Lowest Unoccupied Molecular Orbital Levels Research Articles

Sequential Cyanation of Polythiophenes: Tuning Charge Carrier Polarity in Organic Electrochemical Transistors

AbstractPolythiophenes, built on head‐to‐head (HH) linked 3,3′‐oligomer ethylene glycol‐2,2′‐bithiophene (gT2), have simple chemical structure, good backbone planarity, and relatively high‐lying the lowest unoccupied molecular orbital (LUMO) energy levels, hence showing excellent p‐type performance in organic electrochemical transistors (OECTs) with µC* of several hundred F cm−1 V−1 s−1. In this study, sequential cyano substitution is utilized to enable transformation of charge carrier polarity from p‐type to n‐type in OECTs, based on the parent polythiophene g4T2‐T2. With the increase of cyano group number, the polythiophenes exhibit gradually lowered LUMO levels from −2.55 to −3.90 eV. As a result, from g4T2‐T2 to CNg4T2‐CNT2, the p‐type performance dramatically diminishes accomplished by the enhancement of n‐type one when applied in OECTs. To the authors' delight, polymer CNg4T2‐CNT2 with the highest content of cyano groups exhibits a remarkable n‐type µC* of 27.01 F cm−1 V−1 s−1 and high gm,norm of 6.75 S cm−1 with negligible p‐type character. This study demonstrates that sequential cyano substitution provides a powerful approach for developing high‐performance n‐type polymers for OECT applications.

Manipulating cation-water chemistry to inhibit hydrogen evolution of zinc metal anodes

The electrochemical behavior of zinc metal anodes is dominated by the interfacial chemistry between the anode and the electrolyte. The local enrichment of cation-solvent complexes with strong interactions in the Helmholtz plane can lead to the evolution of hydrogen on the Zn anode surface during cycling. To tackle this issue, we propose the addition of low doses of N-dimethylformamide (DMF) solvent into 2 M of ZnSO4 electrolyte (D-ZS) to inhibit hydrogen evolution from water splitting and dendrite growth. Molecular dynamics simulation and nuclear magnetic resonance spectroscopy reveal that high donor-number DMF molecules replace part of H2O molecules, thereby reducing the amount of H2O in the solvated sheath of Zn2+. Furthermore, density functional theory calculations demonstrate that DMF increases the lowest unoccupied molecular orbital level of bound water owing to reduced Zn2+ cation-water interaction, preventing the breakage of O−H bond and reducing side reactions of Zn anodes. In addition, the desolvated DMF molecules prefer to adsorb on the zinc anode surface, hindering the migration of Zn2+ to low surface-energy positions that could lead to dendritic formation. As a result, a symmetric cell with D-ZS electrolyte can operate for over 1970 h at 0.5 mA cm−2 for 0.25 mAh cm−2, and Zn-ion hybrid supercapacitors achieve an exemplary cycle performance (93% of capacity retention rate after 30,000 cycles at 2 A g−11). The present work provides a roadmap for regulating electrolyte structure to inhibit hydrogen evolution of zinc metal anodes by manipulating Zn2+ cation-water chemistry.

D-A-D type small molecule donors based on BODIPY skeleton for bulk heterojunction organic solar cells

In recent years, structural modification for boron-dipyrromethene (BODIPY) derivatives has been proven to be an effective way to improve their performance of organic solar cells (OSCs). Here, three D-A-D type small molecule donors BDP-1, BDP-2 and BDP-3 were synthesized based on 3,5,8-trimethyl BODIPY dye as the core by the introduction of 4-(diethylamino)phenyl, 4-diethylamino-2-methoxyphenyl and 4-(diphenylamino)phenyl at the 3, 5 and 8 positions, respectively, and used for the construction of organic solar cells. BDP-1, BDP-2, and BDP-3 are synthesized. Due to the strong electron-donating ability of the substituted groups at 3, 5, and 8 positions of BODIPY core, three donors exhibit extended π-conjugation and effective intramolecular charge transfer, resulting in the strong absorption in the range of 500–––900 nm with narrow band gaps below 1.52 eV. BDP-1, BDP-2 and BDP-3 display complementary absorption spectra and suitable highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital levels (LUMO) with fullerene derivative PC71BM. These molecules (BDP-1, BDP-2 or BDP-3) as electron donor were applied into BHJ solar cells by using PC71BM as electron acceptor. The performance of BHJ organic solar cells is systematically investigated and compared. The optimized solar cells based on BDP-1, BDP-2 and BDP-3 show high power conversion efficiencies (PCEs) of 9.52%, 11.83% and 5.27%, respectively. The OSCs based on BDP-2: PC71BM exhibited excellent short-circuit current density (Jsc), high electron mobility and open-circuit voltage (Voc). This work demonstrates that BODIPY derivatives with simple-structured show great potential for high performance OSCs.

Fine‐Tuning Alkyl Chains on Quinoxaline Nonfullerene Acceptors Enables High‐Efficiency Ternary Organic Solar Cells with Optimizing Molecular Stacking and Reducing Energy Loss

AbstractMaterial design of guest acceptor is always a big challenge for improving the efficiency of ternary organic solar cells (OSCs). Here, a pair of isomeric nonfullerene acceptors based on quinoxaline core, Qx–p‐C7H8O and Qx–m‐C7H8O, is designed and synthesized. By moving the alkoxy chain attached on side phenyl from meta‐position to para‐position, both π–π stacking distance and crystallinity are enhanced simultaneously. They obtain the uplifted lowest unoccupied molecular orbital level. Compared to Qx–m‐C7H8O, Qx–p‐C7H8O exhibits wider absorption spectrum and higher extinction coefficient. Using D18‐Cl:N3 as host materials, the addition of guest acceptor Qx–p‐C7H8O significantly improves the power conversion efficiency (PCE) from 17.61% to 18.49% because of higher open‐circuit voltage (0.875 V) and short‐circuit current density (27.85 mA cm−2). This can be attributed to the faster exciton dissociation, more balanced carrier mobility, fine fiber morphology, and lower energy loss in the ternary devices. However, Qx–m‐C7H8O‐based ternary device achieves relatively low PCE of 17.17% because this device shows extremely low electron mobility. The results indicate that molecular stacking, film morphology, etc., can be effectively modulated by fine‐tuning the side chains of guest materials, which may be an effective design rule for further improving the PCE of OSCs.

Role of cations in the photovoltaic performance optimization of ternary stannates, MSnO3 (M = Ca, Sr, and Ba) and N2SnO4 (N = Ca, Sr, Ba, and Zn)

Given the tremendous interest in next-generation photoelectric conversion devices, such as tandem solar cells, indoor photoelectric conversion devices, and image sensors, the development of device-specific electron transport materials is essential for achieving high performance. Sn-based semiconductors are one of the edge materials with the potential to satisfy these requirements. In particular, ternary stannate compounds have shown high potential as electron transport electrodes in several impressive studies. However, systematic research on the electronic band structure and chemical bonding characteristics of these materials remains insufficient. In this study, we present the effect of cations on the crystal structure and electronic band structure of ternary stannate compounds, MSnO3 (MSO, M = Ca, Sr, and Ba) and N2SnO4 (N2SO, N = Ca, Sr, Ba, and Zn). Additionally, their potential as electron transport electrodes is verified by application in dye-sensitized solar cells (DSCs). Notably, although BaSnO3 (BSO) and Zn2SnO4 (Z2SO) had inferior dye adsorption properties, they exhibited higher overall performance in DSCs than the other stannate compounds. The origin of the excellent performances of BSO and Z2SO is attributed to their supportive electronic band features, including the lower flat-band potential and conduction band than that of the lowest unoccupied molecular orbital level of N719 dye and excellent charge transport property. This study highlights the decisive impact of the electronic band structure and charge transport ability of stannate compounds on the photoelectric conversion performance and provides design guidelines for developing new charge transport materials.

Effects of HAT-CN Layer Thickness on Molecular Orientation and Energy-Level Alignment with ZnPc.

Efficient energy-level alignment is crucial for achieving high performance in organic electronic devices. Because the electronic structure of an organic semiconductor is significantly influenced by its molecular orientation, comprehensively understanding the molecular orientation and electronic structure of the organic layer is essential. In this study, we investigated the interface between a 1,4,5,8,9,11-hexaazatriphenylene hexacarbonitrile (HAT-CN) hole injection layer and a zinc-phthalocyanine (ZnPc) p-type organic semiconductor. To determine the energy-level alignment and molecular orientation, we conducted in situ ultraviolet and X-ray photoelectron spectroscopies, as well as angle-resolved X-ray absorption spectroscopy. We found that the HAT-CN molecules were oriented relatively face-on (40°) in the thin (5 nm) layer, whereas they were oriented relatively edge-on (62°) in the thick (100 nm) layer. By contrast, ZnPc orientation was not significantly altered by the underlying HAT-CN orientation. The highest occupied molecular orbital (HOMO) level of ZnPc was closer to the Fermi level on the 100 nm thick HAT-CN layer than on the 5 nm thick HAT-CN layer because of the higher work function. Consequently, a considerably low energy gap between the lowest unoccupied molecular orbital level of HAT-CN and the HOMO level of ZnPc was formed in the 100 nm thick HAT-CN case. This may improve the hole injection ability of the anode system, which can be utilized in various electronic devices.

Nonfullerene Agent Enables Efficient and Stable Tin‐Based Perovskite Solar Cells

Tin‐based perovskites have emerged as the most promising lead‐free perovskite materials due to the characteristics of the environmentally friendly and more suitable bandgap. However, the power conversion efficiency still lags behind the lead perovskite owing to unfavorable defects of the perovskite film and poor interface contact between tin perovskite and electron transporting layers. Herein, an n‐type conjugated nonfullerene molecule, termed IO‐4Cl, is developed for efficient and stable tin perovskite solar cells (TPSCs). The IO‐4Cl possesses electron‐donating functional groups that can enlarge grain size and passivate defect states of perovskite films through Lewis acid–base interactions. Besides, the IO‐4Cl exhibits an appropriate lowest unoccupied molecular orbital level and interface‐modification ability, enabling superior electron extraction and transport ability of TPSCs. More importantly, the hydrophobic characteristic of IO‐4Cl prevents the decomposition of perovskite films under moisture conditions, improving the moisture stability of TPSCs. Consequently, the TPSCs with IO‐4Cl achieve a champion efficiency of 11.49% with an open‐circuit voltage increase of 100 mV, and excellent moisture stability under ambient air condition with 30% relative humidity, without encapsulation. The work provides a new inspiration for the introduction of functional nonfullerene materials to obtain highly efficient and stable TPSCs.

Achieving 17.7% Efficiency of Ternary Organic Solar Cells by Incorporating a High Lowest Unoccupied Molecular Orbital Level and Miscible Third Component

A ternary strategy has been demonstrated as being an effective method to improve the power conversion efficiency (PCE); however, general rules for materials selection are not fully comprehended. Herein, nonfullerene acceptor ITIC‐M and fullerene acceptor PC70BM possessing higher lowest unoccupied molecular orbital (LUMO) and good miscibility with nonfullerene acceptor Y7 are incorporated as third components in the state‐of‐the‐art of PM6:Y7 binary blend. As a result, the device PCE for both ternary devices improves from 16.46% for binary host to 17.73% and 17.67% for ITIC‐M‐ and PC70BM‐based ternary devices, respectively. The higher LUMO of the guest acceptor can play multiple roles to elevate the open‐circuit voltage such as reducing energy‐loss and reverse saturation current, creating less‐localized shallow trap sites along with suppressing charge recombination, and decreasing Urbach energy. Moreover, the good miscibility facilitates an alloy‐like phase in acceptors domain for efficient exciton dissociation and electron transport, which leads to improved short‐circuit current density and fill factor in ternary devices. The results provide a promising approach to realize high‐performance ternary organic solar cells by synergizing the compatible third component with host acceptor.

Third component with a high LUMO energy level enables 17.69% efficiency in ternary organic solar cells

The ternary strategy is thought to be a practical method for improving the power conversion efficiency (PCE) of organic solar cells (OSCs). To create ternary OSCs, a PM6:BTP-BO-4F host blend was combined with the non-fullerene small molecule acceptor BTA3, which has a high lowest unoccupied molecular orbital (LUMO). BTA3 demonstrated complementary absorption spectra and excellent compatibility with BTP-BO-4F, which made it easier to modulate the ternary film's morphology and catch more photons. As a result, the short circuit current density increased. Additionally, BTA3's LUMO level was marginally higher than BTP- BO-4F's, which led to a higher open-circuit voltage of the TOSCs. The addition of BTA3 improved the OSCs' energy transfer and phase separation, which in turn sped up charge transfer and reduced exciton recombination. The PCE of the TOSCs included with 10 wt% BTA3 rose to 17.69% in comparison to a PCE of 16.46% based on host binary OSCs. This research demonstrated that the incorporation of BTA3 with a high LUMO level enabled the fabrication of ternary OSCs with good performance.

Liver Injury Traceability: Spatiotemporally Monitoring Oxidative Stress Processes by Unit-Emitting Carbon Dots.

Exploring the etiology of liver injury is critical to fundamental science and precise treatment, which has not yet been achieved by molecule imaging techniques. Herein, we manage to conquer this challenge by spatiotemporally monitoring oxidative stress processes using the proposed unit-emitting carbon dots (UE-C-dots) as fluorescent probes. We discover and reveal that the UE-C-dots can specifically determine hypochlorous acid (HClO) molecules, one of the important reactive oxygen/nitrogen species (ROS/RNS) in liver injury, by an excited state oxidation mechanism. Other ROS/RNS do not interfere with the assay even if their concentrations are 1000 times higher than that of HClO due to the lowest unoccupied molecular orbital level mismatch. Real-time tomographic imaging demonstrates that different stimuli cause distinctly different HClO bursts in both temporal and spatial dimensionalities. Therefore, the measurement and analysis of temporal information substantially extend our understanding on the relationships of hepatic oxidative stress and corresponding physiological/pathological behaviors.

Naphthoquinoneoxime-Sensitized Titanium Dioxide Photoanodes: Photoelectrochemical Properties.

Naphthoquinoneoxime derivatives, viz., LwOx, 3-hydroxy-4-(hydroxyimino)naphthalen-1 (4H)-one; PthOx, 3-hydroxy-4-(hydroxyimino)-2-methylnaphthalen-1(4H)-one; and Cl_LwOx, 2-chloro-3-hydroxy-4-(hydroxyimino)naphthalen-1(4H)-one, are used in fabrication of dye-sensitized solar cells (DSSCs). The photophysical and electrochemical properties of the sensitizers were studied. The HOMO-LUMO energy gaps of the sensitizers (LwOx, PthOx, and Cl_LwOx) calculated by using the intersection of UV-visible and fluorescence spectra are 2.85, 2.71, and 2.87 eV, respectively. The energy band alignment energy level of the sensitizer, that is, the lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO), should match with the energy level of the TiO2 conduction band and the redox potential of iodine/triiodide electrolyte to allow smooth electron transfer. The electrochemical characterization of sensitizers was done to find the LUMO and HOMO level of the sensitizer. It shows that the LUMO level of (LwOx, PthOx, and Cl_LwOx) is above the conduction band position of TiO2. Electrochemical impedance spectroscopy was used to study the charge transport resistance and electron lifetime of DSSCs. The charge transport resistance at the TiO2 |electrolyte|counter electrode interface was reduced in the Cl_LwOx device; thus, the electron lifetime of Cl_LwOx was enhanced compared to LwOx and PthOx sensitizers. The fabricated device was characterized using photocurrent density-voltage (J-V) measurement. It is observed that there was an enhancement in the overall power conversion efficiency (η) of the DSSCs fabricated by using Cl_LwOx sensitizers as compared to LwOx and PthOx sensitizer-loaded photoanodes. Enhancement in power conversion efficiency, that is, photovoltage and photocurrent, is achieved due to the chlorine substituent. Thus, the chlorine substituent naphthoquinoneoxime pushes the electron density, enhancing the pushing nature and facilitating the lone pair present in the N-OH moiety to attach to TiO2 more strongly.

Dipole Engineering through the Orientation of Interface Molecules for Efficient InP Quantum Dot Light-Emitting Diodes.

InP-based quantum dot (QD) light-emitting diodes (QLEDs) provide a heavy-metal-free route to size-tuned LEDs having high efficiency. The stability of QLEDs may be enhanced by replacing organic hole-injection layers (HILs) with inorganic layers. However, inorganic HILs reported to date suffer from inefficient hole injection, the result of their shallow work functions. Here, we investigate the tuning of the work function of nickel oxide (NiOx) HILs using self-assembled molecules (SAMs). Density functional theory simulations and near-edge X-ray absorption fine structure put a particular focus onto the molecular orientation of the SAMs in tuning the work function of the NiOx HIL. We find that orientation plays an even stronger role than does the underlying molecular dipole itself: SAMs having the strongest electron-withdrawing nitro group (NO2), despite having a high intrinsic dipole, show limited work function tuning, something we assign to their orientation parallel to the NiOx surface. We further find that the NO2 group─which delocalizes electrons over the molecule by resonance─induces a deep lowest unoccupied molecular orbital level that accepts electrons from QDs, producing luminescence quenching. In contrast, SAMs containing a trifluoromethyl group exhibit an angled orientation relative to the NiOx surface, better activating hole injection into the active layer without inducing luminescence quenching. We report an external quantum efficiency (EQE) of 18.8%─the highest EQE among inorganic HIL-based QLEDs (including Cd-based QDs)─in InP QLEDs employing inorganic HILs.

Energy band alignment engineering in nanostructured hybrid materials based on PbS QDs and reduced graphene oxide: Enhancing photodetector device performance

Hybrid materials based on semiconductor quantum dots (QDs) and reduced-graphene-oxide (rGO) have attracted a lot of attention from the scientific community. RGO is compatible with accessible and affordable solution-process methods; additionally, the reduction process allows to manipulate its energy-levels and electrical properties, including majority charge carrier type. These facts, in combination with the known tunability of Eg and the Highest Occupied Molecular Orbital and Lowest Unoccupied Molecular Orbital levels in semiconductor QDs, open novel pathways to improve optoelectronic performance of such nanostructured-hybrid-materials via band alignment engineering. Based on this concept, the effects of different reduction processes on rGO’s electronic properties were studied by Ultraviolet Photoelectron Spectroscopy and Hall effect measurements; additionally, synthesized PbS QDs’ energy levels were measured using electrochemical techniques. Photodetector devices (PD) were fabricated based on the nanostructured-hybrid-materials that showed an improved energy band alignment between rGO and PbS QD’s energy levels; such energy band alignment allowed to improve charge carrier transfer from PbS QDs to rGO upon light absorption, producing photodetectors with improved performance. The devices based on the nanostructured hybrid material reduced thermally at 400 °C displayed the best energy band alignment between the component materials and the lowest resistivity; consequently, showing the highest photocurrent, responsivity, and specific detectivity from all the samples studied.

Efficient ternary organic solar cells with BT-rhodanine-based nonfullerene acceptors in a PM6:Y6-BO blend

A series of benzothiadiazole (BT)–rhodanine-based nonfullerene acceptors (NFAs) were designed and synthesized. These BT–rhodanine-based NFAs were composed of BT as a central core with a 3-octylthienothiohene π-bridge and two different rhodanine end groups (BT-rho for octylrhodanine and BT-rhoCN for dicyano-octylrhodanine end groups). The highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels were −5.53/−3.78 eV for a BT-rho film and −5.63/−3.90 eV for a BT-rhoCN film. The strong electron-withdrawing dicyano-octylrhodanine end group of BT-rhoCN downshifted the HOMO and LUMO levels by ∼0.1 eV compared with those of BT-rho. The 10% BT-rho in a PM6:Y6-BO ternary organic solar cell exhibit a maximum power conversion efficiency (PCE) of 16.17% with a short-circuit current density ( J SC ) of 25.29 mA cm −2 , open-circuit voltage ( V OC ) of 0.84 V, and fill factor ( FF ) of 76.13%. The hole/electron mobilities of the PM6:Y6-BO, 10% BT-rho, and 10% BT-rhoCN ternary devices were 2.20 × 10 −4 /1.85 × 10 −4 cm 2 V −1 s −1 , 3.66 × 10 −4 /3.77 × 10 −4 cm 2 V −1 s −1 , and 1.34 × 10 −4 /0.94 × 10 −4 cm 2 V −1 s −1 , respectively. The crystal coherence lengths ( L C s) of the (010) peaks for the PM6:Y6-BO, 10% BT-rho, and 10% BT-rhoCN ternary films are 43.2, 60.8, and 33.7 Å, respectively. The superior PCE of the 10% BT-rho ternary devices compared with that of the PM6:Y6-BO binary device is attributed to the enhanced hole and electron mobilities and the high crystallinity of the blend films, which contribute to the enhancement of the J SC and PCE. • New BT-based acceptors, BT-rho and BT-rhoCN, were synthesized by inserting rhodanine terminal groups. • Rhodanine end groups of BT-rho lowered the LUMO level and thus increased the open-circuit voltage. • An enhanced PCE of 16.17% was achieved using BT-rho as a third component in a PM6:Y6-BO blend.

Recent development in electron transport layers for efficient tin-based perovskite solar cells

Hybrid organic-inorganic tin (Sn)-based perovskite materials became a promising choice as an alternative to lead-free perovskite solar cells (PSCs) due to their outstanding optical and electrical properties. But, so far, a power conversion efficiency (PCE) of only 13% has been achieved for Sn-based PSCs. To achieve highly efficient and stable PSCs, not only the properties of the active layer but the charge selective contacts (electron and hole transport layers) should be selected wisely. The interfaces between the perovskite active layer and charge transport layers play an important role in achieving the better performance of PSCs. In the present review, the spotlight is on the recent developments made on the optimization of electron transport layers (ETLs) for the efficient Sn-based hybrid organic-inorganic PSCs. Further, we comprehensively discuss the significance and the impact of the lowest unoccupied molecular orbital level of electron transport material on the charge transport, which additionally affects the photovoltaic performance of the device. In summary, with continuous research on the Sn-based hybrid organic-inorganic perovskite materials as an absorbing layer, conventional ETLs (metal oxides) cannot be used. Thus, the optimum candidate for befitted ETLs must be explored and investigated in detail for efficient PSCs.

Hole-Transport Layer-Free Tin-Based Perovskite Solar Cells: Improving Their Performance from a Simulation Perspective

Perovskite solar cells are in rapid development, however, PCE (photovoltaic conversion efficiency) of HTL-free (Hole Transport Layer free) perovskite solar cells is reported to be low. In this work, we propose a tin-based perovskite solar cell with an ETL (Electron Transport Layer) structure of a novel electron transport layer material. We simulated the device with SCAPS-1D (Solar Cells Capacitance Simulator) and found that all of the absorber layer thickness, band gap, doping concentration, defect density, device working temperature, ETL layer thickness, and doping concentration have different degrees of influence on the device performance. After optimization, (Open Circuit Voltage) is 1.07 V, (Short-circuit Current Density) is 24.25 FF (Filling Factor) is 87.87%, and PCE arrives up to 22.72%. There is a significant performance improvement compared to pre-simulation (initially, is 0.97 V, is 24.21 FF is 69.12%, and PCE is 16.22%). It is shown that when the thickness of the absorber layer is larger than the diffusion length, it leads to harmful photogenerated carrier complexes and increases the diffusion length before reaching the electrode, which affects the carrier transport migration. Defect density bears the brunt of the performance impact, the greater it is the worse the performance, so it should be controlled to a small range. When the operating temperature higher than 300 K, the device performance tends to deteriorate and it is recommended to keep the operating temperature around 300 K. The ETL layer doping concentration and thickness have little influence on performance. Finally, we also analytically simulate and compare several traditional ETL PSCs, and the matching of LUMO (Lowest Unoccupied Molecular Orbital) and absorber energy levels of different ETL materials was found to be different, and the higher the matching is, the more favorable the electron transport and the higher the PCE. This work provides new strategy to design PSCs.

Determination of the Formation Potential of Solid-Electrolyte Interphase in Some Amide-Type Ionic Liquids Containing Metal Salts

The formation potentials of the solid-electrolyte interphase (SEI) on a platinum and glassy carbon electrode in two amide-type ionic liquids, 1-butyl-1-methylpyrrolidinium bis(fluorosulfonyl)amide (BMPFSA) and 1-butyl-1-methylpyrrolidinium bis(trifluoromethylsulfonyl)amide (BMPTFSA), containing some metal salts were investigated by cyclic voltammetry in the presence of ferrocene (Fc). It is considered that the SEI with the thickness of less than a few nm can be observed by monitoring the shift in the peak potential of Fc+/Fc. The SEIs began to form at ~2.4 V vs. Li|Li(I) in the presence of lithium salts because of a lowering of the lowest unoccupied molecular orbital level of the anion by the interaction with lithium ion. In the case of BMPTFSA containing Ni(TFSA)2, the SEI began to form at –0.7 V vs. Ag|Ag(I), suggesting that the deposition of Ni may be hindered by the SEI formation.

The Effect of Silicon Substitution in Indacenodithiophene‐Based A2‐A1‐D‐A1‐A2‐Type Nonfullerene Acceptors on the Performance of High‐Voltage Organic Solar Cells

In recent years, the organic solar cells (OSCs) research hotspot is the modification of end groups and alkyl side chains of A‐DA’D‐A‐type nonfullerene acceptors (NFAs). However, the development of novel NFAs by changing the different bridged atom substitution of the central core is lagging behind. Herein, two wide‐bandgap A2‐A1‐D‐A1‐A2‐type NFAs with different central cores, namely BTA501 and BTA502, are developed to improve the photovoltaic performance of high‐voltage OSCs. BTA501 adopted an indacenodithiophene (IDT) core, whereas BTA502 applied a silaindacenodithiophene (SiIDT) core. Expectedly, the SiIDT‐based BTA502 exhibits a higher lowest unoccupied molecular orbital level and wider bandgap than BTA501, which thus enhances the open‐circuit voltage (V OC) but slightly decreases the short‐circuit current density (J SC) of OSCs. Moreover, the stronger self‐aggregation characteristics and weaker π–π stacking of BTA502 severely affect the exciton dissociation and charge transport. When blended with two classic p‐type polymers J52‐F and PTB7‐Th, both combinations based on BTA502 exhibit inferior device performance compared with BTA501. Excitingly, the device of J52‐F: BTA501 achieves a V OC of 1.037 V with a power conversion efficiency of 11.82% and a J SC of 15.89 mA cm−2, which are among the highest values for high‐voltage OSCs with V OC above 1.0 V.

Enhancing the Backbone Coplanarity of n-Type Copolymers for Higher Electron Mobility and Stability in Organic Electrochemical Transistors.

Electron-transporting (n-type) conjugated polymers have recently been applied in numerous electrochemical applications, where both ion and electron transport are required. Despite continuous efforts to improve their performance and stability, n-type conjugated polymers with mixed conduction still lag behind their hole-transporting (p-type) counterparts, limiting the functions of electrochemical devices. In this work, we investigate the effect of enhanced backbone coplanarity on the electrochemical activity and mixed ionic-electronic conduction properties of n-type polymers during operation in aqueous media. Through substitution of the widely employed electron-deficient naphthalene diimide (NDI) unit for the core-extended naphthodithiophene diimide (NDTI) units, the resulting polymer shows a more planar backbone with closer packing, leading to an increase in the electron mobility in organic electrochemical transistors (OECTs) by more than two orders of magnitude. The NDTI-based polymer shows a deep-lying lowest unoccupied molecular orbital level, enabling operation of the OECT closer to 0 V vs Ag/AgCl, where fewer parasitic reactions with molecular oxygen occur. Enhancing the backbone coplanarity also leads to a lower affinity toward water uptake during cycling, resulting in improved stability during continuous electrochemical charging and ON–OFF switching relative to the NDI derivative. Furthermore, the NDTI-based polymer also demonstrates near-perfect shelf-life stability over a month-long test, exhibiting a negligible decrease in both the maximum on-current and transconductance. Our results highlight the importance of polymer backbone design for developing stable, high-performing n-type materials with mixed ionic-electronic conduction in aqueous media.

A new perspective to develop regiorandom polymer acceptors with high active layer ductility, excellent device stability, and high efficiency approaching 17%

AbstractThe recently reported efficient polymerized small‐molecule acceptors (PSMAs) usually adopt a regioregular backbone by polymerizing small‐molecule acceptors precursors with a low‐reactivity 5‐brominated 3‐(dicyanomethylidene)indan‐1‐one (IC) end group or its derivatives, leading to low molecular weight, and thus reduce active layer mechanical properties. Herein, a series of newly designed chlorinated PSMAs originating from isomeric IC end groups are developed by adjusting chlorinated positions and copolymerized sites on end groups to achieve high molecular weight, favorable intermolecular interaction, and improved physicochemical properties. Compared with regioregular PY2Se‐Cl‐o and PY2Se‐Cl‐m, regiorandom PY2Se‐Cl‐ran has a similar absorption profile, moderate lowest unoccupied molecular orbital level, and favorable intermolecular packing and crystallization properties. Moreover, the binary PM6:PY2Se‐Cl‐ran blend achieves better ductility with a crack‐onset strain of 17.5% and improved power conversion efficiency (PCE) of 16.23% in all‐polymer solar cells (all‐PSCs) due to the higher molecular weight of PY2Se‐Cl‐ran and optimized blend morphology, while the ternary PM6:J71:PY2Se‐Cl‐ran blend offers an impressive PCE approaching 17% and excellent device stability, which are all crucial for potential practical applications of all‐PSCs in wearable electronics. To date, the efficiency of 16.86% is the highest value reported for the regiorandom PSMAs‐based all‐PSCs and is also one of the best values reported for the all‐PSCs. Our work provides a new perspective to develop efficient all‐PSCs, with all high active layer ductility, impressive PCE, and excellent device stability, towards practical applications.