Chapter IV. Experimental part. Cyclic voltammetry.
Cyclic voltammetry. Cyclic voltammetry (CV) measurements were performed under nitrogen atmosphere in a freshly prepared anhydrous and deoxygenated solution of tetra-n-butylammonium hexafluorophosphate in acetonitrile (0.1 M) containing small amounts of the analytes. The three-electrode setup consisted of a platinum disc working electrode and platinum wires as counter electrode and pseudo-reference electrode. The voltammograms were recorded at a scan rate of 50 mV·s-1 and referenced to the half-wave potential of the added internal standard ferrocene, adjusted here to 0.00 V. From the formal potential of the ferrocene redox couple E0’(Fc/Fc+) ≈ E1/2(Fc/Fc+) = 0.40 V vs. SCE287 (saturated calomel electrode) and the conversion constant SCE to NHE +0.25 V288 (normal hydrogen electrode) follows E0’(Fc/Fc+) = 0.65 V vs. NHE. The transfer to the Fermi energy was based on Trasatti’s report289 about -4.44 eV being equivalent to 0.0 V vs. NHE which results in E(Fc/Fc+) = -5.09 eV and, consequently for our procedure, in the equation for calculating LUMO energies of the dopants showing electrochemical reversible redox steps and HOMO energies of the polymers without a reversible electrochemical redox behavior290.
For dopants: ELUMO = –(5.09 + E1/2,red vs. Fc/Fc+)[eV]
For polymers: EHOMO = –(5.09 + Eonset,ox vs. Fc/Fc+)[eV]
Electrochemical characterization. Different methods, such as CV, PYS, UPS, PESA etc., can be used for determination of HOMO and LUMO energies and these mesurements frequently give very different values. To bypass this problem and avoid any uncertainties, in this work we investigated the energy levels by using the same method, CV. Electrochemical properties of the dopant molecules HAT-CN6, F4TCNQ, F6TCNNQ and CN6-CP in solution state as well as of PDPP(6-DO)2TT in thin film state were investigated by cyclic voltammetry (CV) in acetonitrile with n-Bu4NPF6 as electrolyte. By recording voltammograms of the single analytes and as mixture with ferrocene as internal standard all voltammograms could be referenced to the formal potential of the ferrocene redox couple Fc/Fc+. The energies of the lowest unoccupied molecular orbitals (LUMOs) and highest occupied molecular orbital (HOMO) were estimated from the reduction and oxidation potentials transferred to the Fermi energy scale (see experimental section).
Chapter IV. Experimental part. Cyclic voltammetry.
Page 138 The recorded cyclic voltammogram of PDPP(6-DO)2TT as thin film (Figure E2) presents an irreversible oxidation processes. By using the onset oxidation value the HOMO energy was estimated to be -5.49 eV (see Table E3). Thus, an efficient doping is only enabled with dopants providing LUMO energies below this value.
Various research groups applied HAT-CN6 in optoelectronic devices and investigated the localization of the LUMO energy level by different methods, like ultraviolet photoelectron spectroscopy (UPS) of thin films on Au substrates287 or photoelectron yield spectroscopy and UV-Vis spectroscopy (PYS)289. Nevertheless, the reported values are not consistent, but varying significantly between 4.4 – 4.8 eV289,290 and 5.6 – 6.1 eV287,288. Our cyclic voltammetry measurements show an electrochemically reversible one-electron redox step from the neutral HAT-CN6 to the radical anion and vice versa. Based on the half-wave potential the LUMO energy was determined to be -4.58 eV confirming the reports by Chiba289 and Chen290. A second redox step to the dianion was electrochemically irreversible and about 0.3 eV shifted. Thereby, our observations are also in good agreement with the original paper by Kanakarajan291 introducing the synthesis of HAT-CN6.
As reported by Li71 and Koech72, F4TCNQ and F6TCNNQ show two reversible one-electron redox steps. Using the halfwave potentials LUMO1/LUMO2 energies were calculated to be -5.28 eV/-4.74 eV for F4TCNQ, and -5.34 eV/-4.96 eV for F6TCNNQ. The determined LUMO levels are thereby in a good agreement with published values for F4TCNQ (from CV:
-5.23 eV71, -5.35 eV72, -5.33 eV133 as well as ultraviolet photoelectron spectroscopy UPS: -5.24 eV292) and for F6TCNNQ (from CV: -5.3772). Furthermore, our measurements also confirm theoretical studies predicting a 60 mV deeper LUMO1 for F6TCNNQ compared with F4TCNQ72.
The hexacyano-trimethylene-cyclopropane based compounds CN6-CP/Na2, CN6-CP/K and CN6-CP/TBA undergo also two electrochemical reversible one-electron redox steps (difference in peak potentials close to 59 mV and ratio of oxidation and reduction current about 1) and are consistent with each other in their electrochemical behavior within the applied potential range (see Figure E1, Table E3). For this reason, the present cationic species do not affect the electrochemical behavior of CN6-CP anions to a significant extend. In contrast to HAT-CN6, F4TCNQ and F6TCNNQ after the addition of ferrocene not all three signals appear in the voltammograms, but only two what is simply caused by an overlap of the ferrocene signal with the redox step 2 of CN6-CP where the oxidation of CN6-CP2- occurs or the reduction of the CN6-CP radical anion takes place, respectively. Based on the measured half-wave potentials LUMO energy levels of -5.88 eV and -5.09 eV were estimated for CN6-CP.
A visual comparison is given on Figures R14, R15 of the main text and Figure E2 by compiling voltammograms and LUMO1/HOMO levels into one respective diagram. Thus, compared to the HOMO energy of PDPP(6-DO)2TT only CN6-CP offers a deeper LUMO level (∆E = -0.39 eV) allowing an efficient doping process, whereas LUMOs of HAT-CN6 (∆E = +0.91 eV), F4TCNQ (∆E = +0.21 eV) and F6TCNNQ (∆E = +0.15 eV) are partly significantly higher. For this reason, CN6-CP can be used as a p-dopant where HAT-CN6,
Chapter IV. Experimental part. Cyclic voltammetry.
F4TCNQ and F6TCNNQ are not suitable for doping PDPP(6-DO)2TT. Additionally, CN6-CP offers a much higher driving force for an efficient p-type doping process with other donor/host materials already dopable by F4TCNQ and F6TCNNQ possibly allowing an improvement of the achieved properties. In conclusion, we highly suspect with regard to our investigations that CN6-CP is the most efficient dopant for similar p-type semiconducting polymers known so far.
Chapter IV. Experimental part. Cyclic voltammetry.
Figure E1. Comparison of cyclic voltammograms of single dopants before and after addition of ferrocene referred to E1/2(Fc/Fc+) = 0.0V.
Chapter IV. Experimental part. Cyclic voltammetry.
Figure E2. Cyclic voltammograms of dopants (in solution) and polymer (as thin film) referenced to ferrocene (E1/2 = 0.0 V) in 0.1 M n-Bu4NPF6 acetonitrile solution at 50 mV·s-1 scan rate.
Chapter IV. Experimental part. Cyclic voltammetry.
Page 142 Table E3. Electrochemical properties, reduction potentials and LUMO energies of HAT-CN6, F4TCNQ, F6TCNNQ and CN6-CP species
Table E4. Optoelectronic and electrochemical properties of PDPP(6-DO)2TT
Compound
Chapter IV. Experimental part. GIWAX data.