Energy-resolved Electrochemical Impedance Spectroscopy Research Articles

Electrochemically induced charge injection in disordered organic conductive polymers

This paper deals with the electrochemically induced charge injection in the conductive polymer (CP), exemplified by well examined archetypal CP—poly(3-hexylthiophene-2,5-diyl). The polar solvent of acetonitrile with salt tetrabutylammonium hexafluorophosphate was used to transport electrons in the electrolyte. The decisive mechanism is the recombination current at the electrolyte/CP interface taking place at the Fermi energy of CP, whose energy position is determined by the externally applied voltage. The corresponding mechanism of the charge carrier transport in the polymer bulk is the space-charge limited current (SCLC) by holes or electrons (or more precisely positive and negative polarons) at the respective transport paths of HOMO and LUMO bands. The charge transport mechanisms and the occupation statistics are the basis of the energy-resolved electrochemical impedance spectroscopy for the mapping of the density of electronic states of conductive organic semiconductors [F. Schauer, V. Nádaždy, and K. Gmucová, J. Appl. Phys. 123, 161590 (2018)]. From the application point of view, the major message of the paper is that it is possible to pass high current densities of the order of 0.1 A cm−2 via electrochemical systems with the CP, induced by means of doping processes of both CP surface and its bulk, leading to the charge injection and SCLC in CP.

Colloidal cobalt-doped ZnO nanoparticles by microwave-assisted synthesis and their utilization in thin composite layers with MEH-PPV as an electroluminescent material for polymer light emitting diodes

Abstract CoxZn1-xO (x = 0.01, 0.05 and 0.1) nanoparticles were prepared by microwave-assisted polyol method from zinc acetate dihydrate and Co(II) acetylacetonate. The reactions were performed in diethylene glycol (DEG) at 250 °C with the use of oleic acid as a surfactant. Resulting nanoparticle (ca. 10 nm) precipitates were washed with methanol and dried or kept as colloidal solutions redispersed in toluene. Colloidal solutions were mixed with the Poly [2-methoxy-5-(2-ethylhexyloxy)-1,4-phenylenevinylene] (MEH-PPV) polymer to produce thin nanocomposite layers with specific optoelectronic properties. The electronic structure in terms of density of states (DOS) of MEH-PPV and MEH-PPV/nanocomposite layers was investigated by recently proposed energy resolved electrochemical impedance spectroscopy method. The MEH-PPV polymer and its nanocomposites with ZnO or CoxZn1-xO nanoparticles were used as thin active layers in polymer light emitting diodes (PLED). The nanocomposite layers exhibited optoelectronic properties which were found to be beneficial as the active layer in PLEDs, exhibiting an order of magnitude enhancement in electroluminescence intensity. A pronounced effect on the opening bias voltage of final devices with CoxZn1-xO nanoparticles was also observed.

Thickness Effect on Structural Defect-Related Density of States and Crystallinity in P3HT Thin Films on ITO Substrates

We report on a study of thickness effect on the formation of structural defect-related density of states (DOS) in the band gap of poly(3-hexylthiophene-2,5-diyl) (P3HT) thin films spincoated on ITO substrates. The energy-resolved electrochemical impedance spectroscopy and grazing-incidence wide-angle X-ray scattering were used to correlate the DOS with the degree of crystallinity in P3HT thin films. We found an exponential increase of the defect DOS in the band gap with increasing fraction of the amorphous phase when decreasing the film thickness. The exponent increases abruptly when reducing the thickness down to 30 nm, which indicates two thickness regions with different dynamics of the defect DOS formation driven by increasing the fraction of the amorphous phase. Moreover, we observed the co-existence of two P3HT polymorphic crystalline phases with different backbone spacings, which results in the appearance of a peculiar DOS satellite peak above the highest occupied molecular orbital. The volume of the minor, more dense, crystalline phase exhibits a thickness dependence with a maximum plateau around 40 nm. These results suggest an important effect of the substrate roughness on the crystallinity and polymorphism of P3HT thin films depending on the film thickness with general implications for polymer thin films.

Measuring the Electronic Structure of Nanocrystal Thin Films Using Energy-Resolved Electrochemical Impedance Spectroscopy.

Use of nanocrystal thin films as active layers in optoelectronic devices requires tailoring of their electronic band structure. Here, we demonstrate energy-resolved electrochemical impedance spectroscopy (ER-EIS) as a method to quantify the electronic structure in nanocrystal thin films. This technique is particularly well-suited for nanocrystal-based thin films as it allows for in situ assessment of electronic structure during solution-based deposition of the thin film. Using well-studied lead sulfide nanocrystals as an example, we show that ER-EIS can be used to probe the energy position and number density of defect or dopant states as well as the modification of energy levels in nanocrystal solids that results through the exchange of surface ligands. This work highlights that ER-EIS is a sensitive and fast method to measure the electronic structure of nanocrystal thin films and enables their optimization in optoelectronic devices.

Electrochemical impedance spectroscopy for study of electronic structure in disordered organic semiconductors—Possibilities and limitations

There is potential in applying conjugated polymers in novel organic optoelectronic devices, where a comprehensive understanding of the fundamental processes and energetics involved during transport and recombination is still lacking, limiting further device optimization. The electronic transport modeling and its optimization need the energy distribution of transport and defect states, expressed by the energy distribution of the Density of States (DOS) function, as input/comparative parameters. We present the Energy Resolved-Electrochemical Impedance Spectroscopy (ER-EIS) method for the study of transport and defect electronic states in organic materials. The method allows mapping over unprecedentedly wide energy and DOS ranges. The ER-EIS spectroscopic method is based on the small signal interaction between the surface of the organic film and the liquid electrolyte containing reduction-oxidation (redox) species, which is similar to the extraction of an electron by an acceptor and capture of an electron by a donor at a semiconductor surface. The desired DOS of electronic transport and defect states can be derived directly from the measured redox response signal to the small voltage perturbation at the instantaneous position of the Fermi energy, given by the externally applied voltage. The theory of the ER-EIS method and conditions for its validity for solid polymers are presented in detail. We choose four case studies on poly(3-hexylthiophene-2,5-diyl) and poly[methyl(phenyl)silane] to show the possibilities of the method to investigate the electronic structure expressed by DOS of polymers with a high resolution of about 6 orders of magnitude and in a wide energy range of 6 eV.

Effect of crystallinity on UV degradability of poly[methyl(phenyl)silane] by energy-resolved electrochemical impedance spectroscopy

Low stability and degradability of polymers by ambient air, UV irradiation or charge transport are major problems of molecular electronics devices. Recent research tentatively suggests that the presence of a crystalline phase may increase polymer stability due to an intensive energy trapping in the ordered phase. Using the UV degradability, we demonstrate this effect on an archetypal model σ bonded polymer - poly[methyl(phenyl)silane] (PMPSi) - with partially crystalline and amorphous-like layers. UV degradation with 345 nm, derived from the branching state generation rate, was inversely proportional to the crystalline phase content, changing from 4.8x1011 s-1 (partially crystalline phase) to 1.8x1013 s-1 (amorphous-like phase). A model is proposed where crystallites formed by molecular packing act as effective excitation energy traps with a suppressed nonradiative recombination improving thus PMPSi film stability. The molecular packing and higher crystalline phase proportion may be a general approach for stability and degradability improvement of polymers in molecular electronics.

Electronic structure of UV degradation defects in polysilanes studied by Energy Resolved – Electrochemical Impedance Spectroscopy

The white photo luminescence after UV degradation in long wavelength range 400–600 nm was examined on the prototypical polysilane, poly[methyl(phenyl)silane], using both photoluminescence spectroscopy and a new method of Energy Resolved - Electrochemical Impedance Spectroscopy (ER-EIS). Two groups of defect states, situated at approximately 440 nm (ΔE1 = 2.8 eV with respect to electron transport energy) and 520 nm (ΔE2 = 2.4 eV with respect to electron transport energy) were found by both spectroscopic methods. The white radiative recombination is ascribed to the recombination from trapping sites following the extreme energy migration. The forming of the crosslinking and bridging defects after photochemical scission of SiSi via the series of various kinds of intermediates is feasible (silyl R3Si −380 nm, silylene Si2H4 − 480 nm, silene and silylsilylene −550 nm emissions). On the grounds of the IR absorption spectroscopy results we suppose the presence of the bonding by methylene bridging and carbosilane unit SiCH2Si creation after SiSi Si σ sp3 bond scission. The ER-EIS method turned out to be extremely suitable for elucidation of the electronic structure and its changes in organic semiconductors due to its great resolving power and wide range both in the energy and the density of electronic states.

Electronic structure mapping of branching states in poly[methyl(phenyl)silane] upon exposure to UV radiation

The origin of white photoluminescence in polysilanes has long been disputed, and this emission is closely connected with information recording in nanotechnologies. We elucidated UV degradation of an archetypal model polymer poly[methyl(phenyl)silane] by using a new method for electronic structure mapping of organic semiconductors, energy-resolved electrochemical impedance spectroscopy (ER-EIS) and photoluminescence spectroscopy. UV exposure at 345 nm resulted in two defect bands above the highest occupied molecular orbital (HOMO) in the energy region from −5.5 eV to −3.5 eV with respect to the zero vacuum energy level. The respective density of states was 1016 − 1017 cm−3eV−1, and the total integrated concentration was 0 − 1017 cm−3. The photoluminescence in the long-wavelength region gave wide bands with photon energies from 2.2 eV to 3.2 eV (corresponding to wavelengths from 600 nm to 390 nm). The observed bands were interpreted by assuming the formation of energetically distributed Si branching radiative states, whose distribution in the HOMO − lowest unoccupied molecular orbital (LUMO) gap was observed by using ER-EIS. The quantum efficiency of defect state formation increased from Φ(x)345 nm = 0.0045 to Φ(x)290 nm = 0.053. The obtained results may contribute to the production of effective polysilane nanomasks and to information recording.

Electrochemical Spectroscopic Methods for the Fine Band Gap Electronic Structure Mapping in Organic Semiconductors

Functionality of organic photonic devices is markedly influenced by the electronic band structure of the used materials. An easy and quick determination of the density of states function (DOS) in the whole energy range from HOMO to LUMO, including the presence of defect states in the band gap, is a prerequisite to a successful design of photonic devices. In this study we present the fine band gap electronic structure mapping in P3HT with two electrochemical spectroscopic methods: the energy-resolved electrochemical impedance spectroscopy (ER-EIS) and the kinetic sensitive voltcoulometry (VCM). We showed that the P3HT exposition to air results in the change of light-induced polaron states in the band gap. The electrochemically measured data are compared with those from the literature, obtained with combined optical spectroscopic methods, electrical methods, or first-principles calculations. The ER-EIS method has been shown as capable of providing valuable information on the DOS in the whole energy range fr...

Energy resolved electrochemical impedance spectroscopy for electronic structure mapping in organic semiconductors

We introduce an energy resolved electrochemical impedance spectroscopy method to map the electronic density of states (DOS) in organic semiconductor materials. The method consists in measurement of the charge transfer resistance of a semiconductor/electrolyte interface at a frequency where the redox reactions determine the real component of the impedance. The charge transfer resistance value provides direct information about the electronic DOS at the energy given by the electrochemical potential of the electrolyte, which can be adjusted using an external voltage. A simple theory for experimental data evaluation is proposed, along with an explanation of the corresponding experimental conditions. The method allows mapping over unprecedentedly wide energy and DOS ranges. Also, important DOS parameters can be determined directly from the raw experimental data without the lengthy analysis required in other techniques. The potential of the proposed method is illustrated by tracing weak bond defect states induced by ultraviolet treatment above the highest occupied molecular orbital in a prototypical σ-conjugated polymer, poly[methyl(phenyl)silylene]. The results agree well with those of our previous DOS reconstruction by post-transient space-charge-limited-current spectroscopy, which was, however, limited to a narrow energy range. In addition, good agreement of the DOS values measured on two common π-conjugated organic polymer semiconductors, polyphenylene vinylene and poly(3-hexylthiophene), with the rather rare previously published data demonstrate the accuracy of the proposed method.