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..