Fluorinated Benzothiadiazole-Based Conjugated Polymers for High-Performance Polymer Solar Cells without Any Processing Additives or Post-treatments

A High-Mobility n-Type Noncovalently-Fused-Ring Polymer for High-Performance Organic Thermoelectrics

Conjugated polymers are emerging as competitive candidates for organic thermoelectrics (OTEs). However, to make the device truly pervasive, both p- and n-type conjugated polymers are essential. Despite great efforts, no n-type equivalents to the p-type benchmark PEDOT:PSS exist to date mainly due to the low electrical conductivity (σ). Herein, a near-amorphous n-type conjugated polymer, namely pDFSe, is reported with high σ by achieving the synergy between charge transport and doping efficiency. The polymer pDFSe is synthesized based on an acceptor-triad moiety of diketopyrrolopyrrole-difluorobenzoselenadiazole-diketopyrrolopyrrole (DFSe), which has the noncovalently-fused-ring structure to reinforce the backbone rigidity. Furthermore, an axisymmetric thiophene-selenophene-thiophene donor is introduced, which enables the formation of near-amorphous microstructures. The above merits ensure good doping efficiency without scarifying efficient intrachain charge-carrier transport. Thus, pDFSe-based n-type transistors exhibit high electron mobility up to 6.15 cm2 V-1 s-1, much higher than its reference polymer pDSe without the noncovalently-fused-ring structure (0.77 cm2 V-1 s-1). Further upon n-doping, pDFSe demonstrates excellent σ of 62.6 S cm-1 and maximum power factor of 133.1 μW m-1 K-2, which are among the highest values reported for solution-processed n-type polymers. The results demonstrate the great potential of near-amorphous n-type conjugated polymers with noncovalently-fused-ring structure for the next-generation OTEs.

Intramolecular Palladium Catalyst Transfer on Benzoheterodiazoles as Acceptor Monomers and Discovery of Catalyst Transfer Inhibitors

Intramolecular catalyst transfer on benzoheterodiazoles was investigated in Suzuki-Miyaura coupling reactions and polymerization reactions with t Bu3 PPd precatalyst. In the coupling reactions of dibromobenzotriazole, dibromobenzoxazole, and dibromobenzothiadiazole with pinacol phenylboronate, the product ratios of monosubstituted product to disubstituted product were 0/100, 27/73, and 89/11, respectively, indicating that the Pd catalyst undergoes intramolecular catalyst transfer on dibromobenzotriazole, whereas intermolecular transfer occurs in part in the case of dibromobenzoxazole and is predominant for dibromobenzothiadiazole. The polycondensation of 1.3 equivalents of dibromobenzotriazole with 1.0 equivalent of para- and meta-phenylenediboronates afforded high-molecular-weight polymer and cyclic polymer, respectively. In the case of dibromobenzoxazole, however, para- and meta-phenylenediboronates afforded moderate-molecular-weight polymer with bromine at both ends and cyclic polymer, respectively. In the case of dibromobenzothiadiazole, they afforded low-molecular-weight polymers with bromine at both ends. Addition of benzothiadiazole derivatives interfered with catalyst transfer in the coupling reactions.

Synthesis and Characterization of the Donor-Acceptor Conjugated Polymer PBDB-T Implementing Group IV Element Germanium

Here, we synthesized and characterized a novel two-dimensional (2D) conjugated electron donor-acceptor (D-A) copolymer (PBDB-T-Ge), wherein the substituent of triethyl germanium was added to the electron donor unit of the polymer. The Turbo-Grignard reaction was used to implement the group IV element into the polymer, resulting in a yield of 86%. This corresponding polymer, PBDB-T-Ge, exhibited a down-shift in the highest occupied molecular orbital (HOMO) level to -5.45 eV while the lowest unoccupied molecular orbital (LUMO) level was -3.64 eV. The peaks in UV-Vis absorption and the PL emission of PBDB-T-Ge were observed at 484 nm and 615 nm, respectively.

Effects of Different Lengths of Oligo (Ethylene Glycol) Side Chains on the Electrochromic and Photovoltaic Properties of Benzothiadiazole-Based Donor-Acceptor Conjugated Polymers

In recent years, donor-acceptor (D-A)-type conjugated polymers have been widely used in the field of organic solar cells (OSCs) and electrochromism (EC). Considering the poor solubility of D-A conjugated polymers, the solvents used in material processing and related device preparation are mostly toxic halogenated solvents, which have become the biggest obstacle to the future commercial process of the OSC and EC field. Herein, we designed and synthesized three novel D-A conjugated polymers, PBDT1-DTBF, PBDT2-DTBF, and PBDT3-DTBF, by introducing polar oligo (ethylene glycol) (OEG) side chains of different lengths in the donor unit benzodithiophene (BDT) as side chain modification. Studies on solubility, optics, electrochemical, photovoltaic and electrochromic properties are conducted, and the influence of the introduction of OEG side chains on its basic properties is also discussed. Studies on solubility and electrochromic properties show unusual trends that need further research. However, since PBDT-DTBF-class polymers and acceptor IT-4F failed to form proper morphology under the low-boiling point solvent THF solvent processing, the photovoltaic performance of prepared devices is not ideal. However, films with THF as processing solvent showed relatively desirable electrochromic properties and films cast from THF display higher CE than CB as the solvent. Therefore, this class of polymers has application feasibility for green solvent processing in the OSC and EC fields. The research provides an idea for the design of green solvent-processable polymer solar cell materials in the future and a meaningful exploration of the application of green solvents in the field of electrochromism.

Efficient side-chain engineering of thieno-imidazole salt-based molecule to boost the optoelectronic attributes of organic solar cells: A DFT approach

This study focused on modeling and density functional theory (DFT) analysis of reference (AI1) and designed structures (AI11-AI15), based on the thieno-imidazole core, in order to create profitable candidates for solar cells. All the optoelectronic properties of the molecular geometries were computed using DFT and time dependent-DFT approaches. The influence of terminal acceptors on the bandgaps, absorption, hole and electron mobilities, charge transfer capabilities, fill factor, dipole moment, etc. Of the recently designed structures (AI11-AI15), as well as reference (AI1), were evaluated. Optoelectronics and chemical parameters of newly architecture geometries were shown to be superior to the cited molecule. The FMOs and DOS graphs also demonstrated that the linked acceptors remarkably improved the dispersion of charge density in the geometries under study, particularly in AI11 and AI14. Calculated values of binding energy and chemical potential confirmed the thermal stability of the molecules. All the derived geometries surpassed the AI1 (Reference) molecule in terms of maximum absorbance ranging from 492 to 532 nm (in chlorobenzene solvent) and a narrower bandgap ranging from 1.76 to 1.99eV. AI15 had the lowest exciton dissociation energy of 0.22eV as well as lowest electrons and hole dissociation energies, while AI11 and AI14 showed highest VOC, fill factor, power conversion efficiency (PCE), IP and EA (owing to presence of strong electron pulling cyano (CN) moieties at their acceptor portions and extended conjugation) than all the examined molecules, implying that they could be used to build elite solar cells with enhanced photovoltaic attributes.

Preparation and Electrochromic Properties of Benzodithiophene-Isoindigo Conjugated Polymers with Oligoethylene Glycol Side Chains

Functional polymers featuring good processability in non-halogenated, benzene-free green solvents are highly desired due to health and environmental concerns. Herein, a series of novel D-A type conjugated polymers, PBDT-IIDs, are designed and successfully prepared by "green" functionalization of the polymers with highly hydrophilic, highly polar, highly flexible, and biocompatible oligoethylene glycol (OEG) side chains in order to improve the processability. These series polymers are named PBDT-IID2, PBDT-IID3, and PBDT-IID4, respectively, according to the number of oxygen atoms in the side chain. After confirmation by structural characterization, the basic properties of PBDT-IIDs are also investigated. With the increase in the OEG side chain length, the polymer PBDT-IID4 not only has good solubility in the halogen solvent chlorobenzene, but also exhibits excellent solubility in the green halogen-free solvent methyltetrahydrofuran (Me-THF). As a result, the green solvent Me-THF can also be applied to prepare PBDT-IIDs' electrochromic active layers, except for chlorobenzene and toluene. The electrochromism of PBDT IIDs under both positive and negative voltages has a practical application potential. The several controllable switches between dark green and khaki (0-0.6 V) are expected to show great potential in the field of military camouflage. Furthermore, according to the principle of red, green, and blue (RGB) mixing, light blue-green in the reduced state (-1.6 V) can be used in the preparation of complementary ECDs to provide one of the three primary colors (green).

Fluorination effects on bithiophene unit in benzodithiophene-4,8-dione based D-A type alternating copolymers for highly efficient polymer solar cells

In this study, two D-A polymers consisting of benzodithiophene-4,8-dione and tetrathiophene with or without fluorination were synthesized to reveal the photovoltaic properties of fluorination effect on the polymer backbone. Polymer PDFTB with two fluorine atoms substituted on the backbone exhibits an enhanced π-π stacking effect, deeper HOMO energy level and better backbone planarity than PDTB without fluorine atom substitution. Devices based on PDFTB:ITIC show a power conversion efficiency of 4.39%, which is 15% higher than that of PDTB-based devices due to the higher hole mobility, optimized surface morphology and homogeneous phase separation of the active layer. These results suggest that the fluorination strategy is a facile way to design polymeric donors for solvent-processed polymer solar cells.

Photophysical characterization and fluorescence cell imaging applications of 4-N-substituted benzothiadiazoles

In this work, a series of fluorescent 2,1,3-benzothiadiazole derivatives with various N-substituents in the 4-position was synthesized and photophysically characterized in various solvents. Three compounds emerged as excellent fluorescent probes for imaging lipid droplets in cancer cells. A correlation between their high lipophilicity and lipid droplet specificity could be found, with log P ≥ 4 being characteristic for lipid droplet accumulation.

New medium bandgap donor D-A1 -D-A2 type Copolymers Based on Anthra[1,2-b: 4,3-b":6,7-c"'] Trithiophene-8,12-dione Groups for High -Efficient non -fullerene Polymer Solar Cells

We synthesized a new acceptor unit anthra[1,2-b: 4,3-b': 6,7-c'']trithiophene-8,12-dione (А3Т) (A2) and then used it to design D-A₁ -D-A₂ medium bandgap donor copolymers with same thiophene (D) and A2 units but different A1 i.e., fluorinated benzothiadiazole (F-BTz) and benzothiadiazole (BTz) denoted as P130 and P131, respectively. Their detailed optical and electrochemical properties were examined. The copolymers show good solubility in common organic solvents, broad absorption in the visible spectral region from 300 nm to 700 nm, and deeper HOMO levels of -5.45 and -5.34 eV for P130 and P131, respectively. Finally, an optimized polymer solar cell based on P131 as the donor and narrow bandgap non-fullerene small molecule acceptor Y6 demonstrated a PCE of more than 11.13%. To further improve the efficiency of the non-fullerene PSC, we optimized the P130 by introducing a fluorine atom into the BTz unit, F-BTz acceptor unit, PCE PSC based on P130: Y6 active layer increased to more than 15.28 %, which is higher than that for non-fluorinated analog P131:Y6. The increase in the PCE for former PSC is attributed to the more crystalline nature and compact π-π stacking distance, leading to more balanced charge transport and reduced charge recombination. These remarkable results demonstrate that A3T-based copolymer P130 with F-BTz as the second acceptor is a promising donor material for high-performance PSCs. This article is protected by copyright. All rights reserved.

Controlling Energy Gaps of π-Conjugated Polymers by Multi-Fluorinated Boron-Fused Azobenzene Acceptors for Highly Efficient Near-Infrared Emission

We demonstrate that multi-fluorinated boron-fused azobenzene (BAz) complexes can work as a strong electron acceptor in electron donor-acceptor (D-A) type π-conjugated polymers. Position-dependent substitution effects were revealed, and the energy level of the lowest unoccupied molecular orbital (LUMO) was critically decreased by fluorination. As a result, the obtained polymers showed near-infrared (NIR) emission (λ_PL =758-847 nm) with high absolute photoluminescence quantum yield (Φ_PL =7-23%) originating from low-lying LUMO energy levels of the BAz moieties (-3.94 to -4.25 eV). Owing to inherent solid-state emissive properties of the BAz units, deeper NIR emission (λ_PL =852980 nm) was detected in film state. Clear solvent effects prove that the NIR emission is from a charge transfer state originating from a strong D-A interaction. The effects of fluorination on the frontier orbitals are well understandable and predictable by theoretical calculation with density functional theory. This study demonstrates the effectiveness of fluorination to the BAz units for producing a strong electron-accepting unit through fine-tuning of energy gaps, which can be the promising strategy for designing NIR absorptive and emissive materials.

Abdullah S, Brza MA, Abdulwahid RT. Influence of Fluorine Substitution on the Optical, Thermal, Electrochemical and Structural Properties of Carbazole-Benzothiadiazole Dicarboxylic Imide Alternate Copolymers

In this work four novel donor-acceptor copolymers, PCDTBTDI-DMO, PCDTBTDI-8, P2F-CDTBTDI-DMO and P2F-CDTBTDI-8, were designed and synthesised via Suzuki polymerisation. The first two copolymers consist of 2,7-carbazole flanked by thienyl moieties as the electron donor unit and benzothiadiazole dicarboxylic imide (BTDI) as electron acceptor units. In the structures of P2F-CDTBTDI-DMO and P2F-CDTBTDI-8 copolymers, two fluorine atoms were incorporated at 3,6-positions of 2,7-carbazole to investigate the impact of fluorine upon the optoelectronic, structural and thermal properties of the resulting polymers. P2F-CDTBTDI-8 possesses the highest number average molecular weight (Mn = 24,200 g mol-1) among all the polymers synthesised. PCDTBTDI-DMO and PCDTBTDI-8 show identical optical band gaps of 1.76 eV. However, the optical band gaps of fluorinated copolymers are slightly higher than non-fluorinated counterparts. All polymers have deep-lying highest occupied molecular orbital (HOMO) levels. Changing the alkyl chain substituents on BTDI moieties from linear n-octyl to branched 3,7-dimethyloctyl groups as well as substituting the two hydrogen atoms at 3,6-positions of carbazole unit by fluorine atoms has negligible impact on the HOMO levels of the polymers. Similarly, the lowest unoccupied molecular orbital (LUMO) energy levels are almost comparable for all polymers. Thermogravimetric analysis (TGA) has shown that all polymers have good thermal stability and also confirmed that the fluorinated copolymers have higher thermal stability relative to those non-fluorinated analogues. Powder X-ray diffraction (XRD) studies proved that all polymers have an amorphous nature in the solid state.

Design and Synthesis of Annulated Benzothiadiazoles via Dithiolate Formation for Ambipolar Organic Semiconductors

Substituted 2,1,3-benzothiadiazole (BTD) is a widely used electron acceptor unit for functional organic semiconductors. Difluorination or annulation on the 5,6-position of the benzene ring is among the most adapted chemical modifications to tune the electronic properties, though each sees its own limitations in regulating the frontier orbital levels. Herein, a hitherto unreported 5,6-annulated BTD acceptor, denoted as ssBTD, is designed and synthesized by incorporating an electron-withdrawing 2-(1,3-dithiol-2-ylidene)malononitrile moiety via aromatic nucleophilic substitution of the 5,6-difluoroBTD (ffBTD) precursor. Unlike the other reported BTD annulation strategies, this modification leads to the simultaneous decrease in both frontier orbital energies, a welcoming feature for photovoltaic applications. Incorporation of ssBTD into conjugated polymers results in materials boasting broad light absorption, dramatic solvatochromic and thermochromic responses (>100 nm shift and a band gap difference of ∼0.28 eV), and improved crystallinity in the solid state. Such physical properties are in accordance with the combined electron-withdrawing effect and significantly increased polarity associated with the ssBTD unit, as revealed by detailed theoretical studies. Furthermore, the thiolated ssBTD imbues the polymer with ambipolar charge transport property, in contrast to the ffBTD-based polymer, which transports holes only. While the low mobilities (10^-4 to 10^-5 cm² V^-1 s^-1) could be further optimized, detailed studies validate that the thioannulated BTD is a versatile electron-accepting unit for the design of functional stimuli-responsive optoelectronic materials.

In silico prediction of annihilators for triplet-triplet annihilation upconversion via auxiliary-field quantum Monte Carlo

The energy of the lowest-lying triplet state (T1) relative to the ground and first-excited singlet states (S0, S1) plays a critical role in optical multiexcitonic processes of organic chromophores. Focusing on triplet-triplet annihilation (TTA) upconversion, the S0 to T1 energy gap, known as the triplet energy, is difficult to measure experimentally for most molecules of interest. Ab initio predictions can provide a useful alternative, however low-scaling electronic structure methods such as the Kohn-Sham and time-dependent variants of Density Functional Theory (DFT) rely heavily on the fraction of exact exchange chosen for a given functional, and tend to be unreliable when strong electronic correlation is present. Here, we use auxiliary-field quantum Monte Carlo (AFQMC), a scalable electronic structure method capable of accurately describing even strongly correlated molecules, to predict the triplet energies for a series of candidate annihilators for TTA upconversion, including 9,10 substituted anthracenes and substituted benzothiadiazole (BTD) and benzoselenodiazole (BSeD) compounds. We compare our results to predictions from a number of commonly used DFT functionals, as well as DLPNO-CCSD(T0), a localized approximation to coupled cluster with singles, doubles, and perturbative triples. Together with S1 estimates from absorption/emission spectra, which are well-reproduced by TD-DFT calculations employing the range-corrected hybrid functional CAM-B3LYP, we provide predictions regarding the thermodynamic feasibility of upconversion by requiring (a) the measured T1 of the sensitizer exceeds that of the calculated T1 of the candidate annihilator, and (b) twice the T1 of the annihilator exceeds its S1 energetic value. We demonstrate a successful example of in silico discovery of a novel annihilator, phenyl-substituted BTD, and present experimental validation via low temperature phosphorescence and the presence of upconverted blue light emission when coupled to a platinum octaethylporphyrin (PtOEP) sensitizer. The BTD framework thus represents a new class of annihilators for TTA upconversion. Its chemical functionalization, guided by the computational tools utilized herein, provides a promising route towards high energy (violet to near-UV) emission.

Preparation and Characterization of Quinoxaline-Pyrene-Based Conjugated Copolymers for Organic Photovoltaic Devices

In this study, two novel conjugated polymers, poly(4,5,9,10-tetrakis((2-ethylhexyl)oxy]pyrene-alt-2,3-bis(3-(octyloxy)phenyl)-5,8-di(2-thienyl)-6,7-difluoroquinoxaline) (PPyQxff) and poly(4,5,9,10-tetrakis((2-ethylhexyl)oxy)pyren-alt-2,3-bis(3-(octyloxy)phenyl)-5,8-di(2-thienyl)quinoxaline) (PPyQx), consisting of quinoxaline units with and without fluorine substituents, as electron-accepting moieties and pyrene flanked with dithienyl units as electron-donating moieties were prepared via Stille polymerization reactions for use as electron donor materials in bulk heterojunction (BHJ) solar cells. PPyQxff and PPyQx were characterized by X-ray powder diffraction (XRD), gel permeation chromatography (GPC), thermogravimetric analysis (TGA), cyclic voltammetry (CV), UV−VIS absorption, and nuclear magnetic resonance (NMR) spectroscopy. PPyQxff and PPyQx revealed excellent solution processability in common organic solvents. PPyQxff and PPyQx presented decomposition temperatures above 300 °C. The inclusion of F atoms to the quinoxaline moiety made a slight reduction in the highest occupied molecular orbital (HOMO) level, relative to the unfluorinated polymer, but had no impact on the lowest unoccupied molecular orbital (LUMO) level. PPyQxff and PPyQx exhibited similar physical properties with strong and broad absorbance from 400 to 700 nm and an optical band-gap energy of 1.77 eV. The X-ray powder diffraction study indicated that PPyQxff possessed a reduced π–π stacking distance relative to PPyQx.

Single-Junction Polymer Solar Cells with 16.35% Efficiency Enabled by a Platinum(II) Complexation Strategy

A new strategy of platinum(II) complexation is developed to regulate the crystallinity and molecular packing of polynitrogen heterocyclic polymers, optimize the morphology of the active blends, and improve the efficiency of the resulting nonfullerene polymer solar cells (NF-PSCs). The newly designed s-tetrazine (s-TZ)-containing copolymer of PSFTZ (4,8-bis(5-((2-butyloctyl)thio)-4-fluorothiophen-2-yl)benzo[1,2-b:4,5-b']dithiophene-alt-3,6-bis(4-octylthiophen-2-yl)-1,2,4,5-tetrazine) has a strong aggregation property, which results in serious phase separation and large domains when blending with Y6 ((2,2'-((2Z,2'Z)-((12,13-bis(2-ethylhexyl)-3,9-diundecyl-12,13-dihydro-[1,2,5]thiadiazolo[3,4-e]thieno[2″,3″:4',5']thieno[2',3':4,5]pyrrolo[3,2-g]thieno[2',3':4,5]thieno[3,2-b]indole-2,10-diyl)bis(methanylylidene))bis(5,6-difluoro-3-oxo-2,3-dihydro-1H-indene-2,1-diylidene))dimalononitrile)), and produces a power-conversion efficiency (PCE) of 13.03%. By adding small amount of Pt(Ph)₂ (DMSO)₂ (Ph, phenyl and DMSO, dimethyl sulfoxide), platinum(II) complexation would occur between Pt(Ph)₂ (DMSO)₂ and PSFTZ. The bulky benzene ring on the platinum(II) complex increases the steric hindrance along the polymer main chain, inhibits the polymer aggregation strength, regulates the phase separation, optimizes the morphology, and thus improves the efficiency to 16.35% in the resulting devices. 16.35% is the highest efficiency for single-junction PSCs reported so far.

Synthesis of Bithiophene-Based D-A₁-D-A₂ Terpolymers with Different A₂ Moieties for Polymer Solar Cells via Direct Arylation

A series of bithiophene (2T)-based D-A₁-D-A₂ terpolymers with different A₂ moieties were prepared via direct arylation reaction. In these terpolymers, pyrrolo[3,4-c]pyrrole-1,4-dione (DPP) was selected as the first electron-accepting (A₁) moiety, 2,1,3-benzothiadiazole (BT) or fluorinated benzothiadiazole (FBT) or octyl-thieno[3,4-c]pyrrole-4,6-dione (TPD) or 2,1,3-benzoselendiazole (SeT) was selected as the second electron-accepting (A₂) moiety, while bithiophene with hexyl side chain was used as the electron-donating moiety. The UV-vis absorption, electrochemical properties, blend film morphology, and photovoltaic properties were studied to explore the effects of the A₂ moiety. It is shown that these terpolymer films exhibit broad absorption (350⁻1000 nm), full width at half-maximum of more than 265 nm and ordered molecular packing. Varying the A₂ moiety could affect the energy levels and blend film morphology leading to different polymer solar cell (PSC) performances of these (2T)-based D-A₁-D-A₂ terpolymers. As a result, the highest Jsc of 10.70 mA/cm² is achieved for Polymer 1 (P1) with BT as A₂ moiety, while the higher highest occupied molecular orbital (HOMO) level limits the open circuit voltage (Voc) and leads to a power conversion efficiency (PCE) of 3.46%.

Synthesis of indacenodithienothiophene-based conjugated polymers containing electron-donating/accepting comonomers and their phototransistor characteristics

IDTT-based conjugated polymers with electron-accepting comonomers exhibit higher hole mobility (10-fold) and photoresponsivity (2-fold) than those with electron-donating comonomers.

Functionalizing benzothiadiazole with non-conjugating ester groups as side chains in a donor–acceptor polymer improves solar cell performance

Side chain ester substitution on donor–acceptor based conjugated polymers used as solar harvesters in a bulk-heterojunction (BHJ) polymer solar cell (PSC) can improve harvesting properties, phase separation in the active layer and PSC performance.

New Fused Pyrrolopyridine-Based Copolymers for Organic Solar Cell

A fused pyrrolopyridine core having substituents on the nitrogen atom instead of the carbon atom of the indoloindole unit is developed as a new donor unit for organic electronics. The new donor-acceptor copolymers, PDHPHBT, PDHPFBT, and PDHP2FBT, are synthesized using the new donor unit, well-known benzothiadiazole derivatives containing fluorine atoms as the acceptor. The thermal, optical, and electrochemical properties of these novel copolymers are reported. A solar cell using PDHPFBT with diphenyl ether has an open-circuit voltage, short-circuit current, fill factor, and power conversion efficiency of 0.86 V, 11.32 mA cm^-2 , 0.59%, and 5.68%, respectively, under AM 1.5G illumination (100 mW cm^-2 ) in the absence of annealing.

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.

Effects of fluorination and thermal annealing on charge recombination processes in polymer bulk-heterojunction solar cells

We investigate the effect of fluorination on the photovoltaic properties of an alternating conjugated polymer composed of 4,8-di-2-thienylbenzo[1,2-b:4,5-b']dithiophene (BDT) and 4,7-bis([2,2'-bithiophen]-5-yl)-benzo-2-1-3-thiadiazole (4TBT) units in bulk-heterojunction solar cells. The unsubstituted and fluorinated polymers afford very similar open-circuit voltages and fill factor values, but the fluorinated polymer performed better due to enhanced aggregation which provides a higher photocurrent. The photovoltaic performance of both materials improved upon thermal annealing at 150-200 °C as a result of a significantly increased fill factor and open-circuit voltage, counteracted by a slight loss in photocurrent. Detailed studies of the morphology, light intensity dependence, external quantum efficiency and electroluminescence allowed the exploration of the effects of fluorination and thermal annealing on the charge recombination and the nature of the donor-acceptor interfacial charge transfer states in these films.

Organic Solar Cell Materials toward Commercialization

Bulk-heterojunction organic solar cells (OSCs) have received considerable attention with significant progress recently and offer a promising outlook for portable energy resources and building-integrated photovoltaics in the future. Now, it is urgent to promote the research of OSCs toward their commercialization. For the commercial application of OSCs, it is of great importance to develop high performance, high stability, and low cost photovoltaic materials. In this review, a comprehensive overview of the fundamental requirements of photoactive layer materials and interface layer materials toward commercialization is provided, mainly focusing on high performance, green manufacturing, simplifying device fabrication processes, stability, and cost issues. Furthermore, the perspectives and opportunities for this emerging field of materials science and engineering are also discussed.

Efficient Non-fullerene Organic Solar Cells Enabled by Sequential Fluorination of Small-Molecule Electron Acceptors

Three small-molecule non-fullerene electron acceptors containing different numbers of fluorine atoms in their end groups were designed and synthesized. All three acceptors were found to exhibit relatively narrow band gaps with absorption profiles extending into the near-infrared region. The fluorinated analog exhibited enhanced light-harvesting capabilities, which led to improved short-circuit current densities. Moreover, fluorination improved the blend film morphology and led to desirable phase separation that facilitated exciton dissociation and charge transport. As a result of these advantages, organic solar cells based on the non-fullerene acceptors exhibited clearly improved short-circuit current densities and power conversion efficiencies compared with the device based on the non-fluorinated acceptor. These results suggest that fluorination can be an effective approach for the molecular design of non-fullerene acceptors with near-infrared absorption for organic solar cells.

Effect of fluorination of naphthalene diimide-benzothiadiazole copolymers on ambipolar behavior in field-effect transistors

Two naphthalene diimide (NDI)-benzothiadiazole (BT) based conjugated polymers with high molecular weight, P1 and P2, were synthesized by introducing F atoms to modulate the electron-donating ability of the BT moiety. 3-Decyl-pentadecyl branched alkyl side chains were employed and expected to improve the molecular organization and device performance. Both polymers have excellent solubility in common organic solvents. UV-vis-NIR absorption and cyclic voltammetry indicate that the maximum absorption wavelength of P2 is blue-shifted and the HOMO energy level of P2 is decreased in comparison with P1. Two dimensional wide angle X-ray scattering of thin films revealed a similar organization of both polymers. A less balanced transport in field-effect transistors with increased electron mobility of 0.258 cm2 V-1 s-1 and lowered hole transport of 2.4 × 10-3 cm2 V-1 s-1 was found for P2. Polymer devices of P1 exhibited a balanced ambipolar transport, with a hole mobility of 0.073 cm2 V-1 s-1 and electron mobility of 0.086 cm2 V-1 s-1.

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.

Synthesis and photovoltaic properties of a non-fullerene acceptor with F-phenylalkoxy as a side chain

A new acceptor–donor–acceptor (A–D–A) non-fullerene acceptor (ITIC-FOR) was synthesized and applied in non-fullerene solar cells with a PCE of up to 7.33%.

Access to small molecule semiconductors via C–H activation for photovoltaic applications

Reporting ruthenium carboxylate-catalysed single step oxidative cross coupling that challenges the conventional Stille and Suzuki coupling reactions, to afford BT and MFBT derivatives.

Investigations into the flexibility of the 3D structure and rigid backbone of quinoline by fluorine addition to enhance its blue emission

Addition of fluorine to the quinoline structure was found to decrease its intermolecular interactions and influence its 3D structure.