Time and frequency domain optoelectronic techniques such as transient photovoltage/photocurrent spectroscopy (TPV/TPC) and electrochemical impedance spectroscopy (EIS) are often used to study transport and recombination in a range of solar cell architectures. Predominantly the techniques have been used to study organic based devices such as dye-sensitized, perovskite and bulk-heterojunction organic PV devices. It is common to use either time domain or frequency domain measurements, but not necessarily the two in conjunction.

In this work, we show that a more in depth understanding of device operation can be obtained when both time domain (TPV) and frequency domain (EIS) measurements are employed together. The two types of technique are in many ways equivalent, and similar information can be obtained from both. However, there are subtle differences between the two which enable certain processes to be studied in more detail using one or the other. In this particular example the techniques have been combined to study the effect the interlayer between the active layer and metal contact has upon OPV device stability.

The devices under test were based on the non-fullerene acceptor, ITIC, and the polymer donor, PCE10. The interlayers tested included Ca, LiF, PEIE and PFN. Accelerated aging under illumination revealed significant differences in terms of stability, with the most stable device containing the PEIE interlayer and the least stable containing LiF. TPV measurements were used to study the varying levels of degradation and revealed slight differences in terms of trap formation and recombination dynamics for the devices with different interlayers. However, EIS measurements showed more pronounce differences in terms of the resistance of the different interlayers upon aging. Although EIS measurements were performed at the open-circuit potential under illumination, a small amount of current flows during the measurement which means it can be used to probe charge transport throughout the entire device and not just recombination (predominantly in the active layer) as is the case for TPV measurements. Whilst recombination behaviour can be observed using EIS measurements, it is obscured by the charge transport through the interlayer. TPV/TPC measurements are therefore more suited to studying recombination and charge accumulation in the active layer. We show that the presence of two arcs in the EIS spectra are related to recombination in the active layer and charge transport through the interlayer. The interlayer is shown to get more resistive during degradation which is responsible for the main loss in device performance.