Wide Band Gap Organic Semiconductor Research Articles

Highly-Polarized Solar-Blind Ultraviolet Organic Photodetectors.

Developing high-performance polarization-sensitive ultraviolet photodetectors is crucial for their application in military remote sensing, detection, bio-inspired navigation, and machine vision. However, the significant absorption in the visible light range severely limits the application of polarization-sensitive ultraviolet photodetectors, such as high-quality anti-interference imaging. Here, based on a wide-bandgap organic semiconductor single crystal (trans-1,2-bis(5-phenyldithieno[2,3-b:3',2'-d]thiophen-2-yl)ethene, BPTTE), high-performance polarization-sensitive solar-blind ultraviolet photodetectors with a dichroic ratio close to 4.26 are demonstrated. The strong anisotropy of two-dimensional grown BPTTE single crystals in molecular vibration and optical absorption has been characterized by various techniques. Under voltage modulation, stable and efficient detection of polarized light has been demonstrated, attributed to the intrinsic anisotropy of transition dipole moment in the bc crystal plane, rather than other factors. In final, high-contrast polarimetric imaging and anti-interference imaging are successfully demonstrated based on BPTTE single crystal photodetectors, highlighting the potential of organic semiconductors for polarization-sensitive solar-blind ultraviolet photodetectors. This article is protected by copyright. All rights reserved.

High‐Efficiency Perovskite–Organic Blend Light‐Emitting Diodes Featuring Self‐Assembled Monolayers as Hole‐Injecting Interlayers

AbstractThe high photoluminescence efficiency, color purity, extended gamut, and solution processability make low‐dimensional hybrid perovskites attractive for light‐emitting diode (PeLED) applications. However, controlling the microstructure of these materials to improve the device performance remains challenging. Here, the development of highly efficient green PeLEDs based on blends of the quasi‐2D (q2D) perovskite, PEA2Cs4Pb5Br16, and the wide bandgap organic semiconductor 2,7 dioctyl[1] benzothieno[3,2‐b]benzothiophene (C8‐BTBT) is reported. The presence of C8‐BTBT enables the formation of single‐crystal‐like q2D PEA2Cs4Pb5Br16 domains that are uniform and highly luminescent. Combining the PEA2Cs4Pb5Br16:C8‐BTBT with self‐assembled monolayers (SAMs) as hole‐injecting layers (HILs), yields green PeLEDs with greatly enhanced performance characteristics, including external quantum efficiency up to 18.6%, current efficiency up to 46.3 cd A−1, the luminance of 45 276 cd m−2, and improved operational stability compared to neat PeLEDs. The enhanced performance originates from multiple synergistic effects, including enhanced hole‐injection enabled by the SAM HILs, the single crystal‐like quality of the perovskite phase, and the reduced concentration of electronic defects. This work highlights perovskite:organic blends as promising systems for use in LEDs, while the use of SAM HILs creates new opportunities toward simpler and more stable PeLEDs.

Revealing Suppressed Intermolecular Coupling Effects in Aggregated Organic Semiconductors by Diluting the Crystal: Model System Perfluoropentacene:Picene.

In order to investigate the effects of intermolecular interactions on the optical properties of organic semiconductors, we employ mixing of the organic semiconductor perfluoropentacene (PFP; C22F14) with the wide band-gap organic semiconductor picene (PIC; C22H14). The binary mixed thin films are prepared by simultaneous coevaporation of PIC and PFP in vacuum. We determine the optical properties of the blends by differential reflectance spectroscopy (absorption) and photoluminescence (emission). PFP:PIC thin films are a rare case of mixed thin films with a known molecular packing. The formation of equimolar mixed domains with a crystal structure clearly different from that of the pure compounds is, in the case of nonequimolar blends, accompanied by pure domains of the excess compound. Due to the wide band gap of PIC, the effect of reduced intermolecular interactions between PFP molecules can be studied in detail without any direct contributions of PIC to the spectra. We find a strongly enhanced emission from PFP in the mixed thin films, which can be explained by decoupling. Real-time investigations of the absorption spectra during growth provide further insight into intermolecular coupling effects on optical properties.

Efficient Non-polymeric Heterojunctions in Ternary Organic Solar Cells

We describe the use of a near IR non-fullerene low molecular weight organic semiconductor molecule, as electron acceptor, in combination with a wide band gap organic semiconductor electron donor, in efficient bulk heterojunction organic solar cells. The electron acceptor is a 2,5-dihydropyrrolo[3,4-c]pyrrole-1,4-dione-based molecule while the electron donor is a carbazole derivative. These two small molecules have complementary absorption spectra, and, thus, the heterojunction absorbs from 450 to 950 nm. The optimized photoactive layer shows power conversion efficiency of 7.44% under standard measurement conditions with an estimated energy loss as low as 0.55 eV. Identical solar cells, but using fullerene derivative, PC71BM, result in lower efficiency values (PCE = 5.07%) and greater energy losses (0.86 eV). The better performance of the non-fullerene-based small molecule organic solar cells is due to the higher LUMO energy level for the electron acceptor. Moreover, to improve further the efficiency of th...

Photoluminescence and semiconductor properties of two novel lanthanide-mercury compounds with one-dimensional chain-like structures

Two novel lanthanide-mercury compounds [Ln(IA)3(H2O)2]n(Hg3Cl9)n·4nH2O (Ln = Eu (1) and Tb (2); IA = isonicotinic acid) have been synthesized under solvothermal conditions and structurally characterized by single-crystal X-ray diffraction. Both complexes exhibit one-dimensional (1D) chain-like structure with a novel isolated (Hg3Cl9)3– anion. Solid-state diffuse reflectance spectroscopy reveals the presence of wide optical band gaps of 2.81 eV and 3.47 eV, suggesting that they are potentially wide band gap organic semiconductor materials. Solid-state photoluminescence measurements uncover that they show red or green light emission bands, respectively. The emission bands can be assigned to the characteristic emission of the 4f electrons intrashell transitions of 5D0 → 7FJ (Eu3+, J = 2 and 4) and 5D4 → 7FJ (Tb3+, J = 6, 5, 4 and 3), respectively. Energy transfer mechanism is explained by the energy level diagram of Eu3+ ion, Tb3+ ion and isonicotinic acid ligand. The title compounds have remarkable CIE chromaticity coordinates of (0.6438, 0.3559) and (0.3172, 0.4773). This suggests that they may be potential candidates with red or green light emitting materials for white light emitting diodes (LEDs).

The mixed-ligand strategy to assemble a europium metal-organic framework with a 2-fold-interpenetrated network

A novel europium complex with the mixed ligands of 1-methyl-3,5-bis(4′-carboxy-phenyl)-1,2,4-triazole (mbcpt) and p-phthalic acid (pa), [Eu2Cl2(H3O)2(μ4-mbcpt)2(μ4-pa)]n (1), has been synthesized under the solvothermal conditions and structurally characterized by single-crystal X-ray diffraction. Complex 1 exhibits a three-dimensional (3-D) metal-​organic framework. The 3-D open framework features a 2-fold-interpenetrated network. Solid-state diffuse reflectance spectroscopy uncovers the presence of a wide optical band gap of 3.56 eV, indicating that it is a wide band gap organic semiconductor material. Solid-state photoluminescence measurement reveals that complex 1 shows red light emission bands locating at 602, 611, 642 and 690 nm, which can be assigned to the 5D0 → 7FJ (Eu3+) with the value of J = 1, 2, 3 and 4, respectively. Magnetic measurements reveal that complex 1 shows an antiferromagnetic behavior.

Fully transparent conformal organic thin-film transistor array and its application as LED front driving.

A fully transparent conformal organic thin-film field-effect transistor array is demonstrated based on a photolithography-compatible ultrathin metallic grid gate electrode and a solution-processed C8-BTBT film. The resulting organic field-effect transistor array exhibits a high optical transparency of >80% over the visible spectrum, mobility up to 2 cm2 V-1 s-1, on/off ratio of 105-106, switching current of >0.1 mA, and excellent light stability. The transparent conformal transistor array is demonstrated to adhere well to flat and curved LEDs as front driving. These results present promising applications of the solution-processed wide-bandgap organic semiconductor thin films in future large-scale transparent conformal active-matrix displays.

Simple Way to Engineer Metal–Semiconductor Interface for Enhanced Performance of Perovskite Organic Lead Iodide Solar Cells

A thin wide band gap organic semiconductor N,N,N',N'-tetraphenyl-benzidine layer has been introduced by spin-coating to engineer the metal-semiconductor interface in the hole-conductor-free perovskite solar cells. The average cell power conversion efficiency (PCE) has been enhanced from 5.26% to 6.26% after the modification and a highest PCE of 6.71% has been achieved. By the aid of electrochemical impedance spectroscopy and dark current analysis, it is revealed that this modification can increase interfacial resistance of CH3NH3PbI3/Au interface and retard electron recombination process in the metal-semiconductor interface.

Highly transparent thin-film transistors using wide-bandgap organic semiconductors and multilayer transparent electrodes

Highly transparent organic thin-film transistors operable at low voltages were demonstrated using a wide-bandgap organic semiconductor for channel layers, metal-based multilayers for transparent source/drain (S/D) electrodes, and an ultrathin fluoropolymer for gate dielectric layers. The devices optimized for high transparency based on characteristic-matrix formalism exhibited transmittances over 85% for the channel region and over 70% for the S/D region in almost the entire visible range. The validity of the multilayer S/D contact and the ultrathin fluoropolymer dielectrics on indium tin oxide (ITO) gate electrodes for favourable electrical characteristics were also verified. The resultant transparent thin-film transistors showed a typical p-channel operation switchable under 2 V with ca. 0.72 cm2/V s mobility.

Cadmium-free copper indium gallium diselenide hybrid solar cells comprising a 2-(4-biphenylyl)-5-(4-tert-butylphenyl)-1,3,4-oxadiazole buffer layer

Copper indium gallium diselenide (CIGS) solar cells are the most efficient thin film photovoltaic devices today. In this work, we investigate CIGS/organic hybrid solar cells comprising a semi-transparent metal top electrode and a wide band gap organic semiconductor as buffer layer. Depositing the organic semiconductor from solution, we fabricate Cd-free solar cells exhibiting about the same efficiency as their counterparts comprising CdS and significantly higher open-circuit voltages as compared to buffer-free devices. Although the organic molecules do not cover the CIGS surface homogeneously, their use enables prolonged charge carrier lifetimes according to impedance spectroscopy measurements.

A new wide bandgap organic semiconductor and its application in organic UV sensors with tunable response wavelength

A triphenylbenzene-containing carbazole derivative (PCP) with wide bandgap, good hole mobility, and excellent thermal stability was synthesized and used as electron donor to construct planar heterojunction organic ultraviolet sensors (UVSs), while bis(4-(4,6-diphenyl-1,3,5-triazine-2-yl)phenyl)diphenylsilane (NSN) was used as electron acceptor. The UVS ITO/PEDOT:PSS/PCP/NSN/LiF/Al/Ag showed good responses to illumination from both ITO and cathode sides. When a semitransparent cathode was applied, the response region could be further extended to deep-ultraviolet (DUV) region. When the semitransparent Al (9nm)/Ag (30nm) cathode was replaced by Al (13nm), the blue-shift of the response spectra of the UVS could be found. It was especially interesting that the cutoff of response could be changed by using different kind of cathodes. This study may provide a useful guideline to achieve low-cost organic deep-ultraviolet sensors (DUVSs) with tunable spectral response.

Improvement of subthreshold current transport by contact interface modification in p-type organic field-effect transistors

The charge injection efficiency of organic field-effect transistors (OFETs) is found to be a critical factor determining the subthreshold characteristics of these devices. OFETs fabricated using a wide band gap organic semiconductor and gold source/drain contacts display large threshold voltage and poor subthreshold characteristics. Insertion of a metal-oxide charge injection layer at the contact/semiconductor interface lower the injection barrier height, resulting in marked improvements in threshold voltage and subthreshold slope and strong suppression of the short-channel effect. The improved subthreshold characteristics are attributed to enhanced charge injection and the consequent promotion of charge accumulation.

Two-step exciton dissociation in poly(3-hexylthiophene)/fullerene heterojunctions

Charge and energy transfer are competing processes in photoexcited organic donor/acceptor systems. By inserting a thin interlayer of a wide band gap organic semiconductor in a poly(3-hexylthiophene)/fullerene heterojunction, we were able to selectively suppress electron transfer. External quantum efficiency measurements show that device performance is unaffected by the presence of the interlayer. These results can be explained by two-step exciton dissociation where energy transfer of excitons from the donor to the acceptor is followed by hole transfer to the donor. The same interlayer was shown to decrease the photocurrent in a heterojunction where this energy transfer pathway is forbidden.