Advanced Electronic MaterialsVolume 2, Issue 1 1500179 Communication Gap States in Small Molecule Thin-Film Transistors Anita Risteska, Anita Risteska Research Center for Functional Materials and Nanomolecular Science, Jacobs University Bremen, 28759 Bremen, GermanySearch for more papers by this authorJavier Bedolla, Javier Bedolla Research Center for Functional Materials and Nanomolecular Science, Jacobs University Bremen, 28759 Bremen, GermanySearch for more papers by this authorJohn E. Northrup, John E. Northrup Palo Alto Research Center, Electronic Materials and Devices Laboratory, Palo Alto, CA, 94304 USA DeceasedSearch for more papers by this authorWerner Bergholz, Werner Bergholz Research Center for Advanced Systems Engineering, Jacobs University Bremen, 28759 Bremen, GermanySearch for more papers by this authorVeit Wagner, Veit Wagner Research Center for Functional Materials and Nanomolecular Science, Jacobs University Bremen, 28759 Bremen, GermanySearch for more papers by this authorDietmar Knipp, Corresponding Author Dietmar Knipp Research Center for Functional Materials and Nanomolecular Science, Jacobs University Bremen, 28759 Bremen, Germany Research Center for Advanced Systems Engineering, Jacobs University Bremen, 28759 Bremen, GermanyE-mail: [email protected]Search for more papers by this author Anita Risteska, Anita Risteska Research Center for Functional Materials and Nanomolecular Science, Jacobs University Bremen, 28759 Bremen, GermanySearch for more papers by this authorJavier Bedolla, Javier Bedolla Research Center for Functional Materials and Nanomolecular Science, Jacobs University Bremen, 28759 Bremen, GermanySearch for more papers by this authorJohn E. Northrup, John E. Northrup Palo Alto Research Center, Electronic Materials and Devices Laboratory, Palo Alto, CA, 94304 USA DeceasedSearch for more papers by this authorWerner Bergholz, Werner Bergholz Research Center for Advanced Systems Engineering, Jacobs University Bremen, 28759 Bremen, GermanySearch for more papers by this authorVeit Wagner, Veit Wagner Research Center for Functional Materials and Nanomolecular Science, Jacobs University Bremen, 28759 Bremen, GermanySearch for more papers by this authorDietmar Knipp, Corresponding Author Dietmar Knipp Research Center for Functional Materials and Nanomolecular Science, Jacobs University Bremen, 28759 Bremen, Germany Research Center for Advanced Systems Engineering, Jacobs University Bremen, 28759 Bremen, GermanyE-mail: [email protected]Search for more papers by this author First published: 18 November 2015 https://doi.org/10.1002/aelm.201500179Citations: 11 Read the full textAboutPDF ToolsRequest permissionExport citationAdd to favoritesTrack citation ShareShare Give accessShare full text accessShare full-text accessPlease review our Terms and Conditions of Use and check box below to share full-text version of article.I have read and accept the Wiley Online Library Terms and Conditions of UseShareable LinkUse the link below to share a full-text version of this article with your friends and colleagues. Learn more.Copy URL Graphical Abstract The influence of environmental effects on the stability of organic thin-film transistors (TFTs) is studied. The formation of moisture and oxygen-induced gap states in pentacene TFTs is investigated by electrical measurements and numerical simulations. An atomistic model is proposed, providing a consistent picture of the formation of electrically active gap states and their influence on the operation of TFTs. Citing Literature Volume2, Issue1January 20161500179 RelatedInformation

The current status of UV lasers and laser diodes (LDs) based on group III-nitrides is reviewed. The focus is on the design, fabrication, and performance of AlGa(In) N laser heterostructures grown by metal-organic vapor phase epitaxy (MOVPE) on high-quality bulk AlN substrates. The review begins with the fundamentals of laser diode operation and identifies the challenges to realize short-wavelength devices with wide band gap materials. In particular, simultaneously achieving high material quality and good p-type conductivity becomes increasingly challenging with higher aluminum concentrations in the epitaxial films. Using low defect density bulk AlN substrates is a good strategy to realize high internal quantum efficiencies and, ultimately, high gain within the active zone. Polarization-assisted hole generation with a short-period superlattice for the cladding layer is a viable approach to overcome the limitations of thermally activated p-type doping. Topics include LD processing considerations that are relevant for the high band gap materials, issues related to efficient carrier injection at the high current densities required for LD operation, and specific approaches to improve the functionality of the electron blocking layer. Next, results are presented for optically pumped UV lasers, with wavelengths down to lambda = 237 nm and low lasing thresholds, and design options are described to manipulate the polarization of the emitted laser light. The review concludes with a discussion of alternative laser designs to realize deep-UV laser emission with nitride semiconductors.

We present a model to estimate the mobility in polymers and to gauge the effect of increased interconnectivity at the level of polymer segments. The model takes into account the finite length of the polymers and the two-dimensional nature of the transport. These results show that the presence of polymer segments extending from one ordered domain into another can enhance the mobility significantly provided the π-π coupling is sufficiently large.

Side‐chain engineering is increasingly being utilized as a technique to impact the structural order and enhance the electronic properties of semiconducting polymers. However, the correlations drawn between structural changes and the resulting charge transport properties are typically indirect and qualitative in nature. In the present work, a combination of grazing incidence X‐ray diffraction and crystallographic refinement calculations is used to determine the precise molecular packing structure of two thiophene‐based semiconducting polymers to study the impact of side‐chain modifications. The optimized structures provide high‐quality fits to the experimental data and demonstrate that in addition to a large difference in interchain spacing between the two materials, there exists a significant disparity in backbone orientation as well. The calculated structures are utilized in density functional theory calculations to determine the band structure of the two materials and are shown to exhibit a dramatic disparity in interchain dispersion which accounts for the large observed difference in charge carrier mobility. The techniques presented here are meant to be general and are therefore applicable to many other highly diffracting semicrystalline polymers.

We describe the properties of in-well pumped blue InGaN/GaN vertical-external-cavity surface-emitting lasers. The laser hetero-structures were deposited on bulk GaN substrates by using metal–organic vapor phase epitaxy near atmospheric pressure. The active zones are comprised of up to 20 InGaN quantum wells distributed in a resonant periodic gain configuration. High-reflectivity dielectric distributed Bragg-reflectors were used as mirrors. Lasing was achieved at a wavelength of about 440–445 nm by exclusively exciting the quantum wells with the 384 nm emission line of a dye/N2 laser. The laser threshold was about 240 kW/cm2. The small pump spot diameter of about 20 µm and the usage of dielectric mirrors result in a rather high thermal resistance, which was experimentally determined by using an all optical measurement technique based on the temperature-dependent change of the refractive index of the device.

We present a comparison based on first-principles calculations of the electronic structure of nonalkylated and alkylated dinaphtho[2,3-b:2',3'-f]thieno[3,2-b]thiophene: DNTT and DNTT-C10. The calculations show that the addition of alkyl chains decreases intermolecular distances, in agreement with experiments. Calculations indicate that effective masses are reduced by the addition of alkyl chains, and within a simple deformation potential model, this translates into higher mobility for DNTT-C10. The shorter intermolecular distances found in DNTT-C10 are attributed to van der Waals interactions between alkyl chains.

In-well-pumped blue InGaN/GaN vertical-external-cavity surface-emitting lasers are demonstrated. The laser structures were grown on bulk GaN substrates by using metal-organic vapor phase epitaxy near atmospheric pressure. The active zone consisted of up to 20 InGaN quantum wells distributed in a resonant periodic gain configuration. High-reflectivity dielectric distributed Bragg-reflectors were used as mirrors. Laser emission with a single longitudinal mode at 440 nm was achieved by exclusively pumping the quantum wells with the 384 nm emission line of a dye-/N2-laser.

Defects caused by radiation induced hydrogen rearrangement may be a source of recombination centers in organic solar cells. It is important to determine the minimum energy required to form such defects. We present density functional calculations of the energy barriers that must be surmounted to form hydrogen related defects in polymers such as poly(3-alkylthiophene). Pathways corresponding to interpolymer and intrapolymer H rearrangements were explored. The calculations indicate that radiation induced gap state production in polymers such as poly(3-alkylthiophene) becomes possible when the energy of the incident radiation exceeds a threshold value in the range from 2.7 to 3.2 eV. (C) 2013 The Japan Society of Applied Physics

Improved p-type conductivity is demonstrated in AlGaN:Mg superlattice (SL) cladding layers with average Al composition ∼60%. The vertical conductivity ranges from 6.6 × 10−5 S/cm at a DC current of 1 mA to ∼0.1 S/cm at 550 mA and approaches the lateral conductivity that was obtained from Hall-effect measurements. The effective acceptor activation energy (EA) in the SL was determined to be 17 meV, nearly 10× smaller than EA in homogeneous p-GaN. The devices sustain current densities of 11 kA/cm2 under DC and up to 21 kA/cm2 under pulsed operation.

Recombination in bulk heterojunction organic solar cells is explored by observing the result of prolonged white light illumination and thermal annealing. The photocurrent spectral response, the steady state photocurrent-voltage characteristics and transient photoconductivity measurements on PCDTBT:PCBM and P3HT:PCBM solar cells provide information about recombination centers. Illumination generates deep traps while thermal annealing broadens the band tail localized states. Both types of state act as recombination centers. The evidence suggests that hydrogen abstraction, migration and re-bonding is responsible for the creation and recovery of recombination centers.

For AlGaN-based multi-quantum-well light emitters grown on c-plane substrates there is a tendency for the polarization of the emitted light to switch from transverse electric (TE) polarization to transverse magnetic (TM) polarization as the wavelength decreases. This transition depends on various factors that include the strain in the quantum well. Experimental results are presented that illustrate the phenomenon for nitride light emitting diodes (LEDs) grown on sapphire and on bulk AlN. Model calculations are presented which quantify the dependence of the TE/TM switch on the quantum well strain and the Al composition in the barriers surrounding the well.

AlGaN based heterostructures exhibit photo-pumped stimulated emission out to 237 nm with a switched from TE to TM polarized observed. PL lifetimes reflect the stronger exciton binding energy of the higher Al content material.