Determining the density of in-gap states in organic semiconductors: A pitfall of photoelectron yield spectroscopy

TL;DR

This work tackles the challenge of measuring the density of in-gap states in organic semiconductors, a key factor for device performance, by showing that conventional PYS and derivative analyses are confounded by BEE and anion-related SQEPE when using low-energy photons. The authors combine photon-energy dependent HS-UPS with high-kinetic-energy CFS-YS to disentangle the various photoemission channels and to reveal the true DOS, demonstrated on Alq3 and C60 films. They show that derivative PYS and standard CFS-YS can misrepresent the DOS due to BEE, while high-Ek CFS-YS reliably recovers an exponential in-gap DOS and the SOMO peak, and they quantify the anion SOMO and reorganization effects. The findings provide practical guidelines for DOS measurements and highlight BEE as a potential carrier-generation and degradation pathway in OLEDs and possibly solar cells, with implications for interpreting interfacial energetics and injection barriers.

Abstract

Accurate determination of low-density electronic states in the bandgap (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, 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 six orders of magnitude, clarifying why the Alq3 layer works effectively in organic light-emitting diodes. In the devices, BEE can act as carrier-generation and degradation processes, and CFS-YS can also probe it. We provide the practical guidelines of low-energy photon measurements for DOS determination, such as measurements of photon-flux dependency.

Figures (7)

• Figure 1: Photoelectron yield spectra of (a) C$_{60}$ and (b) Alq$_\mathrm{3}$ thin films. I and $\textit{I}_\mathrm{th}$ denote the ionization energy and the subthreshold of the photoelectron yield, respectively.
• Figure 2: (a) $h\nu$-dependent HS-UPS spectra of a C$_\mathrm{60}$ thin film for $h\nu = 8.2$--6.0 eV. Intensities are normalized to the peak value. (b) Plots of the onset kinetic energy versus photon energy from the $h\nu$-dependent HS-UPS spectra.
• Figure 3: (a) $h\nu$-dependent HS-UPS spectra of the Alq$_3$ thin film for $h\nu = 7.7$--2.3 eV. Intensities are normalized to the peak value. (b) Expanded view of the onset region for $h\nu = 5.2$--3.5 eV. (c) Plots of onset kinetic energy versus photon energy from the $h\nu$-dependent HS-UPS spectra.
• Figure 4: Comparison of the DOS determined by DPYS and CFS-YS for C$_{60}$ shown in (a) linear and (b) logarithmic scales. CFS-YS was performed at the peak of the secondary-electron cutoff ($E_\mathrm{k} = 0.20$ eV).
• Figure 5: Comparison of the DOS determined by DPYS and CFS-YS for Alq$_{3}$ in (a) linear and (b) logarithmic scales. CFS-YS was performed at different kinetic energies, $E_\mathrm{k} = 0.20$ and 1.74 eV. Peaks arising from biphotonic electron emission (BEE) are indicated by red arrows.