Spectral Signatures of a Negative Polaron in a Doped Polymer Semiconductor: Energy Levels and Hubbard U Interactions

Themodern picture of negative charge carriers on conjugated polymersinvokes the formation of a singly occupied (spin-up/spin-down) levelwithin the polymer gap and a corresponding unoccupied level abovethe polymer conduction band edge. The energy splitting between thesesublevels is related to on-site Coulomb interactions between electrons,commonly termed Hubbard U. However, spectral evidencefor both sublevels and experimental access to the U value is still missing. Here, we provide evidence by n-doping thepolymer P(NDI2OD-T-2) with [RhCp*Cp](2), [N-DMBI](2), and cesium. Changes in the electronic structure after dopingare studied with ultraviolet photoelectron and low-energy inversephotoemission spectroscopies (UPS, LEIPES). UPS data show an additionaldensity of states (DOS) in the former empty polymer gap while LEIPESdata show an additional DOS above the conduction band edge. TheseDOS are assigned to the singly occupied and unoccupied sublevels,allowing determination of a U value of similar to 1eV.

Automated summary

Evidence is provided by n-doping the polymer P(NDI2OD-T-2) with [RhCp*Cp](2), [N-DMBI](2), and cesium, and studying changes in the electronic structure using ultraviolet photoelectron and low-energy inverse photoemission spectroscopies (UPS, LEIPES). The UPS data shows an additional density of states (DOS) in the former empty polymer gap, while the LEIPES data shows an additional DOS above the conduction band edge. These DOS are assigned to the singly occupied and unoccupied sublevels, allowing determination of a U value of approximately 1eV.