Quantifying the Extent of Contact Doping at the Interface between High Work Function Electrical Contacts and Poly(3-hexylthiophene) (P3HT).
We demonstrate new approaches to the characterization of oxidized regioregular poly(3-hexylthiophene-2,5-diyl) (P3HT) that results from electronic equilibration with device-relevant high work function electrical contacts using high-resolution X-ray (XPS) and ultraviolet (UPS) photoelectron spectroscopy (PES). Careful interpretation of photoemission signals from thiophene sulfur atoms in thin (ca. 20 nm or less) P3HT films provides the ability to uniquely elucidate the products of charge transfer between the polymer and the electrical contact, which is a result of Fermi-level equilibration between the two materials. By comparing high-resolution S 2p core-level spectra to electrochemically oxidized P3HT standards, the extent of the contact doping reaction is quantified, where one in every six thiophene units (ca. 20%) in the first monolayer is oxidized. Finally, angle-resolved XPS of both pure P3HT and its blends with phenyl-C61-butyric acid methyl ester (PCBM) confirms that oxidized P3HT species exist near contacts with work functions greater than ca. 4 eV, providing a means to characterize the interface and "bulk" region of the organic semiconductor in a single film.
†Department of Chemistry and Biochemistry, University of Arizona, 1306 East University Boulevard, Tucson, Arizona 85721, United States.
R Clayton
RC
Shallcross
‡Institute of Materials for Electronics and Energy Technology (I-MEET), Friedrich-Alexander-University Erlangen-Nuremberg, Martensstrasse 7, Erlangen 91058, Germany.
Tobias
T
Stubhan
∥Department of Materials Science and Engineering, University of Arizona, 1235 East James E. Rogers Way, Tucson, Arizona 85721, United States.
Erin L
EL
Ratcliff
§Department of Electrical Engineering, Princeton University, Olden Street, Princeton, New Jersey 08544, United States.
Antoine
A
Kahn
‡Institute of Materials for Electronics and Energy Technology (I-MEET), Friedrich-Alexander-University Erlangen-Nuremberg, Martensstrasse 7, Erlangen 91058, Germany.
Christoph J
CJ
Brabec
†Department of Chemistry and Biochemistry, University of Arizona, 1306 East University Boulevard, Tucson, Arizona 85721, United States.
Neal R
NR
Armstrong
P3HT; band bending; contact doping; electrochemistry; photoelectron spectroscopy