Reduction of oxide charge and interface-trap density in MOS capacitors with ito gates

The electrical properties of MOS capacitors with an indium tin oxide (ITO) gate are studied in terms of the number density of the fixed oxide charge and of the interface traps N/sub f/ and N/sub it/, respectively. Both depend on the deposition conditions of ITO and the subsequent annealing procedures. The fixed oxide charge and the interface-trap density are minimized by depositing at a substrate temperature of 240 degrees C at low power conditions and in an oxygen-rich ambient. Under these conditions, as-deposited ITO films are electrically conductive. The most effective annealing procedure consists of a two-step anneal: a 45-s rapid thermal anneal at 950 degrees C in N/sub 2/, followed by a 30 min anneal in N/sub 2//20% H/sub 2/ at 450 degrees C. Typical values obtained for N/sub it/ and N/sub f/ are 4.2*10/sup 10/ cm/sup -2/ and 2.8*10/sup 10/ cm/sup -2/, respectively. These values are further reduced to 1.9*10/sup 10/ cm/sup -2/ and &gt

Reduction of reflection losses in solid-state image sensors

\u3cp\u3eThe whole image area of frame transfer charge coupled device ( FT-CCD ) image sensors is light sensitive. The sensitivity for short wavelengths, however, is limited by the presence of polycrystalline silicon ( poly-Si ) electrodes. This paper describes elements contributing to the improvement of the light sensitivity. It will focus on the reduction of reflectance losses.\u3c/p\u3

Influence of annealing on the optical properties of indium tin oxide

\u3cp\u3eThe effect of annealing on the optical properties of indium tin oxide films prepared by d.c. magnetron sputtering was investigated. The influence of annealing can be described in terms of a change in free electron concentration. With an increase in carrier concentration the absorption edge due to the direct and the indirect transition shifts to the near UV. The intrinsic band gap is found to be 3.53 eV, the intrinsic indirect transition energy is 1.80 eV. The effective electron mass is 0.31 m\u3csub\u3e0\u3c/sub\u3e and 1.0 m\u3csub\u3e0\u3c/sub\u3e for electrons in the conduction and the valence band respectively.\u3c/p\u3

Multifunctional oxides for integrated manufacturing of efficient graphene electrodes for organic electronics

Using multi-functional oxide films, we report on the development of an integration strategy for scalable manufacturing of graphene-based transparent conducting electrodes (TCEs) for organic electronics. A number of fundamental and process challenges exists for efficient graphene-based TCEs, in particular, environmentally and thermally stable doping, interfacial band engineering for efficient charge injection/extraction, effective wetting, and process compatibility including masking and patterning. Here, we show that all of these challenges can be effectively addressed at once by coating graphene with a thin (>10 nm) metal oxide (MoO3 or WO3) layer. We demonstrate graphene electrode patterning without the need for conventional lithography and thereby achieve organic light emitting diodes with efficiencies exceeding those of standard indium tin oxide reference devices

Effect of the alkali metal content on the electronic properties of PEDOT:PSS

\u3cp\u3eThe effect of the sodium and cesium ion surface concentration on the electronic properties of spin-coated poly(3,4-ethylenedioxythiophene)- poly(styrenesulfonic acid) films, known as PEDOT:PSS, has been studied by means of ultraviolet and X-ray photoelectron spectroscopy. The sodium and cesium concentration in the film has been varied by the addition of NaOH or CsOH to the PEDOT:PSS dispersion. Hydrogen ions of the acid PSSH are exchanged for sodium or cesium ions, resulting in the salt PSSNa or PSSCs without changing the oxidation state of PEDOT, i.e., without doping/dedoping the material. The work function changes from 5.1 to 4.0 eV with increasing alkali surface concentration. The ionization potential remains constant at 5.0 eV above 1 at% alkali metal content and coincides with the work function below 1 at%. Thus, the material changes from a semiconductor-like to a metal-like state.\u3c/p\u3

Insights into fullerene passivation of SnO2 electron transport layers in perovskite solar cells

Interfaces between the photoactive and charge transport layers are crucial for the performance of perovskite solar cells. Surface passivation of SnO2 as electron transport layer (ETL) by fullerene derivatives is known to improve the performance of n–i–p devices, yet organic passivation layers are susceptible to removal during perovskite deposition. Understanding the nature of the passivation is important for further optimization of SnO2 ETLs. X‐ray photoelectron spectroscopy depth profiling is a convenient tool to monitor the fullerene concentration in passivation layers at a SnO2 interface. Through a comparative study using [6,6]‐phenyl‐C61‐butyric acid methyl ester (PCBM) and [6,6]‐phenyl‐C61‐butyric acid (PCBA) passivation layers, a direct correlation is established between the formation of interfacial chemical bonds and the retention of passivating fullerene molecules at the SnO2 interface that effectively reduces the number of defects and enhances electron mobility. Devices with only a PCBA‐monolayer‐passivated SnO2 ETL exhibit significantly improved performance and reproducibility, achieving an efficiency of 18.8%. Investigating thick and solvent‐resistant C60 and PCBM‐dimer layers demonstrates that the charge transport in the ETL is only improved by chemisorption of the fullerene at the SnO2 surface

Enhancement-Mode PEDOT:PSS Organic Electrochemical Transistors Using Molecular De-Doping

Organic electrochemical transistors (OECTs) show great promise for flexible, low-cost, and low-voltage sensors for aqueous solutions. The majority of OECT devices are made using the polymer blend poly(ethylenedioxythiophene):poly(styrene sulfonate) (PEDOT:PSS), in which PEDOT is intrinsically doped due to inclusion of PSS. Because of this intrinsic doping, PEDOT:PSS OECTs generally operate in depletion mode, which results in a higher power consumption and limits stability. Here, a straightforward method to de-dope PEDOT:PSS using commercially available amine-based molecular de-dopants to achieve stable enhancement-mode OECTs is presented. The enhancement-mode OECTs show mobilities near that of pristine PEDOT:PSS (≈2 cm2 V−1 s−1) with stable operation over 1000 on/off cycles. The electron and proton exchange among PEDOT, PSS, and the molecular de-dopants are characterized to reveal the underlying chemical mechanism of the threshold voltage shift to negative voltages. Finally, the effect of the de-doping on the microstructure of the spin-cast PEDOT:PSS films is investigated

Quantitative predictions of photoelectron spectra in amorphous molecular solids from multiscale quasiparticle embedding

We present a first-principles-based multiscale simulation framework for quantitative predictions of the high-energy part of the ultraviolet photoelectron spectroscopy (UPS) spectra of amorphous molecular solids. The approach combines a deposition simulation, many-body Green's function theory, polarizable film embedding, and multimode electron-vibrational coupling and provides a molecular-level view on the interactions and processes giving rise to spectral features. This insight helps bridging the current gap between experimental UPS and theoretical models as accurate analyses are hampered by the energetic disorder, surface sensitivity of the measurement, and the complexity of excitation processes. In particular, this is relevant for the unambiguous determination the highest occupied molecular orbital energy (HOMO) of organic semiconductors, a key quantity for tailoring and engineering new optoelectronic devices. We demonstrate the capabilities of the simulation approach studying the spectrum of two isomers of 2-methyl-9,10-bis(naphthalen-2-yl)anthracene as archetypical materials showing a clearly separated HOMO peak in experiment. The agreement with experiment is excellent, suggesting that our approach provides a route for determining the HOMO energy with an accuracy better than 0.1 eV

Polyanionic, alkylthiosulfate-based thiol precursors for conjugated polymer self-assembly onto gold and silver

Anionic, conjugated thiophene- and fluorene-based polyelectrolytes with alkylthiosulfate side chains undergo hydrolysis under formation of alkylthiol and dialkyldisulfide functions. The hydrolysis products can be deposited onto gold or silver surfaces by self-assembly from solutions of the anionic conjugated polyelectrolyte (CPE) precursors in polar solvents such as methanol. This procedure allows solution-based surface modifications of gold and silver electrodes using environmentally friendly solvents and enables the formation of conjugated polymer bilayers. The herein presented alkylthiosulfate-substituted CPEs are promising candidates for increasing the work function of gold and silver electrodes thus improving hole injection from such electrode assemblies into organic semiconductors

On the origin of dark current in organic photodiodes

\u3cp\u3eMinimizing the reverse bias dark current while retaining external quantum efficiency is crucial if the light detection sensitivity of organic photodiodes (OPDs) is to compete with inorganic photodetectors. However, a quantitative relationship between the magnitude of the dark current density under reverse bias (J\u3csub\u3ed\u3c/sub\u3e) and the properties of the bulk heterojunction (BHJ) active layer has so far not been established. Here, a systematic analysis of J\u3csub\u3ed\u3c/sub\u3e in state-of-the-art BHJ OPDs using five polymers with a range of energy levels and charge transport characteristics is presented. The magnitude and activation energy of J\u3csub\u3ed\u3c/sub\u3e are explained using a model that assumes charge injection from the metal contacts into an energetically disordered semiconductor. By relating J\u3csub\u3ed\u3c/sub\u3e to material parameters, insights into the origin of J\u3csub\u3ed\u3c/sub\u3e are obtained that enable the future selection of successful OPD materials.\u3c/p\u3