A'-Site Dipole Magnitude and Direction Dominate the Ionization Energy and Electron Affinity of Layered Metal-Halide Perovskites.

Layered metal halide perovskites (LHPs), often referred to as 2D HPs, are promising materials for developing optoelectronics due to their tunable optoelectronic properties and improved stability compared to nonlayered (3D) metal halide perovskites. For integration into electronic devices, it is critical to appropriately adjust the work function (WF) and transport energies of the LHPs to promote efficient charge transfer between materials in the device stack. The transport energies of LHPs can be modified by changing the A'-site cation structure, inorganic sheet thickness, and the metal cation or halide anion. Here, we investigate how the A'-site cation structure influences the WF, ionization energy (IE), and electron affinity (EA) of n = 1 Sn- and Pb-based LHPs with a series of ortho- and para-functionalized phenethylammonium (PEA) iodide derivatives. To accurately assign the IE and EA, we develop a fitting method where the instrumental broadening, σIB, in ultraviolet and low-energy inverse photoemission spectroscopy (UPS and LEIPS, respectively) is accounted for. Density functional theory calculations combined with UPS and LEIPS measurements show that the dipole magnitude and direction of the A'-site cation exert a dominant influence on the WF, IE, and EA. Here, the direction and magnitude of the dipole, as manipulated through the strength and position of the electron-withdrawing or -donating substituent on PEA, can tune the WF by up to 1.2 eV, the IE by up to 0.9 eV, and the EA by up to 1.2 eV. The crystal structures indicate that the Sn-I-Sn bond angles have a clear influence over the optical gap; however, the influence of these Sn-I-Sn bond angles on the transport energies is dwarfed by the effect of the A' dipole. These results provide insight into how to tune the WF and transport energies of LHPs for optoelectronic device integration.