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

Mapping the Density of States Distribution of Organic Semiconductors by Employing Energy Resolved–Electrochemical Impedance Spectroscopy

Although the density of states (DOS) distribution of charge transporting states in an organic semiconductor is vital for device operation, its experimental assessment is not at all straightforward. In this work, the technique of energy resolved–electrochemical impedance spectroscopy (ER-EIS) is employed to determine the DOS distributions of valence (highest occupied molecular orbital (HOMO)) as well as electron (lowest unoccupied molecular orbital (LUMO)) states in several organic semiconductors in the form of neat and blended films. In all cases, the core of the inferred DOS distributions are Gaussians that sometimes carry low energy tails. A comparison of the HOMO and LUMO DOS of P3HT inferred from ER-EIS and photoemission (PE) or inverse PE (IPE) spectroscopy indicates that the PE/IPE spectra are by a factor of 2–3 broader than the ER-EIS spectra, implying that they overestimate the width of the distributions. A comparison of neat films of MeLPPP and SF-PDI2 or PC(61)BM with corresponding blends reveals an increased width of the DOS in the blends. The results demonstrate that this technique does not only allow mapping the DOS distributions over five orders of magnitude and over a wide energy window of 7 eV, but can also delineate changes that occur upon blending. © 2020 The Authors. Advanced Functional Materials published by Wiley-VCH GmbHVEGA ProjectVedecka grantova agentura MSVVaS SR a SAV (VEGA) [2/0081/18]; Research & Innovation Operational Programme - ERDF [313021T081

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. © 2018 Author(s).Slovak Research and Development Agency [APVV-0096-11]; Scientific Grant Agency VEGA [1/0501/15, 2/0163/17]; Research and Development Operational Program - ERDF under the ASFEU project Centre for Applied Research of Advanced Photovoltaic Cells, ITMS [26240220047

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. Nadazdy, and K. Gmucova, 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. Published by AIP Publishing.Slovak Research and Development Agency [APVV-14-0891, APVV-0096-11]; Scientific Grant Agency VEGA [1/0501/15, 2/0163/17, 2/0081/18]; project Efficient controlling of the production and consumption of energy from renewable sources, ITMS [26240220028]; Research and Development Operational Programme - ERD

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. © 2018 American Chemical Society.P3HT, SAS Institute; COFORD, Programme of Competitive Forestry Research for Development; APVV-0096-11, APVV, Agentúra na Podporu Výskumu a Vývoja; 2/0092/18; 1/0501/15; 2/0163/17; 26240220047; FEDER, European Regional Development FundSlovak Research and Development Agency [APVV-0096-11]; Scientific Grant Agency VEGA [1/0501/15, 2/0163/17, 2/0092/18]; Research and Development Operational Programme - ERDF [26240220047

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 from HOMO to LUMO, and the VCM method opens the possibility to study separately the charge transfer (redox) processes with different kinetics. © 2015 American Chemical Society.Slovak Research and Development Agency [APVV-0096-11]; Scientific Grant Agency VEGA [2/0165/13, 1/0501/15

An experimental and theoretical study of the structural ordering of the PTB7 polymer at a mesoscopic scale

Our extensive study based on optoelectronic and electric measurements (which consisted of: UV–Vis absorption, photoluminescence, surface photovoltage measurement, charge extraction by linearly increasing voltage, and energy-resolved electrochemical impedance spectroscopy) revealed the fundamental role of the thickness of the formation of intra- and interchain interaction in poly({4,8-bis[(2-ethylhexyl)oxy]benzo[1,2-b:4,5-b′]dithiophene-2,6-diyl}{3-fluoro-2-[(2-ethylhexyl)carbonyl]thieno[3,4-b]thiophenediyl}) (PTB7) films. We have shown that the final optoelectronic and electronic properties of PTB7 films are governed by the structural ordering development of the transition from nano-to submicroscale. The ordering of polymer chains and competition between the formation of J- and H-aggregates results in a non-trivial dependence of luminescence, exciton diffusion length, transport band gap, and defect concentration. According to a theoretical analysis, the driving forces responsible for the observed phenomena are associated with the thickness threshold dependence of the thin film drying mode which can proceed with or without the polymer skin formation on the surface of forming film. © 2019 Elsevier LtdMinistry of Education, Youth and Sports of the Czech Republic - Program NPU I [LO1504]; Operational Program Research and Development for Innovations - European Regional Development Fund (ERDF); national budget of the Czech Republic [CZ.1.05/2.1.00/19.0409]; Internal Grant Agency of TBU [IGA/CPS/2016/007, IGA/CPS/2017/008, IGA/CPS/2018/007]; Scientific Grant Agency VEGA [2/0081/18]; Slovak Research and Development Agency [APVV SK-CN-RD-18-0006]; [SVV 260 444/2018