Influence of Moisture on the Energy-Level Alignment at the MoO₃/Organic Interfaces

Abstract

MoO 3 is widely used in polymer-based organic solar cells as an anode buffer layer because of its high workfunction and formation of a strong dipole at the MoO 3 /polymer interface facilitating charge transfer across the MoO 3 /polymer interface. In the present work, we show that exposure of the MoO 3 /polymer interface to moisture attracts water molecules to the interface via diffusion. Because of their own strong dipole, water molecules counter the dipole at the MoO 3 /polymer interface. As a consequence, the charge transfer across the MoO 3 /polymer will reduce and affect the charge transport across the interface. The outcome of this work thus suggests that it is critical to keep the MoO 3 /polymer interface moisture-free, which requires special precautions in device fabrications. The composition of the MoO 3 /P3HT:PC 61 BM interface is analyzed with X-ray photoelectron spectroscopy and the depth profiling technique, neutral impact collision ion scattering spectroscopy. The results show that the concentration of oxygen increases upon exposure but leaves the oxidation state of Mo unchanged. The valence electron spectroscopy technique shows that the dipole across the MoO 3 /P3HT:PC 61 BM interface decreases even for short-time exposure to atmosphere because of the diffusion of water molecules to the interface. The far-ranging consequences for organic electronic devices are discussed.

High Efficiency Perovskite‐Silicon Tandem Solar Cells: Effect of Surface Coating versus Bulk Incorporation of 2D Perovskite

“…The WF was taken by calculating the difference between the excitation energy of UV light, which was 21.22 eV used for UPS, and the peak cut‐off along the base line of the spectra. The value of valence band was determined by approximating the base line and the onset of valence electron peak of UPS with a linear curve and determining their intersections . The same procedure was applied for determining the value of conductive band upon the conduction electron peak of IPES.…”

“…The value of valence band was determined by approximating the base line and the onset of valence electron peak of UPS with a linear curve and determining their intersections. [43] The same procedure was applied for determining the value of conductive band upon the conduction electron peak of IPES. When quantifying the peaks of XPS spectra, the intensity of peaks of various elements were normalized with atomic sensitive factor.…”

Correlation of Changes in Electronic and Device Properties in Organic Photovoltaic with Exposure to Air

“…The method was applied to similar systems before. [ 6,10a ] The result from the procedure will allow reconstructing the energy level diagram at the MoO 3 /BHJ interface. We have applied two methods which show similar results.…”

“…As described in Refs. [ 6,10a ] these intermediate energy levels facilitate the transport of holes generated in the BHJ to the conduction band of the MoO 3 . The thickness for an optimum transfer of holes across the MoO 3 /BHJ interface is reached when the shifted BHJ energy levels are on the same energy level as the conduction band of the MoO 3 which in the present case is achieved with a MoO 3 thickness of ≈0.5 nm.…”

“…IPES [ 6,10b ] allows probing the energy states across a region of unoccupied electronic structure and determining the CBM/LUMO. The technique based on electron in/photon out mechanism and the detected photon emission can be linked to the kinetic energy of the electrons.…”