Determining the Density of In-Gap States in Organic Semiconductors: A Pitfall of Photoelectron Yield Spectroscopy

Accurate determination of low-density electronic states in the band gap (in-gap states) is crucial for optimizing the performance of organic optoelectronic devices. Derivative photoelectron yield spectroscopy (PYS) is employed to estimate the density of states (DOS) of in-gap states. However, low-energy photons in PYS can generate excitons and anions in organic semiconductors, raising questions about whether derivative PYS spectra truly represent the DOS. We revealed that PYS signals originate from the single-quantum external photoelectron effect (SQEPE) of in-gap states, the SQEPE of the singly occupied molecular orbital (SOMO) of anions, and the biphotonic electron emission (BEE) effect via exciton fusion. Because BEE signals mask the DOS contribution, derivative PYS misestimates the DOS of in-gap states. In contrast, constant final state yield spectroscopy (CFS-YS) reliably determines the DOS by separating these components. For a tris(8-hydroxyquinoline) aluminum (Alq3) film, CFS-YS revealed the DOS of in-gap and SOMO states over 6 orders of magnitude, clarifying why the Alq3 layer works effectively in organic light-emitting diodes. In the devices, BEE can act as a degradation process, and CFS-YS is a useful method to also probe it. We provide practical guidelines for low-energy photon measurements for DOS determination, such as measurements of photon-flux dependency.