Top Contact Research Articles

Melanin-Based Thin Films for Sustainable Electronic Devices

Melanin is a natural (bio-sourced) and bio compatible material, attracting attention because of its physicochemical properties. Melanin is a family of pigments and the brown-black eumelanin pigment is part of this. It can be found in skin, hair, eyes, and brain [1-2]. Considering its quinone-based and conjugated molecular structure, eumelanin is an interesting candidate for bioelectronics. Transport physics in eumelanin-based thin films and pellets need to be better understood to optimize its technological applications. The main challenge for the use of eumelanin in bioelectronics is to increase the conductivity and detailed study is required to clarify and quantify the electronic and protonic contribution to charge transport.High quality thin films of eumelanin material on substrates such as ITO (Indium Tin Oxide) and FTO (Fluorine Tin Oxide) substrates were prepared to study the electronic transport properties of the biopigment. The electrodeposition method, spray pyrolysis, and spin coating were used for the preparation of uniform thin films. The structural, optical, and morphological properties of these thin films were studied using X-ray diffraction (XRD), atomic force microscopy (AFM), field emission electron microscopy (FESEM) and transmission electron microscopy (TEM). For the metal contacts both top contacts using a through shadow masking and photolithography were used. Preliminary results on the dependence of electronic transport on temperature, relative humidity, and interelectrode distance range were collected.Reference[1] Julia Wunsche, Yingxin Deng, Prajwal Kumar, Eduardo Di Mauro, Erik Josberger, Jonathan Sayago, Alessandro Pezzella, Francesca Soavi, Fabio Cicoira, Marco Rolandi, and Clara Santato, Chem. Mater. 2015,27, 436−442.[2] Prajwal Kumar, Eduardo Di Mauro, Shiming Zhang, Alessandro Pezzella, Francesca Soavi, Clara Santato and Fabio Cicoira, J. Mater. Chem. C, 2016, 4, 9516.

Fabrication of Highly Stable Non-Volatile Memory Device Using Plasma-Polymerisation of Hexamethyldisiloxane with Graphene Quantum Dots

We demonstrated the fabrication of a highly stable non-volatile memory (NVM) device using pulsed radiofrequency (rf) plasma polymerization and a simple solution route. The two-terminal NVM devices were fabricated based on a metal-insulator-metal (MIM) structure consisting of graphene quantum dots (GQDs) embedded in hexamethyldisiloxane (HMDSO) dielectric layers. GQDs, the charge trapping layer, and the top contacts were formed by spin coating and spray coating methods. Whereas, the dielectric layers were deposited using pulsed rf plasma polymerization as it is a no thermal stressed induced method to deposit the insulator layers and does not disturb the deposited charge trapping nanoparticles. The current-voltage (I-V) curves showed highly reproducible bistable current behavior with the presence of a hysteresis window when measured at room temperature. The fabricated NVM memory devices were reprogrammable for multiple times with a distinct ON/OFF ratio of 103. Various conduction mechanisms were proposed based on the obtained I-V data.

The impact of the charge barrier height on Germanium (Ge) detectors with amorphous-Ge contacts for light dark matter searches

Germanium (Ge) detectors with ability of measuring a single electron–hole (e–h) pair are needed in searching for light dark matter (LDM) down to the MeV scale. We investigate the feasibility of Ge detectors with amorphous-Ge (a-Ge) contacts to achieve the sensitivity of measuring a single e-h pair, which requires extremely low leakage current. Three Ge detectors with a-Ge contacts are used to study the charge barrier height for blocking electrons and holes. Using the measured bulk leakage current and the Döhler–Brodsky model, we obtain the values for charge barrier height and the density of localized energy states near the Fermi energy level for the top and bottom contacts, respectively. We predict that the bulk leakage current is extremely small and can be neglected at helium temperature (sim 4 K). Thus, Ge detectors with a-Ge contacts possess the potential to measure a single e–h pair for detecting LDM particles.

(Invited) Fundamentals and Process Technologies of Current Aperture Vertical Ga2O3 MOSFETs

Ga2O3 has exploded onto the semiconductor landscape for next-generation power electronics because its enticing material properties, most notably a large critical field strength stemming from its ultra-wide bandgap of 4.5 eV, promise miniaturized circuits and systems with high conversion efficiency. High-quality native substrates produced by melt-growth methods offer an economical platform to this technology. Ga2O3 transistors reported to date have predominantly taken on a lateral geometry, yet vertical switching devices are desirable for applications that demand high voltage ratings and high power levels since they allow for large current drives, simplified thermal management, and superior field termination. Capitalizing on cost-effective ion implantation technologies for shallow donor (silicon) and deep acceptor (nitrogen) doping of Ga2O3 with efficient dopant activation at a low thermal budget, we developed both depletion-mode (D-mode) [1] and enhancement-mode (E-mode) [2] current aperture vertical Ga2O3 metal-oxide-semiconductor field-effect transistors (MOSFETs).The vertical Ga2O3 MOSFETs were fabricated on low-doped (~1016 cm-3) halide vapor phase epitaxial drift layers grown on single-crystal β-Ga2O3 (001) substrates [3]. In these devices, silicon ion-implanted top source contacts were electrically isolated from the bottom drain contact by a nitrogen ion-implanted current blocking layer (CBL) [4] except at an aperture bounded by CBLs through which drain current was conducted. The CBL simultaneously served as a back-barrier for a top-gated lateral channel defined by another silicon ion implantation step. D-mode devices delivered a drain current density of 0.42 kA/cm2, a specific on-resistance of 31.5 mΩ·cm2, and an output current on/off ratio of over 108. E-mode devices, whose positive threshold voltage was attained by appropriately designing the channel doping concentration such that the channel was fully depleted at 0-V gate bias, showed a high output current on/off ratio of 2×107 despite a nonideal MOS interface that limited the maximum drain current density to <0.1 kA/cm2. Both D-mode and E-mode devices did not exhibit current collapse in pulsed measurements when subjected to off-state voltage stress; however, their hard breakdown occurred prematurely owing to leakage through the CBLs, a deficiency expected to be readily overcome with an optimized nitrogen implantation process. Further details regarding fabrication, performance, and physics of the devices, as well as potential directions for future development, will be discussed in the talk. The demonstration of planar-gate vertical Ga2O3 transistors based on a highly manufacturable all-ion-implanted process greatly enhances the prospects for Ga2O3-based power electronics applications.[1] M. H. Wong, K. Goto, H. Murakami, Y. Kumagai, and M. Higashiwaki, IEEE Electron Device Lett. 40, 431 (2019).[2] M. H. Wong, H. Murakami, Y. Kumagai, and M. Higashiwaki, Proc. 77th Device Research Conference (2019).[3] H. Murakami, K. Nomura, K. Goto, K. Sasaki, K. Kawara, Q. T. Thieu, R. Togashi, Y. Kumagai, M. Higashiwaki, A. Kuramata, S. Yamakoshi, B. Monemar, and A. Koukitu, Appl. Phys. Express 8, 015503 (2015).[4] M. H. Wong, C.-H. Lin, A. Kuramata, S. Yamakoshi, H. Murakami, Y. Kumagai, and M. Higashiwaki, Appl. Phys. Lett. 113, 102103 (2018).

Numerical Study of Graphene Heat Spreaders for a THz Quantum Diode Based on a G-MGIM Junction

In the context of a numerical study about mid-infrared receivers based on a metal–insulator–metal junction, we study how a top multilayer of graphene is capable to remove heat from the thin metal layer in which the laser impinges on. Due to its extremely high thermal conductivity, the graphene film (made of several layers) extracts the heat and injects it inside bulky lateral metal paddings of the receiver. Therefore, the lateral metal contacts not only detect the electromagnetic field transforming it into a current through this quantum diode, but they can also help to drain the heat. A metal–insulator–metal junction, which combines Kretschmann illumination and a distributed light on the junction (or are illuminated through a grating), allows introducing the electromagnetic field into the junction, biasing in this manner the insulator all along its entire length. The main challenge is the relationship between the insulator thickness and the electromagnetic wavelength. In the mid-infrared region, the insulator thickness of a few nanometers is several orders of magnitude under the diffraction limit, and the electromagnetic field cannot penetrate directly into the structure. In this way, the described technique delivers the infrared radiation into the junction by means of surface-plasmon-polariton traveling wave (SPP-TW). Therefore, a right illumination improves the diode responsivity considerably. However, rectification demands an extremely asymmetrical current–voltage curve. A layered metal–graphene–insulator–metal makes this feasible especially when an SPP-TW, between the graphene and the insulator, establishes a Seebeck effect resulting in the desired asymmetric characteristic. When sampling, the rectified direct tunneling current is desirable to avoid thermionic emission (inherently slow), as well as to induce the SPP-TW in a better form from the outside of the junction. A cooling mechanism that preserves a high electric conductivity on the top metal contact is required. In this theoretical work, we performed a simulation study of how a sheet of graphene is able to enhance the thermal behavior of the receiver under study.

Novel design strategy for GaAs‐based solar cell by application of single‐walled carbon nanotubes topmost layer

AbstractAttempts to improve solar cells efficiency touch all its constituents and are directly related to their fabrication protocols. While the most promising material platform for high efficiency photovoltaic devices is still III‐V semiconductors, introduction of novel materials like single‐walled carbon nanotubes (SWCNTs), which are characterized by unique combination of conductivity and transparency, might greatly yield the device performance. Here, for the first time, we present the results of the fabrication and characterization of a thin‐film GaAs solar cell with a SWCNT top contact. We examine the contact between the SWCNT film and the semiconductor structure by means of the optical and electron beam‐induced current techniques. The fabricated device demonstrates better performance, that is, increased power conversion efficiency from 10.6% to 11.5% when compared to the cell with the traditional metal contact grid, stemming from the enhanced photocurrent collection efficiency and low parasitic light absorption in the emitter layer. We envision future prospects to exploit the multifunctionality of the SWCNTs in fabrication of highly efficient photovoltaic devices including flexible solar cells.

Improved photovoltaic performance of graphene-based solar cells on textured silicon substrate

Abstract Graphene has attracted much interest as an active layer in heterojunction solar cells due to its outstanding properties such as flexibility, transparency, mechanical strength and elevated carrier mobility. In this research, a new technique was presented in order to enhance the efficiency of graphene–based heterojunction solar cells by employing a textured silicon (Si) substrate. Here, two sets of devices were fabricated based on flat and pyramidal structure of Si and the photovoltaic properties of graphene/Si heterojunction solar cells were compared. Selective chemical dissolution of Si wafers was carried out in order to produce pyramidal skeleton. Reduced graphene oxide (rGO) was then transferred on pyramidal Si through electrophoretic deposition (EPD) technique. The evidence of graphene layers on Si substrates was studied using Raman spectroscopy, X–ray diffractometry (XRD) and atomic force microscopy (AFM) analysis. The morphology of samples indicated an enhancement in rGO/Si interface area when the pyramidal structure is applied. Moreover, the enhanced surface area of this sample which is due to elevated roughness of pyramidal structure and wrinkles of graphene layers promotes its antireflective behavior which was proven using reflectance spectroscopy. The average reflectance of the graphene layer on the textured Si was ∼14% in the wavelength range of 400–800 nm which is lower than that of rGO on flat Si. The improved optical properties of graphene on pyramidal silicon can broaden its potential applications in optoelectrical devices such as high-efficiency solar cells. In order to study the photovoltaic properties of rGO/Si samples, a passive layer was formed on Si substrate and a square frame of Ag was coated on it which was acted as a top contact. The current–voltage characteristics showed that the efficiency of rGO/Si heterojunction solar cells was improved when textured silicon was applied.

Reliable inkjet contact metallization on printed polymer semiconductors for fabricating staggered TFTs

Even though the fundamental benefits of the staggered bottom-gate top-contact geometry in organic thin-film transistors (TFTs) have been fully demonstrated for enhancing the charge injection efficiency, most printed organic TFTs with inkjet-printed source/drain electrodes have only been fabricated in bottom-contact configurations, probably due to the difficulty of printing with metal-nanoparticle ink on soluble organic semiconductor (OSC) without film deformation/dissolution. Here, we present the printing process and the electrical characteristics of inkjet-printed top contact OSC in bottom-gate TFTs. We first fabricated polymer bottom-gate TFTs by printing in two different top- and bottom-contact configurations. The physical carrier mobility of the two TFTs was extracted from a gated-contact organic TFT model to exclude the effect of geometric contact resistance RC. When compared to assess the OSC film damage from the printing metallization, the two mobility values were almost identical. This result indicates that the metal nanoparticle ink used in this work formed a top metal contact on the OSC film without significant chemical damage. Furthermore, the printed top-contact TFT exhibited I–V characteristics almost identical to those of a thermally evaporated Ag top-contact TFT. This study suggests the possibility of inkjet-printed top metal contacts for organic thin-film devices such as transistors, solar cells, and diodes.

FingerprintContacts: Predicting Alternative Conformations of Proteins from Coevolution.

Proteins are dynamic molecules which perform diverse molecular functions by adopting different three-dimensional structures. Recent progress in residue-residue contacts prediction opens up new avenues for the de novo protein structure prediction from sequence information. However, it is still difficult to predict more than one conformation from residue-residue contacts alone. This is due to the inability to deconvolve the complex signals of residue-residue contacts, i.e., spatial contacts relevant for protein folding, conformational diversity, and ligand binding. Here, we introduce a machine learning based method, called FingerprintContacts, for extending the capabilities of residue-residue contacts. This algorithm leverages the features of residue-residue contacts, that is, (1) a single conformation outperforms the others in the structural prediction using all the top ranking residue-residue contacts as structural constraints and (2) conformation specific contacts rank lower and constitute a small fraction of residue-residue contacts. We demonstrate the capabilities of FingerprintContacts on eight ligand binding proteins with varying conformational motions. Furthermore, FingerprintContacts identifies small clusters of residue-residue contacts which are preferentially located in the dynamically fluctuating regions. With the rapid growth in protein sequence information, we expect FingerprintContacts to be a powerful first step in structural understanding of protein functional mechanisms.

The analytical model of frequency response and bandwidth for coupled multilayer graphene nanoribbon interconnects

This paper applies the multilayer graphene nanoribbons (MLGNR) as a prospective candidate material to replace the conventional copper (Cu) for next generation on-chip interconnects. Based on the equivalent single conductor (ESC) modeling, a distributed transmission line model for coupled MLGNR interconnects is presented to obtain the transfer gain and 3-dB bandwidth at the global level of 14 ​nm technology node, which take the in-phase and out-of-phase crosstalk modes into consideration, and verified with the Hspice simulation. It is demonstrated that the MLGNR and Cu interconnects under in-phase mode have a larger transfer gain and a wider 3-dB bandwidth than that under out-of-phase mode, and MLGNR interconnects can more effectively improve the transfer gain and 3-dB bandwidth compared with Cu interconnects at the same conditions. The numerical calculation results show that, decreasing interconnect temperature, increasing line space and increasing driver and load sizes are efficient ways to enhance the transfer gain and 3-dB bandwidth for MLGNR and Cu interconnects. Moreover, the top contact MLGNR (TC-MLGNR) also can be used as an emerging material for on-chip interconnects to replace the conventional Cu. In addition, it is found that the results obtained by the proposed model are in good matching with Hspice simulation and the maximum relative deviation for them is less than 4%, meanwhile the efficiency of CPU runtime is improved over 90% by using the proposed model, as compared to Hspice simulation. The results presented in this paper would be useful to design the on-chip interconnects with better bandwidth characteristics.

Near-infrared lateral photoresponse in InGaAs/GaAs quantum dots

Infrared photodetectors with In(Ga)As quantum dot (QD) active element functioning on interband and intersubband transitions are currently actively investigated, however, the vertical sensors were mostly reported. In the current study, a multilayer In0.4Ga0.6As/GaAs QD photodetector structure allowing the lateral photocurrent detection at normal incidence has been prepared depositing top contact. In order to have a comparison, a heterostructure with only a stack of In0.4Ga0.6As/GaAs wetting layers (WL) has been grown. In-depth photoelectrical characterization shows an effective broad-band photodetection related to the interband transitions between quantum-confined levels in QDs ranging from 1.03 to 1.38 eV (0.9–1.2 μm) that covers much wider infrared range in comparison to that from WLs (1.27–1.38 eV). Photoluminescence spectroscopy confirms the existence of QD transitions, observed as intense QD emission which peaked at 1.12 eV (∼1 μm) redshifted in comparison to the WL structure. The mechanisms of photoconductivity are modelled and discussed, comparing both the structures. We also show that our QD stack has an order lower contribution from defects compared to similar QD structures investigated before. At the same time, our structures demonstrate appropriate device characteristics at room temperature, such as the wide dynamic range from 10–2 to 103 μW cm−2 and a high photoresponsivity up to 20 A W−1 at low excitation intensities over 10–2–10–1 μW cm−2, while at higher excitation intensities the responsivity is reduced, exhibiting a strong spectral dependence. Thereby, our results show that the grown multilayer In0.4Ga0.6As/GaAs QD heterostructure is of relevant interest for application in lateral QD photodetectors.

Vertical Ge Gate-All-Around Nanowire pMOSFETs With a Diameter Down to 20 nm

In this work, we demonstrate vertical Ge gate-all-around (GAA) nanowire pMOSFETs fabricated with a CMOS compatible top-down approach. Vertical Ge nanowires with diameters down to 20 nm and an aspect ratio of ~11 were achieved by optimized Cl <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</sub> -based dry etching and self-limiting digital etching. Employing a GAA architecture, post-oxidation passivation and NiGe contacts, high performance Ge nanowire pMOSFETs exhibit low SS of 66 mV/dec, small DIBL of 35 mV/V and a high I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">ON</sub> /I <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">OFF</sub> ratio of 2.1 × 10 <sup xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">6</sup> . The electrical behavior was also studied with temperature-dependent measurements. The deviation between the experimental SS and the ideal kT/q ·ln10 values stems from the density of interface traps (D <sub xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">it</sub> ). Our measurements suggest that lowering the top contact resistance is a key to further performance improvement of vertical Ge GAA nanowire transistors.

Redox Control of Charge Transport in Vertical Ferrocene Molecular Tunnel Junctions

Summary Controlling charge transport through molecular tunnel junctions is of crucial importance for exploring basic physical and chemical mechanisms at the molecular level and realizing the applications of molecular devices. Here, through a combined experimental and theoretical investigation, we demonstrate redox control of cross-plane charge transport in a vertical gold/self-assembled monolayer (SAM)/graphene tunnel junction composed of a ferrocene-based SAM. When an oxidant/reductant or electrochemical control is applied to the outside surface of the neutral single-layer graphene top electrode, reversible redox reactions of ferrocene groups take place with charges crossing the graphene layer. This leads to counter anions on the outer surface of graphene, which balance the charges of ferrocene cations in the oxidized state. Correspondingly, the junctions switch between a high-conductance, neutral state with asymmetrical characteristics and a low-conductance, oxidized state with symmetrical characteristics, yielding a large on/off ratio (>100).

Effects of Annealing on Characteristics of Cu2ZnSnSe4/CH3NH3PbI3/ZnS/IZO Nanostructures for Enhanced Photovoltaic Solar Cells.

This paper presents new photovoltaic solar cells with Cu2ZnSnSe4/CH3NH3PbI3(MAPbI3)/ZnS/IZO/Ag nanostructures on bi-layer Mo/FTO (fluorine-doped tin oxide) glasssubstrates. The hole-transporting layer, active absorber layer, electron-transporting layer, transparent-conductive oxide layer, and top electrode-metal contact layer, were made of Cu2ZnSnSe4, MAPbI3 perovskite, zincsulfide, indium-doped zinc oxide, and silver, respectively. The active absorber MAPbI3 perovskite film was deposited on Cu2ZnSnSe4 hole-transporting layer that has been annealed at different temperatures. TheseCu2ZnSnSe4 filmsexhibitedthe morphology with increased crystal grain sizesand reduced pinholes, following the increased annealing temperature. When the perovskitefilm thickness was designed at 700 nm, the Cu2ZnSnSe4 hole-transporting layer was 160 nm, and the IZO (indium-zinc oxide) at 100 nm, and annealed at 650 °C, the experimental results showed significant improvements in the solar cell characteristics. The open-circuit voltage was increased to 1.1 V, the short-circuit current was improved to 20.8 mA/cm2, and the device fill factor was elevated to 76.3%. In addition, the device power-conversion efficiency has been improved to 17.4%. The output power Pmax was as good as 1.74 mW and the device series-resistance was 17.1 Ω.

Solar cells based on GaAs and carbon nanotubes

This work is devoted to fabrication and characterization of a GaAs solar cell with a CNT top contact. The characteristics of a conventional solar cell with a metal contact grid and a solar cell with a CNT electrode were compared to demonstrate the benefits of a CNT based approach. The photoelectric properties of the solar cells were studied by measuring the IV characteristics and external quantum efficiency. The fabricated CNT solar cell showed better performance due to improved photocurrent collection efficiency. The demonstrated technology for the fabrication of CNT electrodes can be applied to a wide range of semiconductor photovoltaic devices, including flexible thin-film solar cells and solar cells based on nanowires.

Memristive and Memory Impedance Behavior in a Photo-Annealed ZnO–rGO Thin-Film Device

An oxygen-rich ZnO-reduced graphene oxide (rGO) thin film was synthesized using a photo-annealing technique from zinc precursor (ZnO)–graphene oxide (GO) sol–gel solution. X-ray diffraction (XRD) results show a clear characteristic peak corresponding to rGO. The scanning electron microscope (SEM) image of the prepared thin film shows an evenly distributed wrinkled surface structure. Transition Metal Oxide (TMO)-based memristive devices are nominees for beyond CMOS Non-Volatile Memory (NVRAM) devices. The two-terminal Metal–TMO (Insulator)–Metal (MIM) memristive device is fabricated using a synthesized ZnO–rGO as an active layer on fluorine-doped tin oxide (FTO)-coated glass substrate. Aluminum (Al) is deposited as a top metal contact on the ZnO–rGO active layer to complete the device. Photo annealing was used to reduce the GO to rGO to make the proposed method suitable for fabricating ZnO–rGO thin-film devices on flexible substrates. The electrical characterization of the Al–ZnO–rGO–FTO device confirms the coexistence of memristive and memimpedance characteristics. The coexistence of memory resistance and memory impedance in the same device could be valuable for developing novel programmable analog filters and self-resonating circuits and systems.

Enhanced terahertz absorption of quantum wells sandwiched between heavily doped contacts based on micro-cavity resonance

A novel high-efficiency microcavity structure of quantum wells sandwiched between periodic heavily Si-doped GaAs top contact gratings and bottom contact film has been proposed as the optical coupler of a terahertz quantum well photodetector (THz QWP). Similar to metal at visible light, highly doped semiconductors exhibit plasma frequencies at mid- and far-infrared wavelengths. The intersubband absorption spectra and electric field distribution of the microcavity THz QWP are calculated with the finite difference time-domain method. Our results indicate that the frequency of the surface plasmon polariton can be tuned to the microcavity resonant mode under an optimized structure and the intersubband absorption is efficiently enhanced by the microcavity structure. When the doping concentration of the contact exceeds 1018 cm−3, the intersubband absorption of the microcavity THz QWP at the response wavelength is over one order of magnitude higher than that of the standard 45° device. In addition, the angle of the incident light only influences the intensity of the absorptivity, indicating that the designed device was independent of the periodic surface structure.

Solution-Processed Amorphous Zinc Oxide Thin Film Transistor Based NAND Gate

Herein, we discuss the synthesis and deposition of thin films amorphous zinc oxide (a: ZnO) by custom-designed spray pyrolysis unit for Thin Film Transistor (TFT) application towards NAND gate fabrication. Top gate top contact TFT was fabricated on a glass substrate, a: ZnO as the channel layer, PVA as gate dielectrics material and Al as electrodes. Electrical properties of a: ZnO TFT (W/L = 500/200μm) were probed. The individual transistor with a threshold voltage (Vth = 2.1 V), off and on current (Ioff = order of 10-8A; Ion = 10-3) and Ion / Ioff ratio (order of 105). The linear mobility is calculated and obtained as 3 cm-2/Vs. NAND gate is one of the universal and basic building blocks of a digital circuit. The fabricated NAND gate is subjected to the logic operation in the range of 0 to 10 V was tested. The result implies that it can be utilized for logical circuit operation.

Temperature dependence of charge transport in solid-state molecular junctions based on oligo(phenylene ethynylene)s

The ultimate goal of molecular electronics is to achieve practical applications. For approaching the target, we have successfully fabricated solid-state junctions based on oligo(phenylene ethynylene)s (OPEs) and cruciform OPEs with extended tetrathiafulvalene (TTF) (OPE3 and OPE3-TTF) self-assembled monolayers (SAMs) with a diamine anchoring group. SAMs were confined in micropores with gold substrates to ensure well-defined device surface areas. The transport properties were conducted on a double-junction layout, which the rGO films used for top contacts and interconnects between adjacent SAMs. The solid-state devices based on OPE3-TTF SAMs showed the expected higher conductance under ambient conditions because of the incorporation of a TTF moiety. The two devices displayed varying degrees of temperature dependence with decreasing temperature, which resulted from the cross-conjugated OPE3-TTF molecule exhibiting quantum interference while the linear-conjugated OPE3 molecule did not. This study shows the temperature dependence of the electrical properties of molecular devices based on cruciform OPEs, further enriching the research results of functional molecular devices.

Plasmon-Assisted Crystalline Silicon Solar Cell with TiO2 as Anti-Reflective Coating

The present study focuses on the employment of TiO2 (titanium dioxide) film as an anti-reflective coating (ARC) on thin crystalline silicon (Si)-based solar cells along with the incorporation of plasmonic silver nanoparticles (Ag NPs) on its front surface having ITO (Indium Tin Oxide) as top metal contact. The response of solar cell has been studied in context of the external quantum efficiency (EQE) and the photovoltaic current-voltage characteristics. The EQE is enhanced appreciably in the entire wavelength range of 300 to 1100 nm (from 13.26% for bare Si solar cell to 61.93% for solar cell having both TiO2 and Ag nanoparticles at a wavelength of 600 nm), on incorporating anti-reflective TiO2 coating and Ag nanoparticles on the thin Si solar cell. A significant increment in the power conversion efficiency from 9.53% (bare cell) to 16.04% (cell with anti-reflective coating of TiO2 thin film and silver nanoparticles) has also been observed. The presence of TiO2 film reduces the reflection of incident light from the top Si surface and hence appreciably improves solar cell efficiency. Besides this, Ag NPs help in trapping more light due to the plasmonic effect and hence result in further improvement in solar cell efficiency. The achieved results are encouraging for the commercial development of thin Si-based solar cells with TiO2 ARC and Ag NPs.