Recently, monolayers of carbazole derivatives have emerged as a promising replacement for the conventional polymer hole collecting layer materials such as PTAA and PEDOT:PSS in the inverted structure perovskite solar cells[1]. The hole collecting monolayer (HCM) has advantages in electric conductivity, optical transparency and device stability.

 The orientation of the HCM molecules is essential to achieve high hole collection efficiency. Since the HCM molecule has a permanent dipole moment, the molecular orientation affects the energy levels. This also affects the orbital overlap between the p orbitals of HCM molecules responsible for hole correction and the perovskite layer. Further, the orientation of the HCM molecules changes surface free energy leading to the morphology of the perovskite films formed on it. In fact, molecules with extended p-conjugated backbone and those with three anchor groups that bind to the substrate, facilitating parallel alignment, have been developed[2],[3].

 However, it is difficult to determine the molecular orientation on the conductive metal oxide electrode (ITO) due to its roughness. For example, spectroscopic ellipsometry, near edge X-ray absorption fine structure (NEXAFS) and multiple-angle incident resolution spectrometry provide only information on the molecular orientation with respect to the direction of light incidence, rather than the molecular orientation with respect to the inclined surface of the ITO.

 In this study, we measured the molecular orientations of HCM molecules on the ITO substrates using the ultraviolet photoelectron spectroscopy (UPS) and metastable atom electron spectroscopy (MAES). UPS is a standard technique to examine the valence electronic structure, where the electrons are excited by ultraviolet photons and kinetic energy of photoelectrons is analyzed. The probing depth of UPS is a few molecular layers. If the excitation source is replaced by a metastable helium atom, only the outermost molecular orbitals are detected, i.e. MAES is extremely surface sensitive. By comparing the UPS and MAES spectra, we can determine the molecular orientations.  

 We applied UPS/MAES to 2PACz and MeO-2PACz which have one anchor group to bind to the ITO substrate, and 3PATAT-C3[3], which has three anchor groups. At first, we assigned the observed peaks with the aid of the density functional theory calculation. Then, by comparing the spectra peak intensities of the UPS and MAES spectra, we determined the molecular orientations. If the molecules adopt the parallel orientation, the peak of p orbitals are strongly observed in MAES. If the molecules orientate perpendicular to the substrates, the s orbitals are strongly observed in MAES. From the analysis, we determined that 2PACz is tilted, MeO-2PACz is perpendicular and 3PATAT-C3 is parallel to the electrode. In this study, we demonstrate that the combination of UPS and MAES is a powerful technique to determine the molecular orientation on the conductive oxide electrode that could not be detected by other spectroscopic techniques due to the surface roughness.

Although HCM molecules with functional groups designed to alter molecular orientation for enhanced orbital overlap and modulation of surface free energy have been developed, precise measurements have not been achieved, and no definitive evidence linking these characteristics to improvements in power conversion efficiency has been established.