Band Bending in Conjugated Polymer Layers

We use the Kelvin probe method to study the energy-level alignment of four conjugated polymers deposited on various electrodes. Band bending is observed in all polymers when the substrate work function exceeds critical values. Through modeling, we show that the band bending is explained by charge transfer from the electrodes into a small density of states that extends several hundred meV into the band gap. The energetic spread of these states is correlated with charge-carrier mobilities, suggesting that the same states also govern charge transport in the bulk of these polymers.

Figure 1

Work function ${\Phi }_{\mathrm{OSC}}$ of conjugated polymers on various substrates with work function ${\Phi }_{\mathrm{sub}}$. Symbols: KP measurements; solid lines: simulated results with Gaussian model DOS. (a)–(d) vs polymer thickness; (e)–(h) ${\Phi }_{\mathrm{OSC}}$ on 75 nm polymer film (measurements interpolated) vs ${\Phi }_{\mathrm{sub}}$. Valence band onsets measured by UPS are 5.90, 6.10, 5.50, and 5.85 eV, respectively (see 13).

Figure 2

(a),(b) Solid lines: Fitted model DOS of F8BT according to 4 different scenarios described in the text. Dashed lines: PES valence spectrum of F8BT (scaled to fit the model DOS at onset). (a) Linear scale; (b) logarithmic scale. There is a significant fitting uncertainty in the position of the density of states distribution but a small uncertainty in the DOTS below the limit of PES sensitivity. Also shown (c) is the energy diagram of 75 nm F8BT on $\mathrm{Mo}{\mathrm{O}}_3$. Solid lines show the onsets of the HOMO- and LUMO-derived bands (black curves) (HOMO onset energy from UPS; LUMO onset from HOMO onset plus optical absorption maximum) and best-fit Gaussian model DOS at four depths (red curves). The potential $V$ is calculated as described in the text.