The effect of intermediate layers on the internal electric field in organic semiconductor devices (original) (raw)
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The effect of intermediate layers on the internal electric field in organic semiconductor devices
Organic Optoelectronics and Photonics II, 2006
In this work we study the internal electric field (V int) present in devices based on an intrinsically semiconducting polymer. Intermediate layers between the indium-tin-oxide and Al electrodes and the photoactive layer are able to influence and alter this electric field. The two commonly used intermediate layers, namely poly(3,4ethylenedioxythiophene) doped with poly(styrenesulfonate) (PEDOT:PSS) and LiF, are subject of this study. Their influence is studied with Electroabsorption (EA) spectroscopy as well as transient photocurrent measurements under applied bias. While PEDOT:PSS has no significant influence on V int , introducing LiF increases V int close to the bandgap of the studied semiconducting polymer. However, using PEDOT:PSS directly influences the spectral EA response. The interface between PEDOT:PSS and the conjugated polymer is studied by impedance spectroscopy. We interpret the results in terms of the presence of charges at the interface.
Internal electric field in organic-semiconductor-based photovoltaic devices
Applied Physics Letters, 2006
The authors performed transient photocurrent measurements under applied bias and electroabsorption spectroscopy on devices based on a pristine poly͑phenylene vinylene͒ derivative as well as its mixture with 1% of a methanofullerene electron acceptor. Combining both techniques allows us to directly determine the internal electric field and to conclude on its implication on the photovoltaic performance of the devices. The electric field is identified as the driving force of the photocurrent, hence indicating the maximum obtainable photovoltage. Acceptor concentrations as low as 1% shift the energetic alignment of the top electrode to the reduction potential of the acceptor, reducing the internal electric field.
Probing electronic state charging in organic electronic devices using electroabsorption spectroscopy
Synthetic Metals, 1996
In metal/organic-film/metal device structures with different metal contacts there is a built-in electrostatic potential at equilibrium due to the asymmetric contacts. At thermal equilibrium the electrochemical potential is constant across the device structure. The electrochemical potential can be divided into the sum of two parts, the electrostatic potential and the chemical potential. By measuring the built-in electrostatic potential change across a structure at equilibrium, one can determine the change in chemical potential across the structure. Measuring this built-in electrostatic potential for devices with different contact metals provides a way of changing the chemical potential (CL) in the organic material and identifying the values of p at which charged excitations are populated. Such measurements can be used to gain information on the energy spectrum of intrinsic charged excitations, charged trap states and charged interface states. We apply an electroabsorption technique to measure the built-in potentials of metal/organic-film/metal structures fabricated from poly[2-methoxy,5-(2'-ethyl-hexyloxy)-1,4phenylene vinylene] (MEH-PPV), &,doped MEH-PPV, and 1,4,5,8-naphthalene-tetracarboxylic dianhydride (NTCDA) and discuss what is learned about the charged excitations in these materials from the results.
Synthetic Metals, 2000
Ž . Ž X Here we report electroabsorption EA measurements on light-emitting diodes LEDs , fabricated with poly 4-4 -diphenylene . Ž . Ž . Ž . Ž . diphenylvinylene PDPV as the emissive layer in indium-tin oxide ITO rpoly 3,4-ethylene dioxythiophene PEDOT :polystyrene Ž . sulfonic acid PSS rPDPVrCa-Al and ITOrPDPVrCa-Al structures. In the latter structure, the built-in potential, determined from nulling the EA signal, corresponds to the difference between the work functions of the electrodes. By incorporating a PEDOT:PSS film between the ITO electrode and the emissive layer we find that such a built-in voltage increases by 0.5 V. The correspondent lowering of the anodic barrier height at the PDPV interface is likely to be responsible for the improvement in device performance. q 2000 Elsevier Science S.A. All rights reserved.
Thin Solid Films, 2018
The complex dielectric function plays central role in understanding the opto-electronic properties of thin conducting polymer films, and is pertinent to the charge transfer mechanisms in the organic electronic devices. In the present study, we demonstrate the applicability of the variable angle spectroscopic ellipsometry (VASE) in combination with an anisotropic Tauc-Lorentz-Drude model, for the ex-and in-situ determination of the complex dielectric function of the poly(3,4-ethylenedioxythiophene): poly(styrenesulfonate) thin film, spin-coated from the aqueous suspension containing dimethyl sulfoxide as co-solvent and 3glycidoxypropyltrimethoxysilane as cross-linker. For the in-situ VASE measurements, the Kretschmann cell configuration is introduced, which enables extension of the experimentally accessible spectral range into the near infrared region, thus making distinguishable the free charge carrier from the valence band absorptions. We applied this technique for studying the interplay between those two contributions when the equilibrium doping state of the film, deposited at the working electrode in a three-electrode cell, is varied by applying a positive cell potential ("electrochemical doping"). The latter method is designated as electrochemical VASE. As results, we report the substantial dichroism in the complex dielectric function with similar magnitudes for both ex-and in-situ films, as well as the trends of the DC conductivity and the main optical transition versus the applied cell potential. These trends are pertinent to the two electrochemical regimes of doping, which can be identified with the field effect and redox doping operation mechanisms of the organic thin film transistors.
2015
Poly(3,4-ethylenedioxythiophene)-poly(styrenesulphonate) (PEDOT:PSS) is the most used organic hole injecting or hole transporting material. The hole carrying matrix PEDOT is highly doped by the acidic dopant PSS. When coated onto a substrate, PEDOT:PSS makes a highly uniform conductive layer and a thin (<5 nm) overlayer of PSS covers the air interface. Semiconducting polymer layers for organic photovoltaics or light emitting diodes are coated on top. In this article, we demonstrate that the PSS layer will mix with almost all conjugated polymers upon thermal annealing. Depending on the Fermi energy of the polymer an electrochemical reaction can take place, p-type doping the polymer at the interface between the PEDOT:PSS and the semiconducting polymer. We use chemical and spectroscopic analysis to characterize the polymer/PSS interlayer. We show that the stable and insoluble interlayer has a great effect on the charge injection and extraction from the interface. Finally we demonstrate and electronically model organic photovoltaic devices that are fabricated using these mixed interlayers.
Organic Electronics, 2007
Ultraviolet photoelectron spectroscopy (UPS) was used to determine the energy level alignment at organic-organic conductor-semiconductor and semiconductor-semiconductor hetero-interfaces that are relevant for organic optoelectronic devices. Such interfaces were formed by in situ vacuum sublimation of small molecular materials [C 60 and pentacene (PEN)] and ex situ spin-coating of poly(3-hexylthiophene) (P3HT), all on the common substrate poly(3,4-ethylenedioxythiophene)/poly(styrenesulfonate) (PEDOT:PSS). We found that the deposition sequence had a significant impact on the interface energetics. The hole injection barrier (HIB) of C 60 on PEDOT:PSS could be changed from 1.0 eV (moderate hole injection) to 1.7 eV (good electron injection) by introducing a layer of P3HT. The HIB of P3HT/PEDOT:PSS was increased by 0.35 eV due to an interfacial PEN layer. However, PEN deposited on PEDOT:PSS and P3HT/PEDOT:PSS exhibited the same value. These observations are explained by material-dependent dipoles at the interfaces towards PED-OT:PSS and substrate dependent inter-molecular conformation.