Submolecular Electroluminescence Mapping of Organic Semiconductors

Abstract

The electroluminescence of organic films is the central aspect in organic light emitting diodes (OLEDs) and widely used in current display technology. However, its spatial variation on the molecular scale is essentially unexplored. Here, we address this issue by using scanning tunneling microscopy (STM) and present an in-depth study of the electroluminescence from thin C films (<10 monolayers) on Ag(111) and Au(111) surfaces. Similar to an OLED, the metal substrate and STM tip inject complementary charge carriers that may recombine within the molecular film; however, the atomically defined charge injection by the tip enables mapping of the local electroluminescence down to the submolecular scale. We show that the radiative recombination in solid C is restricted to various structural defects, whose emission characteristics can be addressed individually. The emission fine structure reveals a coupling to Jahn-Teller active vibrational modes of C, which implies that its parity-forbidden lowest singlet transition becomes locally allowed at the emission centers. At lateral distances of a few nanometers, only a weak emission from tip-induced plasmons is detectable. Their excitation evidences the injection of both charge carrier types and confirms that they are unable to recombine radiatively at positions far from structural defects. Finally, we demonstrate that the molecular orbital pattern visible in electroluminescence maps enables an unambiguous discrimination between the intrinsic radiative recombination of electron-hole pairs in the organic film and the technique-related emission of tip-induced plasmons. This capability is essential to consolidate STM as a tool to explore the light generation from organic films on the nanoscale.

Bimodal exciton-plasmon light sources controlled by local charge carrier injection

“…Solid C 60 is a wide-bandgap organic semiconductor, whose valence band (VB) and conduction band (CB) are derived from the lowest unoccupied molecular orbital (LUMO) and the highest occupied molecular orbital (HOMO), respectively. For films with five (or more) monolayers grown on coinage metal surfaces, applying a voltage bias, U , below −2.7 V induces electroluminescence in the visible and near-infrared range ( 22 – 24 ). This minimum thickness ensures that the top layers of C 60 being probed are decoupled from the metal.…”

“…The highly localized sharp spectral line (see yellow spectrum in Fig. 1B ) originates from exciton recombination in structural and orientational crystal defects in the C 60 film (see the Supplementary Materials) ( 22 ). Only charge injection near these defects results in charge trapping and exciton formation ( 20 , 25 ).…”

Single Charge and Exciton Dynamics Probed by Molecular-Scale-Induced Electroluminescence

“…These emission centres (ECs) are spatially confined regions that contain both charge and exciton trap states 23,24 and are single photon emitters 22 . By targeting a single exciton system with TR-STML, we show how this powerful technique reveals the details of exciton formation and decay.…”

“…The particular EC defect shown in Fig. 2 is buried subsurface 23 and lacks any detectable electronic in-gap states (see Supplementary Fig. S2) which would otherwise be observable in scanning tunnelling spectra if a defect were located directly at the surface 24 .…”

“…Otherwise, the potential drop within the C 60 film would be insufficient to shift both the LUMO-derived states and the trap to the Fermi energy of the substrate (E F,s ) and enable electron injection from the substrate even after hole trapping as shown in Fig. 2e 23 .…”