A Strategy to Design Substituted Tetraamino-Phenazine Dyes and Access to an NIR-Absorbing Benzoquinonediimine-Fused Quinoxaline

The straightforward access to N- or C-substituted dinitro-tetraamino-phenazines (P1-P5) is enabled in oxidative conditions via formation of two intermolecular C-N bonds from accessible 5-nitrobenzene-1,2,4-triamine precursors. The photophysical studies revealed green absorbing and orange-red emitting dyes, with enhanced fluorescence in the solid state. Further reduction of the nitro functions led to the isolation of a benzoquinonediimine-fused quinoxaline (P6), which undergoes diprotonation to form a dicationic coupled trimethine dye absorbing beyond 800 nm.

Impact of Aryl End Group Engineering of Donor Polymers on the Morphology and Efficiency of Halogen-Free Solvent-Processed Nonfullerene Organic Solar Cells

End group engineering on the side chain of π-conjugated donor polymers is explored as an effective way to develop efficient photovoltaic devices. In this work, we designed and synthesized three new π-conjugated polymers (PBDT-BZ-1, PBDT-S-BZ, and PBDT-BZ-F) with terminal aryl end groups on the side chain of chlorine-substituted benzo[1,2-b:4,5b']dithiophene (BDT). End group modifications showed notable changes in energy levels, dipole moments, exciton lifetimes, energy losses, and charge transport properties. Remarkably, the three new polymers paired with IT-4F (halogen-free solvent processed/toluene:DPE) displayed high power conversion efficiencies (PCEs) compared to a polymer (PBDT-Al-5) without a terminal end group (PCE of 7.32%). Interestingly, PBDT-S-BZ:IT-4F (PCE of 13.73%) showed a higher PCE than the benchmark PM7:IT-4F. The improved performance of PBDT-S-BZ well correlates with its improved charge mobility, well-interdigitated surface morphology, and high miscibility with a low Flory-Huggins interaction parameter (1.253). Thus, we successfully established a correlation between the end group engineering and bulk properties of the new polymers for realizing the high performance of halogen-free nonfullerene organic solar cells.

Side-chain engineering of copolymers based on benzotriazole (BTA) and dithieno[2,3-d;2',3'-d']benzo[1,2-b;4,5-b']dithiophenes (DTBDT) enables a high PCE of 14.6

Dithieno[2,3-d;2',3'-d']benzo[1,2-b;4,5-b']dithiophenes (DTBDT) is a kind of prospective candidate for constructing donor-π-acceptor (D-π-A) copolymer donors applied in organic solar cells but is restricted due to its relatively poor photovoltaic performance compared with benzo[1,2-b;4,5-b']dithiophenes (BDT)-based analog. Herein, three conjugated polymers (PE51,PE52andPE53)-based DTBDT and benzo[d][1,2,3]triazole (BTA) bearing different lengths of alkyl side chain were designed and synthesized. The change in alkyl chain length can obviously affect the energy level distribution, molecular stacking, miscibility and morphology with the non-fullerene acceptor ofY6. PolymerPE52with a moderate alkyl chain realized the highest short-current density (J_SC) and fill factor (FF) of 25.36 mA cm^-2and 71.94%, respectively. Compared with BDT-based analogJ52-Cl, the significantly enhanced crystallinity and intermolecular interaction ofPE52had effectively boosted the charge transport characteristic and optimized the surface morphology, thereby increasing the power conversion efficiency from 12.3% to an impressive 14.6%, which is the highest value among DTBDT-based and BTA-based polymers. Our results show that not only could high efficiency be achieved via using DTBDT as a D unit, but the length of the alkyl chain on BTA has a significant impact on the photovoltaic performance.

Recent Progress in Chlorinated Organic Photovoltaic Materials

ConspectusOver the past few years, the development of new materials has contributed to rapid increases in the power conversion efficiencies (PCEs) of organic photovoltaic (OPV) cells to over 17%, showing great potential for the commercialization of this technology in the near future. At this stage, designing new materials with superior performance and low cost simultaneously is of crucial importance. Chlorinated materials are emerging as new stars with very high PCEs, creating a molecular design trend to replace the most popular fluorinated materials. For example, by using chlorinated non-fullerene acceptors, we recently got a record PCE of 17% for single-junction OPV cells. Firmly based on recent advances, herein we focus on the topic of chlorinated OPV materials, aiming to provide a guideline for further molecular design.In this Account, first, on the basis of most fundamental features of the Cl atom, we highlight the features of chlorinated materials compared with their fluorinated counterparts: (1) Chlorination is more efficient than fluorination in modulating the optical and electrical properties of OPV materials. In many cases, chlorinated materials show lower energy levels and broader absorption spectra than their fluorinated counterparts, which contribute higher output voltages and current densities in the resulting photovoltaic devices. (2) Cl has a large atomic size than F. On one hand, enhanced overlap of π electrons is beneficial for enhancing the intermolecular packing and crystalline property and thus improving the charge transport. On the other hand, if Cl is introduced inappropriately in the backbone or side chain, this feature will cause a more twisted π plane and larger steric hindrance, having negative impacts on the photovoltaic performance of the corresponding materials. (3) Importantly, chlorination is usually chemically cheaper in synthesis, which has the potential to decrease the material cost of OPV cells. Then, we provide a concise review of chlorinated OPV materials, including polymeric and small-molecule donors and non-fullerene acceptors. The photovoltaic performance in various types of OPV cells using chlorinated materials, such as single-junction, tandem, semitransparent, and indoor-light photovoltaic cells is also discussed. For instance, ultranarrow-band-gap chlorinated acceptors can be used to construct highly efficient color-semitransparent OPV cells, and the wide-band-gap chlorinated materials show great potential for fabricating indoor-light photovoltaic devices. Finally, we briefly discuss current questions related to chlorinated OPV materials and highlight the significance of chlorination in future development.

Design Principles and Synergistic Effects of Chlorination on a Conjugated Backbone for Efficient Organic Photovoltaics: A Critical Review

The pursuit of low-cost, flexible, and lightweight renewable power resources has led to outstanding advancements in organic solar cells (OSCs). Among the successful design principles developed for synthesizing efficient conjugated electron donor (ED) or acceptor (EA) units for OSCs, chlorination has recently emerged as a reliable approach, despite being neglected over the years. In fact, several recent studies have indicated that chlorination is more potent for large-scale production than the highly studied fluorination in several aspects, such as easy and low-cost synthesis of materials, lowering energy levels, easy tuning of molecular orientation, and morphology, thus realizing impressive power conversion efficiencies in OSCs up to 17%. Herein, an up-to-date summary of the current progress in photovoltaic results realized by incorporating a chlorinated ED or EA into OSCs is presented to recognize the benefits and drawbacks of this interesting substituent in photoactive materials. Furthermore, other aspects of chlorinated materials for application in all-small-molecule, semitransparent, tandem, ternary, single-component, and indoor OSCs are also presented. Consequently, a concise outlook is provided for future design and development of chlorinated ED or EA units, which will facilitate utilization of this approach to achieve the goal of low-cost and large-area OSCs.

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.

CFBSA: a novel and practical chlorinating reagent

A novel chlorinating reagent was synthesized from easily-obtained materials and it shows great reactivity to a large scope of substrates.