Research ArticleCOMPUTER MODELING

Energy-level alignment at organic heterointerfaces

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Science Advances  27 Nov 2015:
Vol. 1, no. 10, e1501127
DOI: 10.1126/sciadv.1501127
  • Fig. 1 Schematic representation of multilayer organic optoelectronic devices.

    (A) In a typical OLED, hole (h+) and electron (e) injection layers promote the respective charge carriers to accumulate in the emission layer, where they form excitons, which recombine to emit light (wiggly line labeled ). The desired binding energies of the respective HOMOs and LUMOs are indicated by orange and blue horizontal lines as is the VL in black. The question marks and vertical arrows interrupting the VL highlight that significant interfacial shifts may occur upon contact between organic layers. (B) Same for a typical device architecture used in OPVCs. Excitons generated on illumination are separated into free charge carriers at the central organic heterojunctions and selectively extracted by the outer layers. (C) Schematic of an organic/organic interface, where VL alignment prevails because the substrate-imposed Fermi level EF (green) comes to lie well between the occupied (orange) and unoccupied (blue) manifold of electronic states in both semiconductors after mutual contact.

  • Fig. 2 Exemplary case study of the energy-level alignment at an organic heterojunction.

    (A) Initial situation before deposition of CuPc onto Alq3, with the Fermi level (EF = 2.6 eV) indicated by a green dashed line on the binding energy scale. The question mark breaking the VL highlights the possibility for shifts to occur upon completion of the organic heterojunction. (B) Schematic illustrating the Gaussian DOS assumed for the energy distributions of the HOMOs and LUMOs in each discretization interval Δz of both organic semiconductors. Throughout the remainder of the manuscript, respective boxes are drawn, with their upper and lower edges indicating the low and high binding energy onsets of these Gaussians, respectively. (C) Calculated evolution of the local potential energy, −eV(z), and HOMO/LUMO distributions in the completed organic heterojunction compared to the initial situation. Vertical arrows highlight respective changes. (D) Calculated charge-density (ρ) profile across the completed heterojunction. (E) Experimental vacuum-level changes (red crosses) compared to calculated endpoints −eV(d) for increasing thickness d of deposited CuPc (black dots connected with red line). Black lines show the calculated V(z) for different CuPc thicknesses d. (F) Experimental HOMO onsets of Alq3 (bottom) and CuPc (top) upon increasing CuPc coverage (red crosses). Calculated onsets across the entire heterostructure are indicated by orange lines for each CuPc thickness d, and the respective endpoints are connected by red lines.

  • Fig. 3 Numerically calculated energy-level alignment for organic type I heterojunctions.

    (A) The schematic on the very left shows the situation before charge equilibration across the heterostructure and indicates the Fermi-level (EF) positions for which results are shown in the following five panels. There, the top parts show the spatial evolution of the local potential energy −eV(z) as well as of the HOMO (orange) and LUMO (blue) distributions. The according charge-density (ρ) profiles are shown in the bottom panels, with blue shading indicating electron accumulation in LUMOs and orange shading signifying hole accumulation in HOMOs. (B) Same for the reversed stacking sequence. Note that there is, of course, a continuous evolution between the specific scenarios explicitly shown here, but that no qualitatively new cases arise even when the symmetry in the initial energy-level alignment is broken.

  • Fig. 4 Numerically calculated energy-level alignment for organic type II heterojunctions.

    (A) The schematic on the very left shows the situation before charge equilibration across the heterostructure and indicates the Fermi-level (EF) positions for which results are shown in the following five panels. There, the top parts show the spatial evolution of the local potential energy −eV(z) as well as of the HOMO (orange) and LUMO (blue) distributions. The according charge-density (ρ) profiles are shown in the bottom panels, with blue shading indicating electron accumulation in LUMOs and orange shading signifying hole accumulation in HOMOs. (B) Same for the reversed stacking sequence. Note that there is, of course, a continuous evolution between the specific scenarios explicitly shown here and in Fig. 3, but that no qualitatively new cases arise even when the band gaps of the two organic semiconductors differ.

  • Fig. 5 Numerically calculated energy-level alignment for organic type III heterojunctions.

    (A) The schematic on the very left shows the situation before charge equilibration across the heterostructure and indicates the Fermi-level (EF) positions for which results are shown in the following four panels. There, the top parts show the spatial evolution of the local potential energy −eV(z) as well as of the HOMO (orange) and LUMO (blue) distributions. The according charge-density (ρ) profiles are shown in the bottom panels, with blue shading indicating electron accumulation in LUMOs and orange shading signifying hole accumulation in HOMOs. (B) Same for the reversed stacking sequence. Note that there is, of course, a continuous evolution between the specific scenarios explicitly shown here as well as in Figs. 3 and 4, but that no qualitatively new cases arise even when the band gaps of the two organic semiconductors differ.

Supplementary Materials

  • Supplementary material for this article is available at http://advances.sciencemag.org/cgi/content/full/1/10/e1501127/DC1

    Text

    Fig. S1. Exemplary case study of vacuum-level alignment at an organic heterojunction.

    Fig. S2. Energy-level alignment for a more complex organic heterojunction.

    Fig. S3. Energy-level alignment for reversed deposition sequence.

    Fig. S4. Energy-level alignment for an extreme case of a type II heterojunction.

    Fig. S5. Energy-level alignment for a type II heterojunction of lying molecules.

    References (6266)

  • Supplementary Materials

    This PDF file includes:

    • Text
    • Fig. S1. Exemplary case study of vacuum-level alignment at an organic heterojunction.
    • Fig. S2. Energy-level alignment for a more complex organic heterojunction.
    • Fig. S3. Energy-level alignment for reversed deposition sequence.
    • Fig. S4. Energy-level alignment for an extreme case of a type II heterojunction.
    • Fig. S5. Energy-level alignment for a type II heterojunction of lying molecules.
    • References (62–66)

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