Blueshifting the Absorption of a Small‐Molecule Donor and Using it as the Third Component to Achieve High‐Efficiency Ternary Organic Solar Cells

Adding a small‐molecule donor (SMD) to state‐of‐the‐art nonfullerene organic solar cells (OSCs) is demonstrated as a useful strategy to construct ternary organic solar cells, as SMDs typically have high crystallinity and can tune charge transport properties of OSCs. However, the absorption of most SMDs overlaps with typical donor polymers (e.g., PM6), which is against the general guidelines of adopting materials with complementary absorption in ternary OSCs. Herein, the absorption of state‐of‐art SMDs (BTR‐Cl) by linking the beta position of the outer thiophene to the alpha position of the inner thiophene unit is intentionally blueshifted. The resulting molecule β‐S1 shows a maximum absorption peak at 505 nm in the film state, which exhibits wider bandgap and shows complementary absorption with the host system (PM6:Y6). The corresponding ternary OSCs with 20%wt β‐S1 show significantly enhanced efficiency from 16.2% to 17.1% due to the increased short‐circuit current ( J SC ) and improved fill factor (FF). Herein, an effective strategy to design SMDs with both wider bandgaps and higher crystallinity for high‐performance ternary OSCs is presented.