Enhancement of the hole injection into regioregular poly(3-hexylthiophene) by molecular doping

The hole injection in Schottky barriers formed between p -type doped regioregular poly(3-hexylthiophene-2,5-diyl) (rr-P3HT) and silver (Ag) is investigated. The rr-P3HT is controllably doped using the acceptor 2,3,5,6-Tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F4-TCNQ). We demonstrate that only one order of magnitude increase in the background hole density p 0, from 2× 1016 to 2× 1017 cm-3, enhances the hole injection with two orders of magnitude. The hole injection barrier is lowered by 0.5 eV and exhibits a linear dependence on p0, which can be explained by doping induced surface charges

Degradation mechanisms in organic photovoltaic devices

\u3cp\u3eIn the present review, the main degradation mechanisms occurring in the different layer stacking (i.e. photoactive layer, electrode, encapsulation film, interconnection) of polymeric organic solar cells and modules are discussed. Bulk and interfacial, as well as chemical and physical degradation mechanisms are reviewed, as well as their implications and external or internal triggers. Decay in I-V curves in function of time is usually due to the combined action of sequential and interrelated mechanisms taking place at different locations of the device, at specific kinetics. This often makes the identification of specific root causes of degradation challenging in non-model systems. Additionally, constant development and refinement in terms of type and combination of materials and processes render the ranking of degradation mechanisms as a function of their probability of occurrence and their detection challenging. However, it clearly appears that for the overall stability of organic photovoltaic devices, the actual photoactive layer, as well as the properties of the barrier and substrate (e.g. cut of moisture and oxygen ingress, mechanical integrity), remain critical. Interfacial stability is also crucial, as a modest degradation at the level of an interface can quickly and significantly influence the overall device properties.\u3c/p\u3

Sensitive triplet exciton detection in polyfluorene using Pd-coordinated porphyrin

We developed a sensitive spectroscopic method to probe triplet concentration in thin films of polyfluorene (PF) at room temperature. The energy of photoexcited triplet excitons is transferred to the guest metal–organic complex, meso-tetratolylporphyrin-Pd (PdTPP), and detected as phosphorescent emission. The phosphorescence intensity of PdTPP–PF blends is proportional to the independently measured triplet concentration using photoinduced absorption experiments. The high sensitivity of this method allows room temperature detection of triplet excitons in spin-coated polymer films as thin as 10 nm. We found that the triplet lifetime is independent of PdTPP concentration and therefore this method is nearly non-perturbing for the triplet population.

Efficient Gating of Organic Electrochemical Transistors with In-Plane Gate Electrodes

AbstractOrganic electrochemical transistors (OECTs) are electrolyte‐gated transistors, employing an electrolyte between their gate and channel instead of an insulating layer. For efficient gating, non‐polarizable electrodes, for example, Ag/AgCl, are typically used but unfortunately, this simple approach limits the options for multiple gate integration. Patterned polarizable Au gates on the other hand, show strongly reduced gating due to a large voltage drop at the gate/electrolyte interface. Here, an alternative, simple yet effective method for efficient OECT gating by scalable in‐plane gate electrodes, is demonstrated. The fact that poly(3,4‐ethylenedioxythiophene) polystyrene sulfonate (PEDOT:PSS) exhibits a volumetric capacitance in an electrolyte is made use of. As a result, the capacitance of PEDOT:PSS‐based gates can be strongly enhanced by increasing their thickness, thereby reducing the voltage loss at the gate/electrolyte interface. By combining spin coating and electrodeposition, planar electrodes of various thicknesses are created on a multi‐gated OECT chip and their effect on the gating efficiency, examined. It is shown that the gating performed by an in‐plane PEDOT:PSS electrode can be tuned to be comparable to the one obtained by a Ag/AgCl electrode. Overall, the realization of efficient gating with in‐plane electrodes paves the way toward integration of OECT‐based biosensors and "organ‐on‐a‐chip" platforms

Determination of the trap-assisted recombination strength in polymer light emitting diodes

The recombination processes in poly(p-phenylene vinylene) based polymer light-emitting diodes (PLEDs) are investigated. Photogenerated current measurements on PLED device structures reveal that next to the known Langevin recombination also trap-assisted recombination is an important recombination channel in PLEDs, which has not been considered until now. The dependence of the open-circuit voltage on light intensity enables us to determine the strength of this process. Numerical modeling of the current-voltage characteristics incorporating both Langevin and trap-assisted recombination yields a correct and consistent description of the PLED, without the traditional correction of the Langevin prefactor. At low bias voltage the trap-assisted recombination rate is found to be dominant over the free carrier recombination rate.

Organic non-volatile memories from ferroelectric phase-separated blends

New non-volatile memories are being investigated to keep up with the organic-electronics road map(1). Ferroelectric polarization is an attractive physical property as the mechanism for non-volatile switching, because the two polarizations can be used as two binary levels(2). However, in ferroelectric capacitors the read-out of the polarization charge is destructive(3). The functionality of the targeted memory should be based on resistive switching. In inorganic ferroelectrics conductivity and ferroelectricity cannot be tuned independently. The challenge is to develop a storage medium in which the favourable properties of ferroelectrics such as bistability and non-volatility can be combined with the beneficial properties provided by semiconductors such as conductivity and rectification. Here we present an integrated solution by blending semiconducting and ferroelectric polymers into phase-separated networks. The polarization field of the ferroelectric modulates the injection barrier at the semiconductor-metal contact. The combination of ferroelectric bistability with (semi) conductivity and rectification allows for solution-processed non-volatile memory arrays with a simple cross-bar architecture that can be read out nondestructively. The concept of an electrically tunable injection barrier as presented here is general and can be applied to other electronic devices such as light-emitting diodes with an integrated on/off switch.</p

Device physics of polymer:fullerene bulk heterojunction solar cells

Plastic solar cells bear the potential for large-scale power generation based on materials that provide the possibility of flexible, lightweight, inexpensive, efficient solar cells. Since the discovery of the photoinduced electron transfer from a conjugated polymer to fullerene molecules, followed by the introduction of the bulk heterojunction (BHJ) concept, this material combination has been extensively studied in organic solar cells, leading to several breakthroughs in efficiency, with a power conversion efficiency approaching 5 %. This article reviews the processes and limitations that govern device operation of polymer.-fullerene BHJ solar cells, with respect to the charge-carrier transport and photogeneration mechanism. The transport of electrons/holes in the blend is a crucial parameter and must be controlled (e.g., by controlling the nanoscale morphology) and enhanced in order to allow fabrication of thicker films to maximize the absorption, without significant recombination losses. Concomitantly, a balanced transport of electrons and holes in the blend is needed to suppress the build-up of the space-charge that will significantly reduce the power conversion efficiency. Dissociation of electron-hole pairs at the donor/acceptor interface is an important process that limits the charge generation efficiency under normal operation condition. Based on these findings, there is a compromise between charge generation (light absorption) and open-circuit voltage (V-oc) when attempting to reduce the bandgap of the polymer (or fullerene). Therefore, an increase in V-oc of polymer.-fullerene cells, for example by raising the lowest unoccupied molecular orbital level of the fullerene, will benefit cell performance as both fill factor and short-circuit current increase simultaneously

Manipulation of charge carrier injection into organic field-effect transistors by self-assembled monolayers of alkanethiols

Charge carrier injection into two semiconducting polymers is investigated in field-effect transistors using gold source and drain electrodes that are modified by self-assembled monolayers of alkanethiols and perfluorinated alkanethiols. The presence of an interfacial dipole associated with the molecular monolayer at the metal/semiconductor interface changes the work function of the electrodes, and, hence, the injection of the charge carriers. The FET characteristics are analysed with the transfer line method and the hole injection into poly(2-methoxy-5-(2'-ethylhexyloxy)-1,4-phenylene vinylene) (MEH-PPV) and regio-regular poly(3-hexyl)thiophene (rr-P3HT) is investigated. The device parameters are corrected for the contact resistances of the electrodes and the mobilities of the polymers (MEH-PPV, µFET = 4 × 10-4 cm2 V-1 s-1 and rr-P3HT, µFET = (1–2) × 10-2 cm2 V-1 s-1) are determined. The contact resistance obtained for the SAM-modified electrodes is at least one order of magnitude larger than for untreated contacts.

Device operation of conjugated polymer/zinc oxide bulk heterojunction solar cells

Solar cells based on a poly (p-phenylene vinylene) (PPV) derivative and zinc oxide nanoparticles can reach a power conversion efficiency of 1.6%. The transport of electrons and holes in these promising devices is characterized and it is found that the electron mobility is equal to 2.8 x 10(-9) m(2) V-1 s(-1), whereas the hole mobility amounts to 5.5 x 10(-10) m(2) V-1 s(-1). By modeling the current-voltage characteristics under illumination it is found that the performance of PPV/zinc oxide solar cells is limited by the charge-carrier mobilities. Subsequently, how to further improve the efficiency is discussed

Extending the voltage window in the characterization of electrical transport of large-area molecular junctions

A large bias window is required to discriminate between different transport models in large-area molecular junctions. Under continuous DC bias, the junctions irreversibly break down at fields over 9 MV/cm. We show that, by using pulse measurements, we can reach electrical fields of 35 MV/cm before degradation. The breakdown voltage is shown to depend logarithmically on both duty cycle and pulse width. A tentative interpretation is presented based on electrolysis in the polymeric top electrode. Expanding the bias window using pulse measurements unambiguously shows that the electrical transport exhibits not an exponential but a power-law dependence on bias. (C) 2011 American Institute of Physics. [doi: 10.1063/1.3608154