Correlation between electronic and optical properties of materials used in printed organic solar cells (Project 7)

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In this project we apply photoemission spectroscopy (PES) and inverse photoemission spectroscopy (IPES) as well as spectroscopic ellipsometry (SE) for the determination of electronic and optical properties, respectively, of neat non-fullerene acceptor (NFA), donor, and multiblock co-oligomers layers and donor-acceptor blends as-prepared and as a function of various ageing processes. PES and IPES deliver information on the densities of occupied states and unoccupied states, respectively, from which valuable parameters such as the work function, the ionization energy, the electron affinity, and thus the transport or single particle bandgap can be derived. Ellipsometry spectra, on the other hand, are defined by the joint density of states and provide the dielectric function respectively the optical constants, from which the optical bandgap can be derived. Moreover, SE can be employed to determine thin film thicknesses, composition and compositional gradients, optical anisotropies induced by molecular orientation, surface and interface roughnesses, as well as horizontal and vertical inhomogeneities. There are three research foci in this project, namely the investigations of a) neat NFA, donor, and multiblock co-oligomers layers, b) donor-acceptor blends, and c) the influence of degradation/ageing processes. Regarding neat layers we study available NFAs, donors, and novel NFA-containing multiblock co-oligomers synthesized within the Research Unit. The parameters obtained serve as reference data and as input for the device modelling within the Research Unit. The second focus lies on donor-acceptor blends and PES/IPES in combination with SE deliver the same valuable parameters as for neat layers. However, the role of SE becomes more important as it can be employed to investigate the morphology by using effective medium approximation (EMA) approaches. EMA modelling delivers information on the intermixing of donors and acceptors and phase separation. Approaches beyond the common EMA approaches are addressed to detect interaction-induced effects in both spin-coated and printed blends as a function of processing parameters. SE is further employed to investigate inhomogeneities of blended layers both lateral and in depth. Finally, we focus on degradation/ageing processes of blended layers using mainly SE and its in situ monitoring capabilities.