Energy Level Alignment at Metal/Solution‐Processed Organic Semiconductor Interfaces

Abstract

Energy barriers between the metal Fermi energy and the molecular levels of organic semiconductor devoted to charge transport play a fundamental role in the performance of organic electronic devices. Typically, techniques such as electron photoemission spectroscopy, Kelvin probe measurements, and in-device hot-electron spectroscopy have been applied to study these interfacial energy barriers. However, so far there has not been any direct method available for the determination of energy barriers at metal interfaces with n-type polymeric semiconductors. This study measures and compares metal/solution-processed electron-transporting polymer interface energy barriers by in-device hot-electron spectroscopy and ultraviolet photoemission spectroscopy. It not only demonstrates in-device hot-electron spectroscopy as a direct and reliable technique for these studies but also brings it closer to technological applications by working ex situ under ambient conditions. Moreover, this study determines that the contamination layer coming from air exposure does not play any significant role on the energy barrier alignment for charge transport. The theoretical model developed for this work confirms all the experimental observations.

Engineering of the Back Contact between PCBM and Metal Electrode for Planar Perovskite Solar Cells with Enhanced Efficiency and Stability

“…After depositing PCBM, an upward vacuum level shift of 0.13 eV is observed as the WF increases to 5.03 eV, where the VBM of PCBM lies at 0.93 eV below E F . The WF of the bare Al electrode is 4.28 eV, which would create an energy barrier for injection into (bulk) transport states in the adjacent PCBM (the energy difference between the Fermi level of the electrode and the lowest unoccupied molecular orbital (LUMO) of the PCBM) . After inserting the thin Isatin and Isatin‐Cl coated on Al, the values of WF significantly reduce to 3.65 and 3.41 eV, respectively, which are below the LUMO level of PCBM and thus benefits the electron transport.…”

Interfaces of (Ultra)thin Polymer Films in Organic Electronics

“…Furthermore, as noted, they generally do not have the same formation energy as the bulk polarons do, mainly due to strong Coulombic interaction with the opposite charge across the interface (and sometimes different film order at the interface compared to the bulk, also affecting the polarization contribution to the polaron formation energy as noted). There is thus a distribution of these energies connected to specific polymer segments of varying size, orientation, local environment and distance to the substrate, and this interface polaron DOS (sometimes referred to as ICT DOS to highlight the contribution to the polarization energy from the transferred integer charge) typically will differ from the bulk polaron DOS . Hence it is not enough to use the bulk polaron formation energies (IP/EA) to estimate interface energy level alignment, measured (or calculated) values for the interface polaron formation energies are needed as well.…”

Metallated Macrocyclic Derivatives as a Hole – Transporting Materials for Perovskite Solar Cells

“…It should have low electron affinity which will help the electron travel from perovskite to metal electrode . In general, macrocyclic metal complex derivatives have been designed based on a Donor‐π‐Acceptor approach, in which the electron‐withdrawing groups (Acceptor) are substituted with electron donors groups like −NPh 3 , −NR 2 , −OR, −SR, Ar, R. With these substitutions of electron donors groups, the band alignment can be tuned w.r.t perovskite . Additionally, having an extra π‐bridge conjugation between two donors can also significantly change the electronic properties as well as steric geometry.…”