Extended Data Figure 7: Apparent work function evaluated by in situ built-in potential measurements.
From: Doped polymer semiconductors with ultrahigh and ultralow work functions for ohmic contacts
a, Stark-effect electro-absorption spectra of hole-only diodes with p-doped PEDT:PSSH (left), P2 (middle) and P4 (right) as hole contacts, PFOP as the organic semiconductor and Al as the electron-blocking contact. Normalized modulated reflection spectra (ΔR/R) are shown for different d.c. biases Vdc applied to the hole contact, where ΔR is the modulated root-mean-square reflectance in-phase with the forward-bias half-cycle and R is the d.c. reflectance33. Negative ΔR/R therefore indicates induced absorption in-phase with the forward-bias half-cycle. The dotted line gives the null background. Temperature, 30 K; modulation frequency, 535 Hz; root-mean-square voltage Vrms, 0.5 V. b, Apparent energy-level alignment diagrams deduced from the built-in potential Vbi, assuming rigid alignment at the electron contact. Energies are plotted relative to the vacuum level (VL) of the organic semiconductor. The Stark band at 2.87 eV near the π–π* absorption edge of PFOP shows polarity inversion at 1.70 V for PEDT:PSSH, at 2.35 V for P2 and at 2.4 V for P4. Estimated uncertainty, ±0.03 V. Consequently, the apparent work functions are: PEDT:PSSH, 3.4 + 1.7 = 5.1 eV; P2, 3.4 + 2.35 = 5.75 eV; P4, 3.4 + 2.4 = 5.8 eV. In reality, the effective work function at the electron contact decreases in response to an increase in work function at the hole contact, driving it into the pinned regime. This then gives rise to pinning at about 5.6 eV. The apparent work functions therefore confirm that the work functions of P2 and P4 are both larger than 5.6 eV, with the work function of P4 greater than that of P2.
