Low-lying Lowest Unoccupied Molecular Orbital Research Articles

Single-strand and ladder-type polymeric acceptors based on regioisomerically-pure perylene diimides towards all-polymer solar cells

Since dibromo perylene diimides (DBPDIs) contain two isomers that are difficult to separate, isomeric effect on properties and applications of PDI polymers has been rarely studied. Few ladder-type polymers have been used in organic solar cells. Herein, single-strand polymers (PrPDIT) were synthesized by the polymerization of regioisomerically-pure 1,7-dibromo-PDIs (1,7-DBPDIs) and 2,5-bis(trimethylstannyl)thiophene. Furthermore, ladder-type polymers (FPrPDIT) were obtained by the highly efficient photo-induced cyclization of polymers PrPDIT. The effects of isomers and molecular weight on the absorption, electrochemistry, thermal stability, and photovoltaic application of polymers were studied. Pure 1,7-DBPDIs lead to polymers with molecular weight distribution as low as 1.08. These polymers with strong absorption in 300–700 nm, low-lying lowest unoccupied molecular orbital (LUMO) energy levels around −4.0 eV and high thermal stability could be excellent electron acceptors. Regioisomerically-pure polymers demonstrate better photovoltaic performance than that of mixed ones. The highest power conversion efficiency (PCE) of all-polymer solar cells based on PrPDIT and FPrPDIT is up to 5.95% and 4.52% respectively.

Core-expanded naphthalenediimide derivatives as non-fullerene electron transport materials for inverted perovskite solar cells

Abstract Two core-expanded naphthalenediimide (NDI) derivatives NDI3HU-DTYM2 (1) and (DTYM-NDI-DTYA)2 (2) with coplanar and twisted molecular backbones, respectively, were used as low-temperature solution-processed electron transport materials (ETMs) for CH3NH3PbI3-based inverted planar perovskite solar cells (PSCs). The high electron mobility as well as low-lying lowest unoccupied molecular orbital (LUMO) energy levels and good film-formation ability on perovskite layers make 1 and 2 both promising ETMs for PSCs. The PSCs devices based on 1 show a power conversion efficiency (PCE) of 8.7%, while the PSCs devices based on 2 that has twisted molecular backbone exhibit a PCE value of up to 12.9%, a short-circuit current density (Jsc) value of 22.80 mA cm−2 with negligible hysteresis, which is comparable to that of PCBM-based PSCs devices. Our results demonstrate that core-expanded NDI-based n-type organic semiconductors, especially those with twisted molecular backbones, have great potential for high-performance, low-cost, solution-processed non-fullerene ETMs in PSCs.

Molecular Engineering for Large Open-Circuit Voltage and Low Energy Loss in Around 10% Non-fullerene Organic Photovoltaics

Recent efforts in organic photovoltaics (OPVs) have been devoted to obtaining low-bandgap non-fullerene acceptors (NFAs) for high photocurrent generation. However, the low-lying lowest unoccupied molecular orbital (LUMO) level in narrow bandgap NFAs typically results in a small energy difference (ΔEDA) between the LUMO of the acceptor and the highest occupied molecular orbital (HOMO) of the donor, leading to low open-circuit voltage (VOC). The trade-off between ΔEDA and photocurrent generation significantly limits the simultaneous enhancement of both VOC and short-circuit current density (JSC). Here, we report a new medium-bandgap NFA, IDTT-T, containing a weakly electron-withdrawing N-ethyl thiabarbituric acid terminal group on each end of the indacenodithienothiophene (IDTT) core. When paired with a benchmark low-bandgap PTB7-th polymer donor, simultaneous enhancement of both ΔEDA and absorption spectral coverage was realized. The OPV devices yield a VOC of 1.01 V, corresponding to a low energy loss of ...

P-π Conjugated Polymers Based on Stable Triarylborane with n-Type Behavior in Optoelectronic Devices.

p-π conjugation with embedded heteroatoms offers unique opportunities to tune the electronic structure of conjugated polymers. An approach is presented to form highly electron-deficient p-π conjugated polymers based on triarylboranes, demonstrate their n-type behavior, and explore device applications. By combining alternating [2,4,6-tris(trifluoromethyl)phenyl]di(thien-2-yl)borane (FBDT) and electron-deficient isoindigo (IID)/pyridine-flanked diketopyrrolopyrrole (DPPPy) units, we achieve low-lying lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels, high electron mobilities, and broad absorptions in the visible region. All-polymer solar cells with these polymers as electron acceptors exhibit encouraging photovoltaic performance with power conversion efficiencies of up to 2.83 %. These results unambiguously prove the n-type behavior and demonstrate the photovoltaic applications of p-π conjugated polymers based on triarylborane.

P–π Conjugated Polymers Based on Stable Triarylborane with n‐Type Behavior in Optoelectronic Devices

Abstractp–π conjugation with embedded heteroatoms offers unique opportunities to tune the electronic structure of conjugated polymers. An approach is presented to form highly electron‐deficient p–π conjugated polymers based on triarylboranes, demonstrate their n‐type behavior, and explore device applications. By combining alternating [2,4,6‐tris(trifluoromethyl)phenyl]di(thien‐2‐yl)borane (FBDT) and electron‐deficient isoindigo (IID)/pyridine‐flanked diketopyrrolopyrrole (DPPPy) units, we achieve low‐lying lowest unoccupied molecular orbital (LUMO) and highest occupied molecular orbital (HOMO) energy levels, high electron mobilities, and broad absorptions in the visible region. All‐polymer solar cells with these polymers as electron acceptors exhibit encouraging photovoltaic performance with power conversion efficiencies of up to 2.83 %. These results unambiguously prove the n‐type behavior and demonstrate the photovoltaic applications of p–π conjugated polymers based on triarylborane.

Copolymers of Bis-Diketopyrrolopyrrole and Benzothiadiazole Derivatives for High-Performance Ambipolar Field-Effect Transistors on Flexible Substrates.

We develop an "acceptor dimerization" strategy by a bis-diketopyrrolopyrrole (2DPP) for an ambipolar organic semiconductor. Copolymers of 2DPP and benzothiadiazole (BTz) derivatives, P2DPP-BTz and P2DPP-2FBTz, are designed and synthesized. Both of the polymers exhibit narrow optical bandgaps of ca. 1.30 eV. The strong electron-withdrawing property of 2DPP results in low-lying lowest unoccupied molecular orbital (LUMO) energy levels of the polymers, improving the electron mobilities. 2D grazing incident X-ray diffraction and atomic force microscopy indicate that the P2DPP-BTz exhibits a small π-π stacking distance of 3.59 Å and a smooth interface, thus promoting high mobility. To take full advantage of the flexibility of organic semiconductors, flexible field-effect transistors (FETs) were fabricated on poly(ethylene terephthalate) (PET) substrates. The FETs based on P2DPP-BTz show high performance with hole and electron mobilities of 1.73 and 2.58 cm2 V-1 s-1, respectively. Our results demonstrate that the 2DPP acceptor is a promising building block for high-mobility ambipolar polymers.

Low-band gap copolymers based on diketopyrrolopyrrole and dibenzosilole and their application in organic photovoltaics

We report donor-acceptor copolymers incorporating electron-donating dimethylsilole and electron-withdrawing diketopyrrolopyrrole (DPP) units. To investigate effects of alkyl chain attached to the DPP unit, 2-decyltetradecyl (24-alkyl) group and 7-decylnonadecyl (29-alkyl) with linear space group were introduced to DPP units. Two polymers, P24DPP-Silole and P29DPP-Silole, exhibited low band gap of 1.46 and 1.36 eV, respectively, and low-lying lowest unoccupied molecular orbital (LUMO) energy level of −3.85 eV. Photovoltaic devices employing P24DPP-Silole and P29DPP-Silole exhibited the highest power conversion efficiencies of 4.17% and 4.33%, respectively, with 1-chloronaphthalene (CN) as a processing additive. Morphological analyses were conducted with atomic force microscopy (AFM), transmission electron microscopy (TEM), and grazing incidence wide-angle x-ray scattering (GIWAXS) to investigate the effects of different alkyl chain on DPP units and processing additive. The analyses revealed that the addition of CN led to favorable phase separation and increased the degree of crystallinity in blend films. This study demonstrates that the copolymerization of DPP and dibenzosilole units is an effective means of narrowing the optical band gap. Furthermore, the length of alkyl chains on DPP units did not significantly affect on the photovoltaic performances and bulk heterojunction morphologies.

Highly Efficient Solid-State Near-infrared Organic Light-Emitting Diodes incorporating A-D-A Dyes based on \u03b1,\u03b2-unsubstituted \u201cBODIPY\u201d Moieties

We take advantage of a recent breakthrough in the synthesis of α,β-unfunctionalised 4,4-difluoro-4-bora-3a,4a-diaza-s-indacene (BODIPY) moieties, which we symmetrically conjugate with oligothienyls in an unexpectedly stable form, and produce a “metal-free” A-D-A (acceptor-donor-acceptor) oligomer emitting in the near-infrared (NIR) thanks to delocalisation of the BODIPY low-lying lowest unoccupied molecular orbital (LUMO) over the oligothienyl moieties, as confirmed by density functional theory (DFT). We are able to retain a PL efficiency of 20% in the solid state (vs. 30% in dilute solutions) by incorporating such a dye in a wider gap polyfluorene matrix and demonstrate organic light-emitting diodes (OLEDs) emitting at 720 nm. We achieve external quantum efficiencies (EQEs) up to 1.1%, the highest value achieved so far by a “metal-free” NIR-OLED not intentionally benefitting from triplet-triplet annihilation. Our work demonstrates for the first time the promise of A-D-A type dyes for NIR OLEDs applications thereby paving the way for further optimisation.

A Star-Shaped Molecule with Low-Lying Lowest Unoccupied Molecular Orbital Level, n-Type Panchromatic Electrochromism, and Long-Term Stability.

An electron-deficient star-shaped molecule based on anthraquinone imide was synthesized and characterized. It showed high electron accommodating capacity and strong electron-withdrawing ability with a low-lying lowest unoccupied molecular orbital (LUMO) of -4.10 eV. In addition, it exhibited panchromatic electrochromism attributed to the simultaneous presence of π*-π* transitions and intervalence charge transfer (IV-CT) upon one-electron reduction, and revealed long-term stability in electron gain and loss due to the proper LUMO energy level and ordered intermolecular assembly.

Efficient Nonfullerene Polymer Solar Cells Enabled by a Novel Wide Bandgap Small Molecular Acceptor.

A wide bandgap small molecular acceptor, SFBRCN, containing a 3D spirobifluorene core flaked with a 2,1,3-benzothiadiazole (BT) and end-capped with highly electron-deficient (3-ethylhexyl-4-oxothiazolidine-2-yl)dimalononitrile (RCN) units, has been successfully synthesized as a small molecular acceptor (SMA) for nonfullerene polymer solar cells (PSCs). This SMA exhibits a relatively wide optical bandgap of 2.03 eV, which provides a complementary absorption to commonly used low bandgap donor polymers, such as PTB7-Th. The strong electron-deficient BT and RCN units afford SFBRCN with a low-lying LUMO (lowest unoccupied molecular orbital) level, while the 3D structured spirobifluorene core can effectively suppress the self-aggregation tendency of the SMA, thus yielding a polymer:SMA blend with reasonably small domain size. As the results of such molecular design, SFBRCN enables nonfullerene PSCs with a high efficiency of 10.26%, which is the highest performance reported to date for a large bandgap nonfullerene SMA.

Naphthodithiophene Diimide-Based Copolymers: Ambipolar Semiconductors in Field-Effect Transistors and Electron Acceptors with Near-Infrared Response in Polymer Blend Solar Cells

New π-conjugated copolymers based on naphtho[2,3-b:6,7-b′]dithiophene-4,5,9,10-diimide (NDTI) combined with thiophene, thienothiophene, or dithienothiophene units are synthesized and used in field-effect transistors (FETs) and organic solar cells (OSCs). The low-lying lowest unoccupied molecular orbital (LUMO) and high-lying highest occupied molecular orbital (HOMO) levels of the polymers contribute to reducing injection barriers for both electrons and holes, resulting in ambipolar operation of FET devices. The charge mobilities were strongly affected by the molecular orientation of the copolymers, and the highest electron mobility of 0.26 cm2/(V s) was observed for the copolymer with thienothiophene unit with edge-on orientation. On the other hand, OSCs with PTB7 as the electron donor polymer and the copolymers as the acceptor showed a broad photoresponse extending to the near-IR region, and the highest power conversion efficiency of over 3.5% was obtained for the copolymer with dithienothiophene unit th...

Series of Quinoidal Methyl-Dioxocyano-Pyridine Based π-Extended Narrow-Bandgap Oligomers for Solution-Processed Small-Molecule Organic Solar Cells

Small molecules with narrow bandgap of <1.6 eV can harvest the visible and near-infrared solar photons. In this Article, we report a new method to achieve narrow bandgap small molecule donors by using electron-deficient quinoidal methyl-dioxocyano-pyridine (MDP) to induce possible quinoidal resonance structure along the conjugated A-pi-D-pi-A backbone. Practically, two MDP moieties are covalently linked onto an electron-rich benzodithiophene (BDT) through the oligothiophene (OT-5T) pi-bridge. The affording small molecules, namely, nTBM, exhibit broad and strong absorption bands covering the visible and near-infrared region from 400 to 870 nm. The estimated optical bandgap is down to 1.4 eV. The narrow bandgap is associated with the low-lying lowest unoccupied molecular orbital (LUMO) energy level (about -3.7 eV) and the high-lying highest occupied molecular orbital (HOMO) energy level (around -5.1 eV). Density-functional theory calculations reveal that the HOMO and LUMO energy levels, with the increase of the size of the oligothiophene bridge, become localizations in different moieties, i.e., the central electron-donating and the terminal electron-withdrawing units, respectively, which provides necessary driving force for the delocalization of the excited electrons and formation of the quinoidal resonance structure. The quinoidal structure enhances the photoinduced intramolecular charge-transfer, leading to the absorbance enhancement of the low-energy absorption band. With the increase of the size of the oligothiophene from 0 to 5 thienyl units and the change of the direction of the alkyl chains on the bridged thiophene from outward to inward, the crystalline nature, fibril length, and phase size of the blend films as well as the cell performance are all fine-tuned, also. With the inward alkyl chains, the terthiophene bridged molecule is amorphous, while the pentathiophene bridged one is relatively crystalline. Both molecules form nanoscale interpenetrating networks with a phase size of 15-20 nm when blended with PC71BM, showing the higher hole mobility and promising electric performance.

Thienothiophene‐2,5‐Dione‐Based Donor–Acceptor Polymers: Improved Synthesis and Influence of the Donor Units on Ambipolar Charge Transport Properties

A series of donor–acceptor polymers incorporating the thieno[3,2‐b]thiophene‐2,5‐dione (TTD) acceptor unit and different donor units are synthesized. The synthesis of a TTD‐based key monomer, 3,6‐bis(5‐bromo‐4‐alkylthiophen‐2‐yl)thieno[3,2‐b]thiophene‐2,5‐dione, is successfully improved to afford higher total yield with less reaction steps (42%/4 steps) than those previously reported (14%/7 steps). The polymers exhibit low‐lying lowest unoccupied molecular orbital (LUMO) energy levels of around −3.8 eV and highest occupied molecular orbital (HOMO) energy levels ranging from −5.49 to −5.14 eV. Organic field‐effect transistors based on the polymers exhibit ambipolar characteristics with high hole and electron mobilities in the order of 10−1 cm2 V−1 s−1 in air. These high mobilities can be attributed to the formation of highly crystalline lamellar structure with preferential edge‐on orientation of the polymer thin films. Interestingly, the ratio of the electron to hole mobilities decreases with the extension of the donor units. This can be explained by the distribution of HOMOs and LUMOs along the backbones. Additionally, complementary inverters using the polymers with well‐balanced ambipolar characteristics exhibit sharp switching characteristics with high gain of ≈140 at the supply voltage of 40 V.

Facile conversion of polymer organic thin film transistors from ambipolar and p-type into unipolar n-type using polyethyleneimine (PEI)-modified electrodes

We report here a successful polarity conversion of organic thin film transistors (OTFTs) based on several polymer semiconductors with low-lying LUMO (lowest unoccupied molecular orbital) energy levels (⩽−4eV) from ambipolar and even p-type into unipolar n-type devices using an ultrathin layer (∼2–5nm) of polyethyleneimine (PEI) to modify the source and drain contacts. The work function of gold is substantially reduced with the PEI layer on its surface, which effectively suppresses the injection of holes and thus enables electron-only charge transport of these polymers in OTFTs. This general approach of electrode work function modification broadens the scope of available polymer semiconductors for use in printed electronics where n-channel OTFTs are needed.

Efficient approach to electron-deficient 1,2,7,8-tetraazaperylene derivatives.

Using an efficient synthetic strategy, a novel class of 1,2-diazine-embedded perylenes, namely 1,2,7,8-tetraazaperylene derivatives, have been successfully synthesized. These molecules were fully characterized by X-ray diffraction analysis, optical spectroscopy, and electrochemistry. The low-lying lowest unoccupied molecular orbital (LUMO) level of these molecules suggests their potential as good electronic acceptors.

Bandgap Engineering through Controlled Oxidation of Polythiophenes

AbstractThe use of Rozen’s reagent (HOF⋅CH3CN) to convert polythiophenes to polymers containing thiophene‐1,1‐dioxide (TDO) is described. The oxidation of polythiophenes can be controlled with this potent, yet orthogonal reagent under mild conditions. The oxidation of poly(3‐alkylthiophenes) proceeds at room temperature in a matter of minutes, introducing up to 60 % TDO moieties in the polymer backbone. The resulting polymers have a markedly low‐lying lowest unoccupied molecular orbital (LUMO), consequently exhibiting a small bandgap. This approach demonstrates that modulating the backbone electronic structure of well‐defined polymers, rather than varying the monomers, is an efficient means of tuning the electronic properties of conjugated polymers.

Bandgap Engineering through Controlled Oxidation of Polythiophenes

The use of Rozen's reagent (HOF⋅CH3 CN) to convert polythiophenes to polymers containing thiophene-1,1-dioxide (TDO) is described. The oxidation of polythiophenes can be controlled with this potent, yet orthogonal reagent under mild conditions. The oxidation of poly(3-alkylthiophenes) proceeds at room temperature in a matter of minutes, introducing up to 60 % TDO moieties in the polymer backbone. The resulting polymers have a markedly low-lying lowest unoccupied molecular orbital (LUMO), consequently exhibiting a small bandgap. This approach demonstrates that modulating the backbone electronic structure of well-defined polymers, rather than varying the monomers, is an efficient means of tuning the electronic properties of conjugated polymers.

1,2,5]Thiadiazolo[3,4-f]benzotriazole based narrow band gap conjugated polymers with photocurrent response up to 1.1 μm

Three novel donor–acceptor type of narrow band gap conjugated copolymers were synthesized through a palladium-catalyzed Stille copolymerization based on [1,2,5]thiadiazolo[3,4-f]benzotriazole (TBZ) derivatives as acceptor and 4,8-di(2,3-didecylthiophen-5-yl)-benzo[1,2-b:4,5-b′]dithiophene (BDT) as donor. All resulted copolymers exhibited absorbance up to near-infrared region along with relatively narrow band gap in the range of 0.96–1.10eV. Cyclic voltammetry measurements illustrated that the highest occupied molecular orbital energy levels of copolymers lay in the range of −5.04 to −5.13eV, and lowest unoccupied molecular orbital (LUMO) energy levels were in the range of −4.03 to −4.16eV. Photovoltaic performances were evaluated based on the resulted copolymers as donor and [6,6]-phenyl-C60 butyric acid methyl ester (PC61BM) as acceptor with optimized weight ratio of 1:2. All devices displayed comparatively low power conversion efficiencies in the range of 0.1–0.4% due to the low-lying LUMO energy levels. Broad photocurrent response up to near infrared region of 1.1μm was realized for copolymer P2 that containing thiophene unit as the bridge between BDT and TBZ moieties, indicating that it can be potentially applied for near infrared photodetectors.

Ambipolar Low-bandgap Copolymers Consisting of Dithienoketopyrrole for All-Polymer Solar Cells

The new low-band gap copolymers containing electron-accepting thienoisoindigo (TIDG) exhibit the low-lying lowest unoccupied molecular orbital (LUMO) at -3.8 eV. This envisions their use as n-type polymer in combination with typical p-type regioregular poly(3-hexylthiophene) (P3HT) towards all-polymer organic photovoltaics (OPV). The alternating fluorene and TIDG copolymer, PFTIDG, shows the electronic absorption maximum at 744 nm in the film states, of which spectrum is complementary to that of P3HT and thus advantageous for covering the wide solar spectrum. The laser and Xe-flash time-resolved microwave conductivity (TRMC) evaluations were performed for screening the effects of p/n blend ratio and thermal annealing in bulk heterojunction framework. The power conversion efficiency of 0.06% was obtained for pre-annealed P3HT:PFTIDG = 1:1 together with a high open circuit voltage of 0.92 V, which is suggestive of ambipolar characteristics of PFTIDG.

Synthesis and characterization of low bandgap π-conjugated copolymers incorporating 4,7-bis(3,3′/4,4′-hexylthiophene-2-yl)benzo[c][2,1,3]thiadiazole units for photovoltaic application

4,7-Bis(3,3′/4,4′-hexylthiophene-2-yl)benzo[c][2,1,3]thiadiazoles (HT–BT–HT) were used as building blocks to construct a series of low bandgap π-conjugated copolymers for photovoltaic applications. The desired copolymers were obtained by incorporating the HT–BT–HT comonomers together with donor or acceptor units, such as 3,4-ethylenedioxythiophene (EDOT), bis-EDOT, thieno[3,4-b]pyrazine (TP), and 2,3-dimethyl-TP, via a palladium-catalyzed Stille cross-coupling method. A facile synthetic method has also been developed for the synthesis of several EDOT- and TP-based copolymers via direct C–H arylation of EDOT, bis-EDOT, and TP derivatives using the commercially available catalyst Pd(OAc)2 under Heck-type experimental conditions (Jeffery method). For all of the synthesized copolymers, moving the hexyl side chains of the HT unit in the HT–BT–HT comonomers from 3,3′-positions (close to BT, as in P1–P4) to 4,4′-positions (away from BT, as in P5–P8) led to a significant red shift of the UV-vis absorption spectrum, a decrease of the energy bandgap, an increase of the glass transition temperature, and more promising photovoltaic performances. The thin-film copolymer P7 incorporating TP units (–TP–HT–BT–HT–)n exhibited the most extended absorption (beyond 1000 nm) and the lowest optical bandgap (1.24 eV) among the synthesized copolymers. According to time-dependent density functional theory calculations, the TP unit, in contrast to EDOT, has its lowest unoccupied molecular orbital (LUMO) at the same level as BT. An extended π-conjugation along the TP and BT units leads to low-lying LUMO levels of the resulting copolymer P7 and in turn its reduced bandgap. The power conversion efficiencies (PCEs) of organic photovoltaic devices employing copolymers P1–P8 were measured in the configuration of ITO/PEDOT:PSS/copolymer (P1–P8) : PC60BM (1 : 1 w/w)/Al. Copolymer P7 in particular showed the highest PCE of 3.32%.