Shelf Life Degradation of Bulk Heterojunction Solar Cells: Intrinsic Evolution of Charge Transfer Complex

Achievement of long‐term stability of organic photovoltaics is currently one of the major topics for this technology to reach maturity. Most of the techniques used to reveal degradation pathways are destructive and/or do not allow for real‐time measurements in operating devices. Here, three different, nondestructive techniques able to provide real‐time information, namely, film absorbance, capacitance–voltage (C–V), and impedance spectroscopy (IS), are combined over a period of 1 year using non‐accelerated intrinsic degradation conditions. It is discerned between chemical modifications in the active layer, physical processes taking place in the bulk of the blend from those at the active layer/contact interfaces. In particular, it is observed that during the ageing experiment, the main source for device performance degradation is the formation of donor–acceptor charge‐transfer complex (–) that acts as an exciton quencher. Generation of these radical species diminishes photocurrent and reduces open‐circuit voltage by the creation of electronic defect states. Conclusions extracted from absorption, C–V, and IS measurements will be further supported by a range of other techniques such as atomic force microscopy, X‐ray diffraction, and dark‐field imaging of scanning transmission electron microscopy on ultrathin cross‐sections.