be elaborated into higher oligomers. It would be very difficult to prepare these helical PDI building blocks regioselectively without the methods described.

Monodisperse N-Doped Graphene Nanoribbons Reaching 7.7 Nanometers in Length

The properties of graphene nanoribbons are highly dependent on structural variables such as width, length, edge structure, and heteroatom doping. Therefore, atomic precision over all these variables is necessary for establishing their fundamental properties and exploring their potential applications. An iterative approach is presented that assembles a small and carefully designed molecular building block into monodisperse N-doped graphene nanoribbons with different lengths. To showcase this approach, the synthesis and characterisation of a series of nanoribbons constituted of 10, 20 and 30 conjugated linearly-fused rings (2.9, 5.3, and 7.7 nm in length, respectively) is presented.