Jafar Khan1,Neha Chaturvedi1,George Harrison1,Dalaver Anjum1,2,Yuliar Firdaus1,Thomas Anthopoulos1,Stefaan Dewolf1,Frédéric Laquai1
King Abdullah University of Science and Technology1,Khalifa University of Science and Technology2
Jafar Khan1,Neha Chaturvedi1,George Harrison1,Dalaver Anjum1,2,Yuliar Firdaus1,Thomas Anthopoulos1,Stefaan Dewolf1,Frédéric Laquai1
King Abdullah University of Science and Technology1,Khalifa University of Science and Technology2
With the recent reports on power conversion efficiencies surpassing 18%, organic photovoltaic is reaching new heights and is considered as an encouraging contender to other co-existing technologies. The rapid progress is due to the design and emergence of variety of both novel donors and non-fullerene acceptors. Despite this, the necessity of the design combined with efficient systems and the photo-physical processes is still lacking, and also tracking the associated loss mechanisms in devices is still elusive. Herein, we focus on the impact of the ionization energy in selected non-fullerene acceptors IT-4F, NCBDT-4Cl, BTCIC, IEICO-4F, IT-DM and O-IDTBR paired with the PBDB-T-SF and PBDB-T-2Cl donors in bulk heterojunction solar cells. More specifically, the investigation of the impact of the energetic offset on photophysical parameters and losses will be carried out. With combination of time resolved spectroscopy and electro-optical measurements we infer that the PBDB-T-SF:IT-4F with the highest offset exhibit the highest photocurrent (22 mA/cm<sup>2</sup>) and fill factor (64%) due to significantly less geminate - and non-geminate recombination and field independent charge generation. In contrast, the systems with the lowest offset O-IDTBR and IEICO-4F paired with both donors yield lowest photocurrent and fill factor due to considerable geminate recombination, poor charge generation, carrier mobility, and substantial field dependent charge generation. In addition to this, the IT-DM based system, with an intermediate offset displays a photocurrent of 12 mA/cm<sup>2</sup> and a fill factor of 49% caused by substantial non-geminate recombination compared to the IT-4F based device. We specifically infer a good photoluminescence quenching but limited internal quantum efficiency in O-IDTBR and IT-DM based systems when paired with both donors. Our findings provide important performance-charge separation relations for the design of novel polymer:NFA systems.