Scalable Distributed Bragg Reflectors (DBR) for Enhanced Efficiency of Semi-Transparent Non-Fullerene Acceptor Based Organic Photovoltaics

The introduction of non-fullerene acceptors (NFA) has provided several recent record efficiencies in organic photovoltaic (OPV) cells, reaching now above 18%1 for single-junction devices. While these developments have provided a strong boost to the OPV field, more efforts have to be devoted to their application, such as their use in windows to lower the carbon footprint of buildings. To improve the performance of such semi-transparent NFA OPV, a highly reflective Bragg mirror can be formed to reflect selective parts of the sunlight spectrum that match specific parts of the absorption spectrum of the active layer, e.g. the contribution from the near-infrared absorbing NFA molecules. With the progress in the film formation using reactive sputtering, it is possible to tune the thickness, composition, transparency, and uniformity of alternating low and high refractive index oxide thin films, which is needed to form well-performing DBR stacks, making it an ideal technique for this application.

Here, recent progress in adjusting the reflectance of thin film oxide based DBR, e.g. by fine-tuning composition and thickness of the individual layers in order to match the absorption region of specific high performance non-fullerene acceptor molecules, as well as their integration in efficient semi-transparent NFA OPV devices with low visible transmission loss, is demonstrated. Supported by a variety of surface science characterization studies, the importance of the detailed thin film composition and microstructure on the optical properties2 and intrinsic stability of these DBR is discussed. To meet the requirements on scalable OPV development, the up-scaling of these new DBRs is discussed, considering recent results on industrially compatible OPV device development3,4. This includes Roll-to-Roll (R2R) processing of OPV cells and modules using combined solution and vacuum-based techniques including also the reactive sputtering process on R2R scale utilized for DBR development.