Kinetically Driven Near-Unity Charge Generation Yield in Organic Solar Cells

Recent advances in organic solar cell material development based around non-fullerene electron acceptors in bulk heterojunctions have propelled power conversion efficiencies to &gt;18%, with 20% on the horizon and 25% predicted. [1] These efficiencies are close to traditional inorganic semiconductor photovoltaics and thus focus is now turning to manufacturability and creating a viable solar cell technology. In these regards, achieving high power conversion efficiencies in so-called ‘thick junctions’ of order 300 nm and above is an acknowledged prerequisite for the use of high throughout printing techniques. This has proven extremely challenging to achieve with organic semiconductors due to their relatively poor transport properties and in particular photo-generated carrier bimolecular recombination. In this presentation we report the highest efficiency to date (16% with a Fill Factor &gt;70%) in a thick junction binary organic solar cell based upon the PM6:BTP-eC9 system. Using a very accurate approach based upon temperature dependent ultra-sensitive EQE measurements we find that this system (and a similar one based upon PM6:Y6) have near unity charge generation yields (CGY &gt; 99%). In this regime, we observe that a relatively small increase in CGY of only 0.5% leads to a 2.5 times reduction in bimolecular recombination relative to the Langevin limit – clearly demonstrating the subtle link between photo-generation and recombination in excitonic organic semiconductors. We observed a 1000-times-reduced recombination rate constant in the PM6:BTP-eC9 which delivers the superior thick junction performance than PM6:Y6. [2]<br/><br/>References:<br/>[1] Armin et al., “A History and Perspective of NonFullerene Electron Acceptors for Organic Solar Cells”. Advanced Energy Materials, 2003570, 2021.<br/>[2] Li et al., “Organic Solar Cells with Near-unity Charge Generation Yield” Energy & Environmental Science, 10.1039/d1ee01367j<b>, </b>2021.