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

Abstract

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.

Doping-related broadening of the density of states governs integer-charge transfer in P3HT

“…The fact that the modeling of F4TCNQ-doping does not reach 100% is expected because our computational approach focuses on the broadening of the host HOMO-DOS. Since the EA of F4TCNQ is higher than the IE of P3HT, it likely experiences ionization by the next-deeper occupied levels of P3HT as well, which is in line with the shape and position of the occupied states reported in literature, 20 and the landscape of the oxidation states in our P3HT cyclic-voltammogram (Supplementary Material Fig. S1), where the next-to-HOMO peak follows within 0.2 eV.…”

“…S1), where the next-to-HOMO peak follows within 0.2 eV. Most notably, while the reported σ = 63 meV of pristine P3HT, 20 yields <3% ionization for the FTCNQ-…”

“…20,29 The distribution centers of the P3HT HOMO and the dopant LUMO as the most relevant energy levels are deduced from our experimental IE and EA values, as derived from CV. The distribution onsets then lie off the center by twice the standard deviation σ which has been determined for pristine P3HT in a recent study by Bässler et al to 63 meV 20 (for more details, see Methods in the Supplementary Material). We further used molecular volume densities taken from the single-crystal structures of P3HT and the FxTCNQ series.…”

“…This effect is likely further promoted by a broadening of the dopant LUMO-DOS in the case of ICT, where dopant dispersion in the alkyl chain region provides a complex polarizable environment for the dopant (also in its ground state). 20 As deduced from density functional theory (DFT) calculations (see Supplementary Material for methodology, 41,42 )…”

“…This is done for thin films by using 7,7,8,8-tetracyanoquinodimethane (TCNQ) and its derivatives of increasing degree of fluorination (FxTCNQ with x = 0,1,4) with increasing EA. 19 We are particularly interested in this system as, only recently, a very narrow HOMO-DOS of only 63 meV (Gaussian standard deviation σ ) has been reported for rr-P3HT by using energy resolved-electrochemical impedance spectroscopy by the groups of Bässler and Köhler, 20 which is at least four times narrower than that inferred from photoemission. By combining cyclic voltammetry (CV), Fourier-transform infrared spectroscopy (FTIR), conductivity measurements on thin films with theoretical modeling on a semi-classical level, we investigate the interplay between the width of the HOMO-DOS and doping-related charge transfer and show that EA > IE is not a strict condition for ICT in p-type doping.…”