Uncovering the Electronic State Interplay at Metal Oxide Electron Transport Layer/Nonfullerene Acceptor Interfaces in Stable Organic Photovoltaic Devices

The implementation of sputter-deposited TiO_x as an electron transport layer in nonfullerene acceptor-based organic photovoltaics has been shown to significantly increase the long-term stability of devices compared to conventional solution-processed ZnO due to a decreased photocatalytic activity of the sputtered TiO_x. In this work, we utilize synchrotron-based photoemission and absorption spectroscopies to investigate the interface between the electron transport layer, TiO_x prepared by magnetron sputtering, and the nonfullerene acceptor, ITIC, prepared in situ by spray deposition to study the electronic state interplay and defect states at this interface. This is used to unveil the mechanisms behind the decreased photocatalytic activity of the sputter-deposited TiO_x and thus also the increased stability of the organic solar cell devices. The results have been compared to similar measurements on anatase TiO_x since anatase TiO_x is known to have a strong photocatalytic activity. We show that the deposition of ITIC on top of the sputter-deposited TiO_x results in an oxidation of Ti³⁺ species in the TiO_x and leads to the emergence of a new O 1s peak that can be attributed to the oxygen in ITIC. In addition, increasing the thickness of ITIC on TiO_x leads to a shift in the O 1s and C 1s core levels toward higher binding energies, which is consistent with electron transfer at the interface. Resonant photoemission at the Ti L-edge shows that oxygen vacancies in sputtered TiO_x lie mostly in the surface region, which contrasts the anatase TiO_x where an equal distribution between surface and subsurface oxygen vacancies is observed. Furthermore, it is shown that the subsurface oxygen vacancies in sputtered TiO_x are strongly reduced after ITIC deposition, which can reduce the photocatalytic activity of the oxide, while the oxygen vacancies in model anatase TiO_x are not affected upon ITIC deposition. This difference can explain the inferior photocatalytic activity of the sputter-deposited TiO_x and thus also the increased stability of devices with sputter-deposited TiO_x used as an electron transport layer.