Controlling Energy Levels and Blend Morphology for All-Polymer Solar Cells via Fluorination of a Naphthalene Diimide-Based Copolymer Acceptor

Benzotriazole-Based Non-Fused Ring Acceptors for Efficient and Thermally Stable Organic Solar Cells

Non-fused ring acceptors (NFRAs) have attracted significant attention for non-fullerene organic solar cells (OSCs) owing to their chemical tunability and facile synthesis. In this study, a benzotriazole-based NFRA with chlorinated end groups (Triazole-4Cl) is developed to realize highly efficient and thermally stable NFRA-based OSCs; an analogous NFRA with non-chlorinated end groups (Triazole-H) is synthesized for comparison. Triazole-4Cl film exhibits the high-order packing structure and the near-infrared absorption capability, which are advantageous in charge transport and light harvesting of the resulting OSCs. In particular, the strong crystalline behavior of Triazole-4Cl results in enhanced self-aggregation, leading to high charge carrier mobility. Owing to these properties, a PBDB-T(polymer donor):Triazole-4Cl OSC demonstrates a high short-circuit current, fill factor, and power conversion efficiency (PCE = 10.46%), outperforming a PBDB-T:Triazole-H OSC (PCE = 7.65%). In addition, the thermal stability of a PBDB-T:Triazole-4Cl OSC at an elevated temperature of 120°C exceeds that of a PBDB-T:Triazole-H OSC. This is mainly attributed to the significantly higher cold crystallization temperature of Triazole-4Cl (205.9°C). This work provides useful guidelines for the design of NFRAs to achieve efficient and thermally stable NFRA-based OSCs. This article is protected by copyright. All rights reserved.

Naphthalene diimides: perspectives and promise

In this review, we describe the developments in the field of naphthalene diimides (NDIs) from 2016 to the presentday. NDIs are shown to be an increasingly interesting class of molecules due to their electronic properties, large electron deficient aromatic cores and tendency to self-assemble into functional structures. Almost all NDIs possess high electron affinity, good charge carrier mobility, and excellent thermal and oxidative stability, making them promising candidates for applications in organic electronics, photovoltaic devices, and flexible displays. NDIs have also been extensively studied due to their potential real-world uses across a wide variety of applications including supramolecular chemistry, sensing, host-guest complexes for molecular switching devices, such as catenanes and rotaxanes, ion-channels, catalysis, and medicine and as non-fullerene accepters in solar cells. In recent years, NDI research with respect to supramolecular assemblies and mechanoluminescent properties has also gained considerable traction. Thus, this review will assist a wide range of readers and researchers including chemists, physicists, biologists, medicinal chemists and materials scientists in understanding the scope for development and applicability of NDI dyes in their respective fields through a discussion of the main properties of NDI derivatives and of the status of emerging applications.

Dual-Mode Superresolution Imaging Using Charge Transfer Dynamics in Semiconducting Polymer Dots

In a conjugated polymer-based single-particle heterojunction, stochastic fluctuations of the photogenerated hole population lead to spontaneous fluorescence switching. We found that 405 nm irradiation can induce charge recombination and activate the single-particle emission. Based on these phenomena, we developed a novel class of semiconducting polymer dots that can operate in two superresolution imaging modes. The spontaneous switching mode offers efficient imaging of large areas, with <10 nm localization precision, while the photoactivation/deactivation mode offers slower imaging, with further improved localization precision (ca. 1 nm), showing advantages in resolving small structures that require high spatial resolution. Superresolution imaging of microtubules and clathrin-coated pits was demonstrated, under both modes. The excellent localization precision and versatile imaging options provided by these nanoparticles offer clear advantages for imaging of various biological systems.

Effects of Halogenation in B ← N Embedded Polymer Acceptors on Performance of All-Polymer Solar Cells

Halogenation, for example, fluorination and chlorination, is an effective strategy to regulate the performance of organic photovoltaic materials. Although fluorination has been widely applied to polymer acceptors, systematic studies on the comparison of nonhalogenated, fluorinated, and chlorinated polymer acceptors have been a blank to now. Herein, a B ← N embedded electron-deficient unit (A), namely, BNIDT was copolymerized with three electron-rich units (D), that is, benzodithiophene (BDT), fluorinated BDT, and chlorinated BDT to obtain three D-A polymers of BN-BDT, BN-BDT-F, and BN-BDT-Cl, respectively. The three polymers exhibit similar LUMOs of ca. -3.77 eV, whereas the HOMOs are remarkably decreased from BN-BDT (-5.46 eV) to BN-BDT-F (-5.71 eV) and further slightly lowered to BN-BDT-Cl (-5.74 eV). All-polymer solar cells (all-PSCs) were fabricated using PBDB-T as the donor and the three B ← N-based polymers as the acceptors. The efficiencies of all-PSCs were significantly promoted from nonhalogenated BN-BDT (1.60%) to fluorinated BN-BDT-F (3.71%) and further elevated to chlorinated BN-BDT-Cl (4.23%). Device characterizations revealed that halogenation on the polymer acceptors leads to enhanced hole-transfer driving forces and better donor/acceptor miscibility, for example, smaller domain sizes and root-mean-square roughness (rms) values, which further gives rise to higher and more balanced hole/electron mobilities and efficient physical processes, for example, efficient exciton dissociation and collection and weaker recombination losses in halogenated devices. This work demonstrates that the photovoltaic performance of nonhalogenated polymer acceptors can be remarkably boosted by fluorination and further enhanced by chlorination. This is the first systematic study on the halogenated polymer acceptors by comprehensively comparing nonhalogenated, fluorinated, and chlorinated ones.

Understanding of Fluorination Dependence on Electron Mobility and Stability of Naphthalenediimide-Based Polymer Transistors in Environment with 100% Relative Humidity

A family of copolymers (P(NDIOD-T2Fx)) based on naphthalenediimide (NDI) and 2,2'-bithiophene (T2) units with different amounts of 3,3'-difluoro-2,2'-bithiophene (T2F) decoration were synthesized, characterized, and used in n-type organic field-effect transistors (OFETs). With increasing T2F content in the backbone, we observe increased melting and crystallization transitions, blue-shifted absorptions, and deeper-lying highest occupied molecular orbital (HOMO)/lowest unoccupied molecular orbital (LUMO) levels, together with improved hydrophobicity. The highest electron mobility of 4.48 × 10^-1 cm² V^-1 s^-1 was obtained for P(NDIOD-T2F0) without a T2F unit, which is attributed to the larger domain grains and crystallites, as well as a more tightly packed and oriented crystalline structure, as evidenced from the morphological study. In contrast, P(NDIOD-T2F100) with the highest T2F content has superior air stability, showing greater than 25% electron mobility retention after 30 days in wet conditions of 100% relative humidity without encapsulation. Even P(NDIOD-T2F100) is able to operate normally after 30 min of immersion in water, which is due to the synergistic contributions from the deep HOMO/LUMO levels and improved hydrophobicity. This study advances our fundamental understanding of how the morphology/crystallinity, device performance, and device stability of n-type copolymers are tuned by incorporating different concentrations of T2F in the backbone, shedding light on an important modification for air- and water-stable n-type materials for future OFET applications.

Impact of Incorporating Nitrogen Atoms in Naphthalenediimide-Based Polymer Acceptors on the Charge Generation, Device Performance, and Stability of All-Polymer Solar Cells

Substitution of C atoms in a polymer backbone by N atoms allows for the facile tuning of the energy levels as well as the backbone conformation and packing structures of conjugated polymers. Herein, we report a series of three polymer acceptors (P_As) with N atoms introduced at different positions of the backbone and investigate how these N atoms affect the device performances of all-polymer solar cells (all-PSCs). The three P_As, namely, P(NDI2DT-BTT), P(NDI2DT-PTT), and P(NDI2DT-BTTz), are composed of naphthalenediimide (NDI)-based and benzothiadiazole (BT)-based derivatives (dithiophene-BT (BTT), dithiophene-thiadiazolepyridine (PTT), and dithiazole-BT (BTTz)). The PTT and BTTz units are synthesized by replacing the C atoms in BT and thiophene, respectively, with N atoms, which effectively tune the optical, electrochemical, and charge-transporting properties of the corresponding P_As. The all-PSCs using poly[(2,6-(4,8-bis(5-(2-ethylhexyl)thiophen-2-yl))benzo[1,2-b:4,5-b']dithiophene)-co-(1,3-di(5-thiophene-2-yl)-5,7-bis(2-ethylhexyl)benzo[1,2-c:4,5-c']dithiophene-4,8-dione)] (PBDB-T) as a polymer donor and P(NDI2DT-PTT) as P_A exhibit a significantly enhanced power conversion efficiency (PCE) of 6.95%, whereas the all-PSCs based on the other P_As show relatively lower PCEs (6.02% for PBDB-T:P(NDI2DT-BTT) and 1.43% for PBDB-T:P(NDI2DT-BTTz)). The high PCE of the PBDB-T:P(NDI2DT-PTT) device is due to the superior charge transfer and charge dissociation, resulting from the closely matched energy levels between PBDB-T and P(NDI2DT-PTT), as well as a more favorable bulk heterojunction morphology with improved miscibility. Importantly, the P(NDI2DT-PTT)-based all-PSC device shows improved air stability compared to the P(NDI2DT-BTT)-based device, which is most likely due to a decreased lowest unoccupied molecular orbital level of the P_A. Our findings suggest that the incorporation of N atoms into the P_As is an effective strategy for improving the efficiency and stability of all-PSCs.

Recent Advances in n-Type Polymers for All-Polymer Solar Cells

All-polymer solar cells (all-PSCs) based on n- and p-type polymers have emerged as promising alternatives to fullerene-based solar cells due to their unique advantages such as good chemical and electronic adjustability, and better thermal and photochemical stabilities. Rapid advances have been made in the development of n-type polymers consisting of various electron acceptor units for all-PSCs. So far, more than 200 n-type polymer acceptors have been reported. In the last seven years, the power conversion efficiency (PCE) of all-PSCs rapidly increased and has now surpassed 10%, meaning they are approaching the performance of state-of-the-art solar cells using fullerene derivatives as acceptors. This review discusses the design criteria, synthesis, and structure-property relationships of n-type polymers that have been used in all-PSCs. Additionally, it highlights the recent progress toward photovoltaic performance enhancement of binary, ternary, and tandem all-PSCs. Finally, the challenges and prospects for further development of all-PSCs are briefly considered.

Regioisomeric wide-band-gap polymers with different fluorine topologies for non-fullerene organic solar cells

The effects of different fluorine substituent topologies on the morphological and photovoltaic properties are studied for two regioisomeric donor polymer-based nonfullerene organic solar cells.

Designing Cross-Coupling Reactions using Aryl(trialkyl)silanes

Organo(trialkyl)silanes have several advantages, including high stability, low toxicity, good solubility, easy handling, and ready availability compared with heteroatom-substituted silanes. However, methods for the cross-coupling of organo(trialkyl)silanes are limited, most probably because of their exceeding robustness. Thus, a practical method for the cross-coupling of organo(trialkyl)silanes has been a long-standing challenging research target. This article discusses how aryl(trialkyl)silanes can be used in cross-coupling reactions. A pioneering example is Cu^II catalytic conditions with the use of electron-accepting aryl- or heteroaryl(triethyl)silanes and aryl iodides. The reaction forms biaryls or teraryls. This design concept can be extended to Pd/Cu^II -catalyzed cross-coupling polymerization reactions between such silanes and aryl bromides or chlorides and to Cu^I -catalyzed alkylation using alkyl halides.

Comparison of Three n-Type Copolymers Based on Benzodithiophene and Naphthalene Diimide/Perylene Diimide/Fused Perylene Diimides for All-Polymer Solar Cells Application

All-polymer solar cells have gained large attention in recent years because of their tunable energy levels and absorption spectra for both polymeric donor and acceptor. Comparing with the numerous polymeric donors, the development of polymeric acceptors was relatively slow. Rylene diimide-based polymers are regarded as the most promising n-type polymers, which were widely investigated in the past decade, and some novel rylene diimide structures are constantly designed. In this work, three n-type polymers with a donor/acceptor (D/A) alternative backbone structure, named PNDI-BDT, PPDI-BDT, and PFPDI-BDT, were synthesized. In these polymers, naphthalene diimide (NDI), perylene diimide (PDI), and recently developed fused perylene diimide (FPDI) were utilized as electron-withdrawing segment, respectively, and benzodithiophene (BDT) with thiophenes as conjugated side chains was utilized as an electron-rich unit. The optical properties, electron energy levels, charge transport properties, photovoltaic performance, charge recombination loss, and surface morphology were systematically investigated. After optimizing the device fabrication conditions, PNDI-BDT-, PPDI-BDT-, and PFPDI-BDT-based photovoltaic cells realized the power conversion efficiencies of 0.88, 3.74, and 5.65%, respectively. Our results indicate that FPDI is a better electron-deficient segment in comparison with NDI and PDI, for the design of n-type photovoltaic polymers.

Alkoxybenzothiadiazole-Based Fullerene and Nonfullerene Polymer Solar Cells with High Shunt Resistance for Indoor Photovoltaic Applications

We synthesized three semicrystalline polymers (PTTBT_BO, PDTBT_BO, and P2FDTBT_BO) by modulating the intra- and intermolecular noncovalent Coulombic interactions and investigated their photovoltaic characteristics under various light intensities. Low series (R_s) and high shunt (R_sh) resistances are essential prerequisites for good device properties under standard illumination (100 mW cm^-2). Considering these factors, among three polymers, PDTBT_BO polymer solar cells (PSCs) exhibited the most desirable characteristics, with peak power conversion efficiencies (PCE) of 7.52 and 9.60% by being blended with PC₇₁BM under standard and dim light (2.5 mW cm^-2), respectively. P2FDTBT_BO PSCs exhibited a low PCE of 3.69% under standard light due to significant charge recombination with high R_s (9.42 Ω cm²). However, the PCE was remarkably improved by 2.3 times (8.33% PCE) under dim light, showing negligible decrease in open-circuit voltage and remarkable increase in fill factor, which is due to an exceptionally high R_sh of over 1000 kΩ cm². R_s is less significant under dim light because the generated current is too small to cause noticeable R_s-induced voltage losses. Instead, high R_sh becomes more important to avoid leakage currents. This work provides important tips to further optimize PSCs for indoor applications with low-power electronic devices such as Internet of things sensors.

Impact of Acceptor Fluorination on the Performance of All-Polymer Solar Cells

Here, we systematically study the effect of fluorination on the performance of all-polymer solar cells by employing a naphthalene diimide (NDI)-based polymer acceptor with thiophene-flanked phenyl co-monomer. Fluorination of the phenyl co-monomer with either two or four fluorine units is used to create a series of acceptor polymers with either no fluorination (PNDITPhT), bifluorination (PNDITF2T), or tetrafluorination (PNDITF4T). In blends with the donor polymer PTB7-Th, fluorination results in an increase in power conversion efficiency from 3.1 to 4.6% despite a decrease in open-circuit voltage from 0.86 V (unfluorinated) to 0.78 V (tetrafluorinated). Countering this decrease in open-circuit voltage is an increase in short-circuit current from 7.7 to 11.7 mA/cm² as well as an increase in fill factor from 0.45 to 0.53. The origin of the improvement in performance with fluorination is explored using a combination of morphological, photophysical, and charge-transport studies. Interestingly, fluorination is found not to affect the ultrafast charge-generation kinetics, but instead is found to improve charge-collection yield subsequent to charge generation, linked to improved electron mobility and improved phase separation. Fluorination also leads to improved light absorption, with the blue-shifted absorption profile of the fluorinated polymers complementing the absorption profile of the low-band gap PTB7-Th.

Side-chain engineering in naphthalenediimide-based n-type polymers for high-performance all-polymer photodetectors

Optimization of the all-polymer photodetector performance by tuning the size of side chains in NDI-based acceptor polymers.

Effects of incorporating different chalcogenophene comonomers into random acceptor terpolymers on the morphology and performance of all-polymer solar cells

A new family of NDI-based random terpolymers, incorporating a small amount (10%) of different chalcogenophene units (-Fu, -Th, -Se) was synthesized and investigated for all-PSCs.