08-Feb-2023: Talk by Jules T Bertrandie

On the 8th of February, Jules T Bertrandie a PhD in KAUST will give a talk entitled:
Determination of the energy levels of organic semiconductors in solar cells

The electronic structure of organic semiconductors (OSCs) is an essential parameter of the design of organic photovoltaics devices (OPVs). The accurate determination of the energy level of OSCs is crucial to understand underlying mechanisms such as dissociation mechanisms of exciton in OPVs. It is therefore critical to use methods and techniques which allow comparable results and accurate enough to favor correct interpretations of results.
This talk will give a brief introduction of different methods, such as cyclic voltammetry (CV), ultraviolet photoelectron spectroscopy (PES) or photoelectron spectroscopy in air (PESA), used to determine the energetics of OSCs. By comparing the measurement of those techniques with OPV performances, differences between the methods are highlighted. The most commonly technique used in the OPV field to measure the energy levels, CV, is accessible and have simple operation. However, with values obtained via CV, no clear correlation is found when compared with OPV device parameters. On the other hand, values obtained by ultra-violet photoelectron spectroscopy (UPS) and low energy inverted photoelectron spectroscopy (LE-IPES) show better correlations. For instance, the photovoltaics gap Epv, difference between the ionization energy (IE) of the donor and the electron affinity (EA) of the acceptor, correlate better with the open circuit voltage (Voc) when measured with PES compared to CV. Another aspect studied is the blends like PM6:Y6 which have been reported with a low IE offset when measured with CV. It the case of PES method, it not only shows that the reported efficient low IE offset blends with CV appear to have a bigger energy offset with PES values. But it also demonstrates a systematic non-efficient charge generation for devices with low IE offset measured by PES. Those observations can bring insight for the future designs of OPVs but also for a broader community working on OSCs.