Ultra-low Bandgap Research Articles

Sensitive SWIR Organic Photodetectors with Spectral Response Reaching 1.5µm.

The performance of organic photodetectors (OPDs) sensitive to the short-wavelength infrared (SWIR) light lags behind commercial indium gallium arsenide (InGaAs) photodetectors primarily due to the scarcity of organic semiconductors with efficient photoelectric responses exceeding 1.3µm. Limited by the Energy-gap law, ultralow-bandgap organic semiconductors usually suffer from severe non-radiative transitions, resulting in low external quantum efficiency (EQE). Herein, a difluoro-substituted quinoid terminal group (QC-2F) with exceptionally strong electron-negativity is developed for constructing a new non-fullerene acceptor (NFA), Y-QC4F with an ultralow bandgap of 0.83eV. This subtle structural modification significantly enhances intermolecular packing order and density, enabling an absorption onset up to 1.5µm while suppressing non-radiation recombination in Y-QC4F films. SWIR OPDs based on Y-QC4F achieve an impressive detectivity (D*) over 1011 Jones from 0.4 to 1.5µm under 0V bias, with a maximum of 1.68 × 1012 Jones at 1.16µm. Furthermore, the resulting OPDs demonstrate competitive performance with commercial photodetectors for high-quality SWIR imaging even under 1.4µm irradiation.

Sensitive short-wavelength infrared photodetection with a quinoidal ultralow band-gap n-type organic semiconductor

Sensitive short-wavelength infrared photodetection with a quinoidal ultralow band-gap n-type organic semiconductor

Vibration mitigation in porous soil using periodic rock-socketed pile barriers

This study aimed to reveal the vibration mitigation effect of periodic rock-socketed piles installed in porous soil. Biot's theory and the periodic theory from solid-state physics were applied to observe the ultralow and surface-wave bandgaps in rock-socketed pile barriers. The bandgaps of the pile barriers in dry and saturated soils were subsequently compared, indicating that the width of the ultralow bandgap decreased slightly when the porous soil was fully saturated. To overcome the limitations of the sound cone method for calculating the surface-wave bandgap of a periodic rock-socked pile barrier, a critical frequency for Rayleigh waves was defined. Next, the screening effectiveness of pile barriers for both the ultralow and surface-wave bandgaps was validated using three-dimensional numerical simulations. Finally, the influences of the geometric and material parameters of the piles and soil as well as the water table level on the ultralow bandgap were evaluated using a parametric study. The results of this investigation provide a reference for the application of periodic rock-socketed piles in geological conditions with groundwater.

Exceptional Electron-Rich Heteroaromatic Pentacycle for Ultralow Band Gap Conjugated Polymers and Photothermal Therapy.

Stable redox-active conjugated molecules with exceptional electron-donating abilities are key components for the design and synthesis of ultralow band gap conjugated polymers. While hallmark electron-rich examples such as pentacene derivatives have been thoroughly explored, their poor air stability has hampered their broad incorporation into conjugated polymers for practical applications. Herein, we describe the synthesis of the electron-rich, fused pentacyclic pyrazino[2,3-b:5,6-b']diindolizine (PDIz) motif and detail its optical and redox behavior. The PDIz ring system exhibits a lower oxidation potential and a reduced optical band gap than the isoelectronic pentacene while retaining greater air stability in both solution and the solid state. The enhanced stability and electron density, together with readily installed solubilizing groups and polymerization handles, allow for the use of the PDIz motif in the synthesis of a series of conjugated polymers with band gaps as small as 0.71 eV. The tunable absorbance throughout the biologically relevant near-infrared I and II regions enables the use of these PDIz-based polymers as efficient photothermal therapeutic reagents for laser ablation of cancer cells.

Infrared Organic Photodetectors Employing Ultralow Bandgap Polymer and Non-Fullerene Acceptors for Biometric Monitoring.

Recent efforts in the field of organic photodetectors (OPD) have been focused on extending broadband detection into the near-infrared (NIR) region. Here, two blends of an ultralow bandgap push-pull polymer TQ-T combined with state-of-the-art non-fullerene acceptors, IEICO-4F and Y6, are compared to obtain OPDs for sensing in the NIR beyond 1100nm, which is the cut off for benchmark Si photodiodes. It is observed that the TQ-T:IEICO-4F device has a superior IR responsivity (0.03 AW-1 at 1200nm and -2V bias) and can detect infrared light up to 1800nm, while the TQ-T:Y6 blend shows a lower responsivity of 0.01 AW-1 . Device physics analyses are tied with spectroscopic and morphological studies to link the superior performance of TQ-T:IEICO-4F OPD to its faster charge separation as well as more favorable donor-acceptor domains mixing. In the polymer blend with Y6, the formation of large agglomerates that exceed the exciton diffusion length, which leads to high charge recombination, is observed. An application of these devices as biometric sensors for real-time heart rate monitoring via photoplethysmography, utilizing infrared light, is demonstrated.

Design of Persistent and Stable Porous Radical Polymers by Electronic Isolation Strategy

AbstractConjugated organic radical polymers with stable radical features are difficult to design because the π conjugation in the polymer backbones makes the radicals readily delocalize and tend to undergo covalent bonding processes. In this work, we report an electronic isolation strategy to design stable porous radical polymers by homocoupling reaction from a meta‐position active monomer. The meta linkage ensures less conjugation in the polymer skeletons, localizes the resonant radicals, and prevents them from recombination. The resulting porous radical polymer exhibits exceptional radical characters with ultralow band gap of 0.68 eV, strong yet extended UV/Vis‐NIR absorption up to 1800 nm, and high spin density. The above features make the polymer very promising in the photothermal conversion with record‐high photothermal temperature increment of ≈∼240 °C and striking solar‐driven water evaporation efficiency of 96.8 %. Our results demonstrate the feasibility of electronic isolation of radicals for producing outstanding photothermal materials.

Design of Persistent and Stable Porous Radical Polymers by Electronic Isolation Strategy.

Conjugated organic radical polymers with stable radical features are difficult to design because the π conjugation in the polymer backbones makes the radicals readily delocalize and tend to undergo covalent bonding processes. In this work, we report an electronic isolation strategy to design stable porous radical polymers by homocoupling reaction from a meta-position active monomer. The meta linkage ensures less conjugation in the polymer skeletons, localizes the resonant radicals, and prevents them from recombination. The resulting porous radical polymer exhibits exceptional radical characters with ultralow band gap of 0.68 eV, strong yet extended UV/Vis-NIR absorption up to 1800 nm, and high spin density. The above features make the polymer very promising in the photothermal conversion with record-high photothermal temperature increment of ≈∼240 °C and striking solar-driven water evaporation efficiency of 96.8 %. Our results demonstrate the feasibility of electronic isolation of radicals for producing outstanding photothermal materials.

Transformation of Wurtzite ZnO to a New Triclinic Nanoporous ZnO Phase via Hydrothermal Treatment with Metformin for Designing Proton Conducting Material.

Zinc oxide is one of the most widely studied semiconductor metal oxides, which predominantly crystallizes as hexagonal wurtzite and often cubic zinc-blende phases. Here we report the transformation of the highly stable wurtzite ZnO to a new triclinic phase NZO-2 by using metformin as a template during post-synthesis hydrothermal treatment. This crystalline phase of the material NZO-2 has been identified through the refinement of the powder XRD data. NZO-2 possesses porous rod like particle morphology consisting of the self-assembly of 3-7 nm size spherical nanoparticles and interparticle nanoscopic voids spaces. NZO-2 has been surface phosphorylated and the resulting material displayed good proton conductivity. Further, NZO-2 displayed ultra-low band gap of 1.74 eV, thereby responsible for red emission under high energy laser excitation and this may open new opportunities in optoelectronic application of ZnO.

Ultra-wide low-frequency band gap in a tapered phononic beam

We propose a tapered phononic beam with a broad low-frequency band gap for flexural waves. A unit cell of the phononic beam consists of two identical uniform parts and a thickness- and width-varying part sandwiched between them. Thickness and width profiles and uniform beam length are controlled to change the shear and rotational stiffnesses of the phononic beam, changing the starting and ending frequencies of the first band gap. By identifying the effects of those geometrical parameters on band structures, we determine the parameter values that enable the phononic beam to yield an ultra-broad and ultra-low band gap from 3.6 Hz to 237.9 Hz with a small lattice constant of λ/25. For experimental realization, a finite phononic beam with three unit cells is fabricated by wire electric discharge machining and its displacement transmission is measured through impact hammer tests. Particularly low transmission of -35 dB to -10 dB is observed at the band-gap frequency range. We briefly present ongoing works to enhance the structural robustness of the phononic beam by changing geometrical parameters or material of a thin and narrow part.

A new periodic structure composite material with quasi-phononic crystals

Phononic crystals (PCs) provide a new method for vibration insulation and noise reduction. PCs commonly have small bandgap width, and understanding how to expand this bandgap, including the ultra-low band gap, is crucial for applying PCs to vibration reduction in engineering structures. A new periodic structure composite material quasi-PCs is proposed and the high polymer concrete with quasi-PCs structure is conducted. Vibration measurements for the new periodic structure composite material demonstrate that quasi-PC behavior can open the ultra-low frequency band gap. The new periodic structure composite material can be used to reduce vibrations in engineering field.

Trifluoromethylation Enables a 3D Interpenetrated Low-Band-Gap Acceptor for Efficient Organic Solar Cells

Summary Trifluoromethylation has been used for nonfullerene acceptors, resulting in an ultra-narrow band-gap molecule named BTIC-CF3-γ. Compared to its fluorinated and chlorinated analogs, BTIC-CF3-γ possesses the most red-shifted absorptions. The single-crystal structure of BTIC-CF3-γ reveals that the cooperated π-π interactions from H aggregations of central fused cores and J aggregations of end groups lead to a three-dimensional (3D) interpenetrating network, which provides more electron-hopping junctions between different acceptor molecules. After blending with PBDB-TF, a champion power conversion efficiency (PCE) of 15.59% is realized, which is the highest value in reported ultra-narrow band-gap acceptors. Notably, a PCE of 16.50% is also achieved in BTIC-CF3-γ-based ternary devices due to their red-shifted absorptions. Overall, our results provide a visualized understanding of molecule packings of this family of acceptors and also demonstrate that trifluoromethylation is a useful strategy to design highly efficient ultra-low band-gap acceptors for the next nonfullerene solar devices.

Massive Dirac Fermion Behavior in a Low Bandgap Graphene Nanoribbon Near a Topological Phase Boundary.

Graphene nanoribbons (GNRs) have attracted much interest due to their largely modifiable electronic properties. Manifestation of these properties requires atomically precise GNRs which can be achieved through a bottom-up synthesis approach. This has recently been applied to the synthesis of width-modulated GNRs hosting topological electronic quantum phases, with valence electronic properties that are well captured by the Su-Schrieffer-Heeger (SSH) model describing a 1D chain of interacting dimers. Here, ultralow bandgap GNRs with charge carriers behaving as massive Dirac fermions can be realized when their valence electrons represent an SSH chain close to the topological phase boundary, i.e., when the intra- and interdimer coupling become approximately equal. Such a system has been achieved via on-surface synthesis based on readily available pyrene-based precursors and the resulting GNRs are characterized by scanning probe methods. The pyrene-based GNRs (pGNRs) can be processed under ambient conditions and incorporated as the active material in a field effect transistor. A quasi-metallic transport behavior is observed at room temperature, whereas at low temperature, the pGNRs behave as quantum dots showing single-electron tunneling and Coulomb blockade. This study may enable the realization of devices based on carbon nanomaterials with exotic quantum properties.

Photoinduced cationic polycondensation in solid state towards ultralow band gap conjugated polymers

A photoinduced cationic polycondensation towards the synthesis of homopolymer PTT.

Development of Poly (vinylidene fluoride) and Polyaniline blend with high dielectric permittivity, excellent electromagnetic shielding effectiveness and Ultra low optical energy band gap: Effect of ionic liquid and temperature

Electrically conductive polymer blends have attracted profound attention as dielectric materials. However, achieving isotropic behavior, electrical stability and low current leakage is still challenging. Here we have synthesized a conductive polymer blend of Polyaniline (PANI) and Poly (vinylidene fluoride) (PVDF) exhibiting relatively high dielectric permittivity (εʹ) together with low band gap. Ionic liquid (IL) has been incorporated and the effect of IL in the dielectric properties are explored. The interfacial adhesion, morphology and the dielectric polarization of the blends are investigated. The electromagnetic shielding effectiveness (EMISE) of the conductive blends is studied in X-band frequency range. The electrical stability and decrease in current loss of the blends has been analyzed through I–V characteristics. The findings reveal that with increase in PANI loading in the temperature range of 25–100 °C and IL incorporation results in excellent dielectric properties for charge storage applications. The high efficiency of absorption and low transmission of electromagnetic radiation by the conductive blends is the main reason for excellent EMISE possessed by the systems which can be used for shielding applications in electronics. The increase in electrical stability and decrease in current leakage ability appear in the form of narrow band gap.

Resolving the Mechanisms of Photocurrent Improvement in Ternary Organic Solar Cells

The ultralow band gap small-molecule IEICO-4F has been employed as a secondary acceptor in both fullerene-based (PTB7-Th:PC71BM) and nonfullerene-based (PBDB-T:ITIC) ternary organic solar cells (OSCs). Structural characterization methods combined with ultrafast spectroscopy have been applied to resolve the mechanisms, leading to the observed improvement in device efficiency upon addition of IEICO-4F. It is shown that IEICO-4F forms ternary mixed domains in the host systems and improves the device efficiency by broadening the absorption spectral range and enhancing both charge separation and charge transport. The enhanced crystallinity of the semiconductor polymer electron donors in the presence of the EIECO-4 provides additional channels for ultrafast charge transfer and transport compared to binary systems. The optimum ternary blend formulations required to improve device efficiencies are reported. This work provides new insights into the fabrication of high-performance ternary OSCs.

α,β-Unsubstitutedmeso-positioning thienyl BODIPY: a promising electron deficient building block for the development of near infrared (NIR) p-type donor–acceptor (D–A) conjugated polymers

Development of new ultra low bandgap (Eoptg < 1 eV) π-conjugated D–A polymers consisting of α,β-unsubstitutedmeso-positioning thienyl BODIPY.

Polymer solar cells based low bandgap A1-D-A2-D terpolymer based on fluorinated thiadiazoloquinoxaline and benzothiadiazole acceptors with energy loss less than 0.5 eV

We synthesized an ultra low bandgap terpolymer denoted as P containing fluorinated-fluorene attached thiadiazoloquinoxaline and benzothiadiazole acceptors and thiophene as donor in its backbone and investigated its optical and electrochemical properties. This terpolymer is used for as donor along with PC71BM as electron acceptor in solution processed polymer solar cells (PSCs). The P showed a shows strong absorption band from 650 nm to 1100 nm with an optical bandgap of 1.12 eV and highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy levels of −5.25 eV and −3.87 eV, respectively. After the optimization of P to PC71BM weight ratio, the optimized weight ratio 1:2 in chlorobenzene (CB) solution, the PSC showed overall power conversion efficiency of 4.10% (Jsc of 10.96 mA/cm2, Voc of 0.68 V and FF of 0.55). After the solvent additive (3 v% DIO) followed by subsequent thermal annealing (SA-TA) the PCE has been increased up to 7.54% with Jsc of 16.12 mA/cm2, Voc of 0.65 V and FF of 0.72. The increase in the PCE is related with the enhancement in the both Jsc and FF, attributed optimized nanoscale morphology of the active layer for both efficient exciton dissociation and charge transport towards the electrodes and balanced charge transport in the device, induced by the TSA treatment of the active layer. This is the highest PCE of PSCs with an energy loss about 0.47 eV with the low bandgap of 1.12 eV.

Coordination Polymer Framework Based On‐Chip Micro‐Supercapacitors with AC Line‐Filtering Performance

AbstractOn‐chip micro‐supercapacitors (MSCs) are important Si‐compatible power‐source backups for miniaturized electronics. Despite their tremendous advantages, current on‐chip MSCs require harsh processing conditions and typically perform like resistors when filtering ripples from alternating current (AC). Herein, we demonstrated a facile layer‐by‐layer method towards on‐chip MSCs based on an azulene‐bridged coordination polymer framework (PiCBA). Owing to the good carrier mobility (5×10−3 cm2 V−1 s−1) of PiCBA, the permanent dipole moment of azulene skeleton, and ultralow band gap of PiCBA, the fabricated MSCs delivered high specific capacitances of up to 34.1 F cm−3 at 50 mV s−1 and a high volumetric power density of 1323 W cm−3. Most importantly, such MCSs exhibited AC line‐filtering performance (−73° at 120 Hz) with a short resistance–capacitance constant of circa 0.83 ms.

Coordination Polymer Framework Based On‐Chip Micro‐Supercapacitors with AC Line‐Filtering Performance

On-chip micro-supercapacitors (MSCs) are important Si-compatible power-source backups for miniaturized electronics. Despite their tremendous advantages, current on-chip MSCs require harsh processing conditions and typically perform like resistors when filtering ripples from alternating current (AC). Herein, we demonstrated a facile layer-by-layer method towards on-chip MSCs based on an azulene-bridged coordination polymer framework (PiCBA). Owing to the good carrier mobility (5×10-3 cm2 V-1 s-1 ) of PiCBA, the permanent dipole moment of azulene skeleton, and ultralow band gap of PiCBA, the fabricated MSCs delivered high specific capacitances of up to 34.1 F cm-3 at 50 mV s-1 and a high volumetric power density of 1323 W cm-3 . Most importantly, such MCSs exhibited AC line-filtering performance (-73° at 120 Hz) with a short resistance-capacitance constant of circa 0.83 ms.

Ultralow bandgap molecular semiconductors for ambient-stable and solution-processable ambipolar organic field-effect transistors and inverters

New ultralow bandgap semiconductor small molecules were designed and synthesized for ambient-stable and solution-processable ambipolar organic field-effect transistors and high-gain inverters.