Energy level alignment at organic heterojunctions: Role of the charge neutrality level

We present a mechanism that explains the energy-level alignment at organic-organic (OO) semiconductor heterojunctions. Following our work on metal/organic interfaces, we extend the concepts of charge neutrality level (CNL) and induced density of interface states to OO interfaces, and propose that the energy-level alignment is driven by the alignment of the CNLs of the two organic semiconductors. The initial offset between the CNLs gives rise to a charge transfer across the interface, which induces an interface dipole and tends to align the CNLs. The initial CNL difference is reduced according to the screening factor $S$, a quantity related to the dielectric functions of the organic materials. Good quantitative agreement with experiment is found. Our model thus provides a simple and intuitive, yet general, explanation of the energy-level alignment at organic semiconductor heterojunctions.

Figure 1

(Color online) IDIS, CNL, and calculated interface $E_{\mathrm{F}}$ for $\mathrm{CuPc}∕\mathrm{Au}$, for a metal-molecule distance of $3.2\mathrm{\AA }$. Long (short) bars correspond to the $\pi \left(\sigma \right)$ states neglecting the metal-molecule interaction. The value of the initial Au work function, ${\varphi }_{\mathrm{M}}$, taken to be $5.1\mathrm{eV}$, is indicated by a vertical line. The CNL and $E_{\mathrm{F}}$ are related by Eq. (1).

Figure 2

CNL alignment in organic heterojunctions: an interface dipole $\Delta =\left(1\text{−}S\right)$ ${\left(E_{\mathrm{CNL}}^1-E_{\mathrm{CNL}}^2\right)}_{initial}$ is induced, lowering the initial levels of ${\text{organic}}_2$ (dotted lines) and reducing ${\left(E_{\mathrm{CNL}}^1-E_{\mathrm{CNL}}^2\right)}_{initial}$ by a factor $S$.

Figure 3

Transitivity in OO heterojunctions: experimental results for $\mathrm{PTCDA}∕\alpha \text{−}\mathrm{NPD}∕{\mathrm{Alq}}_3∕\mathrm{PTCDA}$ (adapted from Ref. 21). The HOMO and LUMO are represented by black and gray bars, respectively. Interface dipoles, ionization energies, and peak-to-peak transport gaps are shown.