Quantitative electronic structure determination of N-heterotriangulene derivatives adsorbed on Au(111)

Abstract

N-heterotriangulenes (N-HTAs) are promising organic semiconductors for applications in field effect transistors and solar cells. Thereby the electronic structure of organic/metal interfaces and thin films is essential for the performance of organic-molecule-based devices. Here, we studied the electronic properties of two different N-HTAs, the N-HTA-550 and N-HTA-557, the latter containing an additional 7-membered ring, adsorbed on Au(111) using two-photon photoemission spectroscopy. We quantitatively determined the energetic positions of several occupied and unoccupied molecular (transport levels) and excitonic states (optical gap) in detail. The additional –CC– bridge forming the 7-membered ring in N-HTA-557 resulted in a pronounced increase of the electron affinity by 0.92 eV from 2.19 eV in N-HTA-550/Au(111) to 3.11 eV in N-HTA-557/Au(111) due to the increase of the π-conjugated electron system, while the first ionization potential is nearly unaffected. Structural variation or substitution pattern in N-HTAs foster the opportunity for tailoring their electronic properties.