Impact of Different π-Bridges on the Photovoltaic Performance of A-D-D′-D-A Small Molecule-Based Donors

Three small donor molecule materials (<b>S1</b>, <b>S2</b>, <b>S3</b>) based on dithiophene [2,3-d:2',3'-d']dithiophene [1,2-b:4,5-b']dithiophene (DTBDT) utilized in this study were synthesized using the Vilsmeier-Haack reaction, traditional Stille coupling, and Knoevenagel condensation. Then, a variety of characterization methods were applied to study the differences in optical properties and photovoltaic devices among the three. By synthesizing <b>S2</b> using a thiophene π-bridge based on <b>S1</b>, the blue shift in ultraviolet absorption can be enhanced, the band gap and energy level can be reduced, the open circuit voltage (<i>V</i>_OC) can be increased to 0.75 V using the <b>S2</b>:<b>Y6</b> device, and a power conversion efficiency (PCE) of 3% can be achieved. Also, after developing the device using <b>Y6</b>, <b>S3</b> introduced the alkyl chain of thiophene π-bridge to <b>S2</b>, which improved the solubility of tiny donor molecules, achieved the maximum short-circuit current (<i>J</i>_SC = 10.59 mA/cm²), filling factor (FF = 49.72%), and PCE (4.25%). Thus, a viable option for future design and synthesis of small donor molecule materials is to incorporate thiophene π-bridges into these materials, along with alkyl chains, in order to enhance the device's morphology and charge transfer behavior.