The Impact of Benzothiadiazole on the Optoelectronic Performance of Polymer/PC71BM Blend Films and Their Application in NIR Phototransistors

Abstract D‐A conjugated polymers P1 and P2 are synthesized by copolymerizing bisthiophene‐fused diketopyrrolopyrrole (4,11‐bis(2‐decyltetradecyl)‐7H,14H‐thieno[3',2':7,8]‐indolizino[2,1‐a]thieno[3,2‐g]indolizine‐7,14‐dione, BTI) with terthiophene and thiophene‐benzothiadiazole‐thiophene, respectively. Molecular packing is characterized by X‐ray diffraction. The film morphology is characterized by atomic force and scanning electron microscopy. The vertical phase separation is characterized by XPS. The mechanism of film morphology formation is explained from intermolecular interaction, kinetics, and surface free energy. The spin‐coated films of polymers blended with PC71BM are served as semiconductor layers in near‐infrared (NIR) phototransistors. The impact of morphology on optoelectronic performance is analyzed. The mobility of organic field effect transistors based on P1 or P2 thin films remains constant with the addition of PC 71 BM with weight ratio being less than 50%. The main reason is that π–π stacking in both P1 and P2 thin films is at all directions and PC 71 BM does not disturb the charge percolation path. Benzothiadiazole in P2 enables stronger mutual attraction between P2 and PC 71 BM, and therefore large contact area between P2 and PC 71 BM forms in the heterojunction. The photoresponsivity improves by 2 times for NIR phototransistors based on P2/PC 71 BM when decreasing light intensity from 63 to 1.8 µ W cm ‐2 . The above results suggest a feasible conjugated polymer design rule for high sensitivity NIR phototransistors.