Active Layer In Optoelectronic Devices Research Articles

Self-Assembled Monolayers of Push–Pull Chromophores as Active Layers and Their Applications

In recent decades, considerable attention has been focused on the design and development of surfaces with defined or tunable properties for a wide range of applications and fields. To this end, self-assembled monolayers (SAMs) of organic compounds offer a unique and straightforward route of modifying and engineering the surface properties of any substrate. Thus, alkane-based self-assembled monolayers constitute one of the most extensively studied organic thin-film nanomaterials, which have found wide applications in antifouling surfaces, the control of wettability or cell adhesion, sensors, optical devices, corrosion protection, and organic electronics, among many other applications, some of which have led to their technological transfer to industry. Nevertheless, recently, aromatic-based SAMs have gained importance as functional components, particularly in molecular electronics, bioelectronics, sensors, etc., due to their intrinsic electrical conductivity and optical properties, opening up new perspectives in these fields. However, some key issues affecting device performance still need to be resolved to ensure their full use and access to novel functionalities such as memory, sensors, or active layers in optoelectronic devices. In this context, we will present herein recent advances in π-conjugated systems-based self-assembled monolayers (e.g., push–pull chromophores) as active layers and their applications.

Hetero-integrated MoS2/CsPbBr3 photodetector with enhanced performance via combinational modulation of grain boundary passivation and interfacial carrier separation

The distinguished electronic and optical properties of all-inorganic lead halide perovskites (CsPbX3, X = Cl, Br, I) qualify them as ideal candidates for active layer in optoelectronic devices, especially showing more competitive development prospects in photodetection with high performance. However, superior device performance and good long-term stability is still a challenge for perovskite-based photodetectors applying in practical situation. Herein, we demonstrate a novel hetero-integrated MoS2/CsPbBr3 photodetector displaying efficient device performance, where the MoS2 nanocrystals pre-synthesized by hydrothermal route were spin-coated on the surface of CsPbBr3 films with controllable dispersion, providing effective interfacial photo-carriers separation at MoS2/CsPbBr3 van der Waals heterojunction (vdWH) and significant defect passivation for perovskite grain boundaries (GBs) simultaneously, thus boosting optoelectronic performance of our device within ultraviolet (UV)-visible wavelength range, including a higher photoresponsivity (4.79 A/W), enhanced specific detectivity (8.81 × 1012 Jones) and more durable stability than that of the pristine CsPbBr3 photodetector. Furthermore, the fast response speed (0.17/0.22 ms) is obviously more outstanding than that of most CsPbBr3-based and congeneric hybrid photodetectors. These results, combined with our experimental design strategy of the device described herein, provide an efficient approach to achieve stable, high-performance photodetection using perovskite-based hybrid nanomaterial systems, thus opening up new possibilities for two-dimensional (2D) materials/perovskites hetero-integration based photovoltaic and optoelectronic applications in the future.

Physical properties of Zn-Sn-N films governed by the Zn/(Zn + Sn) ratio

At present, the application of ZnSnN2 as an active layer in optoelectronic devices is dramatically limited due to its high carrier concentration. It is suggested that off-stoichiometry of cations might be a promising cure. In this work, Zn-Sn-N films with 0.60, 0.67, and 0.85 Zn/(Zn + Sn) ratios were, respectively, fabricated by DC magnetron sputtering. In spite of off-stoichiometry, the films all exhibited a cation-disordered wurtzitelike ZnSnN2 dominated phase except that the crystallinity was decreased with an increasing Zn/(Zn + Sn) ratio. In agreement with the cation-disordered structure, all the Zn-Sn-N films illustrated Raman spectra of a phonon-glasslike characteristic. The refractive index of the films was increased with the Zn/(Zn + Sn) ratio over a wide wavelength range, for example, from 1.990 to 2.459 at the wavelength of 500 nm. The direct optical bandgap of the films varied from 1.36 to 1.68 eV. Most strikingly, an electron concentration of magnitude down to 1016 cm−3 and a very low resistivity down to 10−2 Ω cm were reached for 0.67 and 0.85 Zn/(Zn + Sn) films, respectively. It is highly desirable that both semiconducting and conducting characteristics can be achieved in the Zn-Sn-N material system, which is highly beneficial to its applications in various optoelectronic devices.

A yellow to magenta multielectrochromic, pH sensor polymer based on 2,5-di(thienyl)pyrrole modified with methyl orange azo dye

The enhancement of optical contrast and color modulation of π-conjugated polymers may be achieved by incorporating organic dyes through modification of the monomer chemical structure. Methyl orange (MO) azo dye is widely used as a pH indicator, changing its color from orange to red depending on the medium, which makes it a promising strategy to attain polymer materials with innovative optoelectronic properties. A 2,5-di(thienyl)pyrrole (SNS) derivatized with MO, namely SNS-MO has been synthesized with 95% yield and characterized by 1H NMR, 13C NMR, FT-IR and mass spectroscopy techniques. Polymer films of PSNS-MO were electrodeposited onto ITO/glass surface by cyclic voltammetry in (C4H9)4NBF4/CH3CN electrolyte. The PSNS-MO films showed yellow orange color in the neutral state (E = 0.00 V) and light magenta in the oxidized state (E = 0.65 V), presenting chromatic contrast (Δ%T) at 496 and 1010 nm of 14.0 and 23.0, respectively, electrochromic efficiency (η) in the range of 83.90 – 308.10 cm2 C−1 and switching time < 3.5 s. The polymer films were also investigated in Phosphate Buffered Saline (PBS) solutions and after exposure to HCl vapors, in which the color varied from reddish brown at pH = 0.0 to yellowish orange at pH = 4.0. These results show the importance of incorporating azo dyes into the structure of the conjugated polymer to obtain original and advanced materials, since PSNS-MO films are promising for application as active layers in optoelectronic devices and pH optical sensors.

Intrinsic Hole Mobility in Luminescent Metal–Organic Frameworks and Its Application in Organic Light‐Emitting Diodes

AbstractMost metal–organic frameworks (MOFs) lack charge mobility, which is crucial for realizing their use in optoelectronic applications. This work proposes the design of a MOF using triarylamine‐based ligands (Zr‐NBP) as the lone pair electron spacer to enhance the hole mobility in the MOF while maintaining its luminescent properties. Zr‐NBP has strong fluorescence with a good hole mobility of 1.05×10−6 cm2 V−1 s−1, which is comparable to organic materials used in optoelectronic devices. We also employed a Zr‐NBP nanofilm in the pure phase as both a non‐doped emissive layer and a hole‐transporting layer within organic light‐emitting diodes (OLEDs). The obtained OLED device produced a bright green light with a low turn‐on voltage of 3.9 V. This work presents an advance in developing the electronic properties of MOFs by modifying the chemical properties of its building blocks, and will likely inspire further design of MOF materials as active layers in optoelectronic devices.

Intrinsic Hole Mobility in Luminescent Metal-Organic Frameworks and Its Application in Organic Light-Emitting Diodes.

Most metal-organic frameworks (MOFs) lack charge mobility, which is crucial for realizing their use in optoelectronic applications. This work proposes the design of a MOF using triarylamine-based ligands (Zr-NBP) as the lone pair electron spacer to enhance the hole mobility in the MOF while maintaining its luminescent properties. Zr-NBP has strong fluorescence with a good hole mobility of 1.05×10-6 cm2 V-1 s-1 , which is comparable to organic materials used in optoelectronic devices. We also employed a Zr-NBP nanofilm in the pure phase as both a non-doped emissive layer and a hole-transporting layer within organic light-emitting diodes (OLEDs). The obtained OLED device produced a bright green light with a low turn-on voltage of 3.9 V. This work presents an advance in developing the electronic properties of MOFs by modifying the chemical properties of its building blocks, and will likely inspire further design of MOF materials as active layers in optoelectronic devices.

A rainbow multielectrochromic copolymer based on 2,5-di(thienyl)pyrrole derivative bearing a dansyl substituent and 3,4-ethylenedioxythiophene

A 2,5-di(thienyl)pyrrole derivative bearing a dansyl substituent (SNSD) was prepared by a simple synthetic route and its copolymerization with 3,4-ethylenedioxythiophene (EDOT) was successfully performed electrochemically in acetonitrile (CH3CN) containing tetrabutylammonium tetrafluoroborate ((C4H9)4NBF4). The electrochromic properties of the P(SNSD-co-EDOT) films were investigated by spectroelectrochemical techniques and, besides high absorption in the near infrared region, such films presented multielectrochromism with rainbow-like colors depending on the applied potential, as can be seen in the track of the CIE 1931 xy chromaticity coordinates. P(SNSD-co-EDOT) films deposited on ITO with 1:5 feed ratio presented chromatic contrast (Δ%T) at 560 and 990 nm of 25.1 and 35.3, respectively, coloration efficiency (η) in the range of 110 – 180 cm2 C−1, fast switching time (< 1.0 s), and stability to redox cycling. Such properties make this copolymer potentially applicable as active layer in optoelectronic devices.

Comparison of interaction mechanisms of lead phthalocyanine and disodium phthalocyanine with functionalized 1,4 dihydropyridine for optoelectronic applications

In the current work, the compound 2-methyl-N-octadecyl-2-(1-((trifluoromethyl)sulfonyl)-1,4-dihydropyridin-4-yl) propanamide was synthesized and characterized. Subsequently, its structure was optimized by DFT (Density Functional Theory) and its interaction with the chosen phthalocyanines: lead phthalocyanine (PbPc) and disodium phthalocyanine (Na2Pc) were also theoretically studied. The energy values of the HOMO (Highest Occupied Molecular Orbital) and the LUMO (Lowest Unoccupied Molecular Orbital), as well as the band gap were obtained from the resulting complex and a semiconductor behavior was found. Based on the theoretical results, bulk heterojunction films were manufactured with the aim of using them as semiconductor active layers in optoelectronic devices. In those films, the bandgap was determined through their absorption coefficients and the energy values of the photon, the results were compared to the theoretical bandgap previously calculated. Finally, the electrical behavior of the electronic donor-acceptor active layers was evaluated in form of simple devices with a heterojunction arrangement. The evaluation of the current-voltage (I–V) was carried out under the presence of different lighting conditions and the devices showed the curve of a typical semiconductor. No variations between the forward and reverse operation zones were shown by the devices, not even when they were exposed to different illumination sources. In general, an ohmic behavior was observed. Results demonstrated that 2-methyl-N-octadecyl-2-(1-((trifluoromethyl)sulfonyl)-1,4-dihydropyridin-4-yl) propenamide can be used as an electron acceptor species for the manufacture of semiconductor active layers of optoelectronic devices.

Two-dimensional transition metal dichalcogenides for lead halide perovskites-based photodetectors: band alignment investigation for the case of CsPbBr3/MoSe2

The distinguished electronic and optical properties of lead halide perovskites (LHPs) make them good candidates for active layer in optoelectronic devices. Integrating LHPs and two-dimensional (2D) transition metal dichalcogenides (TMDs) provides opportunities for achieving increased performance in heterostructured LHPs/TMDs based optoelectronic devices. The electronic structures of LHPs/TMDs heterostructures, such as the band offsets and interfacial interaction, are of fundamental and technological interest. Here CsPbBr3 and MoSe2 are taken as prototypes of LHPs and 2D TMDs to investigate the band alignment and interfacial coupling between them. Our GGA-PBE and HSE06 calculations reveal an intrinsic type-II band alignment between CsPbBr3 and MoSe2. This type-II band alignment suggests that the performance of CsPbBr3-based photodetectors can be improved by incorporating MoSe2 monolayer. Furthermore, the absence of deep defect states at CsPbBr3/MoSe2 interfaces is also beneficial to the better performance of photodetectors based on CsPbBr3/MoSe2 heterostructure. This work not only offers insights into the improved performance of photodetectors based on LHPs/TMDs heterostructures but it also provides guidelines for designing high-efficiency optoelectronic devices based on LHPs/TMDs heterostructures.

Enhanced Tensile Strain in P-doped Ge Films Grown by Molecular Beam Epitaxy Using GaP and Sb Solid Sources

Ge emerges as a good candidate for the active layer in optoelectronic devices and is compatible with complementary metal oxide semiconductor technology. As the Ge band structure exhibits an indirect band gap with a small difference in energy between the direct and the indirect valleys (about 140 meV), the energy band structure can be modified by applying a tensile strain or by doping electrons into the film to turn Ge into a semiconductor material with a direct ban gap structure. In this work, Ge epilayers were grown on Si substrate by molecular beam epitaxy technique, and we show that the tensile strain value of the Ge epilayers increases by a factor of two by doping electrons into the films with two donor elements using GaP and Sb sources. The dopant concentration in Ge epilayers is 5.6 × 1019 at cm−3 for the P dopant and 6.4 × 1018 at cm−3 for the Sb dopant. The activated electron concentration was found to be up to 4.1019 cm−3 in the Ge film. A gain of photoluminescence (PL) intensity three times higher than the sample doped only with P atoms was obtained. This result contributes to the perspective that Ge films can be utilized in functional optoelectronic as well as photonic devices.

(Invited) Metal Halide Perovskites at the Nanoscale: High Quality Optoelectronic Materials with Unique Phase Properties

The newly rediscovered perovskite semiconductor system has the potential to be extremely transformative for all optoelectronic devices, especially photovoltaics (PVs). Perovskite semiconductors of the form APbI3 where A is a large +1 charged cation, typically Cs, methylammonium, or formamidinium have had a huge resurgence among materials scientists for outstanding PV properties despite being overlooked for decades. Semiconductors containing the latter two A-site cations listed are hybrid organic-inorganic materials, and as such, are far less understood compared to conventional all inorganic or even organic material systems. Regardless of this spotty formal understanding, lead-halide perovskites have very rapidly been optimized to power conversion efficiency levels on par with all other materials even with extensive history of research. Perovskites show a unique tolerance to crystalline defects that cause trouble in most other semiconductors. Therefore the potential offered is that very high efficiency PVs can be fabricated in extremely fast and inexpensive ways, thus offering a revolution for the solar industry and a direct route toward producing the world’s energy with a simple and clean technology. Long-term durability of the devices is the critical remaining challenge to be solved.1 Two examples of major instabilities in device performance are the volatility of the organic cation and the specific crystal habit in which the material embodies. Nanoscale versions (often termed quantum dots (QDs)) of the all-inorganic metal halide perovskite (CsPbI3) tend to retain the desired perovskite phase due to strain effects at the surface of the QDs whereas conventional films of the same material “relax” to an orthorhombic non-perovskite structure at room temperature. Therefore these QDs potentially solve both of the instability issues. The cubic CsPbI3 QD cells operate with a rather remarkable open-circuit voltage of >1.2 volts and have produced power conversion efficiencies over 13%.2,3 This customizable new nanomaterial system has incredible potential for many applications in optoelectronics, including photovoltaics, LEDs, displays and lasers. We describe the formation of perovskite phase-CsPbI3 QD films with long range electronic transport that retain the high temperature phase in ambient conditions making up the active layer in optoelectronic devices. Perspectives on how this technology can become transformative will be discussed. References A. Christians, P. Schulz, J.S. Tinkham, T.H. Schloemer, S.P. Harvey, B.J .Tremolet de Villers, A. Sellinger, J.J. Berry, J.M. Luther, Nature Energy, 2018, 3 (1), 68.Swarnkar, A.R. Marshall, E.M. Sanehira, B.D. Chernomordik, D.T. Moore, J.A. Christians, T. Chakrabarti, J.M. Luther, Science, 2016, 354 (6308), 92-95.M. Sanehira, A.R. Marshall, J.A. Christians, S.P. Harvey, P.N. Ciesielski, L.M. Wheeler, P. Schulz, L.Y. Lin, M.C. Beard, J.M. Luther, Science Advances, 2017, 3 (10), eaao4204.

Significant enhancement of photoresponsive characteristics and mobility of MoS2-based transistors through hybridization with perovskite CsPbBr3 quantum dots

Inorganic perovskite CsPbBr3 quantum dots (QDs) are potential nanoscale photosensitizers; moreover, two-dimensional (2-D) molybdenum disulfide (MoS2) has been intensively studied for application in the active layers of optoelectronic devices. In this study, heterostructures of 2D-monolayered MoS2 with zero-dimensional functionalized CsPbBr3 QDs were prepared, and their nanoscale optical characteristics were investigated. The effect of n-type doping on the MoS2 monolayer after hybridization with perovskite CsPbBr3 QDs was observed using laser confocal microscope photoluminescence (PL) and Raman spectra. Field-effect transistors (FETs) using MoS2 and the MoS2–CsPbBr3 QDs hybrid were also fabricated, and their electrical and photoresponsive characteristics were investigated in terms of the charge transfer effect. For the MoS2–CsPbBr3 QDs-based FETs, the field effect mobility and photoresponsivity upon light irradiation were enhanced by ~ 4 times and a dramatic ~ 17 times, respectively, compared to the FET prepared without the perovskite QDs and without light irradiation. It is noteworthy that the photoresponsivity of the MoS2–CsPbBr3 QDs-based FETs significantly increased with increasing light power, which is completely contrary to the behavior observed in previous studies of MoS2-based FETs. The increased mobility and significant enhancement of the photoresponsivity can be attributed to the n-type doping effect and efficient energy transfer from CsPbBr3 QDs to MoS2. The results indicate that the optoelectronic characteristics of MoS2-based FETs can be significantly improved through hybridization with photosensitive perovskite CsPbBr3 QDs.

Measuring the Electronic Structure of Nanocrystal Thin Films Using Energy-Resolved Electrochemical Impedance Spectroscopy.

Use of nanocrystal thin films as active layers in optoelectronic devices requires tailoring of their electronic band structure. Here, we demonstrate energy-resolved electrochemical impedance spectroscopy (ER-EIS) as a method to quantify the electronic structure in nanocrystal thin films. This technique is particularly well-suited for nanocrystal-based thin films as it allows for in situ assessment of electronic structure during solution-based deposition of the thin film. Using well-studied lead sulfide nanocrystals as an example, we show that ER-EIS can be used to probe the energy position and number density of defect or dopant states as well as the modification of energy levels in nanocrystal solids that results through the exchange of surface ligands. This work highlights that ER-EIS is a sensitive and fast method to measure the electronic structure of nanocrystal thin films and enables their optimization in optoelectronic devices.

Morphological consequences of ligand exchange in quantum dot - Polymer solar cells

Mixtures of conjugated polymers and quantum dot nanocrystals present an interesting solution-processable materials system for active layers in optoelectronic devices, including solar cells. We use scanning transmission electron microscopy to investigate the effects of exchanging the capping ligand of quantum dots on the three-dimensional morphology of the film. We created 3D reconstructions for blends of poly((4,8-bis(octyloxy)benzo(1,2-b:4,5-b’)-dithiophene-2,6-diyl)(2-((dodecyloxy)carbonyl)thieno (3,4-b)-thiophenediyl)) (PTB1) and PbS quantum dots capped with oleic acid (OA), butylamine (BA), OA to 3-mercaptopropionic acid (MPA), and BA to MPA. We use these reconstructed volumes to evaluate differences in exciton dissociation and charge transport as a function of ligand processing. We show that the MPA exchange without an intermediate BA treatment results in severe changes to the film structure and a non-ideal morphology for an effective device. We also show that with a BA exchange, the morphology remains largely unchanged with the additional MPA treatment. This quantitative characterization elucidates previously reported device performance changes caused by ligand exchange and should inform future device fabrication protocols.

Characterization of liquid crystalline phthalocyanines for OFET applications

GRAPHICAL ABSTRACT

Investigation and fine-tuning of optoelectrical features of tertiary aminophenyl-activated poly(1,3,4-oxadiazole) conjugated system

A novel π-conjugated polymer with N-alkyl substituted side chains, viz. poly[(2-N,N′-dibutylaminophenyl)1,3,4-oxadiazole] (PNAPO), possessing tunable optoelectronic properties, has been synthesized and characterized. PNAPO exhibited good solubility, indicating its suitability for easy optoelectronic device fabrication. Flexible films of PNAPO have been fabricated by adopting a blending route using poly(methyl methacrylate) or polystyrene. The effect of PNAPO particle size reduction, in the nanoregion, on the optical characterization was evaluated by a re-precipitation strategy. PNAPO has been found to exhibit an emission wavelength in the region of 522–590 nm with good quantum yield, for different particle size dimensions. Interestingly, it has been observed to exhibit solvatochromism. Z-scan experiments, with Nd: YAG laser, reveal that PNAPO possesses a low optical limiting threshold value. Interestingly, it shows a low turn-on voltage (0.23 V), suitable for fabricating the active layers in optoelectronic devices. PNAPO has been found to be stable up to 240 °C. These results highlight the possible utilization of PNAPO as an emissive layer in optoelectronic devices and for nonlinear optical (NLO) processes. The graphical abstract represents the optical and electrical features of PNAPO. It indicates the structure of PNAPO. It shows the bright emissive property of PNAPO in the bluish green region of the electromagnetic radiation upon excitation with UV light. The light-emitting diode fabrication using PNAPO as an emissive layer is highlighted. A linear I–V characteristic of PNAPO has also been indicated through a current–voltage plot.

Newborn 2D materials for flexible energy conversion and storage

Newborn two-dimensional materials (NB2DMs) beyond graphene such as transition metal dichalcogenides (TMDs) exhibit excellent optoelectronic and mechanical properties as well as high theoretical specific capacity, which make them become the promising building blocks of flexible energy devices related to energy conversion and storage. Compared to graphene with zero band gap or traditional friable materials such as Si, these NB2DMs are more suitable to construct flexible devices as active layers of optoelectronic devices or as active materials for batteries. The present review focuses on the recent advances in bendable energy devices based on NB2DMs, including batteries, supercapacitors (SCs), solar cells, photodetectors and nanogenerators (NGs). The NB2DMs pave a new way to construct next-generation flexible energy devices with improved performance and we believe that those devices will be seen in our daily life and change our lifestyle in the immediate future.

Semiconducting ZnSnN2 thin films for Si/ZnSnN2 p-n junctions

ZnSnN2 is regarded as a promising photovoltaic absorber candidate due to earth-abundance, non-toxicity, and high absorption coefficient. However, it is still a great challenge to synthesize ZnSnN2 films with a low electron concentration, in order to promote the applications of ZnSnN2 as the core active layer in optoelectronic devices. In this work, polycrystalline and high resistance ZnSnN2 films were fabricated by magnetron sputtering technique, then semiconducting films were achieved after post-annealing, and finally Si/ZnSnN2 p-n junctions were constructed. The electron concentration and Hall mobility were enhanced from 2.77 × 1017 to 6.78 × 1017 cm−3 and from 0.37 to 2.07 cm2 V−1 s−1, corresponding to the annealing temperature from 200 to 350 °C. After annealing at 300 °C, the p-n junction exhibited the optimum rectifying characteristics, with a forward-to-reverse ratio over 103. The achievement of this ZnSnN2-based p-n junction makes an opening step forward to realize the practical application of the ZnSnN2 material. In addition, the nonideal behaviors of the p-n junctions under both positive and negative voltages are discussed, in hope of suggesting some ideas to further improve the rectifying characteristics.

Effect of Chloride Ligands on CdSe Nanocrystals by Cyclic Voltammetry and X-ray Photoelectron Spectroscopy

Rod-like octadecylphosphonic acid (ODPA) capped CdSe nanocrystals (NCs) produced by hot injection in the presence of chlorinated cosolvents modify their shape and surface properties by incorporation of chloride in the capping ligand shell. Correlated cyclic voltammetry (CV) and X-ray photoelectron spectroscopy (XPS) studies have been performed to address the effect of this incorporation on the NCs surface. In contrast to ODPA capped rod-like NCs, the XPS studies confirm that, during the oxidation of NCs containing chloride, not only the oxidation of Se surface atoms but also of Cd atoms takes place. Furthermore, XPS studies also confirm the partial reversibility of the Se oxidation in the presence of chloride. Both CV and subsequent XPS measurements allows identifying chemical environments and surface site modifications, essential to understand the stability and performance of NCs acting as active layers in optoelectronic devices.

Molecular Origin of Charge Traps in Polyfluorene-Based Semiconductors

The comprehensive control of morphology and structure is of extreme importance in semiconducting polymers when used as active layers in optoelectronic devices. In the work reported here, a systemat...