Modeling Space-Charge Limited Currents in Organic Semiconductors: Extracting Trap Density and Mobility

We have developed and applied a mobility edge model that takes into account drift and diffusion currents to characterize the space charge limited current in organic semiconductors. The numerical solution of the drift-diffusion equation allows the utilization of asymmetric contacts to describe the built-in potential within the device. The model has been applied to extract information of the distribution of traps from experimental current-voltage measurements of a rubrene single crystal from Krellner et al. [Phys. Rev. B, 75(24), 245115] showing excellent agreement across several orders of magnitude of current. Although the two contacts are made of the same metal, an energy offset of 580 meV between them, ascribed to differences in the deposition techniques (lamination vs. evaporation) was essential to correctly interpret the shape of the current-voltage characteristics at low voltage. A band mobility 0.13 cm2/Vs for holes was estimated, which is consistent with transport along the long axis of the orthorhombic unit cell. The total density of traps deeper than 0.1 eV was 2.2\times1016 cm-3. The sensitivity analysis and error estimation in the obtained parameters shows that it is not possible to accurately resolve the shape of the trap distribution for energies deeper than 0.3 eV or shallower than 0.1 eV above the valence band edge. The total number of traps deeper than 0.3 eV however can be estimated. Contact asymmetry and the diffusion component of the current play an important role in the description of the device at low bias, and are required to obtain reliable information about the distribution of deep traps

Contingent liquidity

After the crisis, bank regulators are considering mitigating liquidity risk by introducing quantity limits on liquidity and maturity mismatch. We argue that aggregate liquidity risk can be reduced with little deadweight loss by encouraging banks, through adequate regulatory relief, to satisfy part of their financing needs with a new class of securities. These would include a Roll-Over Option Facility (ROOF) that allows the issuer, for a price, to keep the funds if at maturity a readily observable variable correlated with systemic liquidity risk (e.g. the LIBOR-OIS spread) is above a trigger threshold. At roll-over the yield would reflect the current price of liquidity and credit risk, making ROOFs attractive to investors. The instrument could attenuate a liquidity crisis by reducing banks’ need to roll debt over or sell off assets, and diminish the probability of runs, if markets are convinced that banks can secure sufficient liquidity when needed thanks to the widespread use of this contingent claim.funding, liquidity, contingent claim, financial crisis

Multi-phase semicrystalline microstructures drive exciton dissociation in neat plastic semiconductors

The optoelectronic properties of macromolecular semiconductors depend fundamentally on their solid-state microstructure. For example, the molecular-weight distribution influences polymeric- semiconductor properties via diverse microstructures; polymers of low weight-average molecular weight (Mw) form unconnected, extended-chain crystals, usually of a paraffinic structure. Because of the non-entangled nature of the relatively short-chain macromolecules, this leads to a polycrystalline, one-phase morphology. In contrast, with high-Mw materials, where average chain lengths are longer than the length between entanglements, two-phase morphologies, comprised of crystalline moieties embedded in largely unordered (amorphous) regions, are obtained. We investigate charge photogeneration processes in neat regioregular poly(3-hexylthiophene) (P3HT) of varying Mw by means of time-resolved photoluminescence (PL) spectroscopy. At 10 K, PL originating from recombination of long-lived charge pairs decays over microsecond timescales. Both the amplitude and decay rate distribution depend strongly on Mw. In films with dominant one-phase chain-extended microstructures, the delayed PL is suppressed as a result of a diminished yield of photoinduced charges, and its decay is significantly faster than in two-phase microstructures. However, independent of Mw, charge recombination regenerates singlet excitons in torsionally disordered chains forming more strongly coupled photophysical aggregates than those in the steady-state ensemble, with delayed PL lineshape reminiscent of that in paraffinic morphologies at steady state. We conclude that highly delocalized excitons in disordered regions between crystalline and amorphous phases dissociate extrinsically with yield and spatial distribution that depend intimately upon microstructure.Comment: 19 pages, 4 figure

Consolidation and efficiency in the financial sector: a review of the international evidence

In response to fundamental changes in regulation and technology, the financial industry around the world is undergoing an unprecedented wave of consolidation. A growing body of empirical literature has attempted to measure the efficiency gains from M&As; however there is little sense of how the results might depend on the country, industry and time period analysed. In this paper we review critically works that cover the main sectors of the financial industry (commercial and investment banks, insurance and asset management companies) in the major industrialized countries over the last twenty years, searching for common patterns that transcend national and sectoral peculiarities. We find that consolidation in the financial sector is beneficial up to a relatively small size in order to reap economies of scale, but there is little evidence that mergers yield economies of scope or gains in managerial efficiency.mergers, efficiency, bank mergers

Securitization is not that evil after all

A growing number of studies on the US subprime market indicate that, due to asymmetric information, credit risk transfer activities have perverse effects on banksÂ’ lending standards. We investigate a large part of the market for securitized assets (“prime mortgages”) in Italy, a country with a regulatory framework analogous to the one prevalent in Europe. Information on over a million mortgages consists of loan-level variables, characteristics of the originating bank and, most importantly, contractual features of the securitization deal, including the seniority structure of the ABSs issued by the Special Purpose Vehicle and the amount retained by the originator. We borrow a robust way to test for the effects of asymmetric information from the empirical contract theory literature (Chiappori and Salanié, 2000). Overall, our evidence suggests that banks can effectively counter the negative effects of asymmetric information in the securitization market by selling less opaque loans, using signaling devices (i.e. retaining a share of the equity tranche of the ABSs issued by the SPV) and building up a reputation for not undermining their own lending standards.securitization, asymmetric information, signaling, reputation

Reversible and irreversible trapping at room temperature in poly(thiophene) thin-film transistors

We measured the bias stress characteristics of poly(thiophene) semi-crystalline thin-film transistors (TFTs) as a function stress times, gate voltages and duty-cycles. At room temperature, the bias stress has two components: a fast reversible component and a slow irreversible component. We hypothesize that the irreversible component is due to charge trapping in the disordered areas of the semiconductor film. At low duty-cycle (<2%), the fast bias stress component is reversed during the off-part of the cycle therefore the observed VT shift in only caused by irreversible trapping. Irreversible trapping follows power-law kinetics with a time exponent approximately equal to 0.37. We use these findings to estimate the lifetime of TFTs used as switches in display backplanes

Solid solutions of rare earth cations in mesoporous anatase beads and their performances in dye-sensitized solar cells

Solid solutions of the rare earth (RE) cations Pr3+, Nd3+, Sm3+, Gd3+, Er3+ and Yb3+ in anatase TiO2 have been synthesized as mesoporous beads in the concentration range 0.1-0.3% of metal atoms. The solid solutions were have been characterized by XRD, SEM, diffuse reflectance UV-Vis spectroscopy, BET and BJH surface analysis. All the solid solutions possess high specific surface areas, up to more than 100 m2/g. The amount of adsorbed dye in each photoanode has been determined spectrophotometrically. All the samples were tested as photoanodes in dye-sensitized solar cells (DSSCs) using N719 as dye and a nonvolatile, benzonitrile based electrolyte. All the cells were have been tested by conversion efficiency (J-V), quantum efficiency (IPCE), electrochemical impedance spectroscopy (EIS) and dark current measurements. While lighter RE cations (Pr3+, Nd3+) limit the performance of DSSCs compared to pure anatase mesoporous beads, cations from Sm3+ onwards enhance the performance of the devices. A maximum conversion efficiency of 8.7% for Er3+ at a concentration of 0.2% has been achieved. This is a remarkable efficiency value for a DSSC employing N719 dye without co-adsorbents and a nonvolatile electrolyte. For each RE cation the maximum performances are obtained for a concentration of 0.2% metal atoms. © 2015, Nature Publishing Group. All rights reserved

Why Do Banks Merge?

The banking industry is consolidating at an accelerating pace, yet no conclusive results have emerged on the benefits of mergers and acquisitions. We analyze the Italian market, which is similar to other main European countries. By considering both acquisitions (i.e. the purchase of the majority of voting shares) and mergers we evidence the motives and results of each type of deal. Mergers seek to improve income from services, but the increase is offset by higher staff costs; return on equity improves because of a decrease in capital. Acquisitions aim to restructure the loan portfolio of the acquired bank; improved lending policies result in higher profits.banks, mergers and acquisitions, performance