Energy level alignment and electronic properties at weakly-reactive interfaces for organic electronics
Physical Chemistry of Solids and Surfaces, Material Characterisation
Final Report Abstract
Most important scientific outcomes of the funded project: The applicant has experimentally assessed with an unprecedented level of detail the electronic structure of OSC based heterojunction. We notably revealed that charge transfer can generally occur even at weakly reactive interfaces with favorable energy level offsets before contact. In general, physisorbed charged molecular layers are actually composed of a mixture of charged and neutral molecules as determined by Fermi-Dirac statistics. The electronic structure of the charged molecules was found disturbed by the addition (removal) of an electron as a result of on-site Coulomb potential. The electronic levels of the neutral molecules in the vicinity of charged ones are also shifted in energy because of the inter-site Coulomb potential. This led to a revision of the “textbook” model of polarons embedded into a neutral matrix of molecules. The fundamental phenomena ruling the energy level alignment at weakly reactive OSC-based heterojunction is seemingly independent on the nature of the heterojunction (e.g. OSC/OSC/electrode heterojunction, OSC/Buffer layer/electrode heterojunction, etc.) and the energy level alignment was found driven by Fermi-Dirac statistics and electrostatics in all studied systems. Stronger interfacial interaction (e.g. OSC/electrode interface) can lead to broadening of the molecular states. This can favor the emergence of interface dipoles instead of band-bending as observed at weakly-reactive interfaces. We have directly assessed for the first time the exciton binding energy in 2D-TMDCs deposited on different substrates and have unraveled the electronic structure of a strong molecular acceptor adsorbed on WSe2 supported on a HOPG substrate. We have revealed that the n-type character of the studied HaPs (CH3NH3PbI3−xClx and CH3NH3PbI3) thin films is a surface feature due to the presence of donor levels at the surface. The amount of these donor levels increases over prolonged (white light) illumination time, at least in vacuum condition. The observed surface photovoltage suggests that the Fermi-level is close to mid-gap in the bulk of the freshly prepared HaPs. We have elucidated how the presence of these donor levels (surfaces states) determines the energy level alignment at strong electron acceptor/HaPs interfaces. We have further suggested that different amount of surfaces states might be a reason for the dissimilar energy level alignment reported at sHaPs/OSC interfaces studied by different groups. We have unraveled the physics governing the energy distribution of the SECO by experimentally and theoretically determining the electrostatic landscape above surfaces with defined patterns of Φ. All the above-mentioned fundamental findings will have important implications for rational design from an electronic point of view of OSC-based optoelectronic devices.
Publications
- Probing the energy levels in hole-doped molecular semiconductors. Materials Horizons 2, 427 (2015)
S. Winkler, P. Amsalem, J. Frisch, M. Oehzelt, G. Heimel, N. Koch
(See online at https://doi.org/10.1039/c5mh00023h) - The interface electronic properties of organic photovoltaic cells. Journal of Electron Spectroscopy and Related Phenomena 204, 177 (2015)
P. Amsalem, G. Heimel, M. Oehzelt, N. Koch
(See online at https://doi.org/10.1016/j.elspec.2015.02.012) - Impact of white light illumination on the electronic and chemical structures of mixed halide and single crystal perovskites. Advanced Optical Materials 5, 1700139 (2017)
F.‐S. Zu, P. Amsalem, I. Salzmann, R.‐B. Wang, M. Ralaiarisoa, S. Kowarik, S. Duhm, N. Koch
(See online at https://doi.org/10.1002/adom.201700139) - Reliable work function determination of multicomponent surfaces and interfaces: The role of electrostatic potentials in ultraviolet photoelectron spectroscopy. Advanced Materials Interfaces 4, 1700324 (2017)
T. Schultz, T. Lenz, N. Kotadiya, G. Heimel, G. Glasser, R. Berger, P.W.M. Blom, P. Amsalem, D. M de Leeuw, N. Koch
(See online at https://doi.org/10.1002/admi.201700324) - Direct determination of monolayer MoS2 and WSe2 exciton binding energies on insulating and metallic substrates. 2D Materials 5, 025003 (2018)
S. Park, N. Mutz, T. Schultz, S. Blumstengel, A. Han, A. Aljarb, L.-J. Li, E. J. W. List- Kratochvil, P. Amsalem, N. Koch
(See online at https://doi.org/10.1088/2053-1583/aaa4ca) - Experimental investigation on charge transfer between organic adsorbates and solid surfaces. Encyclopedia of Interfacial Chemistry: Surface Science and Electrochemistry 44, 50 (2018), edited by K. Wandelt (Elsevier, Oxford) (Chapter)
P. Amsalem, G. Heimel, N. Koch
(See online at https://dx.doi.org/10.1016/B978-0-12-409547-2.13035-5)
