Energy-Level Alignment Governs Doping-Related Fermi-Level Shifts in Polymer Films

In thin films of doped organic semiconductors, the position of the Fermi level ( E F ) within the semiconductor fundamental gap depends not only on the dopant loading but also on the energy-level alignment with the substrate. We show that the energy-level alignment of the prototypical conjugated polymer poly(3-hexylthiophene) (P3HT) with the common electrode material indium–tin oxide puts E F far from the midgap and close to the highest occupied molecular orbital (HOMO) level of P3HT already for undoped films, which are intrinsically p-doped through energy-level alignment without dopant admixture. Intentional p-doping with molecular electron acceptors does not necessarily result in notable E F shifts, as we demonstrate by ultraviolet photoelectron spectroscopy (UPS) for the common dopant tetracyanoquinodimethane (TCNQ), which undergoes fractional charge transfer with P3HT. Fluorinated TCNQ derivatives of higher electron affinity (EA), however, do show E F shifts toward the P3HT HOMO, where high EA and dopant loading can put E F into the P3HT-occupied density of states. This renders the conjugated polymer semimetallic and is reminiscent of the degenerate doping scenario found for inorganic semiconductors. By numerical energy-level alignment modeling, which explicitly takes into account the substrate electronic properties, we demonstrate that initial doping-related E F shifts are clearly overestimated if a midgap position of E F is assumed for the undoped organic semiconductor. For P3HT and a series of differently strong p-dopants, we calculate E F positions fully in line with experimental UPS, which also allows for estimating the width of the P3HT HOMO density of states and its doping-related broadening..