Charge-Transfer–Solvent Interaction Predefines Doping Efficiency in p-Doped P3HT Films

Abstract

Efficient electrical doping of organic semiconductors is a necessary prerequisite for the fabrication of high performance organic electronic devices. In this work, we study p-type doping of poly(3-hexylthiophene) (P3HT) with 2,3,5,6-tetrafluoro-7,7,8,8-tetracyanoquinodimethane (F 4 TCNQ) spin-cast from two different solvents. Using electron diffraction, we find strong dopant-induced π−π-stacking for films from the solvent chloroform, but not from chlorobenzene. This image is confirmed and expanded by the analysis of vibrational features of P3HT and polaron absorptions using optical spectroscopy. Here, a red-shifted polaron absorption is found in doped films from chloroform, caused by a higher conjugation length of the polymer backbone. These differences result in a higher conductivity of films from chloroform. We use optical spectroscopy on the corresponding blend solutions to shed light on the origin of this effect and propose a model to explain why solutions of doped P3HT reveal more aggregation of charged molecules in chlorobenzene, whereas more order is finally observed in dried films from chloroform. Our study emphasizes the importance of solvent parameters exceeding the bare solubility of pure dopant and host material for the preparation of highly conductive doped films.

p‐Doping Poly(3‐hexylthiophene) in Solvent Mixtures

“…To establish a reference for the P3HT •+ dispersions in this study, we measured the UV–vis absorption and Raman spectra of p‐doped P3HT •+ using multiple F4TCNQ dopant concentrations—5%, 10%, 15%, 20%, 25%, and 30%—in pure chloroform. The behavior of F4TCNQ‐doped P3HT in good solvents like pure chloroform or chlorobenzene is well established in the literature . For brevity, our analysis of the F4TCNQ‐P3HT dispersions in pure CF, which was in good agreement with earlier reports, is presented in Section SVI, Supporting Information.…”

“…Müller et al. reported different dopant‐induced structural order and π–π stacking behavior in films processed from F4TCNQ/P3HT solution in chloroform and chlorobenzene . In that study, the authors posited that polarity differences of the solvents affect the ordering processes associated with formation of the P3HT + aggregates in the solution phase, which, in turn, leads to variations of the films processed from chloroform and chlorobenzene.…”

“…Salzmann and Heimel stressed that the choice of solvent can drastically impact the observed experimental data and, as a result, the interpretation of molecular‐level mechanisms associated with chemical doping of organic semiconductors . Preliminary studies have demonstrated significant solvent and dopant effects on π–π stacking of polymer chains, as well as a dependence of P3HT solubility on dopant concentration . Müller et al.…”

The Role of Weak Molecular Dopants in Enhancing the Performance of Solution‐Processed Organic Field‐Effect Transistors

“…Similarly, obvious morphology change was also observed in films on the unmodified SiO 2 /Si substrate ( Figure S8, Supporting Information). [13,48] It should be stressed that the insertion of dopant does not disrupt the molecular packing of the host semiconductor. Figure 5a showed that the pristine film had no diffraction peak, suggesting its random molecular orientation and also well illustrating the low mobility of the undoped device.…”

Soft versus hard junction formation for α-terthiophene molecular wires and their charge transfer complexes

“…This is a particularly pertinent issue in the present work; doping of polythiophene organic semiconductors with related (albeit more powerful) acceptors such as tetrafluoro-2,2 -(cyclohexa-2,5-diene-1,4-diylidene) dimalononitrile (F4TCNQ) has been studied in efforts to improve thin film transistor performance with these materials, and it has been shown that alternating donoracceptor stacked aggregates form. 24 Therefore, we wished to test experimentally the possibility that the conductance boost seen for 1 with TCNE might be, at least in part, due to aggregate formation, i.e., that we see single molecule junctions in the absence of TCNE, but multiple molecule junctions owing to donor-acceptor stack interactions in the presence of TCNE.…”