Electrochemical Spectroscopic Methods for the Fine Band Gap

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Electrochemical Spectroscopic Methods for the Fine Band Gap Electronic Structure Mapping in Organic Semiconductors Katarína Gmucová, Vojtech Nádaždy, František (Franz) Schauer, Michal Kaiser, and Eva Majkova J. Phys. Chem. C, Just Accepted Manuscript • DOI: 10.1021/acs.jpcc.5b04378 • Publication Date (Web): 18 Jun 2015 Downloaded from http://pubs.acs.org on June 23, 2015

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The Journal of Physical Chemistry

Electrochemical Spectroscopic Methods for the Fine Band Gap Electronic Structure Mapping in Organic Semiconductors Katarína Gmucová1*, Vojtech Nádaždy1, František Schauer2,3, Michal Kaiser1, Eva Majková1 1

Institute of Physics, Slovak Academy of Sciences, Dúbravská cesta 9, 845 11 Bratislava, Slovak Republic 2

Faculty of Education, Trnava University in Trnava, Priemyselná 4, 918 43 Trnava, Slovak Republic

3

Faculty of Applied Informatics, Tomas Bata University in Zlín, Nad Stráněmi 4511, 760 05 Zlín, Czech Republic

ABSTRACT

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

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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, the VCM method opens the possibility to study separately the charge transfer (redox) processes with different kinetics.

INTRODUCTION Nowadays, a significant effort is paid to the design, synthesis and characterization of low band gap polymers and oligomers. The knowledge of the electronic band structure of the newly synthesized organic semiconductor is a determining factor for its application in molecular electronics, as in photovoltaic and optoelectronic devices. The highest occupied molecular orbital (HOMO) and lowest unoccupied molecular orbital (LUMO) energy level positions and the concomitant band gap energy, as the basic parameters of gap engineering, determine the effectiveness with which solar radiation is absorbed in photovoltaic devices, as well as the color of the emitted light in the optoelectronic devices. The pioneering works on the field of organic photovoltaics were reviewed by Goetzberger et al.1 and an excellent review of the latest progress in this field was published by Facchetti.2 The electronic structure of matter, as a term implying both the wave functions of electrons and their energies, represents the quantum states of electrons in the field of atoms nuclei. Calculations with the “chemical accuracy” were initially limited to the systems of ten to twenty atoms. Later, Kohn has shown that the knowledge of the average number of electrons located at


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any one point in space is sufficient3,4 and it is not necessary to consider the wave functions of all individual electrons. Consequently, the Density Functional Theory (DFT), based on the probability density distribution of electrons in the ground state in the system in question, allowed extensions to much larger systems (up to 1000 atoms). DFT is thus a theory of electronic structure, based on the electron density distribution instead of the electron-electron interaction wave function. The electronic structure is described in terms of the density of states (DOS) function, giving the number of states per unit interval of energy and unit volume at each energy, available to be occupied by electrons. Working with real materials we should keep in mind that electronic states, available to be occupied by electrons, can be present even in the band gap. Such states have profound influence on the efficiency, stability and reproducibility of organic photovoltaic devices. Two classes of defects can be distinguished in organic semiconductors: chemical defects and structural defects. The origin of chemical defects can be ascribed to the ambient atmosphere influence (e.g., oxygen, moisture) and synthesis residuals.5 The structural defects play an important role mainly in small molecular devices due to their presence at the grain boundaries of polycrystalline films.6 Contrary, it was shown that the high-temperature annealing minimizes structural defects in conjugated polymers.7 However, little is still known about the defects nature, i.e., about their position in the band gap, chemical composition, mitigation, etc. The energetics of states, relevant to the process of photoexcitation and charge transport within conjugated polymers, including the energy levels in the ground state, during the charge transport and after the photoexcitation, were described for P3HT by Deibel et al.8 It was shown that the addition of a single positive or negative charge in the HOMO or LUMO level during the charge transport results in the formation of the polaron levels HOMO+ and LUMO-, respectively. The


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knowledge of the transport gap between these polaron levels, the defect states in the transport gap, and the DOS in a broad energy interval, is essential for any industrial applications of organic semiconductors. The criteria imposed on the correct determination of the above mentioned quantities fulfil only the methods, where electrons in localized states directly interact with small perturbation signals without influence of the transport and its peculiar temperature and concentration dependences.9 The total DOS determination of several polymers and small molecule organic semiconductors was performed with the help of the ultraviolet photoelectron spectroscopy and the inverse photoelectron

spectroscopy,8,10

and

by

energy

resolved

electrochemical

impedance

spectroscopy.11 Various electrical methods12-18 and electrochemical methods19-23 were used for the partial elucidation of the DOS. Electrochemical methods provide the possibility to determine the DOS in a broad energy range including HOMO+ and LUMO- and defect states in the band gap. While with the help of the electrical methods the activation energy for the emission from a defect is measured, electrochemical methods yield the potential at which either reduction or oxidation of species under consideration take place. Realizing that oxidation and the reduction describe the loss and gain of electrons, respectively, the emission from electron traps can be regarded as an oxidation process and the capture of electrons in electron traps as a reduction process. Similarly, the emission of holes from hole traps can be regarded as a reduction process and the capture of holes in hole traps as an oxidation process. The determination of HOMO+ and LUMO- energy levels from electrochemical experiments is based on the correlation of the Fermi energy scale with formal potentials of reference electrodes.24 In the conformity with the literature, the terms HOMO and LUMO for the electrochemically determined frontier orbitals will be used in the following text.


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Particularly relevant to the design of organic solar cells, which effectively transport the charge, is the ability of organic semiconductors to form polarons. In organic semiconductors photon absorption at individual molecule produces an exciton, a bound state of the electron and the hole. In general, excitons diffuse along chain segments and deexcite at weaker bonds or defects radiatively or nonradiatively, or even may disrupt the weak bonds. The other process is the exciton dissociation to form polarons. Organic solar cells are mostly fabricated with a bulk heterojunction composed of two materials, the active polymer as a donor, and the electron acceptor, i.e., either a small molecule or a polymer. Illumination of such system by radiation leads to the generation of excitons with subsequent electron transfer from the donor to the acceptor and forming of polarons, susceptible to the transport in the system of donors and acceptors to the electrodes. The exciton dissociation into an electron and a hole can be induced not only by the artificially added acceptor molecules, but also by intrinsic structural defects and/or chemical impurities in the device formed, e.g., by air with ill effect on the photovoltaic performance. The aim of the present paper is to give an analysis of the week and strong points in the implementation of several electrochemical methods (cyclic voltammetry (CV), linear sweep voltammetry (LSV), energy resolved electrochemical impedance spectroscopy (ER-EIS), and voltcoulometry (VCM)) in the fine band gap structure mapping of organic semiconductors. Regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) was chosen as a prototypical photonic material in this study. The reason for the choice was the relative abundance of the data in the literature on the DOS and defect states in the P3HT band gap. The comprehensive analysis of energy levels and binding energies relevant during photoexcitation and charge transport in P3HT have been provided by Deibel at al.8 They also experimentally determined energy levels for the


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transport gap, the absorption gap, the exciton binding energy, the positive and negative polarons, as well as the value for the polaron-pair binding energy using combined methods of optical absorption, photoelectron spectroscopy, photoluminescence quenching and external quantum efficiency measurements. Additional features, present in the bad gap of P3HT, were revealed with combined optical spectroscopic methods by Österbacka et al.25 and Müller et al.26 The electron transport of a number of polymers, including P3HT, covering a wide range of electron affinities was studied by Nicolai et al.27 The authors have shown that it can be described by a common electron-trap distribution centered at approximately - 3.6 eV below the vacuum level. The first-principles studies of oxygen- and water-related defects in P3HT were reported by Volonakis et al.28 The defects presence in encapsulated P3HT:PCBM solar cells and solar cells degraded in air was studied by Khelifi et al.29 Two overlapping defects with the total maximum ten times higher in the degraded sample were observed above the HOMO level of the polymer. The deeper peak was assigned to be related to a reaction of P3HT with oxygen. According to the authors, these defects have a strong effect on the charge transport and the solar cell performance when they are present with a high concentration. The comparison of our results with those from the literature illustrates the power of two electrochemical methods, the recently introduced EREIS11 and the modification of the standard VCM.30

EXPERIMENTAL BACKGROUND We will start with an attempt to provide the reader with the necessary background for understanding the weak and strong points of the electrochemical methods in electronic band structure studies. The electrochemical methods, by virtue of their redox basis, are exceptionally advantageous for the spectroscopic purposes, because the redox processes do not rely on the


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temperature and concentration dependent transport in the bulk of the material. All the quantities concentrations and transfer coefficient, entering the final redox process, are temperature independent and more or less known. As discussed by Cardona et al.,24 the main discrepancies between reported data of HOMO and LUMO levels arise from the experimental differences, i.e., the type of the reference electrode, the used solvent and supporting electrolyte, and the analysis in solution or in film. According to the authors, the correlation of electrochemical potentials with orbital energies requires a series of approximations that are unavoidable even if all measurements are done accurately. The formal potential reflects the relative stabilities of the oxidized and reduced species, and although formal potentials can be correlated with HOMO/LUMO energy levels, it must be understood that the latter can only be obtained with error margins larger than ± 0.1 eV in optimal cases. In cases where CV experiments show lack of electrochemical reversibility, or where measurements are done with polymers deposited on the working electrode surface, the uncertainties involved are even larger. Another source of diversity in HOMO and LUMO energies resides in the different approach in the data evaluation, i.e., from the onset or from the peak of the measured curves. The most problematic issues raised in the various experimental data comparison can be detailed as follows. Influence of reference electrode, solvent, and supporting electrolyte. The chosen electrochemical method gives direct information on the oxidation process corresponding to the removal of electrons from the HOMO level, and the reduction process corresponding to the addition of electrons to the LUMO level of the studied material, as well as on the electronic states present in the band gap. While the semiconductor solid-state physicists have adopted the electron energy in vacuum as a reference for the HOMO and LUMO, the scale of potential vs. a reference electrode is used in the electrochemistry. The choice of the reference electrode is


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predominantly determined by the kind of the solvent, supporting electrolyte and analyte. The comparison of reported redox potentials is complicated by the fact that the recalculation is necessary when different reference electrodes have been used for measurements. A critical review of conversion constants among various reference electrodes reported by Pavlishchuk et al.31 revealed that the comparisons of redox potential values are far from accurate in most cases. The authors proposed conversion constants for redox potentials measured versus different reference electrodes in acetonitrile solutions at 25°C, along with a convention for reporting redox potential values. These constants are based on the measurements with various common reference electrodes in acetonitrile solutions, in compliance with IUPAC recommendations,32 the ferrocene couple is often used as the reference redox system. There is still uncertainty in converting the electrochemical redox potentials measured against ferrocene as internal standard to vacuum level.24 Knowing the position of this reference level vs. vacuum, the position of the HOMO and LUMO levels relative to the vacuum level can be simply calculated. Independently on the solvent nature the most commonly used value is - 4.8 eV.33 This requires the extra-thermodynamic assumption that the redox potential of this couple is invariant with the solvent, which is impossible to be proved inside the realm of the exact thermodynamics. That is why the markedly different values of the ferrocene redox potential vs. vacuum, e.g., - 5.2 eV34 can be found in the literature. Theory vs. experimental data, solution vs. thin film. The theoretical HOMO and LUMO energy levels and band gaps are often calculated by applying DFT method. The criteria and relationships for the prediction of energy data from theoretical ones of chosen conjugated polymers were provided by Longo et al.35 However, the gas-phase single molecule properties are only qualitatively relevant to the standard potentials which are measured for solution-phase


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species or for deposited films. There are two key quantities that are required to move from the gas-phase quantities to the solution-phase electrode potential: the solvation energies of the participants in the redox couple and a term accounting for the difference in energy of the electron in the gas-phase (vacuum actually) compared to the metal in the reference electrode.36 Moreover, the situation when one turns to nonaqueous solvents is quite different from that in aqueous environment. There is a lack in the development of conventional standard free energies of formation of compounds needed to compute redox potentials in solvents typical for molecular electrochemistry (acetonitrile, benzonitrile, tetrahydrofuran, etc.). Different electrochemical behavior as compared to the solution phase should be achieved in the case of surface confined thin films due to both the chain-chain interactions and the presence of a fixed amount of redox active species at the electrode.37,38 Onset or peak. The determination of electrochemical and optical band gaps from peak values rather than from onset values is recommended by many authors, e.g., Berlin et al.39 Although the onset values are argued to be ill-defined, such approach is widely used. This is mainly based on the detailed quantum chemical calculations of Brédas et al.40 who found a linear correlation between ionization potential and oxidation potential and also between electron affinity and reduction potentials. They also indicated that these values refer to the first oxidation and reduction potentials (for removing or adding one electron), which corresponds to the onset values of the electrochemical process. One of the drawbacks for using the onset method is that the onset of oxidation or reduction varies with scan rate.41 Therefore, such approach is correct only at very slow scan rates where the total material amount can be reduced or oxidized.

MATERIALS AND METHODS


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Materials and experimental setup. All solutions and thin films were prepared in glovebox to prevent from oxygen and moisture influence on the sample. Small microreactor with integrated sensor AC1.W2.R2 supplied by BVT Technologies, Inc. (Pt working electrode, Ag/AgCl reference electrode, Pt auxiliary electrode) was used for the electrochemical analysis of ~ 10-3 M P3HT solution prepared in benzonitrile (99%, for spectroscopy) with 0.1 M TBAPF6 (for electrochemical analysis, ≥ 99.0%), PTFE membrane filtered (200 nm pore size) and sonicated for 60 minutes. Thin films were deposited on ITO substrates in the glovebox with spin-coating from 1 wt % solution of P3HT in dichlorbenzene with the spinning rate of 30 rps. Samples were subsequently annealed at 110˚C for 5 minutes. Some of the prepared samples were exposed to air for 3 days to study the influence of the air on the thin film properties. The three days long exposition was chosen to reach reproducible and detectable changes in DOS due to the introduced defects. Electrochemical microcells were formed on the P3HT modified ITO substrates bounding disc working electrodes areas of 12 mm2.11 The solution of 0.1 M TBAPF6 in acetonitrile was used as the supporting electrolyte. The potential of the working electrode with respect to the reference Ag/AgCl electrode was controlled via the potentiostat. Pt wire was used as the counter electrode. The potential recorded with respect to the reference Ag/AgCl electrode was recalculated to the local vacuum level assuming the Ag/AgCl energy vs. vacuum value of 4.66 eV. In order to generate excitons in P3HT, the red Lasiris laser emitting light with the wavelength at the absorption edge of P3HT (670 nm, power density of 100 mW/cm2) was used for the sample illumination. Some irreversible degradation processes can occur at higher positive or negative potentials. Examples include irreversible anodic overoxidation depending on temperature, solvent, and


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electrolyte at the site of the radical cation of the polymer backbone,42 anodic overoxidation of polythiophenes in wet acetonitrile electrolytes or electrolytes containing chloride.43 Anodic overoxidation proceeds beyond the positive limit, as cathodic overreduction is observed at negative potentials. Therefore, the electrochemical measurements of HOMO and LUMO levels were split in two separate sweeps. Each measurement started from the zero potential and was performed with a virgin microcell or microreactor. The whole-energy range scans from ~ -2.4 eV vs. vacuum to ~ - 5.9 eV vs. vacuum enabled us not only the estimation of HOMO and LUMO level positions vs. vacuum, but also the detection of a low-level sample contamination with defect states within the band gap. Linear sweep voltammetry, cyclic voltammetry. Voltammetric methods measure the current flowing through the electrochemical cell as a result of the applied time-dependent voltage. Using LSV, the potential is scanned starting at an initial potential and ending at a final potential. Using CV, an extension of LSV, the potential at a working electrode is swept over a range and back again in the reverse direction while the current is recorded. These two methods are widely used for evaluation of HOMO and LUMO. Electrochemical impedance spectroscopy and energy resolved electrochemical impedance spectroscopy. The EIS method measures dielectric properties of the sample as a function of frequency. An electrochemical system, described by the complex impedance, is disturbed from the steady-state by a harmonic voltage perturbation. An electrochemical cell can be modeled using the Randles equivalent circuit shown in Fig. 1a, where individual components represent various elementary processes, exhibiting different time constants. The individual components of the equivalent circuit can thus be separated and identified by the measurements with varying frequency44,45 the high frequency range gives the solution resistance Rs, the medium frequency


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range gives the double-layer capacitance Cdl and the charge transfer resistance Rct, and the low frequency range gives the Warburg impedance ZW representing the mass transport based on the diffusion processes. The measured data are generally presented as either the Nyquist plot (the imaginary part of the complex impedance vs. the real part of the complex impedance, i.e., both amplitude and phase are displayed on a single plot, using frequency as a parameter) or the Bode plot (the complex impedance vs. a log frequency axis, at which two separate plots display amplitude and phase of the frequency response). The Nyquist plot, obtained on the P3HT thin film under investigation at the zero potential vs. Ag/AgCl, is shown in Fig 1b. The components of the equivalent circuit are also schematically depicted in Figure 1b. Recently, we have introduced ER-EIS11 as a spectroscopic method to map the DOS in organic semiconductor materials. ER-EIS measures DOS as a function of energy vs. vacuum via the semiconductor/electrolyte interface charge transfer resistance Rs at the frequency range where the redox reactions determine the real part of the impedance. The charge-transfer current density j at the semiconductor/electrolyte interface is given by the rate law for interfacial charge transfer46 = et s

(1)

where e is the elementary charge, ket is the charge-transfer rate constant, ns is the electron concentration at the surface of the semiconductor, and [A] is the concentration of the supporting electrolyte in the interphase region of the solid/liquid contact. The equivalence of the Fermi level EF to the redox potential of the electron in both the solid films and solvated organic semiconductors was demonstrated by thermodynamic arguments.47 We can express the DOS function g(E) in the semiconductor at the electrochemical potential , = in terms of the charge transfer resistance Rct measured at the applied voltage U as11


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EF,redox = =

ds d

=

d

et d

=

t ct

(2)

where S is the sample area. The charge transfer resistance Rct = dU/d(jS) is experimentally obtained as the real part of complex impedance measured by the harmonic perturbation with a proper frequency f and the amplitude dU superimposed on the applied voltage U. The sought DOS function g(eU) may be directly derived from the charge-transfer resistance Rct(U), measured at the instantaneous position of the Fermi energy given by the applied voltage U, using eq (2). The impedance/gain-phase analyzer Solartron analytical, model 1260 was used for this purpose. The frequency f was 0.5 Hz, the amplitude of AC voltage was 100 mV. The sweep rate of the DC voltage ramp was 10 mV/s. Voltcoulommetry. The VCM method measures the integrated transient current flowing across the electrochemical cell (charge transient) as a response to a rectangular excitation pulse perturbation superimposed on the swept bias voltage. The charge transient comprises a wide spectrum of relaxations which originate in several processes mentioned in the previous subsection. Depending on the chosen time-domain filtering scheme we can preset so called “ratewindow”48 and extract sought components of the relaxation processes.30 In our case, three values of the transient charge were sampled in the time interval between subsequent excitation pulses and combined according to the equation ∆" = "# + %"# + &"'#

(3)

where t1 is the first sampling event after the end of the sampling pulse, which adjusts the lowest limit of the time constants range for the processes under interest. Constants κ, k, λ, and l, chosen according to the rules published by Crowell and Alipanahi,49 enable to suppress undesired time constants in the transient charge response. The VCM approach thus brings the additional gain


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compared with ER-EIS method, namely, the possibility to discriminate among the charge relaxation processes with different kinetics in the measured DOS. On the other hand, the usage of this method in the HOMO and LUMO determination is limited by the upper limit of the measured current of the apparatus used. The home-made electrochemical analyzer comprising CV, LSV, VCM and chronocoulometry was used during this study. The chronocoulometric data are measured as a time response of the charge transient to the applied sampling pulse at a given potential vs. reference electrode, and stored in the computer. Subsequently, these data are numerically processed to obtain the differential charge according to the eq (3), and plotted vs. the respective potential. The presented results were recorded with the scan rates of 6 mV/s and 100 mV/s in cases of LSV and VCM, and CV, respectively. The period of excitation pulses in case of VCM was set to 280 ms, the excitation pulse duration to 100 ms, the excitation pulse amplitude to - 200 mV, and the first sampling event after the end of the sampling pulse to 15 ms.

RESULTS AND DISCUSSION Frontier orbitals of P3HT. The experimental results of CV, LSV, and ER-EIS for both the solution and thin film are shown in Figures 2 and 3, respectively. The onset currents of the respective voltammograms were used for the frontier orbitals (HOMO, LUMO) determination. The complete irreversibility of the overoxidation/overreduction process is demonstrated by the total lack of currents in the back scan of CV (see Figure 2, upper part). The DOS distribution obtained with ER-EIS and plotted in the linear scale was used to determine the HOMO and LUMO levels.8 Both the HOMO and LUMO positions recalculated to energetic scale and the respective transport gap are summarized in Tables 1 and 2. Small differences between the


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frontier orbitals energies obtained for the solution phase and the thin films can be explained in this case by both the chain–chain interactions of molecules in the film and the presence of a fixed amount of redox active species at the electrode.37,38 The differences in the obtained values are due to typical experimental errors mentioned and explained in the section 2. The transport gap values of the P3HT thin film reported in the literature8,50,51 range from 2.4 eV to 2.6 eV and depend on the used annealing procedure. The transport gap values summarized in Table 2 fall, within the experimental errors, with the reported interval. The average characteristic energy of the HOMO band tail obtained from the P3HT thin film measurements (both the pristine and degraded thin films in dark and under illumination) is 59 meV with the standard deviation of 2 meV. This value is higher than the theoretical one (i.e., 37 meV) derived by Frost et al. for the case of intra-chain and inter-chain couplings.52 However, the obtained value lies within the interval reported by Street et al. for the exponential tail states arising from fluctuations in the π-stacking distance with a Gaussian distribution.53 The obtained average characteristic energy of the HOMO band tail is in a good agreement with the 65 meV value observed by Street in the P3HT:PCBM solar cells and ascribed to the polycrystallinity of the P3HT areas in the thin film.54 Based on the Street et al. data53 we can estimate the standard deviation of the Gaussian distribution of the π-stacking distances in the prepared P3HT thin films to be approximately 0.05 relative to the average spacing of 3.8 Å. The above mentioned small variations in the characteristic energy (± 2 meV) of the HOMO band tail observed on different samples thus probably reflect different degrees of disorder in the prepared thin films. Such an assumption can be proved also by the markedly higher characteristic energy of the HOMO band tail obtained for the disordered solvated P3HT molecules, which in our case reaches the value of 100 meV.


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Electronic states in the P3HT band gap. A low concentration of DOS between the HOMO and LUMO is prerequisite for organic semiconductor application in electronics and photovoltaics, where already defect states with the concentration in the range of 1016 cm-3 are undesirable. Typically, the band gap states concentration should be 4 - 5 orders of magnitude lower than the DOS in the transport paths. Let us demonstrate such a case with the pristine P3HT thin film. The respective DOS distribution measured with the ER-EIS method in dark is shown in Figure 4 as the full line. There are observed additional gap states when singlet excitons are generated in the sample with red laser illumination during measurement. It is worth mentioning that the gap state concentration is dependent on the light intensity (see Figure 4) and the laser switching off during the measurement causes the instantaneous return to the original DOS measured in dark. Since the nature of the ER-EIS method does not allow exciton detection the additional gap states should correspond to polaron states created by excitons dissociation at defects in P3HT. More detailed discussion is given in the next section. Recently, the optical spectroscopic methods were used for the regioregular P3HT band gap study.26 The authors deduced the positions of the respective excitons and polarons with respect to the band gap. The reported free polaronic charge carriers should also be observed by the electrochemical methods under simultaneous illumination. The DOS in the band gap of P3HT thin film prepared in the glovebox, kept in air and measured in the glovebox was compared to that one observed on the P3HT thin film prepared and measured in the glovebox without air exposure. The sensitivity to detect degradation changes in P3HT band gap with ER-EIS, VCM, and LSV methods is shown in Figure 5. CV measurements did not provide any valuable


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information on the presence of electronic DOS in the band gap either in dark or under illumination (not shown in Figure 5). Though the ER-EIS and VCM spectra measured in dark do not show any well-defined structure neither in case of the pristine, nor in case of the degraded film, apparent changes in the band gap can be seen under illumination (see Figure 5). Two defect states distributions at about 1.3 V vs. Ag/AgCl (- 3.4 eV vs. vacuum) and - 0.3 V vs. Ag/AgCl (- 4.4 eV vs. vacuum) can be recognized in the pristine sample. After the degradation in air the respective defect distributions decrease and the other distributions near - 0.9 V vs. Ag/AgCl (- 3.8 eV vs. vacuum) arise in the ER-EIS spectrum. Note that switching off the laser beam during the measurement causes the instantaneous return to the original DOS measured in dark. Oxygen and water related defects caused by the sample degradation in air are molecular impurities in the P3HT matrix (chemical defects) and can act as acceptors. Illumination of such a system by visible light leads to the exciton generation followed by the electron transfer from P3HT to the acceptor (the defect in the P3HT matrix). The polaron states formed by the exciton splitting on such defects differ from those which are observed on a pristine sample, where structural defects and defects from the synthesis residuals prevail. Using LSV, the increased presence of charge states within the P3HT band gap under the sample illumination was observed as a featureless current increase. The LSV was unable to discriminate among several electronic states distributions, which were clearly distinguished by ER-EIS and VCM. The comparison of our ER-EIS data with those obtained by Müller et al.26 with ultraviolet photoelectron spectroscopy (UPS) is shown in Figure 6. The positions of the polaron states depicted in the lower part of the figure are adopted from the above mentioned reference. Our data under the illumination of the pristine sample are in good agreement with results published


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by Müller et al. The energy positions of electronic states due to the degradation in air indicate the presence of the (H2O)2–O2 complex described by Nicolai et al.27 and the COOH related defect in P3HT reported by Volonakis and al.28 Defect states at approximately - 4.8 eV vs. vacuum were observed on both the pristine and degraded sample, in dark and under illumination. The maximum is three times higher for the degraded sample. The peak position on the potential axis is in a good agreement with the total maximum of the overlapping defects observed by Khelifi et al.29 In our case, the growth of the maximum in degraded sample was smaller than reported by Khelifi et al. It can be explained by a different DOS value of the defects present in the pristine sample, which in our case lies at about 2.5 order below the HOMO DOS value, i.e., is markedly higher as that one reported by Khelifi et al.29 The predicted defect statepositions28,29 in the P3HT band gap are labelled by the vertical arrows in Figure 6. The ER-EIS impedance is measured at a chosen frequency at which the redox reactions, expressed by the resistance Rct, determine the real component of the impedance and the reciprocal value of this component is directly proportional to the DOS. The results obtained with VCM are quite similar to ER-EIS spectra which means that the time domain sampling scheme of VCM extracts charge component ∆q related to the charge transfer resistance of ER-EIS measured in frequency domain at f = 0.5 Hz. In case of VCM the value of the differential charge is dependent on the kinetic of redox reaction due to the used filtering of the polynomial form of the system response on the excitation pulse. This dependence arises through the parameter entering the non-Cottrellian diffusion (i.e., Δ" ∝ #* instead of Δ" ∝ # ,.. ) and there is no straightforward correlation ∆q vs. DOS as for 1/Rct vs. DOS. Thus, the differential charge does not directly correspond to the DOS like in case of EREIS. VCM enables to retrieve the chosen part of the overall spectral response on the basis of the “rate window” defined by the sampling events. Some processes in the overall charge transient


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response should be suppressed using different coefficients in eq (3).30 Such a situation is shown in Figure 7. A typical chronocoulometric response at a given potential is shown in insets and the respective sampling events are marked by the arrows. The chronocoulometric data obtained under illumination with the laser on the pristine sample and on the sample degraded in air were processed using two different sampling schemes described by the following equations ∆" = "# − 8"8# + 7"9#

(4)

∆" = "# − 2"5# + "9#

(5)

It can be seen that polaron states denoted as P1 in Figure 6 are almost completely suppressed using eq (5).

SUMMARY The complementary electrochemical methods CV, LSV, ER-EIS, and VCM provide valuable information on the P3HT electronic band structure. The determination of the frontier orbitals using CV, LSV and ER-EIS gave, within experimental errors, identical results. ER-EIS has been shown as a simple tool to spectroscopically resolve not only the positions of HOMO and LUMO on the energy axis, but also the DOS elucidation in the band gap of the examined organic materials. The VCM opens a possibility to study a chosen (as to the kinetic of the redox process) part of the overall charge transient response. The possibility of an easy and quick mapping of the fine band gap electronic structure in the P3HT by ER-EIS and VCM has been proved. The defects identification has been based on the comparison of the energetic positions of the observed defect states with the recently published data.26–29 In case of a pristine sample, polaron states reported by Muller et al.26 prevail in the P3HT thin film. Apparent changes in the DOS induced by the sample degradation in air were observed. The defect states observed at energy - 3.4 eV vs. vacuum are most likely related to the


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common electron trap in semiconducting polymers originating in the presence of (H2O)2–O2 complex.27 The mid-gap defect state position is in a good agreement with the energetic position of the defect computed from the first-principles calculations for the case of COOH complex presence in the P3HT film.28 The defects observed approximately at 0.25 meV above the HOMO level match the position of the two overlapping peaks reported by Khefali et al.29 Additional information on the π-π stacking disorder in the studied samples was obtained from the exponential HOMO band tail based on the analyzis published by Street et al.53


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FIGURES

Figure 1. (a) The electrochemical cell modelled as Randles equivalent electrical circuit. Abbreviations: WE, working electrode; RE, reference electrode; AE, auxiliary electrode, Rs, solution resistance, Rct, charge transfer resistance; Cdl, double-layer capacitance and Zw, Warburg impedance. (b) Nyiquist plot of the prepared P3HT thin film measured in acetonitrile with 0.1 M TBAPF6 at the zero potential vs. Ag/AgCl. Arrows with symbols of the equivalent circuit elements point to the typical features of the plot determined by these elements.


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Figure 2. CV, LSV, and ER-EIS measurements of the frontier orbitals performed on P3HT solvated in benzonitrile with 0.1 M TBAPF6. Dash lines are set at the determined positions of LUMO (left part) and HOMO (right part) levels.


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Figure 3. CV, LSV, and ER-EIS measurements of the frontier orbitals performed on P3HT thin film in acetonitrile with 0.1 M TBAPF6. Dash lines are set at estimated position of LUMO (left part) and HOMO (right part) levels.


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Figure 4. ER-EIS spectra of the pristine P3HT thin film in log DOS scale and measured in acetonitrile with 0.1 M TBAPF6 in dark (full line), under the illumination with the red laser with power density of 10 mW/cm2 (dash line), and with power density of 100 mW/cm2 (short dash line).


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Figure 5. ER-EIS, VCM and LSV measurements in the band gap of the P3HT thin film in acetonitrile with 0.1 M TBAPF6. Pristine sample measured in dark (solid black line) and illuminated during the measurement (dash black line); sample exposed for 3 days to air, measured in dark (solid red line with full dots) and illuminated during the measurement (dash red line with open dots). Red laser (670 nm, 100 mW/cm2) was used for the P3HT thin film illumination.


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Page 26 of 36

Figure 6. A comparison of the P3HT fine band gap structure measured with ER-EIS under the illumination (red laser, 670 nm, 100 mW/cm2) and up-to-date data from literature. ER-EIS data measured on the pristine sample (dash black line); on the sample degraded for 3 days in air (dash red line with open dots); UPS data with bars indicating positions of polaron states of P0, P1, P2, and P3 from Ref. 26 (blue dash-dot line); vertical arrows indicating the predicted positions of the oxygen related defects in the P3HT band gap: (H2O)2-O2 complex reported in Ref. 27, COOH complex reported in Ref. 28, and O2 related defect reported in Ref. 29.


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Figure 7. VCM data for the defect states in the P3HT band gap obtained using various filtering schemes. Upper part - data processed by Eq. (4), lower part, data processed by Eq. (5). Data obtained on the pristine sample (dash black line, dash-dot black line); data obtained on the degraded sample exposed for 3 days to air (dash red line with open dots, dash-dot red line with open squares).


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TABLES Table 1. Experimental values of HOMO and LUMO levels vs. vacuum and the respective transport gap obtained for the solvated P3HT. P3HT solution

EIS

CV

LSV

HOMO vs. vacuum (eV)

-5.04

-5.16

-5.13

LUMO vs. vacuum (eV)

-2.70

-2.85

-2.83

transport gap (eV)

2.34

2.31

2.30

Table 2. Experimental values of HOMO and LUMO levels vs. vacuum and the respective transport gap obtained for the P3HT thin film. P3HT film

EIS

CV

LSV

HOMO vs. vacuum (eV)

-5.09

-5.16

-5.13

LUMO vs. vacuum (eV)

-2.71

-2.78

-2.70

transport gap (eV)

2.38

2.38

2.43

AUTHOR INFORMATION Corresponding Author *E-mail: [email protected] ACKNOWLEDGMENTS The authors would like to acknowledge the financial support of the Slovak Research and Development Agency under Project No. APVV-0096-11 and the Scientific Grant Agency VEGA under Projects Nos. 2/0165/13 and 1/0501/15.


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