Lower HOMO Energy Levels Research Articles

Evidence of the Lewis-Amphoteric Character of Tris(pentafluoroethyl)silanide, [Si(C2 F5 )3

According to a first view on the geometrical and electronic structure of the tris(pentafluoroethyl)silanide, this anion appears as a Lewis base. Quantum chemical calculations on perfluoroalkylated silanides show significantly lower HOMO and LUMO energy levels in comparison to their non‐fluorinated counterparts, which implies reduced Lewis basicity and increased Lewis acidity of the [Si(C2F5)3]− ion. With these findings and a HOMO–LUMO gap of 4.80 eV similar to N‐heterocyclic silylenes (NHSis), perfluoroalkyl silanides are predestined to exhibit Lewis‐amphoteric character similar to silylenes. Deprotonation of Si(C2F5)3H with sterically demanding phosphazene bases afforded thermally stable phosphazenium salts of the [Si(C2F5)3]− anion, which add to benzaldehyde, benzophenone, CS2, and CO2 in various manners. This behavior also mirrors the reactivity of silylenes towards ketones as well as heterocumulenes and is rationalized by Lewis amphotericity being inherent in these silanides.

Naphthalenothiophene imide-based polymer exhibiting over 17% efficiency

Summary Developing novel electron-deficient monomers is crucially important for the future development of non-fullerene organic solar cells. Here, we report a new electron-deficient monomer naphthalenothiophene imide (NTI) and NTI-based copolymers PNTB and PNTB-2T. The strong electron-withdrawing ability of NTI could effectively lower the HOMO level of the polymer that facilitates the tuning of the polymer’s physical properties while simultaneously maintaining the low-lying HOMO value. Polymer chains modification by introducing extra thiophenes results in the closed π−π stacking and ordered polymer chain packing of PNTB-2T and is proven to be an effective strategy to increase the device performance. Thus, solar cell devices for PNTB-2T:Y6 exhibit an efficiency of 16.72%, which is further enhanced to 17.35% by the third component PC71BM, whereas that for PNTB:Y6 is only 3.81%. Importantly, PNTB-2T-based devices exhibit excellent batch-to-batch reproducibility. These features make this family of polymer donors very promising and the design strategies will enlighten future development of high-performance polymer donors for organic solar cells (OSC).

Non-fullerene electron acceptors constructed by simple electron-withdrawing core: Distinct effect of bithiazole vs thiazolothiazole core on photovoltaic properties

Developing simple-structure and high-efficiency non-fullerene small molecule acceptors (NF-SMAs) are crucial for application of organic solar cells (OSCs). To deeply understand the effect of electron-withdrawing central building block on photo-electronic properties and better design simple-structure NF-SMAs, in this work, a novel simple SMA of BTz1 with an A (π-A′-A″)2 framework was synthesized and characterized, in which a unfused-ring weak acceptor (A) of bithiazole and two adjacent strong acceptors of A′-A″ are used as molecular central and terminal units, respectively. And the UV absorption, energy level, crystallinity, mobility, morphology and photovoltaic per-formance of BTz1 were studied systematically. It is found that BTz1 exhibits similar molecular conformations and photophysical properties, but a low HOMO and high LUMO energy level in comparison with its analogue of TTz1 based on thiazolothiazole center. Furthermore, significant different photovoltaic properties are observed in the BTz1 and TTz1 based PSCs using polymer J71 as donor in an inverted device structure. The BTz1-based OSCs exhibit an ascending open-circuit voltage (Voc) of 0.96 V, but TTz1-based devices display a significantly increasing power conversion efficiency (PCE) of 8.77%. The better film morphology of photoactive layer makes the TTz1-based OSCs exhibit higher PCE than the BTz1-based OSCs. This work indicates that tuning electron-withdrawing central structure is crucial to construct simple-structure and high-efficiency NF-SMAs.

Effect of main and side chain chlorination on the photovoltaic properties of benzodithiophene-alt-benzotriazole polymers

Introduction of chlorine in the conjugated side chains significantly improves open-circuit voltage and power conversion efficiency, benefiting from a lower HOMO energy level, well-balanced charge transport and superior nanoscale morphology.

Chain Engineering of Benzodifuran-Based Wide-Bandgap Polymers for Efficient Non-Fullerene Polymer Solar Cells.

Two novel benzo[1,2-b:4,5-b' ]difuran (BDF)-based wide-bandgap polymers, PBDFT-FBz and PBDFF-FBz, featuring a difluorobenzotriazole (FBz) acceptor unit, are designed and synthesized. The first attempt through main-chain engineering to alter thiophene units to furan units in the main chain of PBDFT-FBz, and further side-chain engineering eliminate the 2-ethylthiophenyl side chains of PBDFT-FBz by 2-ethylfuryl side chains to generate the "all-furan" polymer PBDFF-FBz. By taking the benefit of the oxygen atom in furan, both PBDFT-FBz and PBDFF-FBz exhibit lower HOMO energy levels and enhanced polymer chain interactions compared to their benzo[1,2-b:4,5-b' ]dithiophene (BDT)-based counterparts. As a result, while applying both polymers in non-fullerene polymer solar cells with non-fullerene acceptor m-ITIC, both devices exhibit highly promising photovoltaic performance. The power conversion efficiency (PCE) in the PBDFT-FBz device reaches 7.57% with increased open circuit voltage (Voc ) and fill factor (FF) compared to the PCE of 5.98% in its BDT counterpart (J52). A further increased PCE is obtained (8.79%) in the PBDFF-FBz:m-ITIC device, which shows ≈47% enhancement in device performance compared to that of J52. The large increase in photovoltaic performance is attributed to the lower-lying HOMO energy levels and better chain interactions in these BDF-based polymers.

Synthesis and characterization of heterocyclic conjugated polymers containing planar benzo[c]cinnoline and tetraazapyrene structures for organic field-effect transistor application

Abstract Heterocyclic conjugated polymers BZCTVT and TAPTVT containing benzo[c]cinnoline (BZC) and tetraazapyrene (TAP), respectively, were prepared by using Stille coupling polymerization in microwave reactor. BZC and TAP act as acceptors and thiophene-vinylene-thiophene (TVT) acts as donors. Alkylated thiophene is the linker between acceptor and donor. This is the first time that tetraazapyrene was incorporated into conjugated polymers. The films of BZCTVT and TAPTVT showed maximum absorption wavelength at 522 and 534 nm, respectively. From cyclic voltammertry measurement, TAPTVT and BZCTVT exhibited low LUMO (−3.61 and −3.47 eV) and HOMO (−5.64 and −5.63 eV) energy levels. Even though BZC and TAP were planar structures, BZCTVT and TAPTVT showed large dihedral angles between alkylated thiophene and TVT (10.8–22.6°), and between alkylated thiophene and BZC and TAP (22.2–30.3°) based on their simulated energy-minimized geometry. The bottom-gate, top-contact OFETs based on TAPTVT and BZCTVT exhibited only p-channel behavior with hole mobility of 4.63 × 10−4 and 1.04 × 10−4 cm2 V−1 s−1 and current on/off ratios greater than 104 after thermal annealing. Grazing incidence X-ray diffraction (GIXRD) patterns suggested that thin films of TAPTVT and BZCTVT preferred edge-on orientation.

Synergistic Effect of Fluorine Substitution and Thio-Alkylation on Photovoltaic Performances of Alternating Conjugated Polymers Based on Alkylthio-Substituted Benzothiadiazole-Quaterthiophene

Three low bandgap polymers (PSDTBT-DFDT, PSDTfBT-DFDT, and PSDTffBT-DFDT) based on fluorine and alkylthio-substituted benzothiadiazole-quaterthiophene alternating units are synthesized and applied in polymer solar cells. The effects of fluorination and thio-alkylation on photophysical, electrochemical, and photovoltaic properties of the polymers are comparatively investigated. The results indicate that PSDTfBT-DFDT and PSDTffBT-DFDT show better light-harvesting abilities, lower HOMO energy levels, and better morphology than that of PSDTBT-DFDT. Bulk heterojunction (BHJ) organic photovoltaic devices based on the three polymers as donor materials were fabricated. The power conversion efficiencies (PCE) of 6.94% for PSDTfBT-DFDT and 7.50% for PSDTffBT-DFDT were attained, respectively, both of which are higher than that of PSDTBT-DFDT (4.48%) due to the increase in the numbers of fluorine atoms. This work demonstrates that both fluorine substitution and thio-alkylation are efficient to improve photovoltaic pe...

Chlorine substituted 2D-conjugated polymer for high-performance polymer solar cells with 13.1% efficiency via toluene processing

In the past few years, fluorine atom has been widely introduced into organic semiconductor (OS) materials to improve the photovoltaic performance of polymer solar cells (PSCs). In contrast, chlorine atom is rarely concerned, although it is also a halogen element and is more easily introduced into OS materials. Herein, we designed and synthesized a new D-A-type two-dimension (2D)-conjugated polymer, PM7, containing a chlorinated-thienyl benzodithiophene (BDT-2Cl) donor unit and a benzodithiophene-4,8-dione acceptor unit. Compared to the control polymer PBDB-T without chlorine substitution, PM7 shows lower HOMO energy level, higher absorption coefficient, enhanced crystallinity and higher carrier mobility. Moreover, the toluene-processed PSCs based on PM7 as donor and small molecule n-OS IT-4F as acceptor achieved a high power conversion efficiency (PCE) of 13.1% with high open-circuit voltage (Voc) of 0.88 V, short-circuit current density (Jsc) of 20.9 mA cm−2 and fill factor (FF) of 71.1%, while the PBDB-T:IT-4F-based PSC only exhibited a low PCE of 5.8% with low Voc of 0.67 V, Jsc of 15.0 mA cm−2 and FF of 57.6%. The PCE of 13.1% is among the highest values reported for the PSCs to date. These results indicate that the chlorine substitution is a simple and effective strategy to design high-performance conjugated polymer photovoltaic materials.

A Simple but Efficient Small Molecule with a High Open Circuit Voltage of 1.07 V in Solution‐Processable Organic Solar Cells

AbstractAcceptor–donor–acceptor (A–D–A) is an effective molecular design strategy for organic small molecules. In this work, we reported a simple A–D–A small molecule (TBDT‐2HT‐ID), with thiophene‐substituted benzodithiophene (TBDT) as a core, 1H‐indene‐1,3(2H)‐dione (ID) as end‐capped acceptor, and linked by the π‐bridge of 3,6‐dihexylthieno[3,2‐b]thiophene. UV and CV results showed that the molecule exhibited good absorption in the range of 300–700 nm but a very low HOMO energy level (−5.47 eV). Grazing incidence X‐ray diffraction (GIXRD) results demonstrate it has good crystallinity and adopts an edge‐on molecular packing, and consequently its hole mobility could reach 1×10−3 cm2 V‐1S−1. Based on PC71BM as acceptor, TBDT‐2HT‐ID had an excellent power conversion efficiency with 7.4 % without any post‐treatment, and a very high Voc of 1.07 V, which would be one of the highest Voc reported so far.

Diamines as auxiliary ligands for tuning photophysical and electrochemical properties of Ruthenium(II) polypyridyl complexes

The complex with the formula [Ru(bpy)2(2,3-diaminonaphthalene)][PF6] (5) was synthesized and characterized by 1H NMR spectroscopy, mass spectrometry and elemental analysis. A set of previously reported complexes with the formula [Ru(bpy)2(diamine)][PF6]{diamine = 1,2-diaminoethane (2), o-phenylenediamine (3), 1,2-diaminocyclohexane (4) } was synthesized and crystal structures were obtained for complexes 3 and 4. UV–vis absorption spectra of the complexes 2–5 were collected and compared to that of [Ru(bpy)3][PF6]2 (1), showing that the MLCT band is red-shifted upon introducing the diamine ligands in place of bipyridine. Emission spectra, excited-state lifetimes and emission quantum yields were collected at room temperature for the complexes 1–5, showing considerable changes in the photophysical characteristics upon the introductions of the diamine. The emission spectrum of 5 exhibits an intense emission in the far red-NIR region when excited at 510 nm. The cyclic voltammograms of the complexes 1–5 show one oxidation peak between 0.98 V and 1.15 V which is attributed to the Ru(II)/Ru(III) oxidation couple. Calculated HOMO and LUMO energy levels from both electrochemical data and theoretical calculations suggest a lower HOMO energy level for complex 1 than the diamino-containing complexes, presumably due to the stronger ligand field of the bipyridine.

Discotic liquid crystals of transition metal complexes, 55: Novel chlorine-substituted phthalocyanine derivatives showing mesomorphism and low HOMO energy level

We have synthesized a series of novel phthalocyaninato copper(II) (abbreviated as PcCu) compounds, 1,4,8,11,15,18,22,25-octakisalkoxy-2,3,9,10,16,17,23,24-octachloro-phthalocyaninato copper(II) (abbreviated as ([Formula: see text]-C[Formula: see text]O)[Formula: see text]-Cl)[Formula: see text]PcCu (4a–4d): [Formula: see text] 6 (a), 8 (b), 10 (c) and 12 (d)) and, for comparison, another series of PcCu compounds, 1,4,8,11,15,18,22,25-octakisalkoxyphthalocyaninato copper(II) (abbreviated as ([Formula: see text]-C[Formula: see text]O)[Formula: see text]PcCu (1a–1d)). The PcCu derivatives 1a–1d are substituted by alkoxy chains only at the [Formula: see text] positions (1,4,8,11,15,18,22,25). On the other hand, the PcCu derivatives 4a–4d are substituted by alkoxy chains at the [Formula: see text] positions and chlorine atoms at the [Formula: see text] positions (2,3,9,10,16,17,23,24). We have investigated the influence of chlorine atoms substituted at the [Formula: see text] positions of the Pc ring on mesomorphism, spectroscopic and electronic properties for these two series of PcCu derivatives 1 and 4 by using a polarizing optical microscope, DSC, temperature-variable small angle X-ray diffractometer, a UV-Vis spectrophotometer and cyclic voltammetry. Each of the derivatives 1a–1d is crystalline without showing mesomorphism. On the other hand, each of the chlorine-substituted PcCu derivatives 4a–4d shows plural phase transitions, and the longer chain-substituted PcCu derivatives 4c–4d show a rectangular ordered columnar [Col[Formula: see text](P2m)] mesophase. Furthermore, we have revealed that each of the PcCu derivatives 4a–4d shows a Q-band in the near-infrared region and a lower HOMO energy level than conventional phthalocyanine derivatives.

Combining covalent bonding and electrostatic attraction to achieve highly viable species with ultrashort beryllium-beryllium distances: a computational design.

Though ultrashort metal-metal distances (USMMD, dM-M < 1.900 Å) were primarily realized between transition metals, USMMDs between main group metal atoms such as beryllium atoms have also been designed previously using two strategies: (1) formation of multiple bonding orbitals or (2) having favourable electrostatic attraction. We recently turned our attention to the reported species IH → Be2H2 ← IH (where IH denotes imidazol-2-ylidene) because the orbital energy level of its π-type HOMO is noted to be very high, which may result in intrinsic instability. In the present study, we combined the abovementioned strategies to solve the high orbital energy level problem without losing the ultrashort Be-Be distances. It was found that breaking of such π-type HOMO by addition of a -CH2- group onto the bridging position of two beryllium atoms led to the formation of IH → Be2H2CH2 ← IH species, which not only possesses an ultrashort Be-Be distance in the -Be2H2CH2- moiety, but also has a relatively low HOMO energy level. Replacing the IH ligands with NH3 and PH3 resulted in the formation of NH3 → Be2H2CH2 ← NH3 and PH3 → Be2H2CH2 ← PH3 species with similar features. The electronic structure analyses suggest that the ultrashort Be-Be distances in these species are achieved by the combined effects of the formation of two Be-H-Be 3c-2e bonds and having favourable Coulombic attractions between the carbon atom of the -CH2- group and two beryllium atoms. Remarkably, when the IH, NH3, and PH3 ligands were replaced by large ligands with bulky groups, such as 1,3-bis(2,6-diisopropyl phenyl)imidazol-2-ylidene (IDip), triphenylamine (NPh3), and triphenylphoshpine (PPh3), respectively, the resultant species IDip → Be2H2CH2 ← IDip, NPh3 → Be2H2CH2 ← NPh3, and PPh3 → Be2H2CH2 ← PPh3 exhibit good steric protection around the -Be2H2CH2- core. These species are thus examples for the experimental realization of species with ultrashort metal-metal distances between main group metals.

Significantly Enhancing the Efficiency of a New Light‐Harvesting Polymer with Alkylthio naphthyl Substituents Compared to Their Alkoxyl Analogs

AbstractIn this work, a new benzo[1,2‐b:4,5‐b′]dithiophene (BDT) building block containing alkylthio naphthyl as a side chain is designed and synthesized, and the resulting polymer, namely PBDTNS‐BDD, shows a lower HOMO energy level than that of its alkoxyl naphthyl counterpart PBDTNO‐BDD. An optimized photovoltaic device using PBDTNS‐BDD as a donor exhibits power conversion efficiencies (PCE) of 8.70% and 9.28% with the fullerene derivative PC71BM and the fullerene‐free small molecule ITIC as acceptors, respectively. Surprisingly, ternary blend devices based on PBDTNS‐BDD and two acceptors, namely PC71BM and ITIC, shows a PCE of 11.21%, which is much higher than that of PBDTNO‐BDD based ternary devices (7.85%) even under optimized conditions.

New naphtho[1,2-b:5,6-b′]difuran based two-dimensional conjugated small molecules for photovoltaic application

Two new A–D–A small molecules with alkoxyphenyl and alkylthiophenyl-substituted naphtho[1,2-b:5,6-b′]difuran (NDF) as the central building block named NDFPO-DPP and NDFPS-DPP were synthesized and firstly used as donor materials in organic solar cells (OSCs). The effects of the alkoxyphenyl and alkylthiophenyl side chains on the NDF unit have been investigated. With a single atom variation from O to S, NDFPS-DPP exhibited lower HOMO energy levels than its counterpart NDFPO-DPP, which resulted in enhanced Voc. The device based on NDFPO-DPP with thermal annealing exhibited a better PCE of 3.10% due to the higher and more balanced hole and electron mobilities. The investigations show that NDF could be a promising building block in OSCs via rational molecular structure design and device optimizations.

Novel biphenylene-diketopyrrolopyrrole-based A–π–D–π–A molecule: Synthesis, optical, electrochemical and electronical properties

Abstract A novel ethynyl-linked A–π–D–π–A structured molecule DPP 2 Ph 2 composed of biphenylene and diketopyrrolopyrrole has been designed and synthesized. A detailed study was carried out on its optical, electrochemical, electronical, and the photoinduced charge-separation properties by using UV–Vis and fluorescence spectroscopy, cyclic voltammetry, DFT calculations and fluorescence quenching experiment. The optical results display that DPP 2 Ph 2 exhibits strong and broad absorption band covering the whole visible region and has a typical fluorescence of aggregation-caused quenching (ACQ) feature. From the data in electrochemistry and fluorescence quenching experiment, the DPP 2 Ph 2 molecule reveals lower HOMO energy level and proper LUMO energy level to obtain efficient charge separation. These results demonstrated that DPP 2 Ph 2 might be a promising donor material for organic solar cells.

Improving self-assembly behavior and photovoltaic performance of the indacenodithiophene-based small molecules via increasing dipole moment of the terminal group

Two small molecules (SMs) of IDT(BT-T3Cz)2 and IDT(DFBT-T3Cz)2 with a D(A-Ar)2 framework were designed and synthesized as a novel class of photovoltaic donor materials, which contain a donor (D) core of indacenodithiophene (IDT), two acceptor (A) arms of benzothiadiazole (BT) or fluorinated benzothiadiazole (DFBT), and two terminals of 3-carbazole (Cz). The impacts of 3-carbazole and fluorine atom on the optical absorption, electrochemical property, hole mobility, film morphology, and solar cell performance were primarily studied. A broad absorption profile in a range of 350–750 nm and a low HOMO energy level around −5.30 eV were observed for both SMs. In comparison to the reported SM of IDT(BT-Th2)2 with 5-alkyl-thiophene terminal, both SMs with 3-carbazole terminal exhibited stronger molecular packing and better self-assembly behavior owing to the increasing dipole moment of carbazole. Furthermore, the IDT(DFBT-T3Cz)2 showed a higher power conversion efficiency (PCE) of 5.99% with an increasing fill factor (FF) of 59.6% than IDT(BT-T3Cz)2 in their solar cells under the illumination of AM 1.5G at 100 mWcm−2. To the best of our knowledge, this is one of the highest PCE value reported for the IDT based SMs in solution-processed bulk heterojunction solar cells. And this work provides an efficient strategy to improve self-assembly behavior and photovoltaic performance via increasing dipole moment of the terminal groups.

Random copolymers containing tetrafluorophenylene unit with deep HOMO energy levels for solar cell applications

Two Novel terpolymers poly{4,8-di(2-ethylhexyloxyl)benzo[1,2-b:4.5-b']dithiophene-3,6-bis-(thiophen-2-yl)-N,N'-bis(2-octyl-1-dodecyl)-1,4-dioxo-pyrrolo[3,4-c]pyrrole-co-2,6-ditetrafluorophenyl-4,8-di(2-ethylhexyloxyl)benzo[1,2-b:4.5-b']dithiophene} (PBDT-DPP-TFB) are designed and synthesized as donor materials in PSCs by incorporation of tetrafluorophenylene (TFB) unit with different mole ratio of 1% and 5%, namely PBDT-DPP-TFB1% and PBDT-DPP-TFB5%. It is found that through the introduction of small amount tetrafluorophenylene (TFB) into the BDT-DPP backbone, absorbance range, energy levels, the charge carrier mobility and the device performance are significantly manipulated and enhanced. Lower HOMO energy levels are emphasized for PBDT-DPP-TFB1% (−5.39eV) and PBDT-DPP-TFB5% (−5.43eV) compared to pristine PBDT-DPP without TFB unit, due to the strong electron-withdrawing feature of the TFB. Thanks to the more suitable energy levels, and a more favorable morphology with improved excitons dissociation and charge transport in the active layer, the inverted device based on PBDT-DPP-TFB with 1% TFB achieves a dramatic improved PCE of 2.9% with about 200% enhancement to the device with the pristine PBDT-DPP. This work demonstrates the potential of TFB in developing new organic materials for optoelectronic devices, as well as the variation of the third component can be another way to construct D-A polymers with favorable properties.

Positional effects of fluorination in conjugated side chains on photovoltaic properties of donor-acceptor copolymers.

The position at which conjugated side chains were fluorinated, the meta- or ortho-position in phenyl side chains, was varied to investigate the positional effects of fluorination on the energy levels, crystalline ordering, and photovoltaic properties of the polymers. The fluorine in the ortho-position achieved a lower HOMO energy level than that in the meta-position, but reduced the chain rigidity.

D-A copolymers based on lactam acceptor unit and thiophene derivatives for efficient polymer solar cells

Two donor-acceptor (D-A) polymers PTTBDTP and PBTBDTP were developed by copolymerizing lactam acceptor [7,7′-bidithieno[3,2-b:2′,3′-d]pyridine]-5,5′(4H,4′H)-dione (BDTP) with thieno[3,2-b]thiophene (TT) or 2,2′-bithiophene (BT) in good yields. PTTBDTP and PBTBDTP exhibited excellent thermal stability and low HOMO energy levels of −5.52 eV ∼ −5.54 eV. Compared with PTTBDTP, PBTBDTP possessed broader absorption spectra, better crystallinity, and higher hole mobility. Therefore inverted polymer solar cells based on PBTBDTP:PC71BM achieved a higher PCE of 6.86%, with an open-circuit voltage (Voc) of 0.87 V, a short-circuit current (Jsc) of 10.97 mA/cm2 and a fill factor (FF) of 71.85%. Meanwhile, PTTBDTP:PC71BM solar cells gave a PCE of 5.84%. These results have demonstrated that lactam BDTP is a good building block for PSC applications.

Fluorine이 도입된 Quinoxaline과 Fluorene 골격을 가진 고분자의 합성 및 특성분석

New electron deficient moiety, 6,7-difluoro-2,3-dihexylquinoxaline, was developed for the push-pull type copolymer for organic photovoltaics (OPVs). The PFDTQxF with lower HOMO energy level was synthesized using fluorene and 6,7-difluoro-2,3-dihexylquinoxaline by Suzuki polymerization. The PFDTQxF thin film shows two absorption peaks at 368 and 493 nm. The HOMO and LUMO energy levels of PFDTQxF are calculated -5.55 and -3.91 eV, respectively. The device comprising PFDTQxF showed a VOC value of 0.47 V, a JSC value of 4.48 mA/cm, and a FF of 0.32, which yielded PCE of 0.78%, under the illumination of AM 1.5.