Probing the Density of States of Organic and Perovskite Semiconductors By Energy-Resolved Electrochemical Impedance Spectroscopy

Abstract

The performance of thin-film photovoltaic cells is closely related to the energy alignment of the absorber layers and charge collection layers used for constructing the device and is further influenced by the presence of energetic disorder and defects in the photoactive material. This especially holds for novel organic and perovskite semiconductors, where film processing conditions, such as solvents used, thermal or solvent vapor annealing, and aging of the active materials are known to have an influence on the device performance. Knowledge about the energy of the relevant frontier orbitals such as HOMO and LUMO for organic semiconductors,[1]and valence (VB) and conduction (CB) bands for perovskites is key to designing more efficient materials. In addition, the energetic disorder, such as broadening of the band edges and states that appear in the bandgap are strongly affecting solar cell efficiencies. Knowledge about the density of states close to and inside the bandgap is very relevant in this respect.In this work energy-resolved electrochemical impedance spectroscopy (ER-EIS) as advanced by Nádazdyet al.[2]is applied on various organic bulk-heterojunction and lead-halide perovskite semiconductors to map their energy landscape around and in the band gap. ER-EIS provides a direct electrochemical measurement of the HOMO/VB and LUMO/CB energies, but also resolves band tails and sub-bandgap states with a resolution of up to six orders of magnitude. This enables detection of subtle but important effects that are the consequence of layer processing conditions such as spin coat parameters, solvent combinations, thermal annealing time and temperature, and atmospheric exposure (vacuum, inert, air).We will present experiments that aim to quantify relative changes in the energy landscape due to varying processing conditions and assign physical meaning to these changes (e.g. morphology and (un)intentional doping). The systems under investigation are well-known donor-acceptor bulk-heterojunction (e.g., P3HT:PCBM and PM6:Y6) and triple-cation mixed-halide perovskites (CsFAMA). The results shed new light on largely unknown defect states in the bandgap of solution-processed semiconductors.[1] R. E. M. Willems, C. H. L. Weijtens, X. de Vries, R. Coehoorn, and R. A. J. Janssen, Relating frontier orbital energies from voltammetry and photoelectron spectroscopy to the open-circuit voltage of organic solar cells, Adv. Energy Mater. 2019, 9, 1803677.[2] V. Nádazdy, F. Schauer, and K. Gmucová, Energy resolved electrochemical impedance spectroscopy for electronic structure mapping in organic semiconductors, Appl. Phys. Lett. 2014, 105, 142109.