Molecular Orientations and Hole-Collection Efficiency of Monolayer Materials for Hole-Transport Layer of Inverted Perovskite Solar Cells

For the hole collection layer of inverted structure perovskite solar cells, recently monolayers of carbazole derivatives [1] have been proposed to replace the polymer hole transport layer materials such as PTAA and PEDOT: PSS. The monolayers have the advantages of thin film, high transparency, and stability. Large orbital overlap and good interfacial energy alignment at the perovskite/hole collection layer interface are essential to obtain high hole transfer constants. To achieve the sizable orbital overlap, the π-conjugated backbone of the monolayer materials should be aligned face-on to the conductive metal oxide electrode (ITO). However, measuring the molecular orientation is usually difficult owing to the large surface roughness of ITO. For example, polarization-dependence of optical measurements such as ellipsometry gives averaged orientation.<br/>In this study, we employ ultraviolet photoelectron spectroscopy (UPS) and metastable atom electron spectroscopy (MAES) to examine the molecular orientation of monolayers. In UPS, the photoelectron is generated by the excitation of UV light which can penetrate the sample. On the other hand, MAES uses metastable excited atoms (He*) as the excitation source, which only interact with the outermost molecular orbital of the sample material. For example, for a molecule whose π-conjugated backbone is aligned face-on to the ITO surface, only the π orbitals are observed in the MAES spectrum while both pi and sigma orbitals are detected by UPS. The molecular orientation can be evaluated by comparing the UPS and MAES spectra. We chose 2PACz and MeO-2PACz, which have one anchor group to bind to the ITO substrate, and 3PATAT-C3, which has three anchor groups.<br/>The monolayers were prepared by spin-coating on ITO substrates, and the UPS and MAES spectra were measured. We used molecular orbital calculations (B3LYP/6-31G(d)) for the isolated molecules to assign features of UPS and MAES spectra. Comparing UPS and MAES spectra of 3PATAT-C3, the intensity of the σ orbital is weaker in MAES spectra than UPS spectra, whereas the π orbital is stronger in both. Therefore, the π-conjugated backbone of 3PATAT-C3 is aligned face-on to the ITO surface. The results of 2PACz and MeO-2PACz show that the π-conjugated backbone is tilted in 2PACz and edge-on alignment to the ITO surface in MeO-2PACz. The results are consistent with the higher hole transfer rate constant in 2PACz than MeO-2PACz obtained by time-resolved surface photovoltage [3]. To further discuss the hole-collection efficiency, we will report the energy level alignments at the perovskite-monolayer-ITO interfaces.<br/><br/>[1] V. Getautis, S. Albrecht, et al, <i>Adv. Energy Mater. </i>2018, 8, 1801892.<br/>[2] Minh Anh Truong, Aruto Akatsuka, Hiroyuki Yoshida, Atsushi Wakamiya, et al, <i>J. Am. Chem. Soc</i>. 2023, 145, 7528-7539.<br/>[3] Levine et al., <i>Joule.</i> 2021, 5, 2915–2933.