Critical dopant concentrations govern integer and fractional charge-transfer phases in doped P3HT

Abstract

The conjugated polymer poly(3-hexylthiophene) (P3HT) p-doped with the strong acceptor tetrafluorotetracyanoquinodimethane (F4TCNQ) is known to undergo ion-pair (IPA) formation, i.e., integer-charge transfer, and, as only recently reported, can form ground state charge-transfer complexes (CPXs) as a competing process, yielding fractional charge transfer. As these fundamental charge-transfer phenomena differently affect doping efficiency and, thus, organic-semiconductor device performance, possible factors governing their occurrence have been under investigation ever since. Here, we focus on the role of a critical dopant concentration deciding over IPA- or CPX-dominated regimes. Employing a broad, multi-technique approach, we compare the doping of P3HT by F4TCNQ and its weaker derivatives F2TCNQ, FTCNQ, and TCNQ, combining experiments with semi-classical modeling. IPA, CPX, and neutral-dopant ratios (estimated from vibrational absorption spectroscopy) together with electron affinity and ionization energy values (deduced from cyclic voltammetry) allow calculating the width of a Gaussian density of states (DOS) relating to the highest occupied molecular orbital in P3HT. While a broader DOS indicates energetic disorder, we use grazing-incidence X-ray diffraction to assess spatial order. Our findings consider the proposal of nucleation driving IPA formation and we hypothesize a certain host-dopant stoichiometry to be key for the formation of a crystalline CPX phase.

Sterically‐Hindered Molecular p‐Dopants Promote Integer Charge Transfer in Organic Semiconductors

“…An additional challenge in doping can arise from the molecular shape of the dopants. In previous work, we established that ICT does not necessarily happen efficiently upon doping because ground‐state charge‐transfer complexes (CPXs) may form instead, where only fractional charge transfer occurs [9, 10, 14] . This is due to the hybridization of the frontier molecular orbitals of dopant and COM/COP, where the lowest unoccupied molecular orbital (LUMO) of the p ‐dopant and the highest occupied molecular orbital (HOMO) of the COM/COP split into doubly occupied bonding and empty antibonding supramolecular hybrid orbitals.…”

“…This allows investigating whether the shielded LUMO of PFP3CN3‐CP and the openly exposed one of F2TCNQ translate into a different doping phenomenology. To this end, we used both to p ‐dope films of P3HT from common solution, as CPX formation does occur for this COP [10, 18] . We performed ultraviolet‐visible‐near‐infrared absorption spectroscopy (UV/Vis‐NIR), which has been extensively employed before to study doping‐related charge transfer; [10, 11, 14, 15b, 16, 17b, 18c, 26] the results are shown in Figure 3.…”

“… UV/Vis‐NIR absorption spectra of dropcast P3HT films doped with increasing molar ratios of a) F2TCNQ and b) PFP3CN3‐CP in common solution. Data in (a) are taken from an own previous study [10] (normalized to the P3HT maximum) and obtained under identical conditions as the data in (b); P1 and P2 indicate optical transitions assigned to the positive polaron in P3HT, [10, 17a, 27] stars mark transitions assigned to the respective dopant radical anions (F2TCNQ⋅ − and PFP3CN3‐CP⋅ − ; see Refs. [10, 15b] and Figure S12, respectively), the transitions of neutral PFP3CN3‐CP are assigned by supporting DFT calculations (Figure S7).…”

“…In previous work, we established that ICT does not necessarily happen efficiently upon doping because ground-state charge-transfer complexes (CPXs) may form instead, where only fractional charge transfer occurs. [9,10,14] This is due to the hybridization of the frontier molecular orbitals of dopant and COM/COP, where the lowest unoccupied molecular orbital (LUMO) of the p-dopant and the highest occupied molecular orbital (HOMO) of the COM/COP split into doubly occupied bonding and empty antibonding supramolecular hybrid orbitals. The latter becomes the effective acceptor level for electron transfer and takes over the role of the pristine dopant LUMO, but is energetically less favorable, which limits the doping efficiency.…”

“…The latter becomes the effective acceptor level for electron transfer and takes over the role of the pristine dopant LUMO, but is energetically less favorable, which limits the doping efficiency. [9,15] TCNQ and its fluorinated derivatives are fully planar dopants and thus highly susceptible to CPX formation with common COMs [9,14,16] and COPs [9,10,14,17] including the homopolymer poly(3-hexylthiophene-2,5-diyl) (P3HT, IE = 5.21 eV, Figure S1) [10] which has been found to show both ICT and CPX formation even with F4TCNQ of an EA exceeding the IE of P3HT. [10,18] As the energy level splitting between the supramolecular hybrid orbitals upon CPX formation depends on the electronic coupling between dopant and host frontier molecular orbitals, we earlier proposed reducing their coupling via steric hindrance to remedy this issue through directed molecular design.…”