Fused-Ring Electron Acceptors for Photovoltaics and Beyond

Carbazole, Fluorene, and Silafluorene Bearing Non-Fullerene Acceptors: Synthesis, Characterization, and Performance in Organic Photovoltaic Devices

Due to their tunable energy levels, ability for intense light absorption, stability and ease of purification, non-fullerene acceptors (NFAs) have significantly contributed to the progress of organic photovoltaics (OPVs). Herein, a series of newly designed and synthesized NFAs specifically tailored are presented for OPV applications. A new class of NFAs possessing carbazole, fluorene, silafluorene derivatives, and benzothiadiazoles are synthesized. The electrochemical and optical investigations are conducted thoroughly. The utilization of these new materials with commercial donor polymers P3HT and PTB7-Th in OPVs shows their potential applications for future studies. The optimal performance is achieved with an inverted structured OPV device, which demonstrates a power conversion efficiency (PCE) of 3.77%, an open-circuit voltage (VOC) of 1.06 V, a short-circuit current (JSC) of 9.66 mA cm- 2, and a fill factor (FF) of 37.08%.

Development of Dopant-Free N,N'-Bicarbazole-Based Hole Transport Materials for Efficient Perovskite Solar Cells

Efficient and stable hole-transport material (HTM) is essential for enhancing the efficiency and stability of high-efficiency perovskite solar cells (PSCs). The commonly used HTMs such as spiro-OMeTAD need dopants to produce high efficiency, but those dopants degrade the perovskite film and cause instability. Therefore, the development of dopant-free N,N'-bicarbazole-based HTM is receiving huge attention for preparing stable, cost-effective, and efficient PSCs. Herein, we designed and proposed seven distinct small-molecule-based HTMs (B1-B7), which are synthesized and do not require dopants to fabricate efficient PSCs. To design this new series, we performed synergistic side-chain engineering on the synthetic reference molecule (B) by replacing two methylthio (-SCH3) terminal groups with a thiophene bridge and electron-withdrawing acceptor. The enhanced phase inversion geometry of the proposed molecules resulted in reduced energy gaps and better electrical, optical, and optoelectronic properties. Density functional theory (DFT) and time-dependent DFT simulations have been used to study the precise photo-physical and optoelectronic properties. We also looked into the effects of holes and electrons and the materials' structural and photovoltaic properties, including light harvesting energy, frontier molecular orbital, transition density matrix, density of states, electron density matrix, and natural population analysis. Electron density difference maps identify the interfacial charge transfer from the donor to the acceptor through the bridge, and natural population analysis measures the amount of charge on each portion of the donor, bridge, and acceptor, which most effectively represents the role of the end-capped moieties in facilitating charge transfer. Among these designed molecules, the B6 molecule has the greatest absorbance (λmax of 444.93 nm in dichloromethane solvent) and a substantially shorter optical band gap of 3.93 eV. Furthermore, the charge transfer analysis reveals superior charge transfer with improved intrinsic characteristics. Furthermore, according to the photovoltaic analysis, the designed (B1-B7) HTMs have the potential to provide better fill factor and open-circuit voltages, which will ultimately increase the power conversion efficiency (PCE) of PSCs. Therefore, we recommend these molecules for the next-generation PSCs.

Unraveling the structure-property relationship of novel thiophene and furan-fused cyclopentadienyl chromophores for nonlinear optical applications

Development of organic nonlinear optical materials has become progressively more important due to their emerging applications in new-generation photonic devices. A novel series of chromophores based on innovative thiophene and furan-fused cyclopentadienyl bridge with various powerful donor and acceptor moieties were designed and theoretically investigated for applications in nonlinear optics. To unravel the structure-property relationship between this new push-pull conjugated systems and their nonlinear optical property, multiple methods, including density of states analysis, coupled perturbed Kohn-Sham (CPKS) method, sum-over-states (SOS) model, the two-level model (TSM), hyperpolarizability density analysis, and the (hyper)polarizability contribution decomposition, were performed to comprehensively investigated the nonlinear optical and electronic properties of this new π-system. Due to excellent charge transfer ability of new bridge and distinctive structure of donor and acceptor, the designed chromophores exhibit deep HOMO levels, low excitation energy, high dipole moment difference and large hyperpolarizability, indicating the appealing air-stable property and remarkable electrooptic performance of them. Importantly, THQ-CS-A3 and PA-CS-A3 shows outstanding NLO response properties with βtot value of 6953.9 × 10-30 and 5066.0 × 10-30 esu in AN, respectively. The influence of the push-pull strength, the heterocycle and the π-conjugation of new bridge on the nonlinear optical properties of this novel powerful systems are clarified. This new series of chromophores exhibit remarkable electro-optical Pockels and optical rectification effect. More interestingly, PA-CS-A3 and THQ-CS-A2 also show appealing SHG effect. This study will help people understand the nature of nonlinear optical properties of innovative heteroarene-fused based cyclopentadienyl chromophores and offer guidance for the rational design of chromophores with outstanding electrooptic (EO) performance in the future.

Nonfullerene Acceptors Bearing Spiro-Substituted Bithiophene Units in Organic Solar Cells: Tuning the Frontier Molecular Orbital Distribution to Reduce Exciton Binding Energy

The development of nonfullerene acceptors (NFAs), represented by ITIC, has contributed to improving the power conversion efficiency (PCE) of organic solar cells (OSCs). Although tuning the electronic structures to reduce the exciton binding energy (Eb) is considered to promote photocharge generation, a rational molecular design for NFAs has not been established. In this study, we designed and developed two ITIC-based NFAs bearing spiro-substituted bithiophene or biphenyl units (named SpiroT-DCI and SpiroF-DCI) to tune the frontier molecular orbital (FMO) distribution of NFAs. While the highest occupied molecular orbitals (HOMOs) of SpiroF-DCI and ITIC are delocalized in the main π-conjugated framework, the HOMO of SpiroT-DCI is distributed on the bithiophene unit. Reflecting this difference, SpiroT-DCI exhibits a smaller Eb than either SpiroF-DCI or ITIC, and exhibits greater external quantum efficiency in single-component OSCs. Furthermore, SpiroT-DCI shows improved PCEs for bulk-heterojunction OSCs with a donor of PBDB-T, compared with that of either SpiroT-DCI or ITIC. Time-resolved spectroscopy measurements show that the photo-induced intermolecular charge separation is effective even in pristine SpiroT-DCI films. This study highlights the introduction of spiro-substituted bithiophene units that are effective in tuning the FMOs of ITIC, which is desirable for reducing the Eb and improving the PCE in OSCs.

Achieving 19.5% Efficiency via Modulating Electronic Properties of Peripheral Aryl-Substituted Small-Molecule Acceptors

The advancement of acceptors plays a pivotal role in determining photovoltaic performance. While previous efforts have focused on optimizing acceptor-donor-acceptor1-donor-acceptor (A-DA1-D-A)-typed acceptors by adjusting side chains, end groups, and conjugated extension of the electron-deficient central A1 unit, the systematic exploration of the impact of peripheral aryl substitutions, particularly with different electron groups, on the A1 unit and its influence on device performance is still lacking. In this study, three novel acceptors - QxTh, QxPh, and QxPy - with distinct substitutions on the quinoxaline (Qx) are designed and synthesized. Density functional theory (DFT) analyses reveal that QxPh, featuring a phenyl-substituted Qx, exhibits the smallest molecular binding energies and a tightest π···π stacking distance. Consequently, the PM6:QxPh device demonstrates a better power conversion efficiency (PCE) of 17.1% compared to the blends incorporating QxTh (16.4%) and QxPy (15.7%). This enhancement is primarily attributed to suppressed charge recombination, improved charge extraction, and more favorable molecular stacking and morphology. Importantly, introducing QxPh as a guest acceptor into the PM6:BTP-eC9 binary system yields an outstanding PCE of 19.5%, indicating the substantial potential of QxPh in advancing ternary device performance. The work provides deep insights into the expansion of high-performance organic photovoltaic materials through peripheral aryl substitution strategy.

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.

Regulating Intermolecular Interactions and Film Formation Kinetics for Record Efficiency in Difluorobenzothiadizole-Based Organic Solar Cells

The difluorobenzothiadizole (ffBT) unit is one of the most classic electron-accepting building blocks used to construct D-A copolymers for applications in organic solar cells (OSCs). Historically, ffBT-based polymers have achieved record power conversion efficiencies (PCEs) in fullerene-based OSCs owing to their strong temperature-dependent aggregation (TDA) characteristics. However, their excessive miscibility and rapid aggregation kinetics during film formation have hindered their performance with state-of-the-art non-fullerene acceptors (NFAs). Herein, we synthesized two ffBT-based copolymers, PffBT-2T and PffBT-4T, incorporating different π-bridges to modulate intermolecular interactions and aggregation tendencies. Experimental and theoretical studies revealed that PffBT-4T exhibits reduced electrostatic potential differences and miscibility with L8-BO compared to PffBT-2T. This facilitates improved phase separation in the active layer, leading to enhanced molecular packing and optimized morphology. Moreover, PffBT-4T demonstrated a prolonged nucleation and crystal growth process, leading to enhanced molecular packing and optimized morphology. Consequently, PffBT-4T-based devices achieved a remarkable PCE of 17.5 %, setting a new record for ffBT-based photovoltaic polymers. Our findings underscore the importance of conjugate backbone modulation in controlling aggregation behavior and film formation kinetics, providing valuable insights for the design of high-performance polymer donors in organic photovoltaics.

Enhancing Efficiency and Stability of Inverted Perovskite Solar Cells through Solution-Processed and Structurally Ordered Fullerene

The electron transporting layer (ETL) used in high performance inverted perovskite solar cells (PSCs) is typically composed of C60, which requires time-consuming and costly thermal evaporation deposition, posing a significant challenge for large-scale production. To address this challenge, herein, we present a novel design of solution-processible electron transporting material (ETM) by grafting a non-fullerene acceptor fragment onto C60. The synthesized BTPC60 exhibits an exceptional solution processability and well-organized molecular stacking pattern, enabling the formation of uniform and structurally ordered film with high electron mobility. When applied as ETL in inverted PSCs, BTPC60 not only exhibits excellent interfacial contact with the perovskite layer, resulting in enhanced electron extraction and transfer efficiency, but also effectively passivates the interfacial defects to suppress non-radiative recombination. Resultant BTPC60-based inverted PSCs deliver an impressive power conversion efficiency (PCE) of 25.3 % and retain almost 90 % of the initial values after aging at 85 °C for 1500 hours in N2. More encouragingly, the solution-processed BTPC60 ETL demonstrates remarkable film thickness tolerance, and enables a high PCE up to 24.8 % with the ETL thickness of 200 nm. Our results highlight BTPC60 as a promising solution-processed fullerene-based ETM, opening an avenue for improving the scalability of efficient and stable inverted PSCs.

Recent Progress on Cross-Linkable Fullerene-Based Electron Transport Materials for Perovskite Solar Cells

Fullerene-based derivatives are frequently used as electron transport materials (ETMs) and interface buffers for perovskite solar cells (PSCs) due to their excellent properties, including high electron affinity and mobility, low recombination energy, tunable energy levels, and solution processability. However, significant challenges arise because fullerene derivatives tend to aggregate and dimerize, which reduces exciton dissociation and charge transport capacity. Additionally, their chemical compatibility with perovskite absorbers facilitates halide diffusion and degradation of PSCs. This overlap causes delamination and dissolution during device fabrication, hindering the performance enhancement of fullerene-based PSCs. To address these issues, researchers have developed cross-linkable fullerene materials. These materials have been shown to not only significantly improve the power conversion efficiency (PCE) of PSCs but also effectively enhance the device stability. In this review, we summarized recent research progress on cross-linkable fullerene derivatives as ETMs for PSCs. We systematically analyze the impact of these cross-linked ETMs on device performance and long-term stability, focusing on their molecular structures and working mechanisms. Finally, we discuss the future challenges that need to be overcome to advance the application of cross-linkable fullerene materials in PSCs.

Design of Ultra-Narrow Bandgap Polymer Acceptors for High-Sensitivity Flexible All-Polymer Short-Wavelength Infrared Photodetectors

All-polymer photodetectors possess unique mechanical flexibility and are ideally suitable for the application in next-generation flexible, wearable short-wavelength infrared (SWIR, 1000-2700 nm) photodetectors. However, all-polymer photodetectors commonly suffer from low sensitivity, high noise, and low photoresponse speed in the SWIR region, which significantly diminish their application potential in wearable electronics. Herein, two polymer acceptors with absorption beyond 1000 nm, namely P4TOC-DCBT and P4TOC-DCBSe, were designed and synthesized. The two polymers possess rigid structure and good conformational stability, which is beneficial for reducing energetic disorder and suppressing dark current. Owing to the efficient charge generation and ultralow noise current, the P4TOC-DCBT-based all-polymer photodetector achieved a specific detectivity (D * ${{D}^{^{\ast}}}$ ) of over 1012 Jones from 650 (visible) to 1070 nm (SWIR) under zero bias, with a response time of 1.36 μs. These are the best results for reported all-polymer SWIR photodetectors in photovoltaic mode. More significantly, the all-polymer blend films exhibit good mechanical durability, and hence the P4TOC-DCBT-based flexible all-polymer photodetectors show a small performance attenuation (<4 %) after 2000 cycles of bending to a 3 mm radius. The all-polymer flexible SWIR organic photodetectors are successfully applied in pulse signal detection, optical communication and image capture.

Emerging A-D-A fused-ring photosensitizers for tumor phototheranostics

As we all know, cancer is still a disease that we are struggling against. Although the traditional treatment options are still the mainstream in clinical practice, emerging phototheranostics technologies based on photoacoustic or fluorescence imaging-guided phototherapy also provide a new exploration direction for non-invasive, low-risk and highly efficient cancer treatment. Photosensitizers are the core materials to accomplish this mission. Recently, more attention has been paid to the emerging A-D-A fused-ring photosensitizers. A-D-A fused-ring photosensitizers display strong and wide absorption spectra, high photostability and easy molecular modification. Since this type of photosensitizer was first used for tumor therapy in 2019, its application boundaries are constantly expanding. Therefore, in this feature article, from the perspective of molecular design, we focused on the development of these molecules for application in phototheranostics over the past five years. The effects of tiny structural changes on their photophysical properties are discussed in detail, which provides a way for structural optimization of the subsequent A-D-A photosensitizers.

Colorless Near-Infrared Absorbing Dyes Based on B-N Fused Donor-Acceptor-Donor π-Conjugated Molecules for Organic Phototransistors

The introduction of a colorless function to organic electronic devices allows responses to light in the near-infrared (NIR) region and is expected to broaden the applications of these devices. However, the development of a colorless NIR dye remains a challenge due to the lack of a rational molecular design for controlling electronic transitions. In this study, to suppress the π-π* transitions in the visible region, polycyclic donor-acceptor-donor π-conjugated molecules with boron bridges (Py-FNTz-B and IP-FNTz-B) are designed and synthesized, which contain pyrrole or indenopyrrole as donor units with fluorinated naphthobisthiadiazole (FNTz) as an acceptor unit. The pyrrole end-capped Py-FNTz-B shows an absorption band in the NIR region without distinct visible-light absorption, which has led to the establishment of colorless characteristics. The indenopyrrole end-capped IP-FNTz-B shows a narrow optical energy gap of 0.87 eV in films. Time-resolved microwave conductance and field-effect transistors demonstrate the semiconducting characteristics of these molecules, and Py-FNTz-B-based devices function as NIR phototransistors. Theoretical analyses indicate that the combination of a polyene-like electronic structure with orbital symmetry is important to obtain NIR wavelength-selective absorption. This study suggests that a molecular design based on electronic structures can be effective in the development of colorless NIR-absorbing dyes for organic electronics.

Understanding photochemical degradation mechanisms in photoactive layer materials for organic solar cells

Over the past decades, the field of organic solar cells (OSCs) has witnessed a significant evolution in materials chemistry, which has resulted in a remarkable enhancement of device performance, achieving efficiencies of over 19%. The photoactive layer materials in OSCs play a crucial role in light absorption, charge generation, transport and stability. To facilitate the scale-up of OSCs, it is imperative to address the photostability of these electron acceptor and donor materials, as their photochemical degradation process remains a challenge during the photo-to-electric conversion. In this review, we present an overview of the development of electron acceptor and donor materials, emphasizing the crucial aspects of their chemical stability behavior that are linked to the photostability of OSCs. Throughout each section, we highlight the photochemical degradation pathways for electron acceptor and donor materials, and their link to device degradation. We also discuss the existing interdisciplinary challenges and obstacles that impede the development of photostable materials. Finally, we offer insights into strategies aimed at enhancing photochemical stability and discuss future directions for developing photostable photo-active layers, facilitating the commercialization of OSCs.

Systematic Engineering of Near-Infrared Small Molecules Based on 4H-Cyclopenta[1,2-b:5,4-b']dithiophene Acceptors for Organic Solar Cells

Nonfullerene acceptors (NFAs) have emerged as tremendous materials, efficiently advancing bulk-heterojunction organic solar cells (OSCs) technology. Unlike their fullerene counterparts, NFAs offer the unique advantage of finely tunable electronic energy levels and optical characteristics, which correspond to substantial enhancement in power conversion efficiency of OSCs. Herein, we have introduced a new series of near-infrared NFAs (AY1-AY8) to advance this technology further. Our research deeply investigates the structure-property relationship and thoroughly explores the optical, optoelectronics, photophysical, and photovoltaic characteristics of a synthetic reference molecule (R) and the modeled AY1-AY8 NFAs series. We performed advanced quantum chemical simulations using density functional theory (DFT) and time-dependent DFT methods. Additionally, we also estimated key geometric characteristics such as frontier molecular orbitals, hole-electron overlap, density of states, molecular electrostatic potential, molecular excitation and binding energies, transition density matrix, and reorganizational energy of electrons and holes and compared them with those of a synthetic reference molecule (R). Our findings show that all designed materials (AY1-AY8) exhibit red-shift absorption, improved electronic charge mobility, and low binding and excitation energies compared to R. Notably, these designed materials (AY1-AY8) display significantly narrower electronic energy gaps (E g 1.89-1.71 eV), indicating enhanced charge shifting from the highest occupied molecular orbital to lowest unoccupied molecular orbital and broadening of the absorption spectrum. Moreover, we also revealed a comprehensive study of the donor/acceptor complex of PTB7-Th/AY8 to understand charge shifting between donor and acceptor molecules. Therefore, we strongly recommend this designed (AY1-AY8) series to the experimentalists for the future development of highly efficient OSC devices.

Exploration of the synergistic effect of chrysene-based core and benzothiophene acceptors on photovoltaic properties of organic solar cells

To improve the efficacy of organic solar cells (OSCs), novel small acceptor molecules (CTD1-CTD7) were designed by modification at the terminal acceptors of reference compound CTR. The optoelectronic properties of the investigated compounds (CTD1-CTD7) were accomplished by employing density functional theory (DFT) in combination with time-dependent density functional theory (TD-DFT). The M06 functional along with a 6-311G(d,p) basis set was utilized for calculating various parameters such as: frontier molecular orbitals (FMO), absorption maxima (λmax), binding energy (Eb), transition density matrix (TDM), density of states (DOS), and open circuit voltage (Voc) of entitled chromophores. A red shift in the absorption spectra of all designed chromophores (CTD1-CTD7) was observed as compared to CTR, accompanied by low excitation energy. Particularly, CTD4 was characterized by the highest λmax value of 685.791 nm and the lowest transition energy value of 1.801 eV which might be ascribed to the robust electron-withdrawing end-capped acceptor group. The observed reduced binding energy (Eb) was linked to an elevated rate of exciton dissociation and substantial charge transfer from central core in HOMO towards terminal acceptors in LUMO. These results were further supported by the outcomes from TDM and DOS analyses. Among all entitled chromophores, CTD4 exhibited bathochromic shift (685.791 nm), minimum HOMO/LUMO band gap of 2.347 eV with greater CT. Thus, it can be concluded that by employing molecular engineering with efficient acceptor moieties, the efficiency of photovoltaic materials could be improved.

A Theoretical Investigation for Exploring the Potential Performance of Non-Fullerene Organic Solar Cells Through Side-Chain Engineering Having Diphenylamino Groups to Enhance Photovoltaic Properties

The development of ecofriendly fabrication phenomenon is essential requirement for commercialization of non-fullerene acceptors. Recently, end-capped modeling is employed for computational design of five non-fullerene acceptors to elevate various photovoltaic properties. All new molecules are formulated by altering the peripheral acceptors of CH3-2F and DFT methodology is employed to explore the opto-electronic, morphological and charge transfer analysis. From the computational investigation, all reported molecules manifested red shifted absorption with remarkable reduced band gap. Among investigated molecules, FA1-FA3 evinced effectively decreased value of band gaps and designed molecules have low excitation energy justifying proficient charge transference. The lower values of binding energy of FA1 and FA2 suggest their facile exciton dissociation leading to improved charge mobility. By blending with J61 donor, FA4 have sufficiently enhanced value of VOC (1.72 eV) and fill factor (0.9228). Energy loss of the model (R) is 0.57 eV and statistical calculation demonstrate that all our modified molecules except FA3 has profoundly reduced energy loss compelling in its pivotal utilization. From accessible supportive outcomes of recent investigation, it is recommended that our modified chromophore exhibit remarkable noteworthy applications in solar cells for forthcoming innovations.

Non-Fullerene Acceptors Assisted Target Therapy for Interface Treatment Enable High Performance Inverted Perovskite Solar Cells

Targeted treatment of the interface between electron transport layers (ETL) and perovskite layers is highly desirable for achieving passivating effects and suppressing carrier nonradiative recombination, leading to high performance and long-term stability in perovskite solar cells (PSCs). In this study, a series of non-fullerene acceptors (NFAs, Y-H, Y-F, and Y-Cl) are introduced to optimize the properties of the perovskite/ETL interface. This optimization involves passivating Pb2+ defects, releasing stress, and modulating carrier dynamics through interactions with the perovskite. Remarkably, after modifying with NFAs, the absorption range of perovskite films into the near-infrared region is extended. As expected, Y-F, with the largest electrostatic potential, facilitates the strongest interaction between the perovskite and its functional groups. Consequently, champion power conversion efficiencies of 21.17%, 22.21%, 23.25%, and 22.31% are achieved for control, Y-H-, Y-F-, and Y-Cl-based FA0.88Cs0.12PbI2.64Br0.36 (FACs) devices, respectively. This treatment also enhances the heat stability and air stability of the corresponding devices. Additionally, these modifier layers are applied to enhance the efficiency of Cs0.05(FA0.95MA0.05)0.95PbI2.64Br0.36 (FAMA) devices. Notably, a champion PCE exceeding 24% is achieved in the Y-F-based FAMA device. Therefore, this study provides a facile and effective approach to target the interface, thereby improving the efficiency and stability of PSCs.

Wavily Curved Perylene Diimides: Synthesis, Characterization, and Photovoltaic Properties

Solubility enhancement is a key issue for developing the perylene diimide-based functional materials. Introduction of curved structure proved an effective solubilizing method without employing steric repulsion. In this work, wavily curved perylene diimides were developed as a new family of highly soluble curved perylene diimides. Moreover, their conformational dynamics, aggregating properties, electronic properties, and photovoltaic performances were thoroughly examined in comparison to the previously reported isomer exhibiting an arched curvature. The waved isomer demonstrated heightened rigidity and a greater propensity for aggregation compared to the arched isomer, likely attributed to its more planar structure. Each benzoxepin unit played a role in cancelling out the curvature on the opposite side. While the difference in the molecular curvature did not cause significant alterations in the photophysical and electron-accepting properties, we identified that the modulation of the curved structure is effective in controlling the morphology of the photoelectric conversion layer.

Designing efficient materials for high-performance of non-fullerene organic solar cells through side-chain engineering on DBT-4F derivatives by non-fused-ring electron acceptors

CONTEXT

For the advancement in fields of organic and perovskite solar cells, various techniques of structural alterations are being employed on previously reported chromophores. In this study, the end-capped engineering is carried out on DBT-4F (R) by modifying terminal acceptors to improve optoelectronic and photovoltaic attributes. Seven molecules (AD1-AD7) are modeled using different push-pull acceptors. DFT/B3LYP/6-31G along with its time-dependent approach (TD-DFT) are on a payroll to investigate ground state geometries, absorption maxima (λmax), energy gap (Eg), excitation energy (Ex), internal reorganization energy, light harvesting efficiency (LHE), dielectric constant, open circuit voltage (VOC), fill factor (FF), etc. of OSCs. AD1 displayed the lowest band gap (1.76 eV), highest λmax (876 nm), lowest Ex (1.41 eV), and lowest binding energy (0.21 eV). Among various calculated parameters, all of the sketched molecules demonstrated greater dielectric constant when compared to R. The highest dielectric constant was exhibited by AD3 (56.26). AD5 exhibited maximum LHE (0.9980). Lower reorganization energies demonstrated improved charge mobility. AD5 and AD7 (1.63 and 1.68 eV) have higher values of VOC than R (1.51 eV). All novel molecules having outperforming attributes will be better candidates to enhance the efficacy of OSCs for future use.

METHODS

Precisely, a DFT and TD-DFT analysis on all of the proposed organic molecules were conducted, using the functional MPW1PW91 at 6-31G (d,p) basis set to examine their optoelectronic aspects, additionally the solvent-state computations were studied with a TD-SCF simulation. For all these simulations, Guassian 09 and GuassView 5.0 were employed. Moreover, the Origin 6.0, Multiwfn 3.8, and PyMOlyze 1.1 software were utilized for the visual depiction of the graphs of absorption, TDM, and DOS, respectively of the studied molecules. A number of crucial aspects such as FMOs, bandgaps, light-harvesting efficiency, electrostatic potential, dipole moment, ionization potential, open-circuit voltage, fill factor, binding energy, interaction coefficient, chemical hardness-softness, and electrophilicity index were also investigated for the studied molecules.

Dimerized Nonfused Electron Acceptor Based on a Thieno[3,4-c]pyrrole-4,6-dione Core for Organic Solar Cells

In this work, the first dimerized nonfused electron acceptor (NFEA), based on thieno[3,4-c]pyrrole-4,6-dione as the core, has been designed and synthesized. The dimerized acceptor and its single counterpart exhibit similar energy levels but different absorption spectra due to their distinct aggregation behavior. The dimerized acceptor-based organic solar cells (OSCs) demonstrate a higher power conversion efficiency of 11.05%, accompanied by enhanced thermal stability. This improvement is attributed to the enhancement of the short-circuit current density and fill factor, along with an increase in the glass transition temperature. Characterizations of exciton dynamics and film morphology reveal that a dimerized acceptor-based device possesses an enhanced exciton dissociation efficiency and a well-established charge transport pathway, explaining its improved photovoltaic performance. All these results indicate that the dimerized NFEA as a promising candidate can achieve efficiency-stability-cost balance in OSCs.

Key Roles of Interfaces in Inverted Metal-Halide Perovskite Solar Cells

Metal-halide perovskite solar cells (PSCs), an emerging technology for transforming solar energy into a clean source of electricity, have reached efficiency levels comparable to those of commercial silicon cells. Compared with other types of PSCs, inverted perovskite solar cells (IPSCs) have shown promise with regard to commercialization due to their facile fabrication and excellent optoelectronic properties. The interlayer interfaces play an important role in the performance of perovskite cells, not only affecting charge transfer and transport, but also acting as a barrier against oxygen and moisture permeation. Herein, we describe and summarize the last three years of studies that summarize the advantages of interface engineering-based advances for the commercialization of IPSCs. This review includes a brief introduction of the structure and working principle of IPSCs, and analyzes how interfaces affect the performance of IPSC devices from the perspective of photovoltaic performance and device lifetime. In addition, a comprehensive summary of various interface engineering approaches to solving these problems and challenges in IPSCs, including the use of interlayers, interface modification, defect passivation, and others, is summarized. Moreover, based upon current developments and breakthroughs, fundamental and engineering perspectives on future commercialization pathways are provided for the innovation and design of next-generation IPSCs.

End Group Engineering for Constructing A-D-A Fused-Ring Photosensitizers with Balanced Phototheranostics Performance

Phototheranostics continues to flourish in cancer treatment. Due to the competitive relationships between these photophysical processes of fluorescence emission, photothermal conversion, and photodynamic action, it is critical to balance them through subtle photosensitizer designs. Herein, it is provided a useful guideline for constructing A-D-A photosensitizers with superior phototheranostics performance. Various cyanoacetate group-modified end groups containing ester side chains of different length are designed to construct a series of A-D-A photosensitizers (F8CA1 ∼ F8CA4) to study the structure-property relationships. It is surprising to find that the photophysical properties of A-D-A photosensitizers can be precisely regulated by these tiny structural changes. The results reveal that the increase in the steric hindrance of ester side chains has positive impacts on their photothermal conversion capabilities, but adverse impacts on the fluorescence emission and photodynamic activities. Notably, these tiny structural changes lead to their different aggregation behavior. The molecule mechanisms are detailedly explained by theoretical calculations. Finally, F8CA2 nanoparticles with more balanced photophysical properties perform well in fluorescence imaging-guided photothermal and type I&II photodynamic synergistic cancer therapy, even under hypoxic conditions. Therefore, this work provides a novel practicable construction strategy for desired A-D-A photosensitizers.

Impact of length of branched alkyl side chains on thiazolothiazole-based small molecular acceptors in non-fullerene polymer solar cells

It has been reported that the length of branched alkyl side chains on fused-ring electron acceptors confers different impacts on properties versus solubility of BJH blends. However, because this impact on a non-fused acceptor backbone has rarely been studied, we examined the impact of molecular optimization from alkyl chain tuning based on non-fused thiazolothiazole small-molecule acceptors. The length of the side chain on the thiophene bridge was modified from 2-butyloctyl to 2-ethylhexyl, which corresponds to small molecules TTz3(C4C6) and TTz3(C2C4), respectively. Compared with the reported TTz3(C6C8) with long alkyl side chains, TTz3(C4C6) and TTz3(C2C4) exhibited stronger molecular aggregation, higher absorption coefficients, and greater redshifted UV absorption. Unexpectedly, after the alkyl chain was slightly shortened in this type of acceptor system, devices were successfully fabricated, but it was necessary to reduce the blending concentration at low rotation speeds due to the sharp decrease in the solubility of corresponding acceptor materials. Thus, the obtained unfavorable thickness and morphology of the active layer caused a decrease in Jsc and FF. As a consequence, TTz3(C4C6)- and TTz3(C2C4)-based devices showed an unsatisfactory power conversion efficiency of 6.02% and 2.71%, respectively, when donors were paired with the wide bandgap donor J71, which is inferior to that of TTz3(C6C8)-based devices (8.76%). These results indicate that it is challenging to determine the limit of the adjustable range of side chains to modify non-fused thiazolothiazole small-molecule acceptors for high-performance non-fullerene solar cells.

Star-shaped small donor molecules based on benzotriindole for efficient organic solar cells: a DFT study

CONTEXT

The purpose of the S01-S05 series of end-capped modified donor chromophores is to amplify the energy conversion efficiency of organic solar cells. Using quantum chemical modeling, the photophysical and photoelectric characteristics of the S01-S05 geometries are examined.

METHOD

The influence of side chain replacement on multiple parameters, including the density of states (DOS), molecular orbital analysis (FMOS), exciton-binding energy (Eb), molecular electrostatic potential analysis, dipole moment (μ), and photovoltaic characteristics including open circuit voltage (VOC), and PCE at minimal energy state geometries, has been investigated employing density functional theory along with TD-DFT analysis. The molar absorption coefficient (λmax) of all the proposed compounds (S01-S05) was efficiently enhanced by the terminal acceptor alteration technique, as demonstrated by their scaling up with the reference molecule (SR). Among all molecules, S04 has shown better absorption properties with a red shift in absorption having λmax at 845 nm in CHCl3 solvent and narrow energy gap (EG) 1.83 eV with least excitation energy (Ex) of 1.4657 eV. All created donors exhibited improved FF and VOC than the SR, which significantly raised PCE and revealed their great efficiency as OSC. Consequently, the results recommended these star-shaped molecules as easily attainable candidates for constructing extremely efficient OSCs.

Simple-Structured Acceptor with Highly Interconnected Electron-Transport Pathway Enables High-Efficiency Organic Solar Cells

Achieving desirable charge-transport highway is of vital importance for high-performance organic solar cells (OSCs). Here, it is shown how molecular packing arrangements can be regulated via tuning the alkyl-chain topology, thus resulting in a 3D network stacking and highly interconnected pathway for electron transport in a simple-structured nonfused-ring electron acceptor (NFREA) with branched alkyl side-chains. As a result, a record-breaking power conversion efficiency of 17.38% (certificated 16.59%) is achieved for NFREA-based devices, thus providing an opportunity for constructing low-cost and high-efficiency OSCs.

Trap suppression in ordered organic photovoltaic heterojunctions

The high trap density (generally 1016-1018 cm-3) in organic solar cells (OSCs) brings about the localization of charge carriers and reduced charge carrier lifetime, mainly due to the weak intermolecular interactions of organic semiconductors resulting in their relatively poor crystallinity, which leads to low charge carrier mobilities and intense non-radiative recombination, thus impeding the further improvement of power conversion efficiencies (PCEs). Therefore, trap suppression is crucial to boost the performance of OSCs, and improving the crystallinity of donor/acceptor materials and enhancing the molecular order in devices can contribute to the trap suppression in OSCs. In this feature article, we summarize the recent advances of trap suppression in OSCs by material design and device engineering, and further outline possible development directions for trap suppression to enhance PCEs of OSCs.

Isomeric Effect of a π Bridge in an IDT-Based Nonfused Electron Photovoltaic Acceptor

A pair of isomers, IDT-BOF containing S⋅⋅⋅O/F⋅⋅⋅H noncovalently configurational locks and IDT-BFO containing F⋅⋅⋅H/O⋅⋅⋅H noncovalently configurational locks, with an acceptor-π-donor-π-acceptor (A-π-D-π-A) structure have been designed and synthesized by choosing 4,9-dihydro-s-indaceno[1,2-b : 5,6-b']dithiophene (IDT) as the D unit, an F/n-hexyloxy substituted phenyl ring as π bridge, and 3-(dicyanomethylidene)indan-1-one as the A unit. Owing to the S⋅⋅⋅O/F⋅⋅⋅H or F⋅⋅⋅H/O⋅⋅⋅H noncovalently configurational locks, both IDT-BOF and IDT-BFO have a completely planar structure. IDT-BOF exhibits a similar LUMO to IDT-BFO, but higher HOMO energy levels, leading to a smaller optical bandgap and red-shifted absorption. However, IDT-BOF-based bulk-heterojunction organic solar cells (BHJ-OSCs) coupled with PBDB-T, and PCE-10 as donor materials both exhibited a lower PCE than that of IDT-BFO (PBDB-T: 5.2 vs. 6.1 %; PCE-10: 1.7 vs. 3.2 %). Comprehensively comparing and investigating IDT-BOF : PBDB-T and IDT-BFO : PBDB-T OSCs suggested that the large phase separation and serious charge recombination of IDT-BOF-based OSCs contributed to its lower power conversion efficiency. Importantly, ternary solar cells based on PBDB-T : Y5 as control devices with an additional 10 % IDT-BFO exhibited a 5 % enhancement in the PCE compared to the control device (14.3 vs. 13.46 %).

Unfolding the Correlation between Solution Aggregation and Solid-State Crystal Orientation in Donor-Acceptor Copolymers via Solvent Additive Processing

Tailoring the crystal orientation of donor-acceptor (D-A) copolymers is vital for boosting the performance of optoelectronic devices. Despite recent advances in controlling the crystal orientation of D-A copolymers in films, the investigation into their aggregates in solution and the correlation between the solution aggregates and solid-state crystal orientation has been limited. Herein, an effective solvent additive strategy is reported for tuning solution aggregates and the consequent solid-state structures of poly{[N,N'-bis(2-octyldodecyl)-naphthalene-1,4,5,8-bis(dicarboximide)-2,6-diyl]-alt-5,5'-(2,2'-bithiophene)} (P(NDI2OD-T2)). Specifically, the addition of 1-decanethiol (10-thiol) to the P(NDI2OD-T2) chloroform solution promoted the aggregation of P(NDI2OD-T2) chains because of the improved planarization of the backbones, which changed their crystal orientation in the film from coexisting edge-on and face-on to dominant edge-on when produced by drop-casting. The mechanism of this crystal orientation transformation is elucidated based on the interaction between 10-thiol and the side chains of P(NDI2OD-T2). The optical properties of P(NDI2OD-T2) films with different crystalline structures are closely correlated. Notably, the 10-thiol-enabled facile tailoring of the crystal orientation in P(NDI2OD-T2) can be readily applied to other D-A copolymers of interest. The findings of this study highlight a robust solvent additive strategy for regulating solution aggregates and crystal orientation in D-A copolymer films, which have applications in many optoelectronic devices.

Coplanar Conformational Structure of π-Conjugated Polymers for Optoelectronic Applications

Hierarchical structure of conjugated polymers is critical to dominating their optoelectronic properties and applications. Compared to nonplanar conformational segments, coplanar conformational segments of conjugated polymers (CPs) demonstrate favorable properties for applications as a semiconductor. Herein, recent developments in the coplanar conformational structure of CPs for optoelectronic devices are summarized. First, this review comprehensively summarizes the unique properties of planar conformational structures. Second, the characteristics of the coplanar conformation in terms of optoelectrical properties and other polymer physics characteristics are emphasized. Five primary characterization methods for investigating the complanate backbone structures are illustrated, providing a systematical toolbox for studying this specific conformation. Third, internal and external conditions for inducing the coplanar conformational structure are presented, offering guidelines for designing this conformation. Fourth, the optoelectronic applications of this segment, such as light-emitting diodes, solar cells, and field-effect transistors, are briefly summarized. Finally, a conclusion and outlook for the coplanar conformational segment regarding molecular design and applications are provided.

Processability Considerations for Next-Generation Organic Photovoltaic Materials

The evolution of organic semiconductors for organic photovoltaics (OPVs) has resulted in unforeseen outcomes. This has provided substitute choices of photoactive layer materials, which effectively convert sunlight into electricity. Recently developed OPV materials have narrowed down the gaps in efficiency, stability, and cost in devices. Records now show power conversion efficiency in single-junction devices closing to 20%. Despite this, there is still a gap between the currently developed OPV materials and those that meet the requirements of practical applications, especially the solution processability issue widely concerned in the field of OPVs. Based on the general rule that structure determines properties, methodologies to enhance the processability of OPV materials are reviewed and explored from the perspective of material design and views on the further development of processable OPV materials are presented. Considering the current dilemma that the existing evaluation indicators cannot reflect the industrial processability of OPV materials, a more complete set of key performance indicators are proposed for their processability considerations. The purpose of this perspective is to raise awareness of the boundary conditions that exist in industrial OPV manufacturing and to provide guidance for academic research that aspires to contribute to technological advancements.

B─N Covalent Bond-Based Nonfullerene Electron Acceptors for Efficient Organic Solar Cells

The optoelectronic properties and photovoltaic performance of nonfullerene electron acceptors (NFEAs) in organic solar cells (OSCs) are greatly influenced by the rational structure regulation of the central core unit. This study introduces a novel type of six-membered fused electron-donating core containing B─N covalent bonds to construct acceptor-donor-acceptor (A-D-A)-type NFEAs. By modulating the branching alkyl chains on the nitrogen atom, two NFEAs, BN910 and BN1014, are synthesized and characterized. Both molecules exhibit strong near-infrared absorption, narrow bandgaps (≈1.45 eV), appropriate energy levels, and tunable molecular packing behaviors, positioning them as promising candidates for efficient NFEAs in OSCs. The investigation reveals that BN1014, with longer and C2-branched alkyl chains, demonstrates superior intermolecular packing and morphology within active layers, leading to enhanced exciton dissociation, improved charge transfer, and reduced charge recombination in OSCs. As a result, a power conversion efficiency (PCE) of 10.02% is achieved for D18:BN1014-based binary OSCs. Notably, BN1014 can be utilized as the third component in the D18:DT-Y6 binary system to fabricate the ternary OSCs, and a PCE of 17.65% is achieved, outperforming 17.05% of D18:DT-Y6-based binary OSCs. These findings highlight the potential of heteroarenes featuring B─N covalent bonds for constructing high-efficiency NFEAs in OSCs.

Exploration of charge transfer analysis and photovoltaics properties of A-D-A type non-fullerene phenazine based molecules to enhance the organic solar cell properties

To intensify the photovoltaic properties of organic solar cells, density functional theory (DFT) based computational techniques were implemented on six non-fullerene A-D-A type small molecules (N1-N6) modified from reference molecule (R) which consists of phenazine fused with 1,4- Dimethyl-4H-3,7-dithia-4-aza- cyclopenta [α] pentalene on both sides with one of its phenyl rings acting as the central donor unit, further attached with 2-(5,6-Difluoro-2-methylene-3-oxo-indan-1-ylidene)-malononitrile acceptor groups at terminal sites. All proposed compounds have a phenazine base modified with a variety of substituents at the terminals. Transition density matrix, density of states, frontier molecular orbitals, intramolecular charge transfer abilities and optoelectronic properties of these compounds were investigated using B3LYP/6-31G (d, p) and B3LYP/6-31G++ (d,p) level of theory. All six designed compounds exhibited a bathochromic sift in their λmax as compared to the R molecule. All designed molecules also have reduced band gap and smaller excitation energy than R. Among all, N6 exhibited highest λmax and lowest bandgap as compared to reference molecule indicating its promising photovoltaic properties. Decreased hole and electron reorganization energy in several of the suggested compounds is indicative of greater charge mobility in them. PTB7-Th donor was employed to calculate open circuit voltage of all investigated molecules. N1-N5 molecules had improved optoelectronic properties, significant probable power conversion efficiency as evident from their absorption aspects, high values of Voc, and fill factor, compared to R molecule. Designed A-D-A type NF based molecules make OSCs ideal for use in wearable devices, building-integrated photovoltaics and smart fabrics.

P-type Polymers in Semitransparent Organic Photovoltaics

P-type polymers are polymeric semiconducting materials that conduct holes and have extensive applications in optoelectronics such as organic photovoltaics. Taking the advantage of intrinsic discontinuous light absorption of organic semiconductors, semitransparent organic photovoltaics (STOPVs) present compelling opportunities in various potential applications such as building-integrated photovoltaics, agrivoltaics, automobiles, and wearable electronics. The characteristics of p-type polymers, including optical, electronic, and morphological properties, determine the performance of STOPVs, and the requirements for p-type polymers differ between opaque organic photovoltaics and STOPVs. Hence, in this Minireview, recent advances of p-type polymers used in STOPVs are systematically summarized, with emphasis on the effects of chemical structures, conformation structures, and aggregation structures of p-type polymers on the performance of STOPVs. Furthermore, new design concepts and guidelines are also proposed for p-type polymers to facilitate the future development of high-performance STOPVs.

Amplifying the photovoltaic properties of phenylene dithiophene core based non-fused ring by engineering the terminal acceptors modification to enhance the efficiency of organic solar cells

In this study, a series of eight non-fused rings-based semiconducting acceptors (AR1-AR8) were computationally developed by making modifications to the parent molecule (PTICO). In this study, a DFT analysis was conducted at an accurately chosen level of theory to gather a comprehensive inventory of the optoelectronic characteristics of AR1-AR8 and PTICO. The findings indicate that all recently developed molecules exhibit a bathochromic shift in their maximum UV-visible absorbance (λmax) with a smaller band gap (Eg). AR1 has demonstrated the most significant red shift in UV-visible absorbance and possesses the smallest Eg when compared to other recently developed acceptors. AR2 acceptor has shown the best results both as electron and hole-transporting materials owing to its smallest value of reorganization energy for electrons and holes. J61 donor was engaged to calculate the open-circuit voltage (VOC) and the highest VOC with maximum FF % value was observed in AR4. The investigation of charge transfer was also conducted utilizing J61 in conjunction with the AR4 acceptor. Natural transition orbitals (NTO) have also been inspected to recognize the percentage electron transport contribution (% ETC) from the ground state to the first excites state (S0 to S1). The findings of this research suggest that the modified acceptors exhibit potential for practical implementation in the development of organic solar cells that possess improved photovoltaic performance.

Enhancing NLO performance by utilizing tyrian purple dye as donor moiety in organic DSSCs with end capped acceptors: A theoretical study

A series of new organic dyes (T1-T6) with nonfullerene acceptors have been theoretically designed around the chemical structure of tyrian purple (T) natural dye. For their ground state energy parameters, all the molecular geometries of those dyes were optimized by density functional theory (DFT) at its Becke, 3-parameter, Lee-Yang-Parr (B3LYP) level of theory with 6-31G+(d,p) basis sets. When benchmarking against several long range and range separated levels of theory, the Coulomb attenuated B3LYP (CAM-B3LYP) produced most accurate absorption maxima (λmax) value to that of T so it was further employed for further Time dependent DFT (TD-DFT) calculations. Frontier molecular orbitals (FMOs) with natural bond orbital (NBO) studies were used to study their intra molecular charge transfer (ICT). All of the dyes had their energy gaps (Eg) values between their FMOs to range around 0.96-3.39 eV, whereas the starting reference dye had an Eg of 1.30 eV. Their ionization potential (IP) values were ranged to be 3.07-7.25 eV which indicated their nature to loss electrons. The λ max in chloroform was marginally red-shifted with a value 600-625 from T (580 nm). The dye T6 showed its highest linear polarizability (<α>), and first and second order hyperpolarizabilities (β and γ). The synthetic experts can find the present research to design finest NLO materials for current and future uses.

More Is Better: Dual-Acceptor Engineering for Constructing Second Near-Infrared Aggregation-Induced Emission Luminogens to Boost Multimodal Phototheranostics

The manipulation of electron donor/acceptor (D/A) shows an endless impetus for innovating optical materials. Currently, there is booming development in electron donor design, while research on electron acceptor engineering has received limited attention. Inspired by the philosophical idea of "more is different", two systems with D'-D-A-D-D' (1A system) and D'-D-A-A-D-D' (2A system) structures based on acceptor engineering were designed and studied. It was demonstrated that the 1A system presented a weak aggregation-induced emission (AIE) to aggregation-caused quenching (ACQ) phenomenon, along with the increased acceptor electrophilicity and planarity. In sharp contrast, the 2A system with one more acceptor exhibited an opposite ACQ-to-AIE transformation. Interestingly, the fluorophore with a more electron-deficient A-A moiety in the 2A system displayed superior AIE activity. More importantly, all compounds in the 2A system showed significantly higher molar absorptivity (ε) in comparison to their counterparts in the 1A system. Thanks to the highest ε, near-infrared-II (NIR-II, 1000-1700 nm) emission, desirable AIE property, favorable reactive oxygen species (ROS) generation, and high photothermal conversion efficiency, a representative member of the 2A system handily performed in fluorescence-photoacoustic-photothermal multimodal imaging-guided photodynamic-photothermal collaborative therapy for efficient tumor elimination. Meanwhile, the NIR-II fluorescence imaging of blood vessels and lymph nodes in living mice was also accomplished. This study provides the first evidence that the dual-connected acceptor tactic could be a new molecular design direction for the AIE effect, resulting in high ε, aggregation-intensified NIR-II fluorescence emission, and improved ROS and heat generation capacities of phototheranostic agents.

Modulating the organic photovoltaic properties of non-fullerene acceptors by molecular modification based on Y6: a theoretical study

Developing non-fullerene acceptors (NFAs) by modifying the backbone, side chains and end groups is the most important strategy to improve the power conversion efficiency of organic solar cells (OSCs). Among numerous developed NFAs, Y6 and its derivatives are famous NFAs in the OSC field due to their good performance. Herein, in order to understand the mechanism of tuning the photovoltaic performance by modifying the Y6's center backbone, π-spacer and side-chains, we selected the PM6:Y6 OSC as a reference and systematically studied PM6:AQx-2, PM6:Y6-T, PM6:Y6-2T, PM6:Y6-O, PM6:Y6-1O and PM6:Y6-2O OSC systems based on extensive quantum chemistry calculations. The results indicate that introducing quinoxaline to substitute thiadiazole in the backbone induces a blue-shift of absorption spectra, reduces the charge transfer (CT) distance (Δd) and average electrostatic potential (ESP), and increases the singlet-triplet energy gap (ΔEST), CT excitation energy and the number of CT states in low-lying excitations. Inserting thienyl and dithiophenyl as π spacers generates a red-shift of absorption spectra, enlarges Δd and average ESP, and reduces ΔEST and the number of CT states. Introducing furo[3,2-b]furan for substituting one thieno[3,2-b]thiophene unit in the Y6's backbone causes a red-shift of absorption spectra and increases ΔEST, Δd and average ESP as well as CT excitation energy. Introducing alkoxyl as a side chain results in a blue-shift of absorption spectra, and increases ΔEST, Δd, average ESP, CT excitation energy and the number of CT states. The rate constants calculated using Marcus theory suggest that all the molecular modifications of Y6 reduce the exciton dissociation and charge recombination rates at the heterojunction interface, while introducing furo[3,2-b]furan and alkoxyl enlarges CT rates.

A polymer donor based on difluoro-quinoxaline with a naphthalimide substituent unit exhibits a low-lying HOMO level for efficient non-fullerene polymer solar cells

The design and synthesis of polymer donors with a low-lying highest occupied molecular orbital (HOMO) level are crucial for increasing open-circuit voltages (VOC) and achieving high-performance non-fullerene polymer solar cells. Here, we developed two copolymers using non-fluorinated or fluorinated thienyl-conjugated benzodithiophenes as electron donor units, and difluoro-quinoxaline with a naphthalimide substituent (DNB) as the electron acceptor unit. These copolymers, namely PDNB and PDNB-2F, exhibited deep HOMO levels owing to the strong electron-withdrawing ability of the naphthalimide substituent. Density-functional theory calculations demonstrated that the skeletons of the two copolymers featured good coplanarity. Owing to the fluorination, PDNB-2F displayed an increased absorption coefficient and deeper HOMO level than PDNB. Moreover, the blended film based on PDNB-2F:Y6 demonstrated enhanced carrier mobility, decreased bimolecular recombination as well as favorable phase-separation regions. Consequently, the PDNB-2F:Y6-based device yielded a superior power conversion efficiency (PCE) of 12.18%, whereas the device based on PDNB:Y6 showed a comparatively lower PCE of 8.83%. These results indicate that difluoro-quinoxaline with a naphthalimide substituent is a prospective electron-deficient building block to develop donor polymers with low-lying HOMO levels to achieve efficient non-fullerene polymer solar cells.

An In-Situ Cyanidation Strategy To Access Tetracyanodiacenaphthoanthracene Diimides with High Electron Mobilities Exceeding 10 cm2 V-1 s-1

A family of novel highly π-extended tetracyano-substituted acene diimides, named as tetracyanodiacenaphthoanthracene diimides (TCDADIs), have been synthesized using a facile four-fold Knoevenagel condensation strategy. Unlike conventional cyano substitution reactions, our approach enables access to a large π-conjugated backbone with the in-situ formation of four cyano substitutents at room temperature while avoiding extra cyano-functionalization reactions. TCDADIs decorated with different N-alkyl substituents present good solubility, near-coplanar backbones, good crystallinity, and low-lying lowest unoccupied molecular orbital energies of -4.33 eV, all of which contribute to desirable electron-transport performance when applied in organic field-effect transistors (OFET). The highest electron mobility of an OFET based on a 2-hexyldecyl-substituted TCDADI single crystal reaches 12.6 cm2  V-1  s-1 , which is not only among the highest values for the reported n-type organic semiconductor materials (OSMs) but also exceeds that of most n-type OSMs decorated with imide units.

Synthesis of 3,3'-(Ethane-1,2-diylidene)bis(indolin-2-one) Promoted by Thermally-activated Electron Transfer and Photoreduction of CO2 to CH4 and CO

A Sonogashira coupling reaction leads to the formation of a serendipitous product C with the 3,3'-(ethane-1,2-diylidene)bis(indolin-2-one) unit. To our knowledge, our study provides the first example demonstrating that electron transfer between isoindigo and triethylamine can be thermally activated and can be employed in synthesis. The physical properties of C suggest that it possesses decent photo-induced electron-transfer capabilities. Under the illumination of 136 mW cm-2 intensity, C yields ≈2.4 mmol gcat -1 (per gram of catalyst) of CH4 and ≈0.5 mmol gcat -1 of CO in 20 h in the absence of additional metal, co-catalyst, and amine sacrificial agent. The primary kinetic isotope effect suggests that the bond cleavage of water is a rate-determining step in the reduction. Moreover, the CH4 and CO production can be boosted as the illuminance increases. This study demonstrates that organic donor-acceptor conjugated molecules are potential photocatalysts for CO2 reduction.

Synergistic modification of end groups in Quinoxaline fused core-based acceptor molecule to enhance its photovoltaic characteristics for superior organic solar cells

The competence of organic solar cells (OSCs) could be enhanced by improving the light absorption capabilities as well as the open-circuit voltage (V_oc) of utilized molecules. To upgrade overall functionality of OSCs, seven new molecules were designed in this work using an end-cap alteration technique on Quinoxaline fused core-based non-fullerene acceptor (Qx-2) molecule. This technique is known to be quite advantageous in terms of improvement of the effectiveness and optoelectrical behavior of various OSCs. Critical parameters like the absorption maximum, frontier molecular orbitals, excitation energy, exciton binding energy, V_oc, and fill factor of molecules were considered for the molecules thus designed. All newly designed molecules showed outstanding improvement in optoelectronic as well as performance-related properties. Out of all scrutinized molecules, Q1 exhibited highest wavelength of absorption peak (λ_max = 779 nm) with the reduced band gap (1.90 eV), least excitation energy (E_x = 1.59 eV), along with the highest dipole moment (17.982950 D). Additionally, the newly designed compounds Q4, Q5, and Q6 exhibited significantly improved V_ocs that were 1.55, 1.47, and 1.50 eV accordingly, as compared to the 1.37 eV of Qx-2 molecule. These molecules also showed remarkable improvement in fill factor attributed to direct correspondence of V_oc with it. Inclusively, these results support the superiority of these newly developed molecules as prospective constituents of upgraded OSCs.

Electron Transport in Organic Photovoltaic Acceptor Materials: Improving the Carrier Mobilities by Intramolecular and Intermolecular Modulations

High carrier mobility is beneficial to increase the active-layer thickness while maintaining a high fill factor, which is crucial to further improve the light harvesting and organic photovoltaic efficiency. The aim of this Perspective is to elucidate the electron transport mechanisms in prototypical non-fullerene (NF) acceptors through our recent theoretical studies. The electron transport in A-D-A small-molecule acceptors (SMAs), e.g., ITIC and Y6, is mainly determined by end-group π-π stacking. Relative to ITIC, the angular backbone along with more flexible side chains leads to Y6 having a closer stacking and enhanced intermolecular electronic connectivity. For polymerized rylene diimide acceptors, to achieve high electron mobilities, they need to simultaneously increase intramolecular and intermolecular connectivity. Finally, finely tuning the π-bridge modes to enhance intramolecular superexchange coupling is essential to develop novel polymerized A-D-A SMAs.

Effects of Halogenation on Cyclopentadithiophenevinylene-Based Acceptors with Excellent Responses in Binary Organic Solar Cells

In recent years, non-fused non-fullerene acceptors (NFAs) have attracted increasing consideration due to several advantages, which include simple preparation, superior yield, and low cost. In the work reported here, we designed and synthesized three new NFAs with the same cyclopentadithiophenevinylene (CPDTV) trimer as the electron-donating unit and different terminal units (IC for FG10, IC-4F for FG8, and IC-4Cl for FG6). Both halogenated NFAs, i.e., FG6 and FG8, show red-shifted absorption spectra and higher electron mobilities (more pronounced for FG6) in comparison with FG10. Moreover, the dielectric constants of these materials also increased upon halogenation of the IC terminal units, thus leading to a reduction in the exciton binding energy, which is favorable for dissociation of excitons and subsequent charge transfer despite the driving force (highest occupied molecular orbital and lowest unoccupied molecular orbital offsets) being very small. The organic solar cells (OSCs) constructed using these acceptors and PBDB-T, as the donor, showed PCE values of 15.08, 12.56, and 9.04% for FG6, FG8, and FG10, respectively. The energy loss for the FG6-based device was the lowest (0.45 eV) of all the devices, and this may be attributed to it having the highest dielectric constant, which leads to a reduction in the binding energy of exciton and a small driving force for hole transfer from FG6 to PBDB-T. The results indicate that the NFA containing the CPDTV oligomer core and halogenated terminal units can efficiently spread the absorption spectrum to the NIR zone. Non-fused NFAs have a bright future in the quest to obtain efficient OSCs with low cost for marketable purposes.

Upgrading Electricity Generation and Electromagnetic Interference Shielding Efficiency via Phase-Change Feedback and Simple Origami Strategy

Developing ultimate electromagnetic interference (EMI) shielding materials that can simultaneously upgrade the quality of generated electricity and the light-thermal-electric conversion efficiency based on traditional thermoelectric devices is crucially desired. Herein, a series of flexible multilayered phase change films (PCFs) is developed by a simple and novel origami strategy. The PCFs are first reported to improve the light-thermal-electric conversion efficiency by as high as 11.3%. Simultaneously, the PCFs could significantly upgrade the generated electricity on average voltage (27.3%), average current (23.8%), and lasting power outputs by 2010 times from microwatts to milliwatts. Besides, the EMI shielding efficiency of PCFs could be tuned from 39.2 to 71.9 dB by the origami process, the wide-range EMI shielding performance could be suitable for varying occasions. Overall, this work provides a promising solution for both the preparation of multifunctional materials, high-efficiency solar energy harvesting and upgrading electricity generation, which shows broad application prospects in EMI shielding, energy storage, and conversion.

End Group Nonplanarization Enhances Phototherapy Efficacy of A-D-A Fused-Ring Photosensitizer for Tumor Phototherapy

Enhancing the phototherapy efficacy of organic photosensitizers through molecular design is a fascinating but challenging task. Herein, we propose a simple design strategy to first realize the generation of superoxide anion radical (O₂^•-) by A-D-A fused-ring photosensitizers. Through replacing one cyano group of traditional end group with an ester group, we designed a novel nonplanar end group (A unit) to synthesize a novel A-D-A photosensitizer F8CA. In a comparison with its counterpart F8CN with the traditional end group, F8CA displays more loose packing and larger spin-orbit coupling constants. The F8CA nanoparticles showed higher photodynamic activities with the generation capability of singlet oxygen (¹O₂), hydroxyl radical (^•OH), and O₂^•-, while F8CN nanoparticles could only generate ¹O₂ and ^•OH. In addition, F8CA nanoparticles still remain high photothermal conversion efficiency (61%). As a result, F8CA nanoparticles perform well in hypoxia-tolerant tumor phototherapy. This study brings an effective design thought for A-D-A photosensitizers.

A new dithieno[3,2-b:2',3'-d]thiophene derivative for high performance single crystal organic field-effect transistors and UV-sensitive phototransistors

Organic phototransistors (OPTs), as the basic unit for organic image sensors, are emerging as one of the most promising light signal detectors. High performance UV-sensitive phototransistors are highly desired for the detection of UV light. Herein, by introducing the anthracene group to the 2,6-positions of dithieno[3,2-b:2',3'-d]thiophene, we designed and synthesized a new dithieno[3,2-b:2',3'-d]thiophene derivative, 2,6-di(anthracen-2-yl)dithieno[3,2-b:2',3'-d]thiophene (2,6-DADTT). The single crystal structure of 2,6-DADTT presents classical herringbone packing with multiple intermolecular interactions, including S⋯S (3.470 Å), S⋯C (3.304 Å, 3.391 Å, 3.394 Å) and C-H⋯π (2.763 Å, 2.822 Å, 2.846 Å, 2.865 Å, 2.885 Å, 2.890 Å) contacts. Single crystal organic field-effect transistors (SC-OFETs) based on 2,6-DADTT reach a highest mobility of 1.26 cm2 V-1 s-1 and an average mobility of 0.706 cm2 V-1 s-1. 2,6-DADTT-based single crystal organic phototransistors (OPTs) demonstrate photosensitivity (P) of 2.49 × 106, photoresponsivity (R) of 6.84 × 103 A W-1 and ultrahigh detectivity (D*) of 4.70 × 1016 Jones to UV light, which are among the best figures of merit for UV-sensitive OPTs. These excellent comprehensive performances indicate its good application prospects in integrated optoelectronics.

Sensitive photodetection below silicon bandgap using quinoid-capped organic semiconductors

High-sensitivity organic photodetectors (OPDs) with strong near-infrared (NIR) photoresponse have attracted enormous attention due to potential applications in emerging technologies. However, few organic semiconductors have been reported with photoelectric response beyond ~1.1 μm, the detection limit of silicon detectors. Here, we extend the absorption of organic small-molecule semiconductors to below silicon bandgap, and even to 0.77 eV, through introducing the newly designed quinoid-terminals with high Mulliken-electronegativity (5.62 eV). The fabricated photodiode-type NIR OPDs exhibit detectivity (D^*) over 10¹² Jones in 0.41 to 1.2 μm under zero bias with a maximum of 2.9 × 10¹² Jones at 1.02 μm, which is the highest D^* for reported OPDs in photovoltaic-mode with response spectra beyond 1.1 μm. The high D^* in 0.9 to 1.2 μm is comparable to those of commercial InGaAs photodetectors, despite the detection limit of our OPDs is shorter than InGaAs (~1.7 μm). A spectrometer prototype with a wide measurable region (0.4 to 1.25 μm) and NIR imaging under 1.2-μm illumination are demonstrated successfully in OPDs.

Highly Efficient Mixed Conduction in a Fused Oligomer n-Type Organic Semiconductor Enabled by 3D Transport Pathways

Tailoring organic semiconductors to facilitate mixed conduction of ionic and electronic charges when interfaced with an aqueous media has spurred many recent advances in organic bioelectronics. The field is still restricted, however, by very few n-type (electron-transporting) organic semiconductors with adequate performance metrics. Here, a new electron-deficient, fused polycyclic aromatic system, TNR, is reported with excellent n-type mixed conduction properties including a µC* figure-of-merit value exceeding 30 F cm^-1 V^-1 s^-1 for the best performing derivative. Comprising three naphthalene bis-isatin moieties, this new molecular design builds on successful small-molecule mixed conductors; by extending the molecular scaffold into the oligomer domain, good film-forming properties, strong π-π interactions, and consequently excellent charge-transport properties are obtained. Through judicious optimization of the side chains, the linear oligoether and branched alkyl chain derivative bgTNR is obtained which shows superior mixed conduction in an organic electrochemical transistor configuration including an electron mobility around 0.3 cm² V^-1 s^-1 . By optimizing the side chains, the dominant molecular packing can be changed from a preferential edge-on orientation (with high charge-transport anisotropy) to an oblique orientation that can support 3D transport pathways which in turn ensure highly efficient mixed conduction properties across the bulk semiconductor film.