High-Performance Ternary Polymer Solar Cells by Blending Two "Y-Series" Acceptors with Complementary Absorption Bands from Near-Infrared Wavelengths to Medium Wavelengths and Enhancing Crystallinity

Ternary polymer solar cells (PSCs) consist of two structurally similar fullerene-free small molecules as acceptors (Y11 and Y5), and D18-Cl as polymer, which have better photovoltaic performance relative to D18-Cl:Y11 binary PSC. The enhanced photovoltaic performance of the binary PSCs is mainly attributed to the significantly enhanced short circuit current density (J(SC)) and slightly enhanced fill factor. The enhanced J(SC) value is due to the 45 nm blueshift of the absorption peak of Y5 relative to Y11, which facilitates more efficient absorption of photons in the visible light-wavelength band. Another reason for the enhanced J(SC) is the efficient charge transport and reduced exciton coupling. Meanwhile, the Y5 film has higher crystallinity and smaller lamellar spacing and pi-pi stacking distance relative to the Y11 film, and 35% of Y5 instead of Y11 is beneficial to improve the surface morphology, phase separation size, and crystallinity of the ternary photoactive layer. Therefore, the best ternary PSC shows higher open-circuit voltage values relative to the D18-Cl:Y6 binary reference PSC and higher JSC values relative to the D18-Cl:Y11 binary PSC. Thus, blend fullerene-free small molecules with similar molecular structures and different optical and electronic properties as acceptor materials are new insights for the development of ternary PSCs.