Delocalization Enhances Conductivity at High Doping Concentrations

Many applications of organic semiconductors require high electrical conductivities and hence high doping levels. Therefore, it is indispensable for effective material design to have an accurate understanding of the underlying transport mechanisms in this regime. In this study, own and literature experimental data that reveal a power-law relation between the conductivity and charge density of strongly p-doped conjugated polymers are combined. This behavior cannot consistently be described with conventional models for charge transport in energetically disordered materials. Here, it is shown that the observations can be explained in terms of a variable range hopping model with an energy-dependent localization length. A tight-binding model is used to quantitatively estimate of the energy-dependent localization length, which is used in an analytical variable range hopping model. In the limit of low charge densities, the model reproduces the well-known Mott variable range hopping behavior, while for high charge densities, the experimentally observed superlinear increase in conductivity with charge density is reproduced. The latter behavior occurs when the Fermi level reaches partially delocalized states. This insight can be anticipated to lead to new strategies to increase the conductivity of organic semiconductors

Many-body effects in the valence bands of two-dimensional heterostructures based on III/V semiconductors

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Negative capacitances in low-mobility solids

The neg. capacitance as often obsd. at low frequencies in semiconducting devices is explained by bipolar injection in diode configuration. Numerical calcns. are performed within the drift-diffusion approxn. in the presence of bimol. recombination of arbitrary strength. Scaling relations for the characteristic frequency with bias, sample dimensions, and carrier mobilities are presented in the limits of weak and strong recombination. Finally, impedance measurements conducted on a light-emitting diode and photovoltaic cell based on low-mobility org. semiconductors are modeled as a function of bias and temp., resp. [on SciFinder (R)

Large electrically induced height and volume changes in poly(3,4- ethylenedioxythiophene) /poly(styrenesulfonate) thin films

We demonstrate large, partly reversible height and volume changes of thin films of poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS) on the anode of interdigitating gold electrodes under ambient conditions by applying an electrical bias. The height and volume changes were monitored with optical and atomic force microscopy and are found to be independent of initial film thickness. In the first cycle, a relative height change of 950% is observed for a 21 nm thick film. Two regimes are identified. In the first regime, reversible redox reactions occur and reversible height changes can be ascribed to absorption of water via osmotic effects, brought about by an increasing ion concentration on the anode. In the second, irreversible regime, irreversible overoxidation of the PEDOT occurs and mass transport from the channel to the anode becomes important

Origin of Work Function Modification by Ionic and Amine-Based Interface Layers

Work function modification by polyelectrolytes and tertiary aliphatic amines is found to be due to the formation of a net dipole at the electrode interface, induced by interaction with its own image dipole in the electrode. In polyelectrolytes differences in size and side groups between the moving ions lead to differences in approach distance towards the surface. These differences determine magnitude and direction of the resulting dipole. In tertiary aliphatic amines the lone pairs of electrons are anticipated to shift towards their image when close to the interface rather than the nitrogen nuclei, which are sterically hindered by the alkyl side chains. Data supporting this model is from scanning Kelvin probe microscopy, used to determine the work function modification by thin layers of such materials on different substrates. Both reductions and increases in work function by different materials are found to follow a general mechanism. Work function modification is found to only take place when the work function modification layer (WML) is deposited on conductors or semiconductors. On insulators no effect is observed. Additionally, the work function modification is independent of the WML thickness or the substrate work function in the range of 3 to 5 eV. Based on these results charge transfer, doping, and spontaneous dipole orientation are excluded as possible mechanisms. This understanding of the work function modification by polyelectrolytes and amines facilitates design of new air-stable and solution-processable WMLs for organic electronics.Funding Agencies|Dutch program NanoNextNL; Nederlandse Organisatie voor Wetenschappelijk Onderzoek (NWO); Hyet Solar; Ministry of Education, Culture and Science [024.001.035]; Solliance Organic Photovoltaics Programme</p

Scalable electronic ratchet with over 10% rectification efficiency

\u3cp\u3eElectronic ratchets use a periodic potential with broken inversion symmetry to rectify undirected (electromagnetic, EM) forces and can in principle be a complement to conventional diode-based designs. Unfortunately, ratchet devices reported to date have low or undetermined power conversion efficiencies, hampering applicability. Combining experiments and numerical modeling, field-effect transistor-based ratchets are investigated in which the driving signal is coupled into the accumulation layer via interdigitated finger electrodes that are capacitively coupled to the field effect transistor channel region. The output current–voltage curves of these ratchets can have a fill factor &gt;&gt; 0.25 which is highly favorable for the power output. Experimentally, a maximum power conversion efficiency well over 10% at 5 MHz, which is the highest reported value for an electronic ratchet, is determined. Device simulations indicate this number can be increased further by increasing the device asymmetry. A scaling analysis shows that the frequency range of optimal performance can be scaled to the THz regime, and possibly beyond, while adhering to technologically realistic parameters. Concomitantly, the power output density increases from ≈4 W m\u3csup\u3e−2\u3c/sup\u3e to ≈1 MW m\u3csup\u3e−2\u3c/sup\u3e. Hence, this type of ratchet device can rectify high-frequency EM fields at reasonable efficiencies, potentially paving the way for actual use as energy harvester.\u3c/p\u3