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

Three small donor molecule materials (S1, S2, S3) 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 S2 using a thiophene π-bridge based on S1, the blue shift in ultraviolet absorption can be enhanced, the band gap and energy level can be reduced, the open circuit voltage (V_OC) can be increased to 0.75 V using the S2:Y6 device, and a power conversion efficiency (PCE) of 3% can be achieved. Also, after developing the device using Y6, S3 introduced the alkyl chain of thiophene π-bridge to S2, which improved the solubility of tiny donor molecules, achieved the maximum short-circuit current (J_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.