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

Abstract

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.

What is the Binding Energy of a Charge Transfer State in an Organic Solar Cell?

“…Essentially, it is given by the difference between the ionization potential of the donor and the electron affinity of the acceptor, however, not as determined in neat phases but rather as found in a blend and, of course, in the absence of a countercharge. [18,32] It is a conceptually simple way to determine the energies of the charge transporting states and to directly map the distribution of both hole and electron transporting states also in a composite semiconductor. [31] A novel, recently reported way to determine the electrical gap for a CT state in a blend is electrochemical energy resolved impedance spectroscopy.…”

Unifying Energetic Disorder from Charge Transport and Band Bending in Organic Semiconductors

“…These values are now compared to the exponential widths of the DOS distributions from a third technique, namely ER-EIS. [52] The charge-transfer current density between the electrolyte and the semiconductor surface can be written as [51] Recently, a novel ER-EIS method was developed as a way to measure the DOS of organic polymers over wide energy ranges.…”

“…A good agreement is found between the Gaussian widths estimated using the KP and temperature-dependent J-V measurements-with values for both polymers falling within error of each other. [33,49,52] KP and ER-EIS techniques are both based on the band-bending phenomenon happening at a semiconductorelectrode or semiconductor-electrolyte interface, which is independent of any influence of electric field or light intensity. It is worth noting that the correlation observed between the Gaussian and exponential DOS width values obtained from this work for both polymers is equivalent to the correlation reported in the literature.…”

“…[51] Recently, a novel ER-EIS method was developed as a way to measure the DOS of organic polymers over wide energy ranges. [52,53] The ER-EIS technique is based on the reductionoxidation reaction occurring at the interface of a polymer semiconductor film and an electrolyte. In the experimental set-up, an electrochemical cell is placed on a conductive ITO substrate spin-coated with an organic polymer semiconductor film.…”

“…A few assumptions are made in order to derive the DOS from the measured redox current at the semiconductor-electrolyte interface. [52] The charge-transfer current density between the electrolyte and the semiconductor surface can be written as…”