The rising demand for energy generated from renewable resources makes the research on emerging photovoltaic technologies ever more important. Throughout the last years, organic solar cells became competitive with other solar cell techniques by reaching efficiencies higher than 19%. (reference) However, beside their good efficiency, the advantage of thin and lightweight devices and their applicability as semitransparent modules, they have still issues regarding the long-term stability. As several aspects regarding their stability are not fully investigated yet, this work focused on the thermal stability of organic solar cells with absorber layers based on the conjugated polymer PM6 and the non-fullerene acceptor Y6. Concerning this, the optical, morphological and photovoltaic properties of the bulk heterojunction blend at different annealing temperatures were investigated in addition to several long-term temperature stability tests.

We could demonstrate that a temperature treatment of 50 °C (for 1020 min) has almost no influence on those properties. The treatment at 100 °C (for 1020 min) decreased the photovoltage and the fill factor, whilst the short circuit current is hardly affected. This leads to a change of the PCE from 15.9% to 13.7%. Furthermore, the ratio of the hole and electron mobility was strongly affected and increased from 0.7 to 7.9. In the absorption and EQE spectra, a slight red-shift of the Y6 absorption was observed, which can be associated with a higher crystallinity in this phase. Moreover, long-term shelf life tests showed that the T₈₀ value was reached after 3600 h for cells stored at r.t. and below 200 h for cells under a constant temperature of 65 °C.

Based on the performed experiments, the main cause of the PCE reduction is a further crystallization of Y6 in the absorber layer. While we could not observe that the domain sizes of the Y6 phase in the bulk heterojunction becomes significantly larger, atomic force microscopy and grazing incidence small angle X-ray scattering indicated that the phase contrast between the donor and acceptor domains becomes higher. This leads to changes at the interface between the donor and acceptor phase. Furthermore, we assume that the percolation pathways in the Y6 phase are negatively affected because of the significant reduction in the electron mobility observed after the temperature treatment at 100 °C (for 1020 min).