The design and operation of organic photovoltaic devices (OPVs) rely primarily on the electronic structure of organic semiconductors (OSCs). For instance, the dissociation of the exciton at the donor-acceptor (D-A) interface is determined by the energy level offset between the highest energy occupied molecular orbital (HOMO) and the lowest energy unoccupied orbital (LUMO) between the donor and the acceptor. Consequently, the measurement technique and its accuracy to determine the frontier molecular energies of OSCs are critical for designing efficient devices.

Several techniques, such as cyclic voltammetry (CV), ultraviolet photoelectron spectroscopy (UPS), low-energy inverted photoelectron spectroscopy (LE-IPES), or photoelectron spectroscopy in air (PESA), are used to measure the ionization energy (IE) and the electron affinity (EA) of OSCs. CV is more commonly used in OPV field for its accessibility and its simple operation. However, differences between the absolute energy levels measured by CV and photoelectron spectroscopy (PES) have been reported. The range of energy level values for identical material measured with CV shows the uncertainties and the inconsistency of the measurement protocol of this method. Moreover, direct measurement of the EA can be challenging when measured with CV, and indirect methods are used to estimate it by calculation using the IE and the optical gap. Since the optical gap been usually lower than the transport gap (difference between the IE and EA of a same material), that kind of approximation can lead to contradictory conclusion. Several high performing blends have been reported to have negligeable energy offset, but other studies are showing that an IE offset of 0.5eV is necessary for efficient charge transfer.

In this work, we highlight the differences in the IE and EA values for different commonly used OSCs between CV and UPS/LE-IPES measurements. The work on OPV devices made with different D-A blends shows correlation between the energetic properties of the pristine donor and non-fullerene acceptor (NFA) and the OPV devices’ parameters. When measured with PES methods, the photovoltaic gap Epv, difference between the IE of the donor and the EA of the acceptor, corelate better with the open-circuit voltage (Voc), compared to CV measurements. This led to a good path to predict the maximum Voc of OPV devices. A further study on blends like PM6:Y6 reported as high-performing blends with low IE offset, based on CV measurement, reveals a bigger energy offset when measured with PES techniques. In contrast, devices with low IE offset measured with PES methods demonstrate a non-efficient charge generation which can be observed with their low short-circuit current (Jsc). This may bring the community to rethink the behaviors and the correct designs of OPVs. This work establishes a solid base for reliably evaluating material energetics and can be relevant to a broader community working on OSCs for electronic applications.