D. Bruce Buchholz

An all carbon counter electrode for dye sensitized solar cells

This article describes the design and operation of a dye sensitized solar cell with an all carbon counter electrode and plastic electrolyte. For the construction of the counter electrode, the conventional thin platinum catalytic layer was replaced by a novel large-effective-surface-area polyaromatic hydrocarbon (LPAH) film, and the fluorine doped tin oxide substrate was replaced by a graphite film. In this way the internal resistance of the cell was substantially reduced and the cell efficiency can reach nearly 9% using the masked frame measurement technique. To achieve such an efficiency, a series of experiments was carried out to assure that the LPAH layer possesses superior catalytic activity and energy efficiency compared to the commonly used carbon black. To this end a unique LPAH layer synthesis technique was developed. It involved the production of LPAH from a hydrogen arc along with the use of an amphiphilic triblock copolymer (P123) to improve the suspendability of LPAH to form a homogeneous catalytic layer. This layer was then attached to a graphite film to form the counter electrode for the dye sensitized solar cell. Details of the properties of the LAPH and the newly designed solar cell are reported herein.

High-Performance Flexible Transparent Thin-Film Transistors Using a Hybrid Gate Dielectric and an Amorphous Zinc Indium Tin Oxide Channel

[*] Prof. T. J. Marks, Prof. A. Facchetti, J. Liu Department of Chemistry and the Materials Research Center Northwestern University, 2145 Sheridan Road, Evanston, IL 60208 (USA) E-mail: t-marks@northwestern.edu; a-facchetti@northwestern.edu Prof. R. P. H. Chang, Dr. D. B. Buchholz Department of Materials Science and Engineering and the Materials Research Center Northwestern University, 2220 Campus Drive, Evanston, IL 60208 (USA) E-mail: r-chang@northwestern.edu

Control and Characterization of the Structural, Electrical, and Optical Properties of Amorphous Zinc−Indium−Tin Oxide Thin Films

Zinc-indium-tin oxide (ZITO) films are grown by pulsed-laser deposition in which 30% of the indium in the In(2)O(3) structure is replaced by substitution with zinc and tin in equal molar proportions: In(2-2x)Zn(x)Sn(x)O(3), where x = 0.3. Films grown at 25 and 100 degrees C exhibit electron diffraction patterns (EDPs) typical of amorphous materials. At a deposition temperature of 200 degrees C, evidence of crystallinity begins to appear in the EDP data and becomes more evident in films deposited at 400 degrees C. The advent of crystallinity affects the electrical properties of the ZITO film, and the effect is ascribed to the boundaries between phases in the films. The electrical and optical properties of the amorphous ZITO films grown at 25 degrees C are dependent on the oxygen partial pressure (P(O(2))) during film growth, transitioning from a high-mobility (36 cm(2)/V x s) conductor (sigma approximately 1700 S/cm) at P(O(2)) = 5 mTorr to a high-mobility semiconductor at P(O(2)) approximately 20 mTorr. Field-effect transistors (FETs) prepared with as-deposited amorphous ZITO channel layers on p(+)-Si/300 nm SiO(2) substrates yield FETs with on/off ratios of 10(6), off currents of 10(-8) A, and field-effect saturation mobilities of 10 cm(2)/V x s.

All-Amorphous-Oxide Transparent, Flexible Thin-Film Transistors. Efficacy of Bilayer Gate Dielectrics

Optically transparent and mechanically flexible thin-film transistors (TF-TFTs) composed exclusively of amorphous metal oxide films are fabricated on plastic substrates by combining an amorphous Ta(2)O(5)/SiO(x) bilayer transparent oxide insulator (TOI) gate dielectric with an amorphous zinc-indium-tin oxide (a-ZITO) transparent oxide semiconductor (TOS) channel and a-ZITO transparent oxide conductor (TOC) electrodes. The bilayer gate dielectric is fabricated by the post-cross-linking of vapor-deposited hexachlorodisiloxane-derived films to form thin SiO(x) layers (v-SiO(x)) on amorphous Ta(2)O(5) (a-Ta(2)O(5)) films grown by ion-assisted deposition at room temperature. The a-Ta(2)O(5)/v-SiO(x) bilayer TOI dielectric integrates the large capacitance of the high dielectric constant a-Ta(2)O(5) layer with the excellent dielectric/semiconductor interfacial compatibility of the v-SiO(x) layer in a-ZITO TOS-based TF-TFTs. These all-amorphous-oxide TF-TFTs, having a channel length and width of 100 and 2000 microm, respectively, perform far better than a-Ta(2)O(5)-only devices and exhibit saturation-regime field-effect mobilities of approximately 20 cm(2)/V x s, on-currents >10(-4) A, and current on-off ratios >10(5). These TFTs operate at low voltages (approximately 4.0 V) and exhibit good visible-region optical transparency and excellent mechanical flexibility.

The structure and properties of amorphous indium oxide

A series of In2O3 thin films, ranging from X-ray diffraction amorphous to highly crystalline, were grown on amorphous silica substrates using pulsed laser deposition by varying the film growth temperature. The amorphous-to-crystalline transition and the structure of amorphous In2O3 were investigated by grazing angle X-ray diffraction (GIXRD), Hall transport measurement, high resolution transmission electron microscopy (HRTEM), electron diffraction, extended X-ray absorption fine structure (EXAFS), and ab initio molecular dynamics (MD) liquid-quench simulation. On the basis of excellent agreement between the EXAFS and MD results, a model of the amorphous oxide structure as a network of InOx polyhedra was constructed. Mechanisms for the transport properties observed in the crystalline, amorphous-to-crystalline, and amorphous deposition regions are presented, highlighting a unique structure–property relationship.

Ultrafast modulation of the plasma frequency of vertically aligned indium tin oxide rods

Light-matter interaction at the nanoscale is of particular interest for future photonic integrated circuits and devices with applications ranging from communication to sensing and imaging. In this Letter a combination of transient absorption (TA) and the use of third harmonic generation as a probe (THG-probe) has been adopted to investigate the response of the localized surface plasmon resonances (LSPRs) of vertically aligned indium tin oxide rods (ITORs) upon ultraviolet light (UV) excitation. TA experiments, which are sensitive to the extinction of the LSPR, show a fluence-dependent increase in the frequency and intensity of the LSPR. The THG-probe experiments show a fluence-dependent decrease of the LSPR-enhanced local electric field intensity within the rod, consistent with a shift of the LSPR to higher frequency. The kinetics from both TA and THG-probe experiments are found to be independent of the fluence of the pump. These results indicate that UV excitation modulates the plasma frequency of ITO on the ultrafast time scale by the injection of electrons into, and their subsequent decay from, the conduction band of the rods. Increases to the electron concentration in the conduction band of ∼13% were achieved in these experiments. Computer simulation and modeling have been used throughout the investigation to guide the design of the experiments and to map the electric field distribution around the rods for interpreting far-field measurement results.

Transient photoresponse in amorphous In-Ga-Zn-O thin films under stretched exponential analysis

We investigated transient photoresponse and Hall effect in amorphous In-Ga-Zn-O thin films and observed a stretched exponential response which allows characterization of the activation energy spectrum with only three fit parameters. Measurements of as-grown films and 350 K annealed films were conducted at room temperature by recording conductivity, carrier density, and mobility over day-long time scales, both under illumination and in the dark. Hall measurements verify approximately constant mobility, even as the photoinduced carrier density changes by orders of magnitude. The transient photoconductivity data fit well to a stretched exponential during both illumination and dark relaxation, but with slower response in the dark. The inverse Laplace transforms of these stretched exponentials yield the density of activation energies responsible for transient photoconductivity. An empirical equation is introduced, which determines the linewidth of the activation energy band from the stretched exponential param...

Reinforced Self-Assembled Nanodielectrics for High-Performance Transparent Thin Film Transistors

Transparent thin fi lm transistors (TFTs) have stimulated great scientifi c and technological interest due to potential applications in “invisible” electronics, such as transparent touch panels and see-through displays. [ 1–6 ] Since the fi rst demonstration of transparent TFTs using a crystalline ZnO semiconductor, [ 7 ] extensive efforts have sought to enhance performance by increasing the fi eld-effect mobility ( μ FE ) and/or lowering the operating voltage. [ 8–12 ] Principal foci have included the semiconductor and gate dielectric, two essential TFT materials. Among the diverse transparent semiconductors, amorphous transparent oxide semiconductors (a-TOSs) offer distinctive attractions vis-à-vis organics and crystalline TOSs, including good mobility, excellent environmental stability, low-temperature processability, optical transparency, smooth surfaces, and compositional uniformity. [ 4 , 13–15 ] For example, amorphous Zn-In-Sn-O (a-ZITO) fi lms afford moderate TFT performance at operating voltages ≥ 10 V when paired with a SiO 2 gate dielectric. [ 16–20 ]

Quasi-reversible point defect relaxation in amorphous In-Ga-Zn-O thin films by in situ electrical measurements

Quasi-reversible oxygen exchange/point defect relaxation in an amorphous In-Ga-Zn-O thin film was monitored by in situ electrical property measurements (conductivity, Seebeck coefficient) at 200 °C subjected to abrupt changes in oxygen partial pressure (pO2). By subtracting the long-term background decay from the conductivity curves, time-independent conductivity values were obtained at each pO2. From these values, a log-log “Brouwer” plot of conductivity vs. pO2 of approximately −1/2 was obtained, which may indicate co-elimination (filling) of neutral and charged oxygen vacancies. This work demonstrates that Brouwer analysis can be applied to the study of defect structure in amorphous oxide thin films.

Ultra-sharp plasmonic resonances from monopole optical nanoantenna phased arrays

Diffractively coupled plasmonic resonances possess both ultra-sharp linewidths and giant electric field enhancement around plasmonic nanostructures. They can be applied to create a new generation of sensors, detectors, and nano-optical devices. However, all current designs require stringent index-matching at the resonance condition that limits their applicability. Here, we propose and demonstrate that it is possible to relieve the index-matching requirement and to induce ultra-sharp plasmon resonances in an ordered vertically aligned optical nano-antenna phased array by transforming a dipole resonance to a monopole resonance with a mirror plane. Due to the mirror image effect, the monopole resonance not only retained the dipole features but also enhanced them. The engineered resonances strongly suppressed the radiative decay channel, resulting in a four-order of magnitude enhancement in local electric field and a Q-factor greater than 200.